Journal Institution Locomotive Engineers
Volume 42 (1952)
|Steamindex home page Updated 2010-12-28||Key file||The IMechE virtual library is accessible (full papers, all diagrams, photographs, extensive tables, etc).at www.imeche.org.uk.|
Journal No. 225
Visit ro the Willans Works, the English Electric Company, Rugby, 8th April 1952. 28-30.
New electric rolling stock for the Indian Government Railways.
Fifth Ordinary General Meeting of the Session 1951-52 held at the Institution of Mechanical Engineers, London, on Wednesday 16 January 1952 at 5.30 p.m., Mr. J. S. Tritton, President, occupying the Chair. The President then introduced Mr. S. E. Lord (Member), Mr. J. F. Thring (Graduate) and Mr. H. H. C. Barton (Member), who presented their papers on '' New Steel Electric Railway Stock for the Indian Government Railways," which were afterwards discussed, and for which, on the motion of the President, a vote of thanks was accorded to them. Three Papers read :
Lord, S.E. (Paper No. 507)
A quarter of a century of progress in Indian electric stock. 32-43.
Thring, J.F. (Paper No. 508)
Structural design of lightweight steel coaches for Indian Government Railways. 44-58.
Of 112 coaches being supplied to the GIP and the HB&CI Railways, 56 were driving motor coaches being produced by Metropolitan- Cammell Carriage & Wagon Co.
Barton, H.H.C. (Paper No. 509)
New multiple unit rolling stock for India, operational performance and the electrical equipment. 58-68. Disc.: 69-79.
Outlines the character of Bombay's suburban rail traffic and some of the operating features. It describes the electrical equipments of the new multiple unit rolling stock manufactured in this country for the G.I.P., now the Indian Central Railway, and the B.B. & C.J.', now the Indian Western Railway. During WW2 this stock occasionally worked from Bombay over the 1 in 37 grades of the Western Ghats. Excluding the lines beyond Kalyan, but including the Harbour
Sir William Stanier, (70) said that the building of lightweight stock had been very much in his mind for d number of years. On the LMS in 1938, the Liverpool and Southport stock had had to be renewed, and he remembered the late Mr. Fairburn saying that for every ton that could be saved in the weight of the stock, he could save 210 a year in current. That had been an incentive to get some lightweight stock.
In the Derby drawing office there had been a very able young designer, Mr. Moon, who unfortunately died during the war. Mr. Moon had developed a design (taking advantage of the Vierendeel truss) for some lightweight stock which had given some very interesting figures. The motor coach seated 88 and weighed 40 ton 5 cwt. The trailer coach seated 102 and weighed 23 ton 2 cwt. Sir William had been able to give particulars of that construction and the means used to develop it in a paper which he had prepared for presentation to the American Society of Mechanical Engineers and the Institution of Mechanical Engineers' Joint Meeting in September, 1939; he had gone over to America to give the paper, but unfortunately the Conference had been cancelled and he had come back by the next boat. However, the paper had been printed and was in existence.
The intention had been that this principle should be developed for main'line stock, but unfortunately the war had come, and this hampered development.
Sir William emphasised the importance of remembering that weight,was a very important asset or liability when considering costs. The more lightweight stock was developed, the better the services that could be given. Lightweight stock would reduce wear and tear and it would reduce the power required; and, provided it was possible to look after the corrosion, to which Mr. Cock had referred, it would produce very much better stock than existed at present.
Journal No. 226
Marsh, G.C. (Paper No. 510)
Recent developments in vacuum brake equipment. 95-134. Disc.: 134-70. 48 figs.
Second Ordinary General Meeting of the Manchester Centre was held at Engineers Club, Manchester, on Wednesday 28 November 1951, the Chair being taken by D. Patrick.
British Railways had recently confirmed their adherence to the Vacuum Brake for all main line services, and adopted many of the latest devices then available on their new standard locomotives and carriages. Similarly many African and Indian Railways were in the process of modernising their Vacuum Brake Equipment, whilst the Republic of Indonesia had chosen the latest type of vacuum brake as standard on all the new locomotives and rolling stock now being built to rehabilitate the war devastated railways in that country. For many years before WW1, the vacuum auto brake remained almost unaltered in its original form except on the GWR where Churchward introduced many new features such as engine driven vacuum pumps and direct admission valves, some of which had spread to all Regions of British Railways. It was not until the 1930s when large freight locomotives were introduced in India and South Africa for hauling long vacuum fitted trains, that the question of adequate ejector capacity was seriously tackled and the common sense rule established that the size of the maintaining ejector must be related to haulage capacity, the potential leakage factor being obviously higher on longer trains. For long vacuum fitted freight trains the addition of automatic slack adjusters has contributed greatly to efficient braking, whilst the adoption of the lower working vacuum of 16 in. or 18 in. for freight working, coupled with the provision of more powerful ejectors, had enabled brake release times to be much reduced.
See Holcroft (140-2.) page 95 for long contribution by him on the development of vacuum pumps by Churchward and by T.G. Clayton on the Midland Railway. W.A. Tuplin (142-3); E.S. Beavor (144) wished for some form of automatic coupling to incorporarte the vacuum hose; K. Cantlie (146) noted that the only methods for creating vacuum on steam locomotives were the ejector and crosshead pump whereas rotary exhausters were successfully used on diesel and electric locomotives: a turbine might prove more economical in steam. Another experiment might be a vacuum pump similar to the Westinghouse cross-compound pump which he knew to be very economical in steam consumption
E.A. Langridge (160) remarked that many headaches would have been avoided if the Brakes Committee had decided to standardise on air-brakes instead of vacuum brakes at the 1923 amalgamation. He regretted that the fitting of the vacuum pump had been largely dropped. As a matter of interest, it was probably more used by the L.N.W.R. than by the G.W.R. and perhaps the former should have more credit for persistence in using it. The L.M.S. also adopted the pump, but in conjunction with a double ejector and separate drivers valve, and it was easy then to make out a case for its elimination on the basis of excessive equipment causing undue maintenance costs.
He mentioned a further development in the design of brake valves not mentioned by the Author. This took place immediately following the above arrangement when Stanier was appointed C.M.E. of the L.M.S. Stanier designed a single handle operation brake valve in which was incorporated the disc valve and steam brake portion of the combination fitting. This was placed on the back of the firebox with a single ejector outside the cab front and the vacuum pump was crosshead driven. This made a neat arrangement requiring the minimum number of fittings and drivers controls and eliminating much piping and joints such as those described by the Author. In his latest double vacuum system, the Author had in fact returned to single handle operation and the scheme looked most attractive.
Journal No. 227
Harrison, J.F. (Paper 511)
The application of welding to locomotive boiler copper fireboxes. 178-214. Disc.: 214-22.
Forty-First Annual General Meeting of the Institution was held at the Institution of Mechanical Engineers, on Wednesday 19 March 1952, at 5.30 pm., Mr. J. S. Tri!ton, President, occupying the Chair.
The repair and construction of locomotive copper fireboxes by welding is not a recent development, since simple repair work of this kind was being carried out in Germany in 1916, and by the early 1920s more elaborate work, such as the insertion of patches, the repair of tubeplates, and the insertion of half or three-quarter sides, had been successfully accomplished. By 1925 several all-welded copper fireboxes had also been built. In the UK it was not until 1927 that the repair and manufacture of new copper fireboxes by the oxy-acetylene welding process had become an established practice, the Great Western Railway being the first in the field followed some time later by the London Midland and Scottish Railway.
On the London and North Eastern Railway, Great Central Section, the cost of firebox repairs was exceptionally heavy, frequent copper stay and plate renewals being necessary owing to the very bad water conditions met with in that region, possibly the most damaging to boilers in Britain. During the war years great difficulty had been experienced in obtaining the necessary copper plates for renewals, so the Author, then Mechanical Engineer, Gorton, originated at those works serious investigation into the possibilities of effecting repairs to copper fireboxes by the oxy-,acetylene welding process. In 1943 a Technical Assistant, who was a specialist in welding, was engaged to carry out the necessary deve!opmcnt work under the jurisdiction of the Author.
The first objective was to ensuire that any welds produced would have a tensile strength equal to that of the parent plate and successfully bend through 180 degrees when hot. To reach this standard approximately six months of experimental work was found necessary.
Following this stage certain welding repairs were carried out on fireboxes, such as building up wasted radii in tube and doorplate flanges, and in December 1944 a further experiment was carried out by fitting to a Diagram 15 boiler (O4 class 2-8-0 Freight Locomotive) two new lower half wrapper sides by welding. It is of interest to note in passing that this firebox gave a further 4½ years of life, which equals approximately 60 per cent increased life an average firebox. From 1945 onwards a considerable number of half or three-quarter copper sides were fitted and repaircd by welding. These experiments were further extended by the insertion of new plate in and around firehole mouth pieces, the insertion of pieces in tube and doorplate flanges (this latter development being considered from experience a better repair than the building up of worm plate edge laps), patches let into the wrapper sides (lower half) were developed where it was considered to be more economical than the fitting of a complete new half plate, the reinforcing of the radius of firehole flanges of the solid ring type doorplate, the welding of fractures in the tubeplate crowns from both sides simultaneously, and most recently the development of sealing the copper stays in the lower portion of the firebox wrappers.
T. Henry Turner. (202-3.) said that as welding was a borderline subject between metallurgy and engineering, it might be as well to have the metallurgical point of view. Twenty-five years ago all the British railways were using tough pitch arsenical copper for fireboxes, and then the Great Western introduced de-oxidised copper. When Mr. Turner heard that he passed on the information to Sir Nigel Gresley who was not then interested in firebox copper welding. However, Sir Nigel had the foresight to change the specification and introduce the use of de-oxidised arsenical copper so that welding could be done in the future if desired. Thus, when the Author was moved to do something about copper firebox welding, there was a fair quantity of de-oxidised arsenical copper upon which to start. The subject of the paper deserved more consideration by locomotive engineers in this country than elsewhere, because of the many thousands of copper fireboxes in use as compared with the preponderance of steel fireboxes in a number of countries, and the use of steel in stationary and marine boilers.
The Author dealt mainly with his practical development work at Gorton. There was no doubt that had it not been for the Author's initiative, very little copper welding would have been done on the LNER. Mr. Turner had looked into the literature, and submitted a short bibliography on the subject which was not without interest, because it started off forty years ago. At that time, when the Institute of Metals was very young, an Italian, Dr. Carnevali, read a paper in which he spoke of his experiments in the oxy-acetylene welding of copper. In those days the difficulties of welding copper were very great because the great thermal conductivity of copper made it nearly impossible to get the intensity of heat needed in the locality of the weld; the copper absorbed gases readily at high temperatures, and copper oxidised and dissolved its own oxides at high temperatures. With the gassed and overheated copper welds then produced, hammering was sometimes of no use at all, and the resultant welds were full of cracks and blowholes.
What factors of the welding of copper had changed to make the Authors paper of practical value whereas Dr. Carnevali only warned of difficulties?
Firstly as regards the nature of the oxy-acetylene flame; this was now controllable, and if a reducing flame were played on to copper which contained oxide, serious cracking occurred inside the copper, quite unrelated to external stresses. Cracking did not occur, however, with a neutral flame and de-oxidised copper, i.e. one to which phosphorus had been added and of which a residue remained. That lesson was driven home to the speaker by the cracking of almost all the copper cable bonds welded to rails in the Manchester-Sheffield electrification when they were first applied. The cracks burst inside the copper, due to the effect of the hydrogen in the flame and the oxygen in the copper, producing steam under explosive conditions.
Secondly as regards the different types of copper; Dr. Cook read a paper to the Institution in 1938 recording tests on four different types of copper, and if we extract from them the tensile properties at elevated temperatures we may produce a curve of the type shown in _. Fig. 19. It is important to note that all four varieties suffered a similar loss of tensile strength from nearly 15 tons at room temperatures down to below 2 tons/sq. in. at 800°C.
Thirdly as regards the welding rod there had been a development in the introduction of the silver content and a trace of phosphorus from the metallurgical point of view; but there was also an engineering development in the direction of better welding tools. The welding tools were not available in the early days, and the system of training welders had been much improved. It would be interesting to know whether the Author had carried out any practical trials with annular flames for welding rivets.
The inspection ot welds had been helped by the introduction of X-rays which permitted examination below the surface of welds, without destructive testing. However, he agreed with a previous speaker who said that it was not possible to see many little cracks inside the copper by means of X-rays. When he examined one of Mr. Lockleys prototype welds made at Gorton three or more years ago, it was necessary to cut sections from it to find that the welding process had not affected the firebox copper. That method of examination ruined the weld even if it did teach the nature of the metal, so X-rays were of help and the K.E. Research Departments (Metallurgy Division) mobile X-ray coach had been used recently to examine non-destructively welds in copper fireboxes. The carbon arc welding system had been applied to copper using high amperage, say 400 amps and a high voltage of say 50 volts, and Mr. Chaffee in his paper wrote No metal can be fabricated more rapidly or at a lower cost than can copper by this method. . . . So far he understood that method had not been applied to locomotive fireboxes; it might be worth investigating in the workshops.
A short bibliography on copper welding for fireboxes (13 entries)
Meeting of North Eastern Centre at Danum Hotel, Doncaster on 27 March 1952: T. Matthewson-Dick in the Chair.
T. Matthewson-Dick (209) asked about the method of testing the welded seam adding that he knew that to prove the weld equal to the strength of the plate the 180° bend test was usually made, but it was not clear why this vicious test was necessary to prove equality of strength. He asked if there was a definite syllabus for the training of staff in the use of oxy-acetylene torches for copper welding. To indicate the degree of training success he asked what was the expectation of failures in, say, every five men given trajning.
Meeting at Midlands Hotel, Derby on 2 April 1952: M.S. Hatchell in Chair.
C.S. Cocks (210-11) said it was very encouraging to find an engineer who had the courage of his convictions, and having been faced with an unsatisfactory condition, did something about it to improve it, thereby progressing in engineering.
He felt that whilst they were wedded to copper fireboxes, it was imperative to prevent copper stays from leaking. It was perhaps unfortunate that steel fireboxes were not generally adopted, as it would then not be so difficult to perform the welding operation. In connection with the fitting of the copper stays, he asked if they were riveted over on the outside of the steel wrapper before being welded on the inner firebox, or afterwards, since if they were riveted before welding, it would be necessary to leave the stay projecting on the inside of the firebox; it would also be interesting to know how much was the projection.
In view of the Author saying that the copper weld had the equivalent strength of the parent metal, he asked if the ultimate aim was to have a completely welded stay placing complete reliance on the 100 per cent weld.
The Author had said that heat had a lot to do with the question of welding. In view of this Mr. Cocks asked why such a wide angle was used for the weld deposit. The aim surely should be to make the angle of weld as small, as was reasonably possible, less heat would then be needed and less metal deposited.
He asked if 3/16 in. gap per foot run of weld was required, since if the weld was 10 ft. long there would be a gap of 17/8 in. at one end. He thought it was quite clear from the Paper that welds followed each other and there was no question of step back welding being used. Regarding marking off, he suggested it might be better to make a simple template of the staying of the portion of the box in which the stays were to be welded. While the method used by the Author of placing centre pops in convenient places to enable the re-marking of the plate to be done without much difficulty, this method could only be followed where the stays followed a regular pattern, but was of no use for the stays at the forward end of a firebox with a sloping throat plate. In such cases the stays at the forward end followed no regular pattern, but were pitched at regular intervals to stays in the plate between the lap joint and the part of the firebox where regular pattern stays commenced. It seemed to him in either case the job would be more simply and cheaply done by using a suitable template, as such templates could be made to drawing and checked as necessary against each individual firebox.
Fell, L.F.R. (Paper 512)
The Fell diesel mechanical locomotive. 223-71.
Eighth Ordinary General Meeting of the Session 1951-52 was held at the Institution of Mechanical Engineers, Storeys Gate, London, on Wednesday, 16 April 1952, at 5.30 p.m., Mr. Julian S. Tritton occupying the Chair.
This precis is based upon material recorded by Rutherford in Backtrack, 2008, 22, 238 et seq. wherein he noted that "Surprisingly extensive drawing office and technical support manpower was expended on the 2,000hp Fell 4-8-4 diesel-mechanical. In this paper Fell revealed that not only was the wheel arrangement and layout of the locomotive decided by British Railways engineers but that Derby drawing office and works was responsible for the complete design and manufacture of the machine". Fell stated, "The wheel arrangement of 10100 was selected by British Railways as being the most suitable for their purpose, involving the simplest possible arrangement of this transmission, but it was by no means the only possible arrangment." Further, "Dealing with the suggestion that the transmission was a bought out part, Fell pointed out that the whole of the gearbox was made by British Railways. They designed it, made the original drawings, made all the patterns, cast it, machined it themselves. All they did not do was to cut and grind the gears. This was one of the claims in favour of the system - that the steam locomotive men could make the main item of the transmission instead of having to buy the whole thing out. The whole of the control gear was also made by British Railways - drawn by them and made by them, which was very different from the case where electricity was employed." Finally, "... this was British Railways' very own diesel mechanical express locomotive. They had designed it from a clean sheet of paper and had built it.
Journal No. 228
Cock, C.M. [Presidential Address]
Motive power for railways. 281-305.
Delivered to the Institution in London on the 24 September.
"Even today the steam locomotive suffers chronically, as it has done for a century and a quarter, from an unfortunate inability to digest the full substance of its calorific food. Aggravated by a large appetite, this incurable indigestion leads to other ancillary ills in processes of repairing, fuelling, watering and general servicing, and realising all this, railwaymen are seeking for other forms of traction having better qualities in thermal efficiency and availability, always however, with a wary eye on the cost."
Practical alternatives to the steam locomotive, not necessarily in order of precedence:
(1) Self propelled rail cars.
(3) The diesel-electric locomotive.
(4) The gas turbine locomotive
Electrification at 50 cycles: Mercury arc rectifiers: Morecambe/Heysham-Lancaster trial about to start. Mentions Aix-les-Bains to La Roche-sur-Foron in France and even earlier system in Germany (1936) between Freiberg and Seebrugg. "For various reasons, including economic consideration, the British Transport Commission had accepted the 1,500 volt d.c. system as standard for British Railways but the 50 cycles system has not been ruled out for electrification of secondary lines with light traffic" Includes the operating costs of diesel locomotives in the USA.
The Locomotive Railway Carriage & Wagon Review published an extended abstract which follows: His remarks mainly concerned some alternatives to the reciprocating steam locomotive and he ventured to assume, in view of his long association with the alternatives, that members would have been disappointed if he had not addressed them on that basis. The President explained that a very careful assess- ment is necessary to determine real values of traction in respect of cost, and efficient and reliable movement of traffic. The assessment must also take into account the suitability of the tractor to the particular territory. In the President's opinion there is a good deal of misconception regarding railcars, probably arising from a belief that they are mere 'buses on a railway. The fact is that important developments have been made in diesel-powered units in recent years and they have become firmly established in many countries. Multiple unit sets, in a sense, are something between electrification and the ordinary steam hauled passenger train and it was stated that there seemed to be great scope on British Railways for this kind of development.
Although on equation the electric locomotive is easily the most powerful and efficient of all types of locomotive, and the cheapest to maintain, the cost and characteristics of the locomotive itself cannot be excepted in any fair comparison with other forms of traction. Unlike steam and other locomotives it does not contain a prime mover so that a high-priced fixed installation is required to enable it continuously to receive electrical energy. This equipment comprising a contact line, sub-stations and possibly a high-voltage distribution system, is a charge against the running costs of the locomotive, when compared with other forms of traction and the same, of course, applies to multiple unit electric trains. Nevertheless electrification under favourable conditions can be the cheapest and most efficient of all forms of .traction. Reference was made to the three basic electrical systems applied to railway traction, viz., direct current, alternating current single phase, alternating current three phase. As to which of these is the best, no hard and fast dogmatic principles can be laid down; it has been proved beyond any doubt whatever that both the d.c. systems and the a.c single phase low frequency systems can work with maximum reliability and efficiency. There is also promise in the 50 cycle a.c. single phase system. The issue can be decided quite clearly and logically on examination of facts. The justification for the electrification of any railway, and the system to be adopted is primarily, but not entirely, economic; the value of electrification as a capital investment is determined by comparing the working expenses after electrification with those of steam operation under similar conditions, and a reduction in working expenses must be found more than sufficient to meet the additional fixed charges due to electrification. This applies principally to such main line electrification where no increase in traffic due to electrification may be expected. With suburban electrification, however, the track capacity can always be increased. More trains can be run to provide a faster and more frequent service than that provided by the displaced steam service. Although this may result in increased working expenses and so appear speculative, experience has shown, both in this country and abroad, that the improved but more costly services attract substantial increases in traffic with consequent increased nett revenue despite' the greater working expenses due to the improved services.
Electrification has assisted in the development of cheap power in some countries; a good example is South Africa where the primary traction electricity supply system was designed to accommodate the general demand throughout the area traversed by the electric railway. The growth of the traction load, which may be considered the base load, resulted in a reduction of from 0.816d. to 0.48d. in the average price per unit of electrical energy for all purposes on this system between 1927 and 1949 in spite of rising costs of coal used for generation.
The Weir Committee (1931) estimated that complete electrification of British Railways would require some 5,700M units of electrical energy per annum and a recent check confirms that this figure still holds good.
The main items making up first costs of electrification were outlined; in general, the higher the operating voltage the lower are the first costs and the resultant capital charges of the fixed installation. On the other hand, with an increase in operating voltage there is an increase in the cost of the electric locomotives and electrical equipment of coaches; so that to determine the economic effect of the variables on any proposed system of electrification for any particular line or territory the actual case must be worked out, and estimates of costs must be calculated for various voltages. High traffic density tends to favour the adoption of low voltage, and conversely, low traffic density favours a high voltage, but the balance can only be assessed by taking into account the actual conditions of a particular scheme. . The use of single phase SO cycle current was con- sidered and reference given to the progress made. In Britain trials with multiple unit coaches operating on 50 cycles are about to commence on the Lancaster-Morecambe-Heysham line; in this case the d.c. traction motors are fed from rectifiers.
While the 50 cycles system may be attractive insofar as the cost of the fixed installation is concerned, there are on the other harrd disadvantages with the electrical equipment of the vehicles, and the effect of unbalance on the main three phase power network of the single phase traction supply. For various reasons, including economic considerations, the British Transport Commission has accepted the 1,500v. d.c. system as standard for British Railways but the 50 cycles system has not been ruled out for electrification of secondarylines with light traffic.' Considering the diesel electric 'locomotive the President pointed out that the diesel is the most efficient heat engine available at present for practical application in a locomotive but the overall cost of translating efficiency into useful work at the wheel rim must be weighed when determining whether this type of locomotive is indeed more economical than the steam locomotive. There has been a phenomenal growth of diesel traction on the main line railways of North America. During the first six months of 1951 the builders produced an average of 330 units a month of all types above 100 tons and 600 h.p. Reference was made to fuel costs and it was stated that the differential in cost per B. T. U. as between oil and coal is rather more in the U.S.A. than it is in Britain. The average cost per horse power for diesel locomotives in Britain is rather more than double that for steam locomotives whereas in the U.S.A. the contrast is more favourable to diesel locomotives. Of all the factors contributing to the economy of diesel traction in the U.S.A., it would seem that chiefly those concerning capital costs and utilisation might be unfavourable perhaps in Britain.
Taking into account the reiative costs and calorific values of diesel fuel and coal in Britain, a theoretical evaluation indicates that for equivalent work the cost of fuel for the diesel electric locomotive is less than the steam locomotive by about only 10% and this is supported by actual tests. When the respective capital charges are added to the account the higher first cost of the diesel locomotive swings the balance in favour of the steam locomotive to the order of 25% But such conclusions when drawn from particular and individual comparisons are unrealistic. The real general and total costs must take into account the many contributing auxiliary factors when a large number of diesel locomotives displace a larger number of steam engines for equivalent work. So far as steam turbine locomotives are concerned some of the experiments have been costly, but in spite of persistent and patient endeavour nothing so far has emerged as a permanently better substitute for the reciprocating steam locomotive. Attention has been turned to the gas turbine which shows greater promise for locomotive applications, the ultimate hope being that it will enable smaller, cheaper and more powerful locomotives to be built within the limits of existing axle weights and load gauges. The thermal efficiency of gas turbines is limited in practice, at the present stage the best figure yet achieved for the restricted space in a locomotive is 19% at the turbine shaft. The full power efficiency is reduced with electrical transmission to 15.5% at the rail. The French 1,000 h.p. locomotive is claimed to have a thermal efficiency at the turbine shaft, of 33-35% using non-distillate fuel, but the free piston compressor system has not yet been proved in rail service nor are any overall efficiency figures yet available. At the present stage of development the gas turbine locomotive holds some promise of economy in capital and maintenance charges as compared with the diesel locomotive, but until the all day thermal efficiency at the rail can be improved considerably, the margin of overall economy is unlikely to give the gas turbine locomotive superiority over the diesel locomotive. Torque conversion received consideration and the President then dealt with the important subject of energy for traction. The Federation of British Industries estimate that the true shortage of coal in Britain is now between 10M and 20M tons per year and on the present trend will grow to about 50M tons by 1960-65. Apart from conservation, the cost of coal must be a factor of some influence in regard to extravagance in its use and ability to compete with other forms of energy. In 1951 the cost of fuel (exclusive of carriage charges) for operating British Railways was nearly £38M, i.e., 11.2% of the total working expenses.
In conclusion the President stated that he had tried, objectively, to set out the facts as he found them and to clarify some matters of controversy or doubt. concerning the forms of motive power which can be applied to railways today. The steam loco- motive has survived for so long, not by any claim to technical superiority but because it is cheap, sturdy, and simple. For these reasons, and for some time ahead, it will remain on many railways in accordance with the concept of George Stephenson. Our coal must be conserved. Unless the Coal Board magically cap. produce more coal of satisfactory quality from yet unknown fields, the position will degenerate from one of gravity to utmost gravity. Electrification at least will assist in easing the position; complete electrification in this country would save at least 8½M tons. of coal per annum, which is 4% of the present national production. and because low grade fuel would be consumed in central power stations, the saving of best quality coal would be 14 million tons. The use of oil with diesel locomotives would also assist; in the USA for traction purposes, one ton of diesel fuel will do the work of at least 94 tons of coal. If strategic hazards are to be taken into account, it must be remembered that without oil all our defence services would be immobilised; and even in the event of a complete changeover to oil by the railways, the, maximum additional load imposed on the supply organisation would be, if American results are repeated here, of the order of only 5.5%. If and when nuclear energy becomes available, electric traction would appear to be the most convenient way to use it."
Graff-Baker, W.S. (Paper 513)
Considerations on bogie design with particular reference to electric railways. 306-39. Disc.: 339-61.
General Meeting of the Institution of Mechanical Engineers held on 4 January 1952 at Storey's Gate, London S. W.1, to which members of the Institution of Locomotive Engineers had been invited. Mr. A. C. Hartley, C.B.E., BSc. (Eng.), President of the Institution of Mechanical Engineers, took the Chair. Mr. A. W. Manser, B.Sc.(Eng.) (M.) read the paper entitled " Considerations on Bogie Design, with Particular Reference to Electric Railways " in the absence, through illness, of the Author Mr. W. S. Graff-Baker, B.Sc.(Eng) (M.).
An examination of the dynamic characteristics of wheel sets and bogies, and of the various forces which act upon a bogie under service conditions. The fundamentals of bogie design are considered, and particular mention is made of recent developments in methods of body suspension. Problems of frame construction, braking, and power transmission are also considered. The paper concludes with a survey of the development of bogie design on the railways of London Transport Executive and elsewhere, and a restatement of the basic problems in the relation of bogie Paper inntroduces work which would lead to displacement of traditional metal springs by rubber springs which deflected in shear..
Ends paper by suggesting that only one motor should be fitted per bogie and that there should be more motor bogies per unit.
Sir William Stanier (339-40) opened the discussion and sung the praises of the Dean bogie. R.A. Riddles (349-50) Riddles made one of his rare contributions in which he observed that the Authors survey of bogie design indicated that any generally accepted alternative to the conventional bogie was a long way off. The paper emphasised, with particular reference to electric traction, that whatever form a perfect bogie might eventually take, engineers and the travelling public alike had been convinced for a long time that better riding was overdue.
He had travelled on the London-Brighton line in a Pullman car in which the riding was very good, and on looking to see what type of bogie was fitted, he had found that it was the British Railways standard bogie, which had been fitted under a Pullman car to see how it would work. In view of the large amount of experimental bogie design recorded In different parts of the world, little of which so far pointed to conclusive results, it was obvious that any improvement could be only a long-term matter, and since both vehicle and track were equally concerned, there could be no short-cut by using a bogie, designed to run under different track conditions, on British railways. The work was therefore naturally divided into two parts-short-term and long-term policies.
The obvious solution to the short-term policy was to discover the best bogie available, and that had been done very simply by running comparative trials with all existing bogies and with such instrumentation a? was available, and then, by taking the best out of each, producing a bogie that gave the best possible results with existing knowledge. That had occupied a long time, and had consisted in trying out the bogies under standardised conditions, with records, with both new and well-worn tyres. The results of the tests was the present British Railways standard bogie. He was glad that the Author approved of it. It gave a reasonable ride, although they were not finally satisfied with it, and such improvement as had been obtained had entailed increased weight and cost.
The long-term policy was therefore more important, but it would be some time before all the answers were known. Recognising that it was not only a matter of the bogie itself, a committee had been formed to study the interaction between the track and the vehicle, with representatives from the mechanical and civil engineering departments, under the chairmanship of the director of recearch.
The first objective of the committee was to decide how to measure scientifically that elusive quality riding, and to break it down into its different elements. The long-established method in use was not sufficiently analytical for the purpose. There would then be selected a limited number of variations from the conventional designs, and prototypes would be built which could be fully tested against the existing designs, with such improved measuring technique as might be developed.
Research of a more fundamental nature was proceeding, and the universities were assisting the British Railways research department with laboratory work on small-scale models. Motor bogies would be considered equally with trailer bogies; the worn condition was of more importance than the new, and the mileage which could be run before riding conditions became uncomfortable was of the greatest economic importance.
In spite of the most.comprehensive work which had been carried out in the United States, France, and elsewhere, there was no short-cut which could be an alternative to dealing with the problems under British conditions. The factor at present missing, and which was absolutely essential, was the means of accurate measurement; that subject was of most absorbing interest and had great possibilities. Much had been said about the flexible wheel. Only that week he had turned down a project of a fully designed and developed flexible wheel because the cost was practically 50 per cent of the total cost of the coach itself. After hearing the discussion,. he was inclined, in spite of that, to have a set of bogies fitted with such wheels purely for development and experimentation, from which something was certain to be learnt. He was completey satisfied, as were the technical officers of the manufacturers, that a flexible wheel was possible; the design was ready to go into manufacture.
T. Henry Turner (350-1) said that the Author had omitted two conditions of operation which should be mentioned, in view of the statement that the public motor coach did " ride rather well." Surely there was no public motor coach in which a passenger could write at 60 m.p.h., as was done regularly in the ordinary mainline coaches of a train.
The Author had rightly said that the problem must be considered in regard to the bogie plus the rail. The road vehicle never had to go backwards for the same distance and at the same speed, and that was one of the features which had to be considered in the design of the bogie. The "toe-ing in" or castoring action possible with some other types of vehicle could not be considered in bogie design for rail vehicles. A train feature that applied to the electrical bogies for two-thirds of the systems in Britain was that thev must'conduct electricity. Four-rail systems were relatively few in Britain; so that the current was likely to flow through the components of the bogie.
Uneveness over rail joints and lateral track irregularities could both be reduced by butt-welding the rails. At the time he had joined the railway service, civil engineers had been afraid of lateral deformation of the track and catastrophic deformation of the track in hot weather. It had been definitely proved that they were dangers about which there need be no worry, where long welded rails were used. He knew of no case where the long welded rail had been catastrophically deformed. The only rails that had so deformed, in railway experience, were those in which there were expansion joints. From French work which had been done on the subject, it would be seen that use of the expansion joint was courting catastrophic deformation under thermal expansion in the hotter parts of the year. Hence, there was no reason why rail joint bumps should be considered inevitable. The Author had spoken of axles having lasted longer than they should have done according to theory. Probably that was due to the absence of corrosion. Experience had shown that the average mainline axle would fail by corrosion fatigue in a relatively few years, if it weFe machined. Experience equally showed that thorough painting would preserve it for very many years. Corrosion could shorten the service life to a quarter of what it might have been. Before very expensive pneumatic tyres were adopted, with their greatly increased friction, he hoped that Mr. Trittons recommendation would bear fruit, and that the rubber-spring wheel-centre would be considered. With that there was little friction and little unsprung weight. Four or five years previously, when he had approached the biggest rubber undertaking in Britain, they had seen no reason why the success which had been achieved in the tramcars in the United States, Switzerland, and Sweden should not be matched in largerscale wheels, or why the trouble with heat from braking should not be overcome.
The lateral stability of the bogie deserved further experiment. Vertical rigidity was obviously necessary, but by design of the sides so that the one would contract and the other expand (which was possible), he was certain that the winding up which took place in the frame at the expense of abrasion of the rail could be avoided. If roller bearings were used, however, provision would have to be made in the shops to ensure that electric currents did not pass, because it was clear that, in certain of the electrified lines, arcing had been occurring from the race to the rollers.
Loosli, H. (Paper No. 514)
Railway electrification in Switzerland, with special reference to the Swiss Federal Railways and their rolling stock. 362-82. Disc.: 382-7. + 2 folding plates. 15 figs. (illus. & diagrs.)
Eighth Ordinary General Meeting of the North Eastern Centre held at the Great Northern Hotel, Leeds, on 22 May 1952, the Chair being taken by Mr. D.C. Stuart.
The decision to use single-phase alternating current of a frequency of 162/3 cycles p.s. led automatically to the necessity to build railway-owned power stations because it was impossible to be supplied directly with this kind of furrent by existing hydraulic power plants which mainly generated 3-phase a.c. of 50 cycles p.s. It is a great advantage of a high voltage supply system that the voltage drop of the conductors is very small in comparison with the mains voltage. Moreover, the copper losses of the conductors are considerably smaller than would occur with a low pressure as used, for instance, for d.c. supply. It is, therefore, a characteristic feature of the energy supply system that only a small number of substations had to be provided. Altogether there are three main substations, 22 substations, three transformer stations (which transmit the energy in one line only) and one feeder station. In addition, 5 power stations transmit current directly into the overhead conductors of the track so that the whole network is fed at altogether 31 points.
As far as the hydraulic power plants are concerned we can discriminate from the technical point of view between two types, viz. the plants of a high storage capacity and the river plants. The first-named are mainly situated in the Alps and are worked exclusively from late autumn to early spring. The turbines are driven by water which is collected mainly during summer in artificial lakes, situated at a much higher altitude than the actual power stations.
The river power plants are to be found in the Alps and the Midlands as well. They utilize the rate of flow and their output depends in particular on the water level of the corresponding rivers so that these plants are considerably more efficient during summer thah during winter.
The ownership of the hydraulic power plants, supplying the Swiss Federal Railways, can be divided into three main groups. Approximately 65% of the whole required energy is generated by exclusively state-owned power stations, all mentioned before. 22% or so is supplied by two so-called combined plants under the mutual supervision of two owners, the Swiss Federal Railways and a privately-owned concern. In both plants one half of the existing generators produce 3-phase alternating current of 50 cycles p.s. and the other half the special railway current. A great advantage of these power plants is in the possibility of exchanging energy between the two contractors by varying the water supply to the several turbo-generators. Therefore it cannot be said that the system for generating railway current is completely independent of that supplying the normal 3-phase industrial current. Apart from the combined power plants there are some other points of contact between the supply system of the Swiss Federal Railways and the normal 3-phase current network. In a few river power plants special 3-phase industrial current generators are installed which feed the industrial network in the case of water surplus. On the other hand some private power stations are equipped with railway current generators. In addition a small number of converter stations were provided. The energy, which is bought by the Swiss Federal Railways, amounts to approximately 13% of the total consumption.
The motor consists of the exciter, commutation and compensating windings, which are situated in the stator, and the armature winding, all being connected in series. In addition, the commutating winding has, in parallel, a non-inductive shunt. In many respects, this diagram is very similar to that of a series d.c. motor apart from the special commutating winding and its corresponding non-inductive shunt.
The purpose of the exciter winding is to induce by its magnetic flux in the armature windiRg an e.m.f., being a!most inverse to the initial voltage, and to provide in each armature turn a force resulting in the motor torque.
The compensating winding has to comptnsate, as in the,case of the d.c. motor, the armature magnetic field. An uncompensated armature field would cause a considerable. voltage drop and, in addition, the commutation troubles would be increased. AS mentioned before, the main problem of the a c. commutator motor was to overcome the commutation difficulties or, in other words, to provide sparkless running. It is essential for each commutator motor that sparking between the brushes ahd the commutator is avoided with regard to a long lifetime of the expensive commutator. Sparking roughens the commutator surface and may lead to fhe.nasty flashing-over from one commutator segment to anotherdue to the deposited brush dust which acts as a conductor between the commutator segments.
The commutation problem of the a.c. commutator motor caused special difficulties due to the following facts:-
Fig: 1 (b) is a sectional view of the armature winding being in its principle just the same as for the d.c. motor. Each turn of the armature winding has its corresponding commutator segment, all of them forming the commutator, moving along the brushes which are, of course, at standstill. As in the case of the d.c. machine, the currents in the turns of the left and right hand side of the brush IV flow in opposite directions as it is indicated by the arrow?. Therefore the direction of the current is reversed in the coils short-circuited by the brush. The coil participating in the commutation is the one most plainly marked in the diagram. With the current also its proper magnetic field is reversed and this alternating field, in phase with the current induces in the short-circuited coils an e.m.f. e,. Thi? e.m.f. e, is proportional to the speed in r.p.m. of the armature winding and the motor current J.
In the case of the a.c. commutator motor, however, the alternating field of the exciter winding induces in the short-circuited coils a transformer e.m.f. which is directly proportional to the main circuit frequency f.
These two e.m.f.s have to be compensated otherwise they would cause a considerable short-circuited current flowing through the coils and commutator segments in commutation and the brush. The shortcircuited current would lead to heavy sparking and an unbearably high wear and tear of the commutator.
By means of the commutation winding in conjunction with its non-inductive shunt the two e.m.f.s induced in the short-circuited coils can be compensated. The compensation, however, is perfect at higher speeds only and, therefore, sparking at standstill and very low speeds cannot completely be avoided. This is of secondary importance as it occurs only at starting and the collectors of a.c. commutator motors, being turned over on an average every 200,000 miles, are characterised by their long lifetime. Another means to avoid a high short-circuited current is the choice of a low main current frequency which is in the case of the Swiss Federal Railways 163 cycles p.s.
The general behaviour of the motor is plotted on Fig. 2. Formula 1 shows that the product of the motor current J and the speed in r.p.m. is almost constant, a characteristic relation of the series motor. From the formula I11 for the rating in the case of a constant initial voltage, and formula I1 follows the simplified relation between the tractive effort and the locomotive speed. The tractive effortspeed diagram of a 28-wheeled Gothard locomotive of the Swiss Federal Railways may be taken as an illustration. The tractive effort is almost inversely proportional to the speed square of the locomotive. To a low speed corresponds a high tractive effort and vice versa so long as the initial voltage is constant. The electric locomotive equipped with series motors has, therefore, a remarkably stable behaviour. In the case of this Gothard locomotive there are altogether 26 tractive effort speed curves or, in other words, 26 notches each corresponding to a fixed initial voltage or a fixed tapping at the transformer, respectively.
The heaviest line on the diagram shows the course of the starting tractive effort in the case of a goods train of 1,720 tons total weight on a gradient of 1 in 100. The driver operates his control wheel in such a way that whilst changing over from one notch to another in order to accelerate the locomotive, the adhesion limit Ad is ncver exceeded. The course of the tractivr effort, therefore, is a zig-zag line of which the points on the side of the higher tractive effort coincide with the curve of the adhesion limit.
The distance between the ordinate and the curve Z , represents the total resistance of the train in question as a function of the train speed. The distance between this line and the zig-zag line equals the amount of tractive effort which remains over for accelerating the train. Point I1 represents the maximum speed which can be achieved in the case of notch No. 21. Here the total train resistance equals the tractive effort developed by the locomotive so that the whole train is in the position of balance. Point I11 represents the position of balance in the case of notch No. 26 and indicates therefore the maximum speed which can be expected with the train in question on that particular gradient.
Journal No. 229
Jarvis, R.G. (Paper No. 515).
The railways and coal. 390-404. Disc.: 404-24: 1953, 43,724-9. Bibliog.
Joint Meeting was held-with The Institute of Fuel at the Institution of Mechanical Engineers, on 1 May 1952, the Chair being taken by Dr. G. E. Foxwell, President, The Institute of Fuel.
British Railways consumed 15 million tons at a cost of £40m. Noted testing to make savings, plus some anodyne comments on poppet valves and high pressure boilers. On page 414 made some observations on experiments with pulverized fuel. R.A. Riddles (404) noted that a fireman had only to use 11 shovelfuls (1 cwt) more than necessary between Euston and Birmingham to increase coal consumptiion by one pound per mile. Noted that spent much on training enginemen.
On page 725 A.E. Simpson stated that he had suffered in discomfort Robinson's experiments with pulverized coal and with colloidal fuel. The former eventually produced satisfactory results but did not justify the preparatory work of grinding and storage. In the case of the colloid it was difficult to keep the coal particles in suspension.
Ikeson, W.C. (Paper 516)
Development of the oil-fired locomotive. 425-75. Disc.: 475-515.
Second Ordinary General Mecting of the Session 1952-53 was held at the Institution of Mcchanica! Engineers on 22 October 1952, at 17.30 C.M. Cock, President, in the Chair.
Author was Chief Mechanical Engineer of the Iraqi State Railways in Baghdad. Reviewed the Urquhart system (citing Urquhart's IMechE paper), Holden's system used on the Great Eastern Railway (and cites Holden's paper to the International Railway Congress in 1900), W.N. Best's system which was widely adopted in the USA, H.G. Garratts drooling steam jet burner used on the Lima Railways in Peru.
The principal advantagcs in order of importance.
The principal disadvantagcs in order of importance:
The principal methods of oil firing in use were:
The Stanier 8F 2-8-0 type was amongst locomotive types in service and oil-burning. See also this author's discussion on paper by Roosen (Paper 607) in V. 50: pp. 266-70.
Andrews, H.I. (Paper 517)
Stresses in locomotive coupling and connecting rods. 533-79. Disc. 579-603. 35 figs. (mainly diagrs.)
Third Ordinary General Meeting of the Session 1952-53 held at the Institution of Mechanical Engineers, London, on Wednesday 19 November 1952 at 5.30 p.m.: Mr. C. M. Cock, President, occupying the Chair
The design of both coupling and connecting rods was complicated by the considerable inertia forces to which they may be subjected while working, and as speeds and loads continually increased, designs were tending toward the critical, and it became increasingly important, both that the working of such rods was fully understood, and that the loads to which they were subjected in service be ascertained. A theoretical paper with 21 citations to other research on stress, notably that at the University of Illinois.
Discussion: E.S. Cox (579-81) said that thanks were due to the Author for gathering together the available information on rod design and adding something new. He confined his remarks to coupling rods, which represented a more serious problem in British locomotive design than did connecting rods. He divided the subject rather differently from the Author, into two parts, one of which was concerned with making the most intelligent and practical use of the largc amount of information, some of it a little conflicting, which was available, and the other with the problem of filling the gaps in the information available, which were still considerable. The Author would no doubt be the first to agree that the last word on this subject has not yet been said.
On the formation of British Railways there had been an opportunity of seeing how the drawing offices of the different railways had dealt with the subject of rod design. The methods had been extremely diverse. All designers had made use of such basic ideas as the strength of a beam and the well known strut formula, but in trying to apply these to actual design and to connect them with actual practical conditions there had been wide variations in the use of adaptations of the strut formula, such as those of Merriman, Rankine and Fidler.
There had been different treatments of loads and stresses in relation to factors of safety. In some designs the smallest rod section had been taken, and in others the largest, and the final stresses had been related to the yield or to the ultimate strength of the material, so that it had been possible for a classic case to occur where a rod which failed was shown to have a factor of safety of 13 by the method of calculation adopted by the company responsible for its design, whilc the same rod calculated according to the method of another company had a factor of safety of 2½. It was clear that the methods originally adopted, therefore, left something to be desired by leading to a false sense of security. What it meant was that each office had proceeded by a process of trial and error to arrive at a design of rod which in comparison with others would give reasonable and satisfactory service. Whilst it could not be denied that 19,000 locomotives were running about in this country with, on the whole, satisfactory results so far as their rods were concerned, the present unsatisfactory basis did mean that occasionally one received an unpleasant surprise.
Since the present paper became available, they had applied the Perry formula to one or two rods, and, speaking generally, they found that if the rods were short or medium in length it gave what they considered to be a reasonable rod section in relation to other methods of calculation, but it was somewhat weighted against the longer rod, in that it gave an uneconomically deep section when the rod became long, if the same factors of safety were worked to in each case.
The Author and other designers based their calculations on piston thrust and crank effort and the force required to slip the wheels, but rods still buckled from time to time, in spite of the application of high factors of safety, and this tended to indicate that the strength of the rod had been based not on the forces which actually crippled it, but on the forces which had not crippled it. It was clear that there was another and more random factor which was causing the intermittent failures to which they were still subject to-day.
The Author had indicated that the effect of clearances was not decisive. This could be supported by actual experience, because such rod failures as took place often occurred on locomotives in good condition and with relatively tight clearances; they were not confined to sloppy locomotives. The view was coming to be held that the really destructive force was the one associated with the stopping of slipping rather than that associated with the commencement of slipping. That was a force which was not mentioned in the paper and not much considered in the literature on the subject. He would be glad to have the Authors view on that. In other words, the problem was really to know what force it was necessary to design against rather than how to design rods to meet a certain force. He noticed that 20 pages of the paper were devoted to a consideration of stresses in what might be termed the vertical direction the centrifugal, with the strut bending vertically and only half a page to the horizontal strut effect. In such experience as he had bad of railway work, however, he knew of very few cases where rods had failed in the vertical direction, whereas bending of rods outwards or inwards had from time to time caused some preoccupation.
The Author stated that to meet the stresses, both known and unknown, the rod should be designed for the maximum stiffness in all directions. It was perfectly possible to design rods in that way, methods originaly adopted, therefore, left something to be desired because there were many successful rods running trouble free with an I-section which was reasonably stiff, but some designers deliberately introduced an element of flexibility by the use of a very flat rod section, and, indeed, a recent experience of a very flexible rod had drawn attention to the fact that there might be another possible method of design in which this very flexibility was exploited to advantage by making some use of the elastic strain energy of the material of the rod. Mr. Cox hoped that a later speaker that evening would enlarge on that interesting alternative.
On the experimental side, one had to agree with the Author that it was surprising how few attempts had been made to obtain actual stresses and loads in service. With reference to the tests which were described, the low speeds at which those tests had been run was striking, not more than 175 r.p.m., and he wondered whether there was any particular reason for that, having regard to the fact that speeds up to 400 r.p.m. were run every day in this country. It was obviously difficult to set up the conditions in which tests of this kind should be carried out. He could mention the case of a rod which failed in service in connection with high speed slipping, and yet when the same locomotive was tested most rigorously, both on the plant and on the road, through the whole range of its power and speed capacity during which slipping had freely occurred it had been quite impossible to make the rods of that locomotive behave similarly under observed conditions. It would be interesting tp have the Authors views on how, if they were to undertake strain gauge tests similar to those illustrated in the paper, they should set about reproducing their limiting conditions.
Failures could be cured, of course, by putting sufficient metal into the rod. The particular instance which he had in mind was that of a class of engine which had a few failures, and those failures completely disappeared by the addition of some 60 lb. of metal in each leading rod, so that the total addition of weight to the rods as a whole was 216 lb. That was less than one per cent of all the revolving weights on the engine, which made one wonder whether there was not too much preoccupation with reduction in weight in the rods, at any rate if it was at the expense of introducing some unreliability .
In conclusion, rods were still the simplest and cheapest means of coupling to stabilise adhesion, and from Webb in the early days to the designers of the Pennsylvania duplex locomotives in recent days, designers had ignored at their peril the correct application of these rods. To illustrate what the Author had referred to with regard to electric locomotives, onIy the previous week Mr. Cox had been present at some tests on the Manchester and Sheffield electrified road, where locomotives were beinq run up to the very limits of their adhesion under different rail conditions. An electrical engineer who was present said in his hearing what an advantage it would be if there could have been coupling rods on those locomotives. It was clear that there was still a great deal in this subject, and it was worth while persevering with better use of what was known, and continuing to add to the knowledge available.
W.A. Tuplin: (586-90) observed
that the paper brought together very conveniently the accepted formulae for
calculating stresses in rods for known load conditions. It showed, however,
particularly in respect of coupling rods, that the varieties of possible
:oadings were numerous. The number of ways of getting into trouble was quite
large; in fact, a study of these ways might deter anybody from using coupling
rods at all. As was usual in engineering practice, however, what happened
was that somebody used coupling rods and did not calculate the stresses until
things went wrong. That had always been in the past the common practice of
engineering, to make something and hopc that it would be all right, and,
if it was, not to bother to calculate stresses.
Coupling rods usually worked very well indeed, but not always, and it was necessary to take an interest in the occasional failures. Coupling rods themselves were a rather crude, pre-Gcorge Stephenson type of engineering, but they worked. If one tried to work out means of connecting axles in other types of locomotive, as for instance internal combustion engine locomotives, by gears and cardan shafts, one would be astonished at the weight and cost and possibilities of trouble, whereas the old coupling rod accommodated itself to violent conditions, with axles moving up and down and tilting in relation to each other, and alignments which would horrify anybody but the locomotive engineer; coupling rods, though apparently crudc, did work.
It might bc gathered from the available information on the calculation of stresses that the design of coupling rods was completely straight-forward and there ought never to be any failures; nevertheless, failures did take place. It had been stated, that evening, that in general coupling rods seemed to give more trouble than connecting rods. It was interesting to consider what sort of loads could come on to the two types of rod. It had been mentioned that if one designed a coupling rod so that it was strong enough to take all the load necessary to slip the smaller of the group of wheels which it connected, that would seem to be satisfactory. It had also been said that space limitations might make that difficult, but it would be interesting to know whether any rod designed to meet that condition had failed in service.
The parallel limitation for a connecting rod was really the drength of the cylinder cover bolts. To do the same thing with a connecting rod it would have to be strong enough to balance a steam pressure sufficient to burst the cover bolts off. That was bigger than anything which was considered at the present time.
Other loads which might come on to coupling rods were due to errors in manufacture and setting up. If the length of a coupling rod between its centres was different from the centre distance of the axles there might be trouble. If the difference was large it might not be possible to get both rods on, but a smaller error might allow both rods to be put on in the easiest angular positions of the cranks, but on turning through 45° the stresses in the rods might be very great indeed. A comparable error might arise if the crank pins were not properly set, when there could be stresses which rose and fell with the position of the cranks. He believed that with the quartering error permitted at present there could be a tensile stress of about 3 tons/sq. in. in a 7-ft. rod.
In spite of attention to these matters, trouble did occur, and it had become almost a regular practice now in many branches ot engineering that when things broke where they should not one looked for resonant vibration. If one had an elastic system and applied to it an alternating force of the same frequency as the natural vibration, the internal loads might be very much greater than the applied loads. That had been such a common source of failure in sonic branches of engineering that it was regular practice to guard against it.
Taking the coupling rod itself, the natural frequency of vertical vibration is usually too high to coincide with the frequency of applied impulses in an ordinary 2- or 4-cylinder engine with 4 impulscs per revolution. The other type of vibration, lateral vibration, was of lower frequency because the rod was less stiff horizontally than vertically, and that frequency was often so low that it was possible to get a violent lateral vibration by the coincidence of the fiequency of crank impulses with the natural frequency of the rod. That was a possible cause of failure of coupling rods by lateral bending. On the other hand, the same kind of thing might happen in connecting rods, and yet as far as he knew it did not, which might incline one to the belief that the lateral vibration of coupling rods was not the cause of the trouble.
In the locomotive, however, there was another type of vibrating systcm. Taking two pairs of coupled wheels, one had a system which could be an angularly vibrating system. One pair of wheels and axle was one mass and the other was another mass with the coupling rods providing an elastic connection between them. The whole system could get into vibration, and if torques or impulses of the right frequency were applied to it, it was possible for the loads in the coupling rods to be greater than the applied loads. That being a possibility, the question at once arose of how likely it was to occur in practice. He had worked that out roughly for certain classes of locomotive. The type of vibration to be considered was what was called the 4th order, because it could be produced by the 4 impulses in the system per revolution of the wheels. There was a critical speed for the wheel and rod assembly at which the axial loads in the coupling rods might be considerably greater than the piston loads. Of the three locomotives which he had taken, the first was a curiosity, because it was a type of which there had only been one example, and that had not lasted long, namely the G.W.R. North Star as an Atlantic which originally had I-section rods. The next was the L.M.S. Class 5 locomotive with rectangular rods, and the third the present Class 7 with I-section rods. The critical speeds were as follows:
|Locomotive||Coupling rod section||
4th Order Critical Speeds (m.p.h.)
Wheel and road assembly
|G.W.R. " North Star " (4-4-2)||
|L.M.S. Class 5||
|B.R. Class 7||
The first one was not very dangerous, because trains did not often
travel at 103 m.p.h.; but the others, and especially the last, were getiing
near to running conditions. The " North Star " was an interesting example,
because it was an Atlantic with two cylinders driving one axle and two the
other, and on the face of it it hardly needed coupling rods at all, and yet
there were rod failures of the same nature as more recent ones, and the
difficulty had been overcome by replacing the I-section by a rectangular
section. The vibrating system of the Class 5 type, which in main dimensions
was fairly similar to that of the Class 7, had a higher critical speed due
to the rectangular section of the rod.
It might be asked, as speeds of this sort were fairly common, why rods did not break every day. In an ideal vibrating system with no damping at all, the magnification could be infinite at a critical speed, but that never happened in practice, because there were always resistances to keep the magnification down. They might only keep it down to a figure of 100, which was quite dangerous. It was interesting to see what sort of damping conditions came in. When the locomotive was running normally on the track there was no slipping, but if this type of resonant vibration tended to build up, with angular oscillations about a uniformly rotating mean position, there would be slipping; and that slippingeven the small amount due to angular oscillationwas sufficient to keep down the resonant vibration and the maximum stress within reasonable limits. If slipping was already occurring, however, that type of frictional damping did not come in at all, and so, with little damping remaining, there was a danger of severe vibration at these speeds. He had given those critical speeds as if the system had a definite natural frequency. In actual fact it had not, because the effective stiffness of the rods depended on the angular position of the cranks. When one was vertical and the other was horizontal one rod was incffective whereas a 45° displacement could make the rods equally effective. It was ,not a simple vibrating system, but as the full stiffness was effective four times per revolution it was probably fairly near it. When a locomotive was in bad condition, with a good deal of slackness in all the joints, this system fell down again and the danger of resonance was remote, so that one was led to the conclusion that there was a possibility of dangerous loading by resonance in a locomotive in good condition when the wheels were slipping; otherwise the risk of high loads due to resonance was slight. That agreed with what was found in practice.
When slipping took place there was violent oscillation and high loading in the rods, and the maximum load might be great enough to cause failure, which in a coupling rod would always be by lateral bending, because the stiffness laterally was very much less than that vertically. If there was a high compressive load in the rod there was a tendency to bend the pin outwards rather than inwards, and that again lined up with what was observed in practice.
One thing which could be done was to keep these resonant speeds out of the range likely to be experienced. It was not possible to be certain of doing that, because when an engine slipped the actual rotational speed might go up to 150 m.p.h., but at least one ought to aim at a critical speed well above any expected running speed. How should that be done? The dimensions of wheels and axles were fixed by other considerations, and the only way to raise the critical speed was to increase the cross-sectional area of the rods. It did not matter what shape was used; what was necessary was to increase the cross-sectional area to increase the axial stiffness. The weight would have to go up, and that would have to be accepted. As it was purely a revolving weight it could at least be accurately balanced. Because stiffness was the necessity one might as well use a cheap steel as an expensive one, and therefore the cheapest steel which would stand the stresses should be employed. It was necessary. to consider the section from the point of view of resistance to compressive load. A hollow section might be regarded as preferable, and an elliptical tube had been suggested, thus minimising weight for any specified strut strength, but it was possible to make each coupling rod in thg form of two channel-section rods bolted near the middle, which seemed to be a way of getting maximum stiffness for a given weight. What could be done about it?
One other condition which might cause trouble in connecting rods was when something went wrong with the valve gear and caused steam to be pushing on the piston when it ought not to be doing so. When a piston was at the end of a stroke its inertia effect tended to be balanced by piston pressure if the valve movement was correct, but if the valve gear broke down, or if the driver put the engine in reverse gear for an emergency stop, as was sometimes the practice, then steam load could be added to inertia-load and the total load on the connecting rod might be doubled. That was sufficient to cut deeply into a factor of safety of only 24.