The Motor Omnibus World.
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The Theory and Practice of Power Transmission in Public Service Vehicles.'
I find myself to-night in the rather unusual position of having to discourse upon a subject suggested to me, and the title for which was written by somebody else, that somebody being no other than the energetic Vice-president of this society. It is, at any rate, certain that there is no need of an apology for the introduction this evening of such a subject, as it probably occupies a nearly equal prominence at the present time in the minds of omnibus engineers with the vexed question of side-slip itself. The matter is at a stage that makes me think that I shall best serve the interests of this society if I can provoke a thorough and frank discussion to-night between manufacturers on the one hand, and those responsible for the working of motor omnibuses themselves on the other.
The first thing that will doubtless strike every expert is the wide range of ground covered by the title, for all the time at our disposal might be spent in dealing with even one of the numerous points in transmission. We might, for instance, devote the evening to dealing with the vexed question of clutches alone, or in discussing the relative merits of chain and direct drive, but, with a view to some practical results, I shall confine my remarks entirely to the question of methods for changing speed, and I hope that the discussion will be limited to this matter. I have ventured to ask several gentlemen who represent different systems to be present to-night, in order that they may have every opportunity of stating their views and opinions, and with the object of making the discussion as full as possible, I have tried to condense my paper, resisting the temptation to give illustrations and descriptions of a number of most interesting gears. I have, however, prepared diagrams and descriptions of a few cases in which there are novel features, and of which it is necessary to give details in order to elucidate my remarks.
Acceleration and Variable Speed.
The general nature of the problem involved is the moving of a mass, which for convenience let us assume to be 6 tons, under the two following conditions : (1) Acceleration from rest must be rapid and smocth, whether on the level or ascending hills, and (2) gradients, no matter of what inclination or length, must be taken with as little as possible variation from the maximum permissible speed. It is just as well to keep these two ideas separate, because, for a pubic service vehicle running in a crowded city, the former may be of by far the greater importance, whereas, for motor vehicles running in the country, especially in hilly districts, the opposite may be the case. But, beyond this, there are two different ideas involved. On the one hand, it is clear that some form of variable gear may be absolutely necessary and most desirable in varying the speed zero to the maximum running speed at each frequent stoppage of a heavy vehicle. On the other, the variable speed gear used in climbing gradients is a mere device for coping with the difficulty of insufficient motive power. It is thes evident that, if the journeys of art omnibus are on the level,. quite a moderate power, combined with an effective variable speed gear of some kind, may fulfil all the conditions cf the case. If, however, a passenger vehicle is in use in the country, let us say in the Lake district or in many parts of Devonshire, it may be necessary, for economical running, to have sufficient power to take hills at nearly, if not quite, the nominal speed of the vehicle. In order to make the importance of this point more evident, let us take the case of two vehicles running in a district which is all uphill and downhill. One of these vehicles is allowed to run at 19 miles per hour down the hills, but can only go up the hills at 6 miles per hour, while the other maintains a steady speed of 12 miles per hour through the whole journey, which we will assume to he 72 miles. Now, whilst the mean speed of the former is 12 miles an hour, the actual time taken to do the journey is 8 hours, six of these being consumed in climbing hills and two in running down them, as against 6 hours for the total time of the other vehicle. I need not point out which of the two is most likely to attract pas• sengers, who would be, presumably, under these conditions, of the tourist class, or, what is still more important, which veWcle would be likely to run more cheaply in the matter of repairs or depreciation. I will only remark that, to secure hill-clitnbin at. fair speeds is a matter of engine power, and cannot be attained by any mere form of transmission gearing, however excellent. It is this fact which has led to the extraordinary increase in the number of firms turning out six-cylinder engines for touring cars, there being, I believe, more than 30 makers who exhibited at the recent Olympia Show. It is the popularity of steam in this consideration which renders it such a formidable rival to the internal-combustion engine. I have seen the gauge of a steam omnibus, when the call was made upon it to ascend a hill, automatically rise from 200lb. to 1,0001b. per sq. inch, which would represent, in the case of a petrol-driven car, a change of gearing bringing the speed down from 20 to 4 mi.es per hour. In the case of the steam vehicle, the speed was maintained nearly uniform, the rise of pressure affording the necessary increase of force to overcome gravity in cdmbing the hill. I have enlarged upon this point, because it is one of the many instances in the development of self-propelled vehicles, that what may be the best under one class of conditions, even, let us say, for one branch of the industry, such as we are discussing, may not be suitable for another, and success or failure may, respectively, depend upon taking into consideration such matters as these. Thus, having in view town conditions, as distinct from country conditions, there is more than one competent engineer, who, having tried more powerful motors for town work, thinks that the more moderate power is better in every way, providing the variable speed gearing is of a satisfactory nature.
The variable speed gearing is thus a vary important item in a public service vehicle for town work, and, unless it is thoroughly effective from the point of view of easy manipulation, small transmission losses, durability and silence, it puts the petrol or paraffin-driven bus at a great disadvantage in relation to its steam rival. We will, therefore, now consider the different methods of variable transmission which are either at the present moment available or are likely to become available in the future.
Alternative Transmissions.
The ideal transmission gear for the purposes we are considering is one which would enable the change to take place continuously. This result can be obtained; (l) by means of pure mechanism: (2) by the use of a fluid transmission ; and (3) ter electrical methods.
(I) ith regard to pure mechanism, the only real solution that I am aware of is by means of frictional transmission, either, for instance, what is known as the disc and roller, in which a roller is kept in frictional contact with the disc, and the variation of speed is obtained by moving the roller to a greater or less radius, or by some form of expanding and contracting pulley in connection with which belt transmission is 'employed. Both these methods of transmission, although tried over and over again, have soon been abandoned, even for small powers, on account of their practical imperfections. Although hundreds of inventors have tried their hands at some positive mechanical drive, using some form of tooth or ratchet motion, nobody has yet succeeded in producing a continuously variable gear of this type, and those who have thought much over the matter gene. rally seem to agree that the search very much resembles that tor perpetual motion, although it is probably a trifle safer to say that a successful result is impossible in the case of the latter than it would be in the case of the former, but at best the assertion is only a trifle the safer. (2) When we come to a continuously variable gear of the second type—i.e., using fluid transmission—we at once are able to quote practical results. A great many devices have been suggested, but I only know of one which has been actually applied with success to motor vehicles, and that is the Hall hydraulic gear. In this gear an ordinary petrol motor drives a threethrow pump, which circulates oil to a three-throw working cylinder. Assuming the stroke of either pump or cylinder to he varied at the will of the driver, a varied torque can be transmitted to the axle of the road wheels, and hence a continuously variable transmission can he secured. Mr. Hall has kindly provided me with drawings of the very latest type, which is the first he has designed for omnibus work, and from these drawings a wall diagram has been prepared, which I hope will make the action clear without a lengthy description.
The new gear works from a maximum speed right down to zero—i.e., the whole vehicle can be brought to rest through the gear itself and started up again, working from maximum torque at zero velocity up to the full speed which it is capable of giving under the given torque required. Thus the torque can be made to vary inversely as the speed, and instead of, as before, driving the pump shaft and taking the power off a revolving cylinder, the power on the new pattern is supplied to the cylinder and the torque is taken from the pump shaft. The pump cylinders in the new gear are the same size as the motor cylinders, when the motor cylinders are at their full capacity. Consequently, when •the numn shaft is held back by the resistance of the vehicle, and the gear is at the zero point, there is simple transfer of a given volume of oil from t!-e prmp cyli-e-r to tile motor cylinder, from which it exhausts into the casing. This oil is taken up from the casing again by the pump and circulated continuously. It will thus -'be seen that the mechanism is simplified and the external gearing which previously was the cause of much noise is removed. The Hall gearing has, therefore, not
only been improved in its external efficiency, but has also been improved, out of all recognition in the matter of silent working. Mr. Hall states that, in a new machine of about 14h.p. hunt on this principle, he has actually obtained an efficiency of no less than 92.7 per cent. There was present at the test a well known representative of the motor omnibus interest, who, if he is here to-night, might relate his views on the subject, as I have myself not seen the new gear. Inasmuch as I happen to have made the original tests some five or six years ago on the than new invention, and could not obtain an efficiency of much more than 50 per cent., it seems evident that most satisfaritory results have followed the labours of the indefatigable inventor during the period in which be has continued to work upon his scheme. As to the durability of this type of gear, there has now been sufficient evidence to pronounce very decidedly. The gear of a vehicle (one of six) which had been running since 1904, and had never been looked at or examined since then, was taken to pieces the other day, and there was, apparently, not the slightest indication of wear. This would probably surprise people who were not familiar with the extraordinary r sults secured with journals and pistons which run under forced t• cation. If, therefore, Mr. Hall has succeeded in gettinf igh efficiency combined with silent working, omnibus engineers and designers will doubtless consider very carefully the question of applying hydraulic transmission.
The use of air for variable transmission has been suggested by a good many people, and, undoubtedly, a compressible fluid offers many advantages. Its use, however, owing to the well
known properties of thermo-dynamics, involves considerable loss owing to the heat generated in compression, and given out iii expansion. By its means, however, a certain amount of storage power for starting and hill-climbing might be obtained by using
pneumatic accumulators, but it does not take much calculation to show that the storage capacity on or under any ordinary vehicle is limited to the power represented by an extra cylinder or two on the engine working for a very limited fraction of time. The use of variable pneumatic transmiesion is, nevertheless, quite feasible and might at any time be seriously brought forward.
(3) With regard to electrical methods, which have lately been heard of in the form of the Auto-mixte (horrible name l and the Hart-Dartnall and other systems, I prefer to say nothing at the present moment, because a later paper will be on the HartDurtnall system itself, which will probably raise the whole discussion of electrical transmission.
Stepped Gears.
I have, I believe, exhausted the methods of continuously varying the ratio of power and speed, and it is a curious fact that the only successful kind of device which has come into general use is one which is most imperfect on theoretical grounds. This device, the only one in which metal teeth can be practically employed, is that of the step-by-step method, one example of which has been employed from early times in connection with the change speed of lathes. I doubt if any engi neer, had he seriously considered the matter, would have believed that the almost barbarous method of change gear, adopted more than ten years ago on the first Panhard cars, would have survived twelve months. Yet, although the variations of detail in connection with this method are almost bewildering in their multiplicity, the essential teaturee of the method not only survive to-day, but have no serious rival. It would be impossible to give even a list of all these variations, much less describe them, and I do not know that any useful object would be attained by doing so. Inasmuch, however, as the step-by-step method has been brought to such great perfection, and the results at any rate of having four, three, or even only two changes possible on a heavy vehicle are, owing to the elasticity of the motor, recognised, generally, as, without doubt, practically successful, it will be well to consider briefly the chief types and point out the chief features of one or two of the more remarkable of these gears. The gears themselves may be roughly divided as follow :—
(1) The run-through gears, having plain quadrants for actuating them, of which the Peahen-I was the forerunner, and among these may be classed surh well-known gears as Panhard, Ce-ermain, Rover, Brasier, .Armstrong-Whitworth, Deeauville, Chenard-Walcker, Vinot, Spyker, Hitehon, etc. In them, the wheels are simply pushed through with an intermediate neutral space, so that no two sets of gears can engage simultaneously.
(2) Generally known as the gate type, from the fact that the lever must pass through a gate, engaging whichever gear may be required, without having to run through all the other gears. Of this kind are the Daimler, Mercedes, Thornyeroft, James
and Browne, Benz, Austin, Rolls-Royce, Singer, F.L.A.T., Deasy, Berliet, Crossley, etc. There are also special types of gate coming under this head, such as the Argyll, Straker-Squire, Siddeley, and others. (3) Always-in-mesh type, in which the separate sets of gears always mesh, either as in the Durham-Churchill, in which, by means of a mechanical contrivance, the respective gears are locked on the driven shaft as required by temporary keys, or dog or magnetic clutches, as in the new Ravenshaw gear. Epicyclic gear. Types of this kind of gear are the Lanchester, Cadillac, Verclet, Adams-Hewitt, WilsonPitcher, New Engine Co., elc.
(5) Transmission at right angles. In all the previous gears the transmission is parallel. The present type may be illustrated, either by cases, of which there were more than one at the recent Olympia Show, which employed bevel gears of different ratio, mutually concentric with the same pair of axes. An entirety novel gear of this type, and one which at first would seem to contradict the ordinary laws of tooth gearing, is the Lloyd cross-roller gear, also exhibited at that show, in which, from a disc, four concentric lines of rollers, taking the place of ordinary teeth, project. The pinion is of the lantern type, and the change of speed is obtained by causing it to engage in whichever row of concentric teeth on the disc gives the required velocity ratio. There are numerous other variations, but the foregoing represent the five chief classes in which gear may be divided.
Individual Modif ications.
Each maker has, as a role, some slight variation, even
amongst the types in question. As to which type is best, this question is not likely to be decided except by the survival of the fittest. The laws of evolution work slowly, and at present it is open to each maker to say that the particular system employed by him and the particular details of that system give the best results. There are a few points of interest in regard to these varieties which I will, in conclusion, point out. The first one I will take is the variation from the run-through type in the Hitehon gear. I select this gear because it seems to eller peculiar advantages in driving a heavy omnibus in not giving the driver an opportunity of missing gear. I have prepared wall illustrations, which make its action easy to understand, and it has been described in the motor Press, so I will only say that its principle of action is that there is no neutral space in running through, since on the intermediate shaft there is free-wheel driving action on all pinions, and, hence, the one which has to move faster simply overruns, and there is no conflict of forces between the two. I have also prepared a drawing of the Adams gear, which has attracted a good deal of notice by the manceuvring powers which it showed at the Town Carriage Competition, and the present gear has been designed specially for omnibus work and is shown for the first time. The feature which is particularly striking in the Adams gear, and which can also be applied to the Cadillac or other epicyelic gears, is, that, since the action of each gear is obtained by gripping a separate brake band on the outside of a, revolving drum, change of gear can be smoothly effected by merely pressing a foot pedal. Hence the driver need not lean down or take his eye off the road in front, and this, it must be admitted, is a point of no little importance. I do not say that the pedal system of changing gear is applied to the other epi. cyclic gears—but, so far as I can see, it can be done.
I have mentioned a magnetic clutch, and when we know that more than 70 patents have been taken out for such clutches, by, amongst others, Holden, Krebs, and Jenatzy, it is evident that there most at least be latent possibilities in this direction. It is evident that if different gears can be engaged by turning a mere switch to different positions on the driving wheel, a great step onwards will be taken. This it appears can be done in the Sheffield-Simplex car with the clutch of Messrs. Middleton, Townsend. and Ravenshaw ; but of this clutch no description is at present forthcoming.
No kind of gear is more interesting as a mechanical device than epieyclic gears—the theory and 'action are alike fascinating, and one thing of great importance is secured by their use, viz., the distribution and balance of stresses by the number of pinions which can be carried on the spiders, so as to engage on a central wheel. The Adams has two, and the Cadillac three such pinions—though inure could be used if necessary. In order to distribute the stresses, a similar method can be adopted in parallel gears, though, as far as I know, only the ChenardWalcker have yet adopted this device, as it is more expensive, though undoubtedly more satisfactory in action.
Conclusion.
There is one point I cannot omit alluding to, and that is the wonderful perfection to which the material used for wheel teeth has been brought, and, when it is remarked that all wheels are now made of the finest steel, and many are brought
to their correct shape AFTER being hardened, it will be seen that automobile practice demands, and secures, the very best that is possible in modem machine methods, and that this enables the toothed-wheel, step-by-step change speed to survive as still the best practical means of transmitting variable power under the very difficult conditions of public service vehicles. I think you will all agree that, so far from exhausting the subject, I have only. as it were, taken a hasty survey of what will, during the next few years be a fruitful field of invention and improvement, and in which, if the internal-combustion motor is to continue to so largely hold the field against elec tricity and steam, there may be changes in the matter of transmission gears greater than we can at the present moment venture to forecast with any certainty.
[We are pleased to he able to announce that we have arranged with Dr. H. S. Hele-Shaw, F.R.S., the reader of the foregoing paper, that he will write a short description, of each of the principal illustrations employed by him in connection with his paper, exclusively for this journal. We are, therefore, holding over until our next issue certain drawings and diagrams which we had hoped to be able to include this week.—F,n.]
The " Ensign ' omnibuses stopped running on Friday last. We have been informed that the debenture holders have foreclosed, but this information requires confirmation.
The adjourned discussion on Mr. Thomas Clarkson's paper, before the Institution of Mechanical Engineers, will be resumed to-morrow (Friday), at 8 o'clock, at Storey's Gate, S.W.
Rawienstall Town Council, in its Bill which will come up during the ensuing session of Parliament, seeks powers to run motorbuses, both within and beyond the limits of the town.
A few weeks ago, "No. 7 Vanguard" service had the roof-board painted in for the route to be changed via Holborn, the Bank, and Old Kent Road, to High Street, Peckham, but the service has never been run through beyond Oxford Circus. On Monday last, the 26th instant, the service was extended down Regent Street, Piccadilly, to the Strand and Law Courts, so that it is probable that this service will not run via Holborn as originally intended.
The directors of the Bath Electric Tramways, Limited, one of whose electric tramcars got beyond control when descending Locksbrook Hill on Sunday last, have, as will be known to many of our readers, a number of MilnesDaimler and Straker-Squire omnibuses in use upon other and more hilly routes, including the famous Lansdown Hill, along the face of which the construction of a tramline was held to be inadmissible, having regard to the public safety.
Manchester Motorbuses.
Shareholders should put themselves in immediate communication with Mr. Charles Dunderdale, solicitor, 85, London Wall, London, E.C., or 2, Marsden Street, Manchester, who will be prepared to supply the necessary farms of proxy. Particulars of the Ravenshaw system of magnetic clutches, to which a reference is made on our first Editorial page, may be obtained from Messrs. Middleton and Townsend, Electrical Engineering Works, 56, Bedford Road, Edinburgh.
\ The first omnibus of the London Central Omnibus Company has been put into service. It is worked in conjunction with those of the London Suburban Omnibus Company, Limited, between Chalk Farm and Waterloo Station. It is painted dark blue, with the name " Central " on a red label, and is one of the new Leylands which are giving such satisfaction to all who have tried them.
The first Arrol-Johnston motorbus to be added to London's public stage carriages entered service on Saturday last, the 24th instant. The principal features of this machine have been fully described and illustrated in this journal, and many of them should appeal to operating engineers, who will probably watch this trial with great interest. Simplicity of construction and silence in running have been aimed at ; lubrication to the engine is by a force pump in the crankcase, through channels in the casingand a bored crankshaft, whilst that to the gearbox is by syphon wicks to the roller bearings : there are no oil pipes to become choked, or to cause trouble through rattling or breakage. Low-tension magneto ignition is fitted, as well as high-tension coil and accumulator equipment. The drive is by encased side chains, which work in a dust-proof and oil-retaining casing; this casing can be removed in five minutes, whilst the chains can be adjusted without their removal. The company's patent special spring drive is also introduced in front of the differential. Four speeds .forward, and a reverse, are provided, with direct drive on top gear, and all the gearshafts run on roller bearings, which are strictly
interchangeable. The Hele-Shaw clutch is employed, and the flywheel combines the fan in its construction, the radiator being a specially large o va standing high above the bonnet. The vehicle is in the service of the London and District Motor Bus Company, Limited, between Putney and Shored itch,
Judge Woodfall, at the Westminster County Court, on the reth instant, gave judgment against Messrs. Toler Brothers, who sued the London. General Omnibus Company, Limited, for damages to one of their vans. Evidence showed that the driver of the motorbus suddenly deviated from his apparent course, and collided with the van, which was stationary at the time, but the driver stated that he had to choose between knocking down a man, who suddenly stepped from the pavement, or knocking over the cart. The Judge found that the driver was not negligent, and that, by rightly avoiding killing the man., he injured the van as an unavoidable sequence. He, therefore, gave judgment for the defendants with costs. An appeal is pending.
Our Fortnightly Census total recorded, 789.
TABLE A.—OWNERS. London General Omnibus Co., Ltd. ... 163 London Motor Omnibus Co., Ltd., 162