THE FUTURE OF COMMERCIAL VEHICLE DESIGN.
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A Comm-ent Upon Sir John E. Thornycroft's Paper. By Engineer-Designer.
THE paper read by Sir John E. Thornycroft, K.B.E., at the Institute of Civil Engineers on May 18th was interesting, but, in my opinion, the general tone was somewhat noncommittal. It is a pity that Sir John, with his experience, did not go more into details, as he fully recognizes that for the present, we must be content with detail improvements only.
Tinfertunately, he foreshadows nothing in the way of definite advance. Sir John seems to think that it is only in very special cases that we have not beaten the horse. The writer is inclined to think that in the matter of cost we have not beaten the horse as much as many people may imagine.: In the delivery or collection of many things, where the stoppages are frequent and long, the horse has it. still. A class of vehicle is still wanted which will deliver bottled beer to shops situated only a few hundred yards 'apart, where stoppages have to be made while the return bottles are examined and the necessary business gone through. Another class of work where we petrol engineers have not scored is the dust cart. The horse holds his own here unless the distances are long.
Improvements, for the Present, will be in Details.
Sir John points out that, in the general scheme, the petrol vehicle is stereotyped, and it is only in details that we can look for improvements • at any rate, for some time to come. Many would have been glad had Sir John gone a little further in this direction and pointed out what he considered the points in a commercial vehicle which most loudly call for improvement. . That improvements are still urgently needed there is no gainsaying. .
If one reviews the quantity 'of parts which leave the replacement department of any one of the firms who manufacture commercial vehicles, it will be seen that, quite apart from fair wear and tear and damage through accidents, there must be failures of important component parts of such vehicles. The writer is well aware that at least half these replacements would never he needed if such simple things as lubrication and the occasional tightening up of a nut were attended to. Even after making all allowance for neglect, there is a good deal of money spent On each vehicle yearly which could be reduced if more care were given to the design of details.
Factors in Correct Design of Crankshafts.
As an instance of this, the writer, during the war, had to examine many hundreds of broken crankshafts of all kinds and makes of engines. These breakages were not caused by shell-fire, but were developed through use. In many cases the cause of breakage was not difficult to find, and it is strange how it could have escaped the notice of the designer. The commonest cause was insufficiently large radius in the corners of the junction of web and journal. The next commonest was too largo a hole for lubrication having been drilled in the journal. In many eases the hole was out of all proportion to the size of the journal. Yet another cause was the sharp corners just above the radius. The proportion of the journal to web was found to vary to a great extent, in some cases the sectional area of the pin was equal to that of the web, while in others the sectional area of the pin was one-half that of the web. This does not look as if we have reached the
stage of scientific designing.
There are many points in the design of a commercial vehicle which one would have expected to have
seen settled once and for all by now. In spite of this, the last Show revealed a great variety in design. One would have expected the matter of the surroundings of the back axle to have been nearer to some standard than they are. On one model we find a rigid torque tube and radius rods; another has a torque rod fitted with a spring at its front end ;. another has radius rods only; whilst] yet a fourth (and an undeniably successful one, too) has no torque or radius rods, and relies entirely on the springs for both. Sir John mentions the matter of the engine being formed in combination with the gearbox. This con• srtruction may be favoured by some, but the man who has to keep the fleet going is seldom in favour of it after a little experience. In the large engines needed for heavy commercial vehicles and the proportionally large gearboxes which should be fitted, there is a heavy mass of metal of very considerable leingth. Whether this is made of castings securely bolted together or formed of separate units mounted on a sub-frame, it is difficult to mount such a long and heavy part in a frame, which is known not to be rigid, and yet insure that distortion of the frame will not put undue strains on this part. The subframe has its advantages, hut, even with it, it is necessary that two universal joints or their equivalent should be fitted between engine and gearbox.
Bad Economy in Universal Joints.
The writer is surprised to see many vehicles with only one universal joint between engine and gearbox. It is not a correct construction, and has been found beyond all doubt to put undue strains on both engine and gearbox. In some designs there is a single universal joint which has to be the supDort and centring arrangement for the clutch shaft, and it is not surprising that there is a good deal of trouble with this part. The writer hopes to be able at an early date to give particulars of a single universal which will allow for not only an error in alignment, but also for position of shafts.
Sir John does not throw much light on the matter of pneumatic vsirsus solid tyres. He seems, however, to think that only in special cases will pneumatic tyres be used to any great extent on commer. cial vehicles.
The writer is confident that if more scientific attention were given to the design of springs' vibration could be greatly reduced. Sir John points out that by. actual measurement there is not much difference in the amount of vertical motion of a vehicle fitted with pneumatic tyres and of one fitted with solid tyres; yet, as he says, there is undoubtedly greater comfort in riding in a vehicle fitted with pneumatic tyres. Investigation will show that the loss of comfort is due to two causes—namely, sluggish shackle joints and unscientifically designed springs. For commercial vehicles shackle joints should he as free as possible. There is no rebound in the heavier vehicles, so no dampingeffect is required. Only those who have actually tried the difference between the ordinary badly lubricated shackle joint and one fitted with an anti-friction bearing can realize the
importance of free-working shackle joints. .
As to the design of springs, the present form is altogether wrong, as it yields in an even progression, whereas it should give out freely to a light load and gradually increase in resistance as the load increases. The writer hopes to be able at an early date to give some further information on a new system which enables springs to respond more rapidly to the vibrations of the road.
Sir John mentions aluminium wheels, and the writer is glad he calls attention to this matter, as there is much to be hoped for in this new departure.
Rapid Loading and Unloading Devices, Tipping, etc.
Sir John also mentions the use of containers and other arrangements whereby the load can be lifted by means of a crane. Where a crane is available, this is all very well and good ; but, where there is but one man with the lorry and no crane is to be found, matters are worse than if the load were not in a container. The writer's opinion is that all containers should be accompanied by some contrivance whereby the container can be got off the lorry by one man without any undue exertion. The same applies to. tipping devices. There does not seem to
be much reason for costly mechanical devices when with a little ingenuity the load can be made to discharge itself from a simple and inexpensive body which will return to its original position unaided. This class of device might be called the " Sans-Power " class of body. Some practical suggestions appear in The Commerciat Motor, May 30th, pages 451 and 452. In one design there illustrated and described the lorry could be not only unloaded, but actually loaded, without more power than is necessary to lift the centre of gravity some 6 ins., the load being rolled on to the body instead of lifted. There were also described and illustrated tipping bodies of the "Sans-Power " class, whereby the load can discharge itself and the body return to its original position without the application of any power other than that of gravity.