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Two Views About the Worm Drive.

27th April 1911, Page 5
27th April 1911
Page 5
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Page 5, 27th April 1911 — Two Views About the Worm Drive.
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Sharp contioversy over the respective merits of worm and chain drive has been precipitated in the tinned States of America by the appearance of the following article in a recent issue of "Machinery" : - " The compactness and simplicity of the worm gear are generally attractive to mechanical engineers and designers, and many have been led against their better judgment to use it in devices for which it is not suitable Of all ordinary transmission mechanisms the worm gear requires the best workmanship, both in making and in mounting, to secure maximum efficiency ; and even when everything possible has been done to make for the best efficiency, it is still relatively low. When high speed ratios are required in small space, in devices with few parts, or the self-locking feature is important, m i

the worm is the deal transmission to use, but if other transmission gear of high efficiency and durability can be used, why use something inferior ?

" A movement in the design of motor trucks to be deprecated is towards the use of the worm drive in the transmission to the rear axle, a worm and wormwheel taking the place of bevel gears. We believe the move will be disastrous if generally followed by the builders of motor trucks, because reliability is required first of all in cars for commercial uses. Neatness of outline and freedom from noise are prime considerations in pleasure cars, but it appears like poor design to employ a rear axle in a heavy truck containing the differential and a worm drive, in place of the plain axle and a parallel lay shaft carrying the differential and chain sprockets for the intermediate transmission to the wheels. The latter form of drive is easily repaired ; drivers can make shift to get along if one side is badly damaged, and are doing it every day. Hooking the rear wheel into an obstruction with force sufficient to bend the axle does not necessarily put the truck out of running, but it surely would if furnished with the worm drive and differential in -the axle. Another disadvantage of the latter is the greater dead weight—that is, weight not spring supported—carried on the rear wheels as compared with the load carried by the differential countershaft type. The i chain transmission gear may not be pretty, but f encased, as it should be, it is comparatively noiseless, highly effi

cient, and simple to repair."

Ti. was not to be expected that so sweeping a condemnation of the worm drive would be permitted to go unchallenged. Mr. H. Kerr Thomas, our U.S.A. correspondent, and the designer of the Pierce-Arrow

ave-ton motor-wagon chassis, has taken up the defence of the worm gear in the following article, which he has sent round to the principal motor journals of America :—

"The statements made are opposed to the experience of a very considerable number of engineers who have for years been engaged in building automobiles, both pleasure ears and trucks, equipped with wormdriven live axles.

" Let me say at once that unless the best workmanship is available, any kind of worm gear is best left alone, and in this opinion I am entirely in agreement with the editor of Machinery.' He proceeds, however, to state that the efficiency is at best still relatively low.' At this point it becomes evident that modern practice and modern engineering literature have alike been overlooked. The efficiency of worm gears properly designed and correctly mounted is as high as 95 per cent. This statementis made from my own observation and can be calculated from the formula : "Where ri = the coefficient of friction, P = the axial pitch of the worm, and D = the pitch diameter of the worm. Confirmation of this is given by Fred. A. Halsey in his handbook on Worm and Spiral Gearing,' published in New York in 1903, reproduced, I believe, from articles in the AMerican Machinist.' Any engineer may therefore verify this fact for himself.

" In the 'Automobile Trade Directory,' published in January, 1911, on page 746, will be found a table by Worby Beaumont, which gives the efficiency of various types of transmission mechanism. Here it will be found that one set of gears will absorb 5 per cent. of the efficiency and two chains 6 per cent., or a total of 11 per cent., giving for a bevel and chain transmission of the usual type a total efficiency of 89 per cent. against 95 per cent, for a worm gear. " Were it possible to employ for a large truck a single final bevel reduction, the efficiency of the bevel gear would be equal, but not superior, to that of the worm gear ; owing to the size of gear which this would involve, such an arrangement is not possible, however, and chains must be employed, making the further reduction in efficiency referred to. Beaumont's figures, however, are for new chains, and it is a fact well known to all engineers that the efficiency of chains falls off rapidly as the chains become stretched and the sprockets worn in use, so that the 89 per cent. when new is probably as low as 73 per cent. long before the chains are worn out. It is therefore difficult to see in what way the efficiency of worm gear is ' relatively low,' particularly when it is remembered that the wearing of a worm and worm wheel does not reduce its efficiency at all, owing to the fact that the tooth sides are flat and remain so in spite of wear." • With regard to the so-called " self-locking " feature of worm gears, which was a valid objection against older types, Mr. Thomas goes on to say, " It is not a little curious that this point is so frequently brought up—it is obvious that no automobile could exist with a worm gear were it not possible to ' coast ' as freely as with a bevel, that it is difficult to conceive any engineer building such a machine unless he had first proved the fallacy of this idea.

" irreversible worm gears are so common in, for example, elevators and dividing heads of machine tools, that the uninitiated are apt to overlook the geometrical principles on which the operation of any worm gear depends, and which make it an easy matter for the designer to make the gear reversible or otherwise at his option ; it is only a question. of varying the gliding angle, which. is always a function of, never equal to, but necessarily greater than, the spiral angle.

" As to durability, which is another point raised, it. is probably not generally known that heavy trucks fitted with worm gear are In general use in England, and have been for the last seven or eight years. in London omnibus work, where three-ton chassis are employed, worm gears have given satisfactory results for the last five years.

" I have before me as I write a letter from the operating engineer of Thos. Tilling, Ltd., who own arid operate a large fleet of public-service vehicles in London, in which he says :

" The average life of a worm drive on an omnibus is between 28,000 and 30,000 miles ; now that we have re-designed the torque rod the life should be 40,000 miles.'

" Thave yet to find any chain which will in average working approach one-half this distance.

" Reliability is much the same as durability, but as the two are frequently considered separately, I will deal with them in the same way, merely observing that., to take only one example, Dennis Bros., who were the pioneers of the worm drive for trucks, have given a specific guarantee of two years with every worm-driven rear axle they have made, and their range of models has for five years included those of five tons capacity. Has any manufacturer of other forms of drive exceeded this?

"Lastly, the argument of complication and unsprung weight and all the disadvantages thereof is such an old friend, that those of us who have had some experience in the automobile industry have lively recollections of precisely similar objections when the gear drive was first substituted for chains in the lighter vehicle ; such of us as are familiar with present-day practice also know how far they were removed from the truth.

" The analogy between pleasure ears and trucks is largely in favour of the latter, inasmuch as their slower speed is in their favour, size for size, the destructive effect of shocks, etc., being measured by the energy stored in the vehicle (or part of it).

Since energy foot pounds, it follows 64.4 that destructive action will vary directly with the weight. and, directly with the square of velocity ; hence it is an easier thing to make a live axle for a truck than a pleasure car, because of its lower speed.

" With regard to repairs, it is obvious that a broken chain can be easily repaired ; it is equally obvious that such repairs arc unnecessary if no chains exist. If a worm gear will last from 30,000 to 40,000 miles, as it does, it may be assumed that roadside repairs are not a frequent occurrence, and so far as the axle itself is concerned, it becomes a problem of everyday mechanical engineering to design what will be strong enough for the purpose.

"The whole question of worm drive for automobiles seems to be regarded in some quarters as a new and obscure thing—not fit to be understood. That it is so in this country must be the result of accident or prejudice. It was past the experimental stage 10 years ago in England, and such names as Lanehester, Napier, and Daimler in pleasure cars, and Dennis. Halley, and Leyland in trucks, not to mention many others, should he sufficient guarantee that the worm axle is a practical and reliable proposition."

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