WORM versus CHAIN TRANSMISSION.
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A Reply to a Recent Comparison Between the Two Forms of Drive. Superiority of Worm-drive Maintained.
By a Worm-drive Enthusiast
IN an article which appeared in The Commercial Motor (April 14th, 1925) a "Chain Enthusiast," with more enthusiasm than veracity, advanced a formidable array of arguments designed to convince those employing worm drive that they were, to say the least of it, unwise in their choice of a anal drive. It only seems just, therefore, that the WORM drive should have its share in ti,e argument.
In surveying the range of vehicles in which the selection of a final drive lies between shaft and cliain transmission, "Chain Enthusiast" assigns as limits the cycle and the steam wagon. Whilst a very ingenious way of emphasizing the range of application of the chain, however, It hardly seems a rational selection. No one would reasonably question the suitability of the chain drive for the propulsion of two-wheeled vehicles such as cycles and motorcycles, but the conditions of design are so different from those obtaining in the cases of four or more wheeled vehicles that comparison be:. tween chain and other transmissions is quite valueless. Such comparisons can only be fair when both types of drive are considered as applied to the ne kind of vehicle, so that in the downward direction the limit is the light car. In the other direction, however, the limit is not reached with the steam wagon—there are still the electric tramcar and the internal-combustion locomotive.
Having conceded the chain its value as a transmission for cycles, however, its range of application to the multi-wheeled vehicle does not compare so favourably with the worm drive as its protagonist would have one believe. "It is significant," it is said, "that this form of drive (to wit, the chain) is always embodied in attempts to produce a really cheap car." True, 0 King; but how often are these attempts successful? It is in the failures that the real significance of the remark is to be found. There are chain-driven light cars, but their survival is not so much due to, as in spite of, the chain. No one could have been more anxious than Mr. Ford to produce a really cheap car, and he, it is rumoured, appears likely to succeed. Had he thought of the chain drive, one imagines, he might have been quite well off by now. And is it not a deplorable sign of the irrational and stubborn conservatism of car designers that they persistently blind themselves to the cheapness, simplicity and efficiency of the chain? Perhaps it is. Moreover, one may seek in vain for heavier cars propelled by chain. Although early tried, they failed to stay, and now the shaft drive leads the way. But we seem to wander from the subject of worm drives. The point of the foregoing remarks was to show that "Chain Enthusiast" based his sweeping claims on the application of the chain to cycles and heavy steam wagons, but that he left a hiatus extending over the central portion of his range which he discreetly passed over. The worm drive, however, has been successfully employed for the transmissions of vehicles covering a wide and uninterrupted range. From light two-seaters, examples may be met progressively increasing in size, including a 12 h.p. car holding numerous world records for speed and endurance, medium and heavy closed-body cars (in which the quietness of the worm is a great asset), one-ton trucks and commercial motors of every description. To say, therefore, that "the shaft drive is suitable for use over a limited range only . . . and . . . signs are not wanting to show that this range may be still further decreased" demands courage and disregard of veracity which is to be deplored.
The Advance of the Worm in Powertransmission.
As a matter of fact, the worm drive is claiming an ever-increasing amount of attention from those confronted with exceptional power-transmission problems. This is noticeable in the field of the steam wagon, where the chain is being supplanted in favour of the worm drive in many of the latest designs. The worm drive has also been applied with success to the internal-combustion locomotive and electric tram, and the rapid expansion„of its sphere of application would seem to indicate that (once more quoting "Chain Enthusiast ") "the difficulties experienced in the designing and manufacturing of a shaft-driven live axle which will stand up to the loads" have been completely overcome. The developments which have taken place in worm-gear design have been quite as rapid, and of no less importance, than the corresponding and unquestioned improvements which have been made in chain gearing, and if there are, as "Chain Enthusiast" affirms, any makers who are wavering in their allegiance to the worm drive, they will derive equal benefit from a study of developments in the one case as in the other. And what is more, they will probably stick to the worm.
It is not a difficult task to manufacture claims for any mechanical device. Dr. Guillotine in 1789 doubtless claimed that the machine he advocated was simple, accessible, reliable, easily operated by unskilled labour and involved low initial and maintenance cost ; but it is only by being furnished with records of its,popularity and widespread adoption that we can be sure of this. In the same way claims may be made for both chain and worm drive, but a true index to their validity can only be found in the trend of actual practice, afer making allowance for the combined effects of fashion and conservatism. Both these last-named factors have lent their aid to the chain drive, for it attained a wide Popularity which conservatism would tend to retain. But, nevertheless, the worm drive is increasing its hold, and over 70 per cent. of the commercial vehicles in the United States, for example, are worm driven, whilst in this country the proportion is 80 per cent. Moreover, the largest individual axle manufacturer in the U.S.A., who might reasonably be expected to regard the question from every standpoint, adopted the worm drive, and came to England for it.
Double Reduction Favoured for Heavy Vehicles.
For heavy vehicles the choice of transmission seems to be centring again round the double reduction. One of the most notable examples of this is to be found in the latest design of transmission for the London buses. This, it will be remembered, is a combination of a high-efficiency worm gear, offset to give both a low load level and sufficient ground clearance, with internal spur gearing to each wheel. The high-speed, high-efficiency worm drive gives a light and well-mounted differential, efficiently protected and lubricated, whilst the speed of rotation permits light sections to be employed and thus reduces the weight to a point which the chain would have difficulty in equalling. Such a design would never have been considered if there had been any question of its efficiency or reliability or any risk of trouble in repair and maintenance.
A point which is sometimes advanced in favour of the chain drive is the ease with which the ratio may be altered. But is this always so simple? Even on a cycle a new sprocket is not so quickly mounted, whilst the further alteration to the length of the chain and the adjustment of the rear-axle position make it anything but a five-minute job. On the other hand, the worm-spur reduction permits a still easier gear-ratio change, for it is only necessary to substitute a new pair of gears on the intermediate shaft. This is true at least of one commercial design.
There is, in addition to, and perhaps more im,portant than, the points raised above, the question of the precise functions of a chain drive and a worm drive, and the extent to which each complies with theoretical requirements. The chain drive can only connect two parallel shafts, so that when mounted in a vehicle the engine of which has a longitudinal axis of rotation, there must of necessity be an additional right-angle drive including a differential, so that the chain simply becomes a competitor to a spur, helical or internal-gear drive. If, then, the gear ratio is not so great as to demand a double reduction, the cost of a chain drive, and the power which it wastes, is the price to be paid for putting the differential and right-angle drive in another place where it will be only a little lighter, less accessible, and, if two simple universal joints are to be saved, necessarily very accurately mounted. A chain, moreover, will not transmit uniform 'velocity between the driving and driven shafts, and the periodic accelerations and decelerations, occurring with the passage of every link round the sprocket, are conducive to high and rapidly recur ring stresses, which are not good for either the tyres or the transmission. This is apart from local stretching of the chain or wear of the multitudinous badly lubricated little bits which compose it. A worm drive, on the other hand, will transmit absolutely uniform velocity, whilst in addition to the bearings there are only two parts to it—the worm, of case-hardened, ground and polished steel and the wheel of bronze, which, after years of trouble-free service, will still have an appreciable scrap value. Being totally enclosed, the lubrication is maintained as it should be with very little trouble, and since with the most highly developed worm-thread forms of the present day there is always a continuous oil film between the engaging teeth, the rate of wear is negligible.
In the matter of efficiency Chain Enthusiast,, again makes sweeping, but discreetly vague, statements. He maintains that the chain driveprovides the most efficient transmission ; but he disregards the fact that there is still the intermediate right-angle drive causing loss, and this itself, if a bevel reduction, will be equal to the loss in a wormdriven axle. The extra loss involved by the chain Is thus entirely gratuitous, and even in the case of a double-reduction transmission the chain deficiency will still be greater than that of the corresponding spur or internal-spur stage.
The Efficiency and Durability of the Worm.
The difference between laboratory efficiency and road efficiency, which "Chain Enthusiast" so heavily labours, applies, if anything, more forcibly to the chain than to the worm. So far as the worm is concerned, there is very little reason to suppose that the efficiency is seriously affected by practical working conditions. In February of this year Mr. J. W. Hobson read a paper before the North-East Coast Institution, in which he stated that he had made tests on a very large worm gear used in the transmission of an internal-combustion locomotivel and had obtained an efficiency of 96 per cent. This figure will, of course, include bearing, churning and windage losses, so that the loss due to the worm drive proper must be very small indeed. More-' over, the extraqrdinary durability of well-designed worm gear is in itself an indication that the efficiency does not in practice fall off appreciably if .the lubrication be looked after. The worm and wheel, when on the road, are still mounted with considerable rigidity, are completely enclosed, protected and lubricated. The chain, on the other hand, is one of the most difficult things to protect and lubricate when employed as a final drive on a vehicle, and the discrepancy between laboratory and road tests is much more likely to be appreciable. Hence the statement that "the chain drive Itself is inherently" (why inherently?) "able to deal with the practical conditions without any lowering of efficiency" does not sound quite rational, somehow. A statement, too, that the losses in coasting are greater with worm gearing is grossly inaccurate at the present time, although true, perhap, many years ago. In any event, the point is trivial and has no influence on the issue ; It is only of value as exemplifying the vagueness of the knowledge prevailing on the subject of worm-gear performance generally. And it is not everyone who will answer after the manner of Dr. Johnson when asked why he had erred in a definition : "Ignorance—pure ignorance."
The final-drive controversy will continue to rage, and a full discussion, from first principles, as it were, is perhaps impossible at one sitting. The present writer only seeks? at the moment to join issue with the honourable protagonist of the Chain In respect of those matters in which the writer believes him to be in error, as determined by the article hereinbef ore referred to. If there are any other fallacies, or points of view, the more thoroughly they are ventilated the better. • c31