The Differential Lock,
Page 7
Page 8
If you've noticed an error in this article please click here to report it so we can fix it.
Proved to be Desirable Under War Conditions, Is It Necessary for the Commercial Vehicle in Normal Times ? One or Two Examples are Illustrated and Described.
One of the first stumbling blocks to the novitiate in the workshop where motorcars or lorries are made, is undoubtedly the differential gear. What is its use? How does it work ? Why---? Simple enough questions, and easy enough to answer in concrete form, with an example of the gear before one for demonstration purposes, provided the apprentiee be not of a too inquiring nature. Even on paper, the purpose of the differential is simply defined, although an academic discussion of the theory of its mechanism might well prove to be too highly technical for the majority of lay readers. Happily, we are not on this occasion concerned with such discussion.
The Dill. Gear Simply Defined. Stated in simple language, the differential gear is an arrangement of gearwheels, so placed as to divide one driving wheel from its fellow and, whilst still maintaining the sum of the velocities of the two wheels as it would be without a differential, to allow one to overrun the other as the direction of travel may require. For example, it is necessary when rounding a curve, say, to the right hand, for the nearside wheel to travel at a greater rate than that on the offside ; with the aid of the differential, this is possible without the vehicle's suffering, in consequence, any diminution in speed.
The Case for Elimination.
It is not so very long ago, in point of actual time, although it seems ages ago when considered in the light of happenings since, that our sister journal, "The Light Oar and Cyclecar," was discussing the advisability of eliminating the differential gear from the anatomy of the cheapest types of car. A little while before that, and an older member of our journalistic family, "The Motor," was debating its pros and cons as a necessary component of a racing car.
In the case of the former, the expense of its fitting was the basis for argument ; in the latter, economy of tires, under the very special conditions •of racing, formed the principal theme of the deliberations. With neither of these reasons are we for the moment concerned. We are not even considering the question of the removal of the differential from the scheme of things, but rather the advisability of providing means for rendering it inoperative at will. This accessory gear is, at least, necessary on all commercial models. In many, it must be capable of becoming inoperative. Differential Limitations.
In normal times, and with ordinary road surfaces, the gear is a good servant, allowing the power to be usefully expended at the road wheels whilst rounding a curve, without unnecessary abrasion of the tire surfaces. In order automatically to perform this function, as it is usually arranged, it allows the road wheel which offers the least resistance to overrun the other. While this is quite satisfactory for the end in view, the resultant effect, in certain circumstances when the roads are so greasy that the tires are unable to obtain the necessary grip of the surface, is very far from being a desirable one. If, on such an occasion, one wheel fails to grip, the differential allows the • whole of the power to be transmitted to that wheel, and as a consequence none of the driving force is available at the circumference of the other one, which could utilize it to advantage. The result, of course, is that the vehicle loses all power of self-propulsion, for the time being, and recourse has to be had to external and artificial means of preventing the slip. . Such a happening is, fortunately, not so frequent in civilian life as when working under war conditions, with the roads cut up, and practically destroyed, by the rapid and continuous passage of the heavy vehicles employed under these circumstances.
It will readily be understood from what has been stated, that, if it were possible easily to render the inoperative, noperative, progress under the difficult conditions outlined above would be considerably improved. Such facility is already provided on all steam wagons, on which, in the case of the steel-tired ones, the liability to slip is much greater. On some American-made petrol lorries, too, a differential lock is standardized, and the Bayard two-tonner, for instance, as exhibited at the last show at Olympia, was also so fitted. Steamers are invariably so equipped.
Is It Required for Civilian Usz.?
The. fact that its practical application has been accomplished, is, perhaps, a sufficient indication of the need for a differential lock. It is debatable as to whether it is needed for city and town roads, or even for country roads in reasonable condition, and not traversing any exceptional gradient. The necessity for it has not been sufficient to create a demand for its standardization on vehicles for civilian use. It is possible, however, as owners become more exacting,
and more alive to the minor disabilities of the machines they use, that a demand for this additional refinement may arise ; the extended use of the motor in outlying country districts, where heavy gradients and had going are more the rule than the exception, will also create a similar tendency in the requirements of the user.
If we consider for a moment the conditions that hold in the Colonies, and Overseas districts, it will quickly be realized that they are closely related to those outlined at the conclusion of the last paragraph. The need for the provision of some means of rendering the differential inoperative has frequently been insisted upon by correspondents from the various dis tricts concerned. Its inclusion will inevitably, we think, become the rule rather than the exception', on all models specially designed and built for Colonial work.
The Government Will Ask For It.
In the future W.D. specification the differential lock will undoubtedly be demanded ; its omission has been almost invariably deplored whenever a discussion of the merits and demerits of existing models has taken place amongst those who have had experience of actual service conditions.
From a careful consideration it would appear that the policy of the designer, with regard to this important fitment, should be to keep in view the possibility of an occasional request for its provision, from the home market ; a more insistent demand from Overseas users ; and the certainty of its early inclusion in the W.D. specification for lorries entitled to subsidy.
A Few Illustrations.
This article may well be concluded by a brief reference to one or two examples of actual applications of the differential lock, followed by an indication of the methods which could be adopted in constructions which have not hitherto been so treated. The commonest application is that illustrated by our first drawing, which shows how Alley and MacLellan, Ltd., arranges to cut out the differential gear on its steam lorries. It should be noticed that,
in order to put the lock into effect, the driver, or his assistant, must descend from his seat. Referring to• the• first illustration, it will be seen that the wheel shown is driven by the axle sleeve ; the other one is on the axle. In normal circumstances these are connected only through the medium of the differential. By throwing the differential locking pawl into gear with the ratchet wheel keyed to the axle, both sleeve and axle are made to revolve as one. A similar principle underlies most of the steamwagon differential locks, with such modifications in detail as have suggested themselves from time to time to the designers concerned. A notable exception is the Burrell, which can be operated by the driver from his customary position in the cab. Our second illustration is of the differential locking gear as applied to the Knox tractor, an American machine, and petrol-driven. It is operated by similar mechanism to that utilized for changing the gears in the box and is manipulated from the cab by means of a pedal. The engagement of the clutch shown locks the differential case to the crass-shaft and renders the gear inoperative. The third drawing shows an alternative method which may be applied to chain-driven lorries ; in operation it is very similar to that of the Knox.
Locking by an Original Method.
The last figure illustrates a suggested method of locking the differential gear in the case of live-axletype heavy vehicles in which the final drive is by double-reduction gear, bevel and spur. The device consists of a pair of spur pinions, connected together in such a manner that they are bound to revolve together. In the example shown they are made in one piece with a connecting sleeve. They are mounted so as to have a bearing on an eccentric shaft, and, by movement of this shaft, they may be thrown in or out of mesh with the main spur wheels attached to the live-axle shafts. When in gear, they effect the locking of the differential by compelling the two wheels to revolve with equal velocities. When out of mesh they remain idle, and allow the differential to perform its proper function.
A Field for Designers.
Other methods, perhaps equally efficient and simple, may suggest themselves to the designer and user. Modifications to suit the various alternative arrangements of gearing in use will also be necessary. In the case of worm-drive axles, and again of those' in which the final drive is by spur or bevel reduction. in the road wheels, or where the reduction in the rear axle is a single one only, the method referred to in No. 3, with slight alterations, could readily be used.