AT THE HEART OF THE ROAD TRANSPORT INDUSTRY.

Call our Sales Team on 0208 912 2120

Fixed versus Live Axles.

17th August 1905
Page 8
Page 9
Page 8, 17th August 1905 — Fixed versus Live Axles.
Close
Noticed an error?
If you've noticed an error in this article please click here to report it so we can fix it.

Which of the following most accurately describes the problem?

An illustrated explanation of the function of the differential gear in a motor vehicle was concluded in last week's issue. Following up the subject, this article is intended to make clear the distinct advantages of fixed and live axles and the methods of driving with them. A live axle is one which turns with the wheels, whereas a fixed axle does not turn, but the wheels revolve on the ends of the fixed axle and are driven independently. In the article on differential gears, to which allusion has already been made, it was explained that it was necessary to have mechanism to allow the two wheels to revolve at differential speeds when the vehicle was turning a corner or following any path other than a perfectly straight line, and it was further explained that the differential gear for effecting this may be placed on the hind axle, or on a separate differential countershaft, in which latter case each wheel would be driven separately from one end of the countershaft.

In the earlier forms of motor vehicles, always excepting the pre-Victorian motor omnibuses, the drive was by live axle, with the differential gear in the centre of the axle, the "

spider" being driven by a chain. It is. obvious that this would be so when it is remembered that the pleasure motorcar was developed from an ordinary tricycle, which was generally fitted with a live back axle having the differential gear in the centre. In looking back at illustrations of these earlier vehicles one cannot but be struck by the marked family resemblance to the ordinary tricycle. To this day Many of the small runabout cars imported from America employ this type of chain drive with live axle. But whilst the pleasure car was developed on the lines of the tricycle, the heavy motor lorry, which was the first type of commercial motor to become popular, was an offspring of the traction engine. The traction engine of 1896 was generally fitted with a live axle having a differential gear close up to one of the wheels, so that the axle could be a straight one, going right through both wheels. One wheel was fixed to this axle by a driving block or key, and the other turned on it, being driven from one of the bevel spur wheels of the differential gear, which bevel spur wheel was bolted direct Co the toad wheel. It is because of this origin that many of the successful motor lorries we see to this day have such a form of axle. Fig.I shows the latest of these axles as employed in the Bretherton and Bryan motor lorry, which vehicle was fully described in our issue of May 4th. This is a typical traction engine axle, but it differs from the common type in the absence of keys or keyways, the right-hand wheel and its differential bevel spur wheel fitting to squares on the axle, instead of being keyed. It will be noticed that the left-hand road wheel is bushed with a phosphor-bronze bush so that it may work sweetly on the axle. In these figures the road wheels have been sectioned black to gain distinctness, but it is clearly shown how the left bevel spur wheel is bolted to the left road wheel, and how the differential " spider " turns on a bronze bush, half on the left road wheel and half on the right bevel spur wheel. The differential is fitted with three bevel pinions; consequently only one of these is shown in section. This is undoubtedly the best form of live axle, properly so called, because there is no break in the continuity of the axle itself, which is a steel ingot and continuous from end to end.

Fig. 2 shows a live axle as employed in the Wilson-Pilcher pleasure cars, which are made by W. C. Armstrong, Whitworth and Company, Ltd. Although this company has not yet manufactured commercial motors, the axle is typical of many of those employed in delivery vans. It will be observed that the axle and differential are enclosed in an oil-tight casing, one half of which has been removed in the illustration, and each half shaft is provided with two bearings, one as close to the differential as possible, and the other as close to the wheel as possible. The differential, in this instance, is a planetary one, but throughout the mechanism helical gears have been employed instead of plain spur wheels, which make the car silent, Although the planet wheels are clearly visible in the illustration, a casual glance might lead one to think that this was a bevel differential. In reality, the bevel wheels provide the means of driving and reversing. In the illustration the gear is placed for going forward, and the bevel pinion which takes the drive from the crankshaft engages with the left-hand bevel wheel. liv handling the proper lever the bevel pinion can be moved sideways, until it disengages from the left-hand and engages with the righthand bevel wheel, and then it drives the car backwards.

It is interesting to consider the big strain to which a live axle is subjected. The drive is transmitted to the axle from the differential, and the resistance is at the point of contact of the wheels with the road. There is, therefore, a twisting or torsional stress throughout the length of the axle, from the point where the bevel wheel is secured to it to the road wheel at the end. At the same time it is loaded on the bearings with a large part of the weight of the vehicle and its cargo, each bearing being usually about one quarter of the length from the end of the axle. There are, therefore, forces at the two bearings trying to press the axle down, and at the extreme ends the two road wheels are bearing it up. This

means a tendency to buckle the axle so that it sags down in the middle. Now if one wishes to break a stout stick and finds it too strong to break at once across one's knee, one gives it a partial turn, strains it as much as possible, another partial turn and again strains it, and so on until, as a result of the frequent strains, one is able to break it with a sharp snap. This is exactly what is happening with a live axle; the load transmitted through the bearings is trying to break the axle and at the same time the axle is being turned round, so that this breaking stress is acting differently from instant to instant. To use the correct dynamic terms each portion of the axle is alternately in tension and in compression, and it is a well-.known engineering fact, illustrated above by the simple example of the stick, that a shaft will break with a very much lighter load constantly applied and reversed than it will under a much greater continuous load. But the facts are much worse with a live axle, because in addition to the carrying strain, which is being continuously applied and reversed as the axle turns, there is a torsional strain due to the driving. It follows, therefore, that a live axle is very much weaker than a fixed axle, or that to get the same strength a very much larger and heavier live axle must be used.

Fig. 3 shows a photograph of a live axle which broke in two on a steam wagon. It will be observed that the fracture has occurred close to the point at which the left-hand bevel wheel is keyed to the axle. This place is the one at which the torsional strain commences, and the bending strain will be almost at its greatest. It is a curious fact that, with the excellent material generally employed at this day, an axle seldom breaks off completely with one sudden crack. The writer has examined some score of broken steam wagon live axles during the last few years, and in every case there was a portion of the axle, of about one-eighth of its total crosssection, which had been the last to break. The remainder of the metal had evidently fractured on one or more previous occasions, because the broken ends had rubbed against one another and produced a smooth polished surface. For how long the axle had been running with seven-eighths broken through it was impossible to say, but in two specific instances the writer has known the axles to break when the wagon was empty, clearly proving that the mischief had been done at some earlier time,but that the last piece to go had only broken across when bumping home over a bad road with an empty wagon. Although in pleasure cars little objection can be taken to the employment of live axles, they are open to serious objections for commercial vehicles. However much we may preach against the evil practice of overloading, it must he remembered that a commercial vehicle is sure to be overloaded now and again, either intentionally or through ignorance, and it is this which is so fatal to the live axle. Unless it is of abnormal dimensions it is cracked or broken by the overloading, and the vehicle is stranded on the roadside with a broken axle, frequently at the time when its services are r—

most in request, and always involving great expense and trouble to deliver the load with which it started out, and to get the vehicle home.

Tags


comments powered by Disqus