A LONDON BUS TRIAL Oj the MAYBACH GEARBOX F OR some
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reason, hard to explain, the average individual seems to have a natural antipathy towards changing gear more often than is absolutely necessary, and apparently on this account drivers of four-speed vehicles too often fail to take full advantage of the gears at their disposal.
There is, however, an unquestionable advantage in having as large a number of gears as is practicable and in providing that the ratios shall follow each other in easy steps. This is especially important where the economic speed range of the engine is narrow, as is particularly the case with oil engines.
Even in a four-speed box, there is a limit to the closeness of the ratios, as an emergency low must be provided and the obvious solution is to employ five ratios. But then one comes up against the human element and it is found that the driver is disinclined to make full use of the variations in ratio that are available.
In the Maybach gearbox the wide, range of close ratios has been provided, and the human element has been combated by reducing the operation of changing to a simple sequence of movements involving the minimum of effort. There is nothing intricate in the mechanism—it is simply a combination of a more or less conventional constantmesh four-speed box with an additional pair of sliding gears that can be brought into action to provide an emergency bottom, and the application to this gearbox of the patent Mayhach automatically synchronizing dog-clutch, operated by engine suction.
The complete gearbox shown in the s24 drawings on this page is that which has been employed with success on the Lagonda ear and, in principle, is exactly the same as the box made for heavy vehicles.
It will he noticed that the dogs of the clutches have sloping faces, and this simple fact makes it impossible for a pair of dogs to engage with each other until the speeds at which they are rotating are the same.
In view of the small difference in speed between each dog of the various pairs, owing to the close ratios provided, it is, therefore, possible to move any dog out of mesh and another into mesh without noise or shock, and with the minimum of delay.
The Pre-selective Gear.
The principle of the pre-selecting mechanism is to employ a vacuum cylinder to compress or extend a doubleacting spring, which imposes a load upon the selector arm. While power is being transmitted, however, the dog to be shifted will not disengage, as in addition to the end faces being machined at an angle the driving faces are slightly undercut. But the moment the drive ceases, which occurs, of course, when the driver depresses the clutch, the dogs are disengaged, by the spring pressure, and the next pair engaged at the instant the relative motion between them ceases.
Although, as at present used, only five speeds are requisitioned, it is actually possible to obtain eight different ratios from the 3laybach gearbox in its existing form. Reference to the four diagrammatic drawings of the gears will explain clearly how they are Pro
own shaft. The drive from the engine is applied to X and the power to the back axle transmitted from Z.
In top, the drive is through X, 1, 0, 2, Y, F, H, Z. For third, dog 1 is shifted, connecting X to A, and disconnecting X from C. Thus the drive is through X, A, B, D, C, and as before. For second, dog 1 is moved back to engage X with C; dog 2 is disconnected from C, and dog 3 is moved to connect E with D. The drive then goes through X, C, D, E, F, Z. For first, dog 1 links up A and X again, leaving C to idle, and the drive becomes A, B, E, F, Z.
To obtain the emergency low gear, H is moved along to mesh with G, the power travelling via A, B, G, H.
There are still three other ratios available. Were they employed the arrangements would be F, E, 0,H (low top), A, B, D, C, F, Id, G, H (low third), and C, D, 0-, H (low second).
A London General bus with a fivespeed Maybach gearbox installed is at the present time in ordinary daily service between West Hampstead and Plumstead, and we have recently had an opportunity for travelling on the vehicle and observing its performance.
In this case a box of somewhat different type is used and, of course, of considerably heavier construction. Third speed is direct, and top is an overspeed through wheels A, B, 0 and C. The ratios provided by the gearbox, we were informed, are 4.72 to 1, 2.28 to 1, 1.52 to 1, 1 to 1, and 1 to 1.5.
The controls consist of an ordinary gear lever which operates the sliding wheel (H) by hand, and a single lever below the steering column, moving about a vertical axis, in a quadrant having four positions' which controls the vacuum valve, through which the three dog clutches are actuated.
The small auxiliary cone clutch shown in the drawing of the box used on the Lagonda car is not employed in that installed in the bus.
The first procedure in starting away from rest is to declutch and engage the dogs on wheel la with those on wheel P. This gives second gear or "high first," which is the lowest required for all
normal circumstances. Then, as the vehicle gains speed, third is pre-selected by means of the handle under the wheel, the accelerator released, and the clutch disengaged. The gear then changes automatically and the process is repeated from any one gear to another, until it is desired to stop the machine and to put it into "neutral," when the ordinary gear lever is again used.
Movement of this lever also provides the emergency bottom when this is needed, and a third position gives reverse.
In operation it is possible to detect from the gearbox the noise made by the dogs riding over each other during the short space of time occupied in changing gear. The noise, however, is slight and can, we are informed, be eliminated by choosing a more suitable angle for the faces of the dogs. The gearbox in question is, of course, hardly yet out of the experimental stage.
On our journey to Plumstead the bus was in the hands of a driver who appeared not to appreciate fully the capabilities of the box installed, and he made the mistake of revving the engine up too much. before making 'upward changes. The rapidity with which the change could be made was further handicapped by the fact that the clutch-stop appeared to be acting inefficiently, and that the engine speed was slow to fall when the accelerator was released.
In consequence, our first impressions of the box, although they showed what possibilities it had, were not wholly favourable.
At the terminus at Plumstead a new driver took over and at once displayed signs of greater competence than his predecessor. The highest praise one can pay to a gearbox is to say that one is hardly conscious of its existence,. and during this part of the journey we found that it was only with an effort to concentrate on the job in hand that we could pay attentiOn to the gear changes which were continually being made.
Close Ratios Mean Economy.
The close ratios enabled an efficient engine speed to be chosen for all road speeds and the upward changes to be made early. In consequence engine noise and vibration were practically imperceptible, and the changes could be made in rapid succession. This is bound to reduce petrol consumption and
-wear and tear. Further, the change could actually be made while using both hands upon the wheel when turning a corner, or steering with one hand and giving road signals with the other.
At the Hampstead end of the route there is a long ascent up to the terminus. We have made this climb in other buses many times and it seemed
that on this occasion it was accomplished with less fuss and at higher speed than before.
Upon questioning the driver an the end of the trip, he told, us that he would unquestionably prefer driving this bus with the Maybach box to a bus with an ordinary box, given that other conditions were the same. He complained, however, that additional and unnecessary work was caused on this vehicle through the inefficiency of the clutch stop and the heaviness of the clutchoperating gear generally. He also expressed the view that the engine was a bad puller compared with other buses he was driving. The fact that this had not been particularly apparent to us is, perhaps, a further item of praise for the gearbox.
The conclusions we came to from this experience were that the fitting of a Maybach gearbox to a commercial vehicle does not necessarily make a good driver out of a bad one, but without doubt it enables a good driver to take full advantage of the engine power at his disposal and to drive more economically, and with less fatigue.
Perfection with this gearbox and operating gear has not yet been reached, but its possibilities strike us as being great. The results of experiments now . being made and of experiences being gained should enable such minor troubles as those in connection with the operating gear and the choice of clutch-face angles to be overcome and a sound piece of mechanism to be produced.
The sole agent, for Maybach gear. boxes in this country is Ernst Schneider, 18, Victoria Street, London, S.W.1.