New Maybach_ Multi - speed Preselective Gearbox A Transmission Component Neither Heavier
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Nor More Bulky than the Average Four-speed Unit, Affording up to Eight Ratios with Vacuum-operated Automatic Change WE referred last week, in an article on the " Variable Transmission Compromise," to a seven-speed gearbox that was shortly to he introduced by Maybach Gears. Ltd., Lincoln House, 296-302, High Holborn, London, W.C.1. Since then we have had an opportunity of investigating the performance of this unit, which, although designed for commeroial vehicles, has been installed in a Maybach 12-cylinder tar for 'demon
stration purposes. We were very favourably impressed with its behaviour, and regard it as marking a most interesting development. Arrangements are in progress, we understand, for its manufacture in this country.
Preselective and vacuum operated, the mechanism depends basically for its operation upon the well-known Maybach dog clutch. It incorporates a gearbox unit, on which are mounted three vacuum cylinders for shifting the dogs, a control unit, incorporated in which is a rotating-disc valve communicating with the gearbox cylinders, vacuum cylinders for operating the clutch and accelerator pedals, a master valve, a preselecting lever and a lever for engaging forward or reverse drives from a neutral position.
Despite these auxiliaries, it is claimed that the whole outfit is no heavier than an " average " fourspeed box of conventional design.
Before describing the interesting construction of the gearbox, we will outline the sequence of operations in lved in driving a vehicle in which this new Maybach system is installed. Let us start with a stationary vehicle, with the engine running and the large gear lever in the neutral position. Then the clutch pedal is depressed, thus disengaging the clutch through a vacuum servo apparatus, and the lever moved to the forward position, where it remains untouched until neutral or reverse is required.
Assuming the preselective lever (mounted under the steering wheel in the usual position) to be at " first," on letting in the clutch the machine moves off. " Second" is then preselected, the foot removed from the accelerator and the clutch pedal slightly depressed, which action causes second gear to take up the drive. The same operation is required for all other changes, up or down, and any gear can be engaged from any other; for example, one can change directly from " second " in " fifth."
Vacuum Control Arrangements.
Preselecting moves the disc valve, thus causing --,ertain vacuum ports to coincide. The initial clutch-pedal movement opens a master valve which puts into communication with the vacuum tank (or induction manifold) the disc valve, the accelerator cylinder (in the case of a change-down) and the clutch cylinders.
As a result, the appropriate dog clutches in the gearbox are moved, the clutch is disengaged and the engine is speeded up. So soon as the dogs mesh, the vacuum is automatically cut off and air admitted to the cylinders.
Turning now to the gearbox itself, it will be seen from an accompanying illustration that it contains four pairs of wheels. Three of these have helical teeth and are in constant mesh, whilst the fourth has straight teeth, one of the pinions being required to slide for reverse.
A leading feature of the box is that the lowest reduction is not afforded by a single pair of wheels, but by a train of gears. Because of this fact, the shafts can be mounted considerably closer together than would otherwise be possible. This means that all wheels can be of smaller diameter and the width of the box less. Additionally, there is a bearing between every two -wheels, with the result that lighter shafts can be used. In this way the remarkably low weight of the gearbox is explained.
Briefly, the layout qf the shafts and wheels is this: Referring to the illustration showing the gears, the drive is imparted to the left-hand end of the upper shaft, which is a unit with its Wheel. Spigoted to this is a shaft carrying only the dog. Then, on. the same axis, there is an independent wheel. On its right is .another short
shaft carrying a dog and the third wheel on a spline. Finally, there is the tailshaft on which floats the large straight-toothed wheel.
Considering the layshaft, we have On the left a gear fixed on a short shaft, an independent gear, a shaft carrying only a dog, a wheel fixed to a shaft, and on splines on the last-named the small straight-toothed wheel.
Mainshaft dog number one (left to right) has teeth on the right. Number two has teeth on the left, and number three teeth on both faces. On the layshaft the first dog has the right-band face toothed, whilst on the other dog. the teeth are on the left. Ball and roller bearings are used throughout.
In order that the method of obtaining the eight possible ratios may be understood, we will number. the mainshaft wheels 1, 3, 5 and 7, and the layshaft wheels 2, 4, 6 and 8. In first gear the drive is 1, 2, 4, 3, 5, 6, 8, 7; in second 1, 2, 8, 7; in third I, 2, 4, 3; in fourth 5, 6, 8, 7; in fifth 1, 2, 6, 5; in sixth 3, 4, 8, 7; in seventh direct; and in eighth (an overdrive) 3, 4, 6, 5.
Typical Close Ratios Available.
With regard to the actual ratio figures, there is a variety available. A typical six-speed box for a bola chassis gives 5.15, 3.45, 2.3, 1.55 and 1 to 1, and an overdrive of I to 1.48. None of the intersteps exceeds 1.48, whilst the overall interstep-that is, 5.15-to-1 to 1-to-1.48-is 7.5.
For a goods chassis, with an overall interstep of 9.75 and seven speeds, the ratios are 6. 75, 4.65, 3.26, 2.08, 1.45 and 1 to 1, and an overdrive of I to 1.45.
An interesting point in connection with the vacuum cylinders, which is important as it simplifies manufacture, is that the pistons are fitted to these with a big clearance and work without lubrication, the aluminium alloy of which -both cylinders and pistons are made' being treated to give a. high degree of surface hardness.
They cannot seize and an air filter on the intake excludes foreign matters from the system, whilst air leakage. past the . pistons is of little consequence because the vacuum acts only momentarily and is not used to -hold any part of the apparatus in position.
An Illuminating Test.
An interesting comparative test, made prior to the announcement of this new box, struck us as illuminating. A vehicle was driven over a 100kilometre course, first with a four-speed box and then with a six-speed box installed, the latter incorporating an overdrive. The same speed was averaged in both cases and at all points the same speeds were maintained so far as possible. With the fewer ratios the crankshaft made 122,000 revolutions, and, in the other case, only 84,000. It is generally recognised that high engine speed, especially with small throttle openings, is responsible for
heavy oil consumption. In another series of tests the quantities of lubricant used were 5 litres and 1 litre re
spectively, whilst a saving of 16-20 per cent. of fuel was recorded.
In this case three sets of trials were conducted to include all conditions. One was carried out on the German motor highways, another in thick traffic and the third on a road-racing track where hills, sharp bends, etc., are numerous.
An object of the designers of this gear-change system Was to provide a system of control • which, whilst foolpreof and simpledomande:di froth the driver no operations different from. those he would be accustoined to petform.. When handling the -Maybach car ourselves we found, that, although it is impossible to clash the gears or to go wrong in other respeas, a degree
of skill is required to avoid interrupting the smooth progress of the vehicle. It seemed that this was due to the fact that after automatic engine acceleration when changing up, the throttle is suddenly allowed to close, creating a slight jerk. The knack of holding the accelerator pedal with the foot to prevent this, however, is soon acquired.
In other respects, the exactly right ratio is always available, and the ease of changing gear is such that, instead of being tempted to shirk the operation one definitely enjoys inakingsuse of the facilities afforded. There is no doubt that their value on a heavy vehicle is even greater than CM
high-powered car on which we gain d our experience.