Ingenious Principles of de Normanville Gear
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An Interesting Epicyclic Gearbox with Hydraulic Control, that May be Employed for Commercial Vehicles. Excel
lent Efficiency Obtained
ON Wednesday last it was announced that Humber cars are to be equipped with de Normanville epicyclic gearboxes. In view of the association between Humber, Ltd., and Commer Cars, Ltd., of the 'fact that the gearbox has been tested out on a Commer lorry, and of its intrinsic interest, its mechanism merits a brief description in our pages. Moreover, we are informed that there is a possibility of its being offered, at some future date, as a transmission unit for Commer chassis.
An outstanding feature of the gear is the method of control. Employing epicyclic trains of gearing of more or less orthodox type, the gearbox incorporates an hydraulic system of holding and releasing the drums for selection of the desired indirect ratios.
Gear Not Pre-selective.
The device is not pre-selective, the change being effected in the usual manner in conjunction with appropriate use„of the clutch, whic.h is of the normal single-plate foot-operated type. As a result of extensive experiment the designer has come to the conclusion that, in the long run, the automatic clutch is less satisfactory than that which demands the exercise, on the part, of the driver, of a small amount of skill.
In pre-selective arrangements, each gear change requires two operations, chronologically separated, and although 'these may be extremely simple. the mental effort needed for the ordinary system is thought to be smaller.
A similar lever mounted on the steering column, however, is employed, thus obviating the necessity of having a gear lever in a position which probably impedes the driver from entering and leaving his seat.
One of the accompanying illustrations shows how this lever opens and closes the oil valves, whilst the other explains the means for operating the brake shoes by oil pressure. The last-named is maintained by a pump, which is worm-driven from the main shaft, and is stored in a cylindrical-reservoir, provided with a spring-loaded piston. When this piston has travelled far enough down its stroke for a stipply of oil under pressure—sufficient for two series of gear changes—to have been accumulated, it uncovers a port which acts as a relief valve, and allows oil to escape for gearbox-lubrication purposes.
The working of the brake-shoe operating gear is easily comprehended. Oil entering between the pistons causes them to move apart, that on the right to pull on the left-hand shoe and that on the left to apply the right-hand shoe by pressure. The forces on both shoes are equal, and the return spring, working in conjunction with the adjustable stop, positively withdraws them.
It is claimed that each brake can exert double the power required to resist the torque imparted to any drum by the maximum engine output.
It should be appreciated that its function is only to hold the drum stationary, the gradual retardation or acceleration of the rotating members beingeffected by the clutch. There
is, of course, a brake for each unit of the train of gears, and a control valve for each brake, These devices are ingeniously simple. Rotation of the hand-lever spindle, until the cam depresses the plunger, opens the valve, by pushing the ball off its seating, thereby sealing the escape orifice and allowing oil to flow to the operating cylinder. Reverse or further forward motion of the cam allows the spring to replace the ball on its seating, cutting off the supply of fluid, whilst back pressure from the brake, caused by the shoe-return spring, acts upon the shoulder on the plunger, lifting it and so permitting the oil to escape by the central passage back to the pump.
Effortless Control.
In this manner one or more of the epicyclic trains can be brought into use, to afford the desired ratio, with the minimum effort or delay and without the exercise of any driving skill. Top gear is obtained by locking the mechanism and causing it to rotate as a solid mass.
It is claimed that the flywheel effect thus created is beneficial to the smooth running of the vehicle.
The gears are locked by small cone clutches, a feature of which is that they are positively released, when the spring pressure is removed, to eliminate the possibility of their sticking or binding. Another point worthy of mention is that the spring thrust is not resisted or transmitted by a bearing while top gear is engaged. After the fourth-gear position on the controlquadrant, there is a notch for coasting, in addition to the neutral position between " first " and " reverse."
Remarkably high efficiency figures for the de Normanville gear have been obtained in all ratios. These are largely due to the fact that roller bearings are used for every wheel and shaft, whilst the drums do not dip into an oil well, thus there is no oil drag.
Proved by Exhaustive Trials.
The gear should possess excellent lasting qualities, and its capacity for standing up to abuse and hard wear has been proved in a long series of exacting tests carried out under conditions of extreme difficulty. For example, a team of drivers devoted several weeks to subjecting cars, in which it was installed, to gruelling trials in the Swiss Alps, and a fivethousand-mile journey, through the Sahara Desert, was carried out with a caravan having a gross weight of about 3+ tons. During this expedition an average of 150-200 gear changes per hour is estimated to have been made.