The Advantages of the Silent Third.
Page 52
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The Editor, THE COMMERCIAL MOTOR.
[3984] Sir,—We beg to refer to the letter of Mr. Michael Field-Richards on the subject of the "silent third" on commercial vehicles.
As the manufacturer of the vehicle which he probably has in mind, we should like the opportunity to enlighten him as to some of the advantages to be gained by adoption of the gearbox so fitted. There are at least four distinct mechanical advantages, apart from the absence of noise in operation.
In a well-designed box of this type it is possible to obtain approximately 50 per cent, greater gear width and a consequent reduction in tooth pressure on this much-used gear, resulting in considerably longer service.
Secondly, in spite of this added tooth length, it is possible to reduce the overall length of the box,
thereby minimizing the main and layshaft deflections. Thirdly, with double-helical-type gear there is no 17438
tendency for the gear to wander out of mesh and consequently damage the teeth.
Fourthly, probably the greatest advantage is the absence of gear-clashing, which is easily one of the primary causes of third-speed-gear failure. This third speed is probably the most-used ratio in the lorry gearbox, and the constant 'dashing of these gears when changing speed usually produces chipped and deformed teeth.
It may be said that the gear-cutting is more expensive than with a pair of straight pinions, but the extra cost is easily offset by the fact that helical teeth need not be ground.
We can assure your correspondent that, apart from its mechanical advantages, the adoption of the "silent third," or "traffic top," as it is sometimes called, is just as likely to be beneficial, if not more so, to the commercial-vehicle user as to the owner of a private car.
We consider that anything which goes towards minimizing the driving effort of the commercial vehicle is a commendable asset.
P. G. Rost, Chief Designer, For Morris Commercial Cars, Ltd. Birmingham.