The ASYNCHRO transmission

Power transmission

The difficulty in transmitting high power in mechanical gearboxes does not come from the gears, which, when properly dimensioned, have no other limits than that of the peripheral speed of the teeth, but from the difficulty in coupling, when changing combinations, pinions and shafts with high relative speeds and inertias that increase very rapidly with the size of the pinions.

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The principle of Asynchronous Transmission

In the Asynchro transmission, in order to avoid synchronization devices, the gearbox is completely isolated and is stopped when changing combinations by means of :

  • a) the freewheel at the output;
  • b) a Coupler-Brake at the input.

The jawing manoeuvres are thus carried out on immobilized shafts and gears

It should be noted that the gearbox comes to a complete stop in less than a second, although the braking torque is only a small fraction of the engine's maximum torque.

As an example for a gearbox with 8 combinations, transmitting a power of 800 hp, it is sufficient to apply a brake torque equal to 1/12 of the engine torque to obtain the gearbox stopping in one second in the worst case. This is equivalent to braking a single automobile wheel.

Description of an Asynchro transmission

The Asynchro transmission essentially consists of a brake-coupler, placed on the input shaft of the gearbox itself, and a freewheel on the output shaft.


The combination of brake and coupler, controlled by a single pneumatic piston placed at the entrance of the gearbox, allows two positions:

  • 1°/ closed-released
  • 2°/ open-closed.

Brake and coupler are made up of stacks of discs, running dry or in oil depending on the type of machine.


From a constructional point of view, gearboxes generally have 6, 7 or 8 combinations. All constantly meshing gears are cylindrical with ground spur gears. The spur gears and the fixed pinions are mounted directly on the shaft splines, the idle pinions are mounted on roller bearings and have lateral internal toothing for spur gearing. The teeth of the sprockets have a special slope to facilitate engagement, which is always in the same direction.

Lubrication is provided by sprinkling the gears through a vane pump at the end of the gearbox input shaft

The freewheels

The freewheels are specially designed by C.F.D. Their hubs are mounted on the splines of the gearbox output shaft. The rims have a dowel pin and centering corresponding to the Glaenzer cardan plates.

They do not require any maintenance. The oil level is only checked during general machine inspections.

Several hundred of these freewheels have been in service for several years.

Automatic control of the Asynchro transmission

Asynchro transmissions generally have automatic control based on axle speed, engine speed, and the power supplied by the engine.

The mechanic has no worries, since gear selection is done with greater precision than that of a good driver.

Two or even three locomotives can also be operated with a single agent.

This makes the operation identical to that of a locomotive with hydraulic or electric drive.

It is possible for machines with the electro-pneumatic control of the "Asynchro" transmission to be given a controlled automatism, the instant of the gear change being determined by a reflex movement of the mechanic.

source : Article "Hydro-mechanical transmissions for railcars and high-power diesel locomotives" by Pierre ZENS, engineer.

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