The operation of the "Asynchro" box

A single coupler in the hydraulic part

Manufactured in the Montmirail workshops, the "ASYNCHRO" hydromechanical transmission has a single coupling in the hydraulic part, which avoids the use of torque transformers that are not very efficient. The result is an improvement in performance, a reduction in consumption and motor fatigue.

Access to our range of gearboxes

The combination of brake and clutch controlled by a single pneumatic piston placed at the entrance of the gearbox allows 2 positions:

Clutch Brake

  • 1°) engaged and disengaged
  • 2°) disengaged tight

It should be noted in passing that the piston does not require a rotary joint and that the stop works only during the short time the piston is moving.

The gearbox is stopped totally when shifting gears, and it is on a stationary gearbox that the appropriate interlocking for the chosen combination is carried out. There is no trick; in this gearbox the "synchros" are replaced by a single external brake, which can be properly dimensioned and as indisputable as the one that ensures the safety of millions of motorists.

The chosen brake is a dry or wet multi-disc brake with the best guarantee of longevity. A simple device, which blocks the gears until the gearbox comes to a standstill, ensures that the gears are only shifted when the gearbox comes to an absolute stop.


  • 1°) We get rid of the "synchros", and the gearbox is only a "classic" technique, hence the lower cost price.
  • 2°) By properly dimensioning the brake and the gearbox, it becomes possible to shift any power without any limitation and with all the desired "reasons".
  • 3°) Gear changes take place with maximum speed, which is no longer conditioned by the lowering of the engine speed. (With synchros, uneven operation was obtained depending on temperature, oil quality, or degree of wear).
  • 4°) The preselection of gears becomes possible, without false operation it is possible to shift any combination at any time and whatever the speed of the train.
  • 5°) As the change of combinations takes place on a box which is absolutely at rest, there are no more wearing parts in the box such as synchros, forks, crabs and coils .
  • Figures 2 and 3 show, by way of example, the implementation of this principle for 4-speed and 8-speed gearboxes.
  • The application of this transmission seems to be able to be extended both to railcars, because of its low weight and compactness, and to the most powerful locomotives, since it is easy to give them a size that ensures a very low fatigue rate for all its components.
Fig. 2 - 4-speed "Asynchro" gearbox, 200/250 hp
Fig. 3 - "Asynchro" 8-speed gearbox, 400/600 hp


We now describe the control of an 8-speed gearbox for engines from 500 to 600 hp (see fig. 4).

The gearbox is equipped with a pre-selective servo.

The mechanic's working on:

  • 1°) a gear selection lever (S) ;
  • 2°) an impulse button (I).

It places the selector on the chosen combination, and when it impulses: the gear change is carried out in an automatic way with full safety, whatever the number of gearboxes, in the case of twin-engine locomotives, or coupled in multiple elements.

Fig. 4 - Control diagram of the 8-speed "Asynchro" gearbox


On each gearbox there is a distributor that directs air to the Cv cylinders. This distributor is remotely controlled from the mechanic's desk, by means of a three-way valve A, B, C, - which we'll designate when they're supplied with compressed air by AAAp - Bp - Cp or escape by A0 - B0 - C0 - , thus gives the following eight combinations, each corresponding to a speed:

  • 1st A Bp Cp 5th Ao Bp Cp
  • A Bp Co 6th Ao Bp Co Co 6th Ao Bp Co
  • A Bo Cp 7th Ao B C C 7th Ao B C

As the preselection is made on a combination, the impulse has the effect of moving the spool of the two-way valve (D). to the right

This one by its channel 1 supplies the clutch-brake (Ef), the engine idling (Mr) and the direct solenoid valve (EV) on the supply side.

By its track 2 it releases the rod lock (F) which under the action of its spring is released upwards unlocking the three rods.

As soon as the gearbox is stopped, and this about a second to a second and a half after the impulse, the oil pressure of the gearbox pump (P(Pb drops to zero, the pressure switch (M) establishes the current on the solenoid valve (Ev) which admits the air to the gear distributor (Dv). The latter feeds the different cylinders giving the previously chosen combination, while, thanks to the retarder (R), a fraction of a second after opening of the solenoid valve, the air comes via the double valve (Vv to pressurise the rod lock (V) ; as soon as the rods are all in position, this one comes to be pushed down from top to bottom, controlling at the end of the stroke the valve (Z), which brings back towards the left the spool (D) whose track (1) is put at the exhaust, releasing the brake, engaging and putting back the injection of the engine.

While channel (2) comes via the double valve (V) to take over, to keep V locked.

The control of several boxes is identical, and a simple pulse triggers their operation. In order not to interrupt the tractive effort, it is then sufficient to place calibrated retarders on the tracks (I) of the (D), valves, the effect of which is to trigger the operation at different times on each box.

It should be noted that, thanks to the presence of the freewheel, it is possible to engage any combination of gears in the gearbox, whatever the speed of the vehicle, and that this control allows you to switch from any combination to any other.

source : Revue Générale des Chemins de Fer, June 1957

Related Articles

Hydro-mechanical transmissions

for railcars and high-power diesel locomotives

Manufacturers of high-power diesel locomotives generally use either electric ...

Contact us