Control Cartridge Operation........................................................................................................................... 100 à

INCHING VALVE OPERATION (TH 7 Valve on Brake Pedal)

With the foot off the inching pedal, the tappet 1 is fully in and the oil passes unhindered between port P (pilot pressure in) and port A (pilot pressure out). The oil flows freely through the opening 3 in the slide valve 2.

The pilot pressure, injected into the control cylinder of the pump cam plate, is therefore identical to that which would be obtained without this TH 7 valve.

With the foot on the inching pedal for a slow approach, the tappet 1 is partially out; the oil passage is calibrated by the opening 3 in the slide valve 2.

The pilot outlet pressure through port A will therefore be controlled and the value of this pressure will be constant. If the pilot pressure entering through port P were to increase, as this pressure would be applied to the section S of the slide valve 2, the springs 4 and 5 would be slightly compressed and the slide valve 2 would move in the direction of the arrow F. This would reduce the oil passage through the opening 3 and in turn create a drop in pressure between port P and port A. The pilot outlet pressure through port A is therefore controlled and the value of this pressure is constant whatever the level of pilot pressure entering through the port P.

This means that the pressure injected into the control cylinder of the pump cam plate will be constant, and therefore the angle of the cam plate will also be constant. This allows the truck to approach slowly and at a constant speed selected according to the pressure exerted on the inching pedal.

With the inching pedal pressed to the limit of travel before the truck brake control comes into force, the tappet 1 is fully out (this position with the slide valve out is retained when the pedal at the limit of travel controls the truck brakes).

With the tappet 1 in this position, there is no pilot pressure at port A, and therefore no pressure in the control cylinder of the pump cam plate. The pump displacement is therefore nil.

NB : It is understood that the pilot pressure leaving through port A is always guided: • on the one hand towards the pump control cylinder, • and on the other towards the hydraulic motor displacement control unit.

CONTROL CARTRIDGE OPERATION

The pilot pressure is drawn via the control cartridge slide valve into the booster circuit. At maximum IC engine speed, the pilot pressure is a few bars lower than the booster pressure.

On idling (850 rpm), the booster flow which passes through the calibrated opening in the control cartridge slide valve is insufficient to create a drop in pressure capable of displacing the slide valve 1 against the force of the spring 2. The pilot pressure is therefore nil.

When the operator accelerates in order to move forward (to approximately 1050 rpm), the booster flow increases ; the drop in pressure before the calibrated opening is now sufficient to move the slide valve to the left, against the spring, thus revealing the pilot opening.

When the pilot pressure reaches approximately 6 to 7 bar, the servocontrol cylinder begins to move, tilting the plate of the variable capacity pump. The pump begins to discharge (start of control). The greater the engine speed, the more the booster flow increases thus leading to a rise in the pilot pressure and therefore increased influence on the pump and greater machine speed.

To halt this process, the operator stops accelerating, the booster flow drops, the pressure drop is reduced and the spring 2 pushes back the slide valve 1 thus closing the booster passage to the pilot conduit. The residual pilot pressure is discharged towards the housing.

Servocontrol cylinder

Booster pressure relief valve

Item 2

Start of control adjusting screw CONTROL CARTRIDGE OPERATION

Variable capacity pump

FWD/N/RR drive solenoid valve

Item 1

Control Cartridge

Booster Pump Accelerator