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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 the tappet 1 is fully
out.
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.
13
13
INCHING VALVE OPERATION
(TH 7 Valve)
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