Oil pressure control valve assembly for hydrostatic transmissions

An oil pressure control valve assembly for a vehicle hydrostatic transmission which comprises:

• a first port (21) connected to an oil path connecting between a hydraulic pump and hydraulic motor of the hydrostatic transmission;
• a second port (22) connected to an oil-charging path from a charging pump for supplementing working oil to the hydrostatic transmission;
• a differential piston (23) operable as a relief valve member for relieving oil from said first port (21) to said second port (22), said differential piston having at the side of said first port (21) a first pressure-applied surface area (S.sub.1) and at the opposite side a second pressure-applied surface area (S.sub.2) which is larger than said first surface area (S.sub.1);
• a throttled oil passage (24) for conducting oil pressure at said first port (21) to said opposite side of said differential piston (23) having said second surface area (S.sub.2);
• a control piston (26) disposed co-axially with said differential piston (23);
• and a valve spring (25) adapted to bias said differential piston (23) through said control piston (26) to move toward said first port (21), characterized in:
• that there are provided a spring-receiving member (27) for receiving an end of said valve spring (25) at the side opposite to said control piston (26) and a stopper member (28) for limiting a movement of said control piston (26) in a direction away from said differential piston (23) such that said spring-receiving member (27) and said stopper member (28) are adjustable in position independently of each other in a direction toward and away from said control piston (26).