| 摘要 |
<p>PROBLEM TO BE SOLVED: To stably control boosting irrespective of an unevenness of clutch precision or a temperature characteristics of a directional control valve in a oil hydraulic circuit which selectively supplies pressure oil to a plurality of hydraulic clutches by operation of the directional control valve. SOLUTION: A pressure oil discharged from an oil hydraulic pump 44 passes through a pressure reducing circuit 45, and a part of it is supplied to a forward/ backward drive switching clutch 31 via an oil passage 49 and a solenoid-method directional control valve 46. A branch oil passage 52 is arranged in the oil passage 49, and a solenoid-operated proportional relief valve 53 is provided therein. The solenoid-operated proportional relief valve 53 is initially set so that a pressure oil supplied to the forward/backward drive switching clutch 31 is at a minimum operating pressure for a clutch piston when a solenoid 54 is off. The solenoid 54 is not turned on until a fixed time after the directional control valve 46 is switched. After the clutch piston 41 or 43 of the forward/ backward drive switching clutch 31 reaches its stroke end, the solenoid 54 of the solenoid-operated proportional relief valve 53 is turned on to start a boosting control, thereby remarkably reducing a shock of connecting a clutch, and attaining a smooth boosting characteristic.</p> |
