Power transmission control apparatus

Claim 1

A power transmission control apparatus comprising a main change speed control valve (V2), a hydraulic pump (P) for supplying pressure oil to the main change speed control valve (V2), hydraulic cylinders (18a-23a) for controlling clutches, a modulation valve (MV) disposed between the main change speed control valve (V2) and the hydraulic pump (P) for gradually increasing a hydraulic pressure for the hydraulic cylinders (18a-23a) after the hydraulic cylinders (18a-23a) are filled with the pressure oil by operating the main change speed control valve (V2) to connect the hydraulic pump (P) to the hydraulic cylinders (18a-23a), and a backward and forward switch valve (V1) connected to an output port (P1) of the modulation valve (MV) through an inching valve (IV) for controlling the clutches, wherein said inching valve (IV) comprises a valve casing (26), a tubular spool (28) slidable in the valve casing (26), and a spindle (31) slidable in and relative to the spool (28) and operatively connected to a clutch pedal (30), the valve casing (26) and the spool (28) defining therebetween a first oil chamber (A) connected to the output port (P1) of the modulation valve (MV), a second oil chamber (B) in communication with an output port (29) defined in the valve casing (26) and connected to the backward and forward switch valve (V1), and a first oil passage (r1) intercommunicating the first and second oil chambers (A,B), the spool (28) and the spindle (31) including therebetween a third oil chamber (C) and a spring (32) compressible by depression of the clutch pedal (30), the spool (28) defining a throttling portion (28A) for throttling the first oil passage (r1), and a first throttle passage (r2) and a second throttle passage (r3) for connecting the third oil chamber (C) to a first drain port (D1) defined in the valve housing (26) when the clutch pedal (30) is depressed, the first and second throttle passages (r2, r3) being disposed such that sliding of the spool (28) establishes in an alternative way an initial state in which the first throttle passage (r2) is connected to the first drain port (D1) and the second throttle passage (r3) is closed by the valve casing (26) and a declutched state in which the second and third oil chambers (B,C) are placed in communication with each other by the first throttle passage (r2) and the second throttle passage (r3) is connected to the first drain port (D1), and the spindle defining a drain passage (r4) to place the third oil chamber (C) in communication with a second drain port (D2) defined in the valve casing (26) only when the clutch pedal (30) is adjacent an inoperative position.