In a power system comprising a prime mover (10), including a controllable transmission member (18a), and a means (11) for generating a first signal indicative of the power required of said prime mover, a servo mechanism (13) comprising:
- a source of pressurized fluid (72);
- a reservoir (73);
- an actuator (30) including a housing having a bore (35) therein and a piston (36) slidably disposed within said bore so as to thereby define first (37) and second (38) hydraulic chambers within said housing, said piston having large and small hydraulic pressure receiving areas (40, 41) which are exposed to said first and second chambers, respectively, and being connected to said controllable transmission member;
- means (75) for generating a second signal indicative of the actual speed of said prime mover;
- a first passage means (74, 43) for connecting said source of pressurized fluid to said second chamber of said actuator;
- a second passage means (74, 53, 78, 42) for connecting said source of pressurized fluid to said first chamber of said actuator;
- a first spool valve means (31) disposed within said second passage means for controlling the hydraulic communication therethrough;
- means for transmitting said first and second signals to said first spool valve means, whereby a differential signal thereto is provided, said first spool valve means being responsive to said differential signal between said first signal and said second signal;
- said first spool valve means (31) having a housing (44), a spool valve (49) slidably fitted within said housing and responsive to said differential signal between said first signal and said second signal, and first and second ports (46, 47) provided in said housing, said first port being hydraulically connected with said source of pressurized fluid while said second port is hydraulically connected with said first chamber of said actuator, said spool valve being movable to control the flow of pressurized fluid from said first port to said second port;
- a third passage means (79, 61, 60, 42) for connecting said first chamber of said actuator to said reservoir;
- a second spool valve means (32) disposed within said third passage means and having a housing (58), a spool valve (63) slidably fitted within said housing being responsive to said differential signal for controlling the hydraulic communication between said first chamber of said actuator and said reservoir, and first and second ports (61, 60) provided in said housing (58), said first port being hydraulically connected with said reservoir while said second port is hydraulically connected with said first hydraulic chamber of said actuator, said spool valve being movable to control the flow of pressurized fluid from said second port (60) to said first port (61);
- means for transmitting said differential signal to said second spool valve means;
- and
- means (33) connected to said first signal generating means (11) and disposed between said first and second spool valve means (31, 32) for moving said spool valves (49, 63) of said first and second spool valve means simultaneously and in opposite directions in response to movement of said first signal generating means, whereby when one of said source of pressurized fluid and said reservoir is hydraulically connected with said first chamber of said actuator, the other of said source of pressurized fluid and said reservoir is interrupted from hydraulic connection therewith.