Control system for a hydraulic clamping device

Claim 1

A control system for a hydraulically actuated apparatus comprising, at least, a first expansible hydraulic motor for actuating a clamping device and a second hydraulic motor actuatable separately from said first motor, said control system comprising: fluid distribution means including: manually movable valve means for selectively supplying fluid under pressure along a first hydraulic path from a source to said fluid distribution means, said fluid distribution means having, at least, one valve member in said first hydraulic path movable between an actuated position, wherein flow of fluid under pressure to said second hydraulic motor is permitted, and a deactuated position, wherein said flow of fluid to said second hydraulic motor is prevented, said fluid distribution means further including a second hydraulic path communicating with said first expansible hydraulic motor and said first hydraulic path at a point between said manually movable valve means and said one valve member, check valve means in said second hydraulic path, said check valve means being movable between an open and closed position in said second hydraulic path, resilient means for constantly urging said check valve means towards said closed position, a fluid actuated pilot piston in said second hydraulic path movable between a deactivated position, wherein fluid flow through said second hydraulic path to said first expansible motor is controlled by said check valve means, and an activated position, wherein said pilot piston engages and holds open said check valve means, means biasing said pilot piston towards said deactivated position, said check valve means being movable to said open position when both said movable valve means supplies fluid under pressure to said fluid distribution means and said pilot piston is in its said deactivated position so that the pressure of the fluid supplied to said first hydraulic motor will be the same as the pressure of the fluid supplied to said second hydraulic motor through said one valve member, fluid operated locking means for holding said pilot piston in its said activated position when said pilot piston moves into its activated position, said fluid distribution means further including a fluid return path and said pilot piston having means for closing said second hydraulic path and passage means associated therewith for establishing a drain path from said first expansible hydraulic motor to said fluid return path when said pilot piston is in its said activated position, said manually movable valve means being movable to one position to supply fluid under pressure to said pilot piston to move said pilot piston from each deactivated to said activated position and to another position to supply fluid to said locking means wherein said locking means is released and said pilot piston is moved by said biasing means to said deactivated position.