Monitor for double safety valves

Claim 1

An arrangement for sensing a malfunction in a pneumatic control system for presses or the like comprising a pressure inlet, a supply outlet, and an exhaust, a pair of valve means each having three valving parts operated thereby, the first of each valving part being effective to control the communication of pressure from said inlet to a respective intermediate pressure area of each of said valve means, the second of each of said valving parts being effective to control the communication of the intermediate pressure area of the other of said valve means with said supply outlet, and the third of each of said valving parts being effective to control the communication of said supply outlet with said exhaust, said valve means each being movable between a first position wherein said first and second valve parts are closed and said third valve parts are opened and a second position wherein said first and second valve parts are opened and said third valve parts are closed for communicating said inlet with said supply outlet and for closing communication of said supply outlet with said exhaust when both of said valve members are in their second positions, for closing communication of said inlet with said intermediate pressure areas, closing communication of said intermediate pressure areas with said supply outlet and opening communication of said supply with said exhaust when said valve means are both in their first position, and for precluding communication of inlet pressure to said supply outlet when both of said valve means are not in their second position, the improvement comprising means responsive to actual pressure for sensing pressure in either of said intermediate pressure areas for providing a malfunction signal when said valve means are intended to be in their first position.