Apparatus for disabling an obstructed lift mechanism

A hydraulic system for preventing the lowering of a mechanically obstructed lift mechanism and including:

• a first cylinder having its head end connected by a first conductor to a source of pressurized fluid;
• a second cylinder having its head end connected by a second conductor to the rod end of said first cylinder for permitting fluid flow therebetween;
• a first valve movable between a first position for permitting fluid to flow from said first cylinder and a second position for preventing fluid from flowing from said first cylinder, said first valve including a first pilot port for sensing the hydraulic pressure in said first conductor and a second pilot port for sensing the hydraulic pressure in said second conductor;
• a second valve movable between a first position for permitting fluid to flow from said first cylinder and a second position for preventing fluid from flowing from said first cylinder, said second valve including a first pilot port for sensing the hydraulic pressure in said first conductor and a second pilot port for sensing the hydraulic pressure in said second conductor;
• said first valve being normally biased to said first position and piloted to said second position when said hydraulic pressure in said second conductor exceeds the pressure in said first conductor by a first predetermined value;
• said second valve being normally biased to said first position and piloted to said second position when said hydraulic pressure in said first conductor exceeds the pressure in said second conductor by a second predetermined value.

A hydraulic system for preventing the lowering of a mechanically obstructed lift mechanism and including:

• a first cylinder having its head end connected by a first conductor to a source of pressurized fluid;
• a second cylinder having its head end connected by a second conductor to the rod end of said first cylinder for permitting fluid flow therebetween;
• a valve movable between a first position for permitting fluid to flow from said first cylinder and a second position for preventing fluid from flowing from said first cylinder, said valve including a first pilot port for sensing the hydraulic pressure in said first conductor and a second pilot port for sensing the hydraulic pressure in said second conductor;
• said valve being further movable between said first position for permitting fluid to flow from said first cylinder and a third position for preventing fluid from flowing from said first cylinder, said valve being normally maintained at said first position and piloted to said second position or to said third position when said hydraulic pressure in one of said conductors exceeds the pressure in the other of said conductors by a predetermined value.