Plunger is normally extended by light spring force. When the workpiece is loaded, its weight depresses the plunger to the proper height. When fluid pressure is applied, the precision sleeve grips the plunger, locking it securely.
Locking height is repeatable within +/-.0002". Such excellent repeatability is possible because of two design features: 1) a precision sliding-fit pressure sleeve holds the plunger absolutely vertical even while unlocked, avoiding any inaccuracy due to locking a tilted plunger, and 2) locking force is entirely radial, so the plunger remains absolutely stationary during the locking process.
These flange-base Work Supports have a contact force that is adjustable from 2 to 20 lbs. To set contact force, insert a screwdriver into the plunger from above. Force varies somewhat depending on plunger extension (2 lbs minimum when fully extended, 20 lbs maximum when fully retracted).