Lifting large sacks of coal from ground to height of flatbed on truck (ledge on right side in simulator) was one use to save workers backs. Hydraulic circuit shown in simulator.
Current source represents hydraulic pump (electric or gasoline powered).
Springs hold pedal above SPDT(two way hydraulic lever) up until worker on ground places sack on switch pedal sending oil pump pressure into hydraulic piston (shown as resistor) lifting bag and platform for second worker standing on truck flat bed to lift sack onto truck bed.
Lifting sack off allows SPDT or hydraulic control lever springs to move to up position to release pump pressure and complete circuit sending oil to holding tank (ground) and lower automatically to ground for next bag.
Top STST is pressure relief valve to hold sack up if worker on truck bed does not grab sack as soon as it is lifted.
