Compressed air is delivered to rotary directional valves 1 and 2 of the control levers. If valve 2 is in position I (as shown), air from the valve is delivered to the upper end of cylinder 5 of the hoisting unit. The pressure of the air moves cylinder 5 upward with respect to fixed piston 6. Air from the lower end of the cylinder is discharged through port a. If valve 2 is in position II, the upper end of cylinder 5 is connected to the atmosphere. The weight of the grab bucket moves cylinder 5 downward and air from its upper end is discharged through valve 2 to channel 3 which leads to a flow-control valve. The speed at which the cylinder descends is regulated by the flow-control valve. If valve 2 is in position III, the pneumatic hoist is held fixed at any height. If valve 1 of the control levers is in position I (as shown), compressed air is delivered to the upper end of cylinder 7. The pressure of the air moves cylinder 7 upward with respect to fixed piston 8. This closes scoops 9 of the bucket. Air from the lower end of cylinder 7 is discharged to the atmosphere through a hole in piston 8, valve 1 and channel 4 which leads to a flow-control valve. If valve 1 is in position II, compressed air is delivered to the lower end of cylinder 7, moving it downward and opening the bucket scoops. Air from the upper end of cylinder 7 is discharged through valve 1 and a flow-control valve to the atmosphere. The speed at which the scoops are closed and opened is regulated by the flow-control valves. In position III of valve 1, the scoops are held fixed at any required position. By shifting the two control levers to the corresponding positions, the pneumatic hoist and grab bucket operate simultaneously.
$4284$CHP,MH$