Hydraulic control apparatus

摘要

Hydraulic control apparatus serves to operate roof support props of a mineral mining installation. An automatic setting arrangement with a valve device ensures the props become set against the roof with a high uniform force by connecting a prop pressure chamber to a pressure fluid source once a threshold pressure is exceeded signifying the prop has been extended. To disable the automatic setting arrangement during prop retraction the valve device thereof is provided with a control piston which is subjected to pressure during retraction to oppose the action of another control piston which responds to the threshold pressure.