Mine-roof support control mechanism

An hydraulically-movable mine-roof support comprising an hydraulic advancing cylinder which is pressurised on both sides of a piston therein with pressurised fluid by a drive valve, the advancing cylinder being held in a base structure of the mine-roof support, a plurality of hydraulic, vertically-adjustable, props supported on said base structure for supporting a main roof-support canopy provided with a forward, auxiliary, roof-support canopy pivoted thereto and extensible in the direction of the working face in synchronism with longitudinal extension of the advancing cylinder by an auxiliary roof-support cylinder pressurisable on both sides of a piston therein with pressurised fluid, the said synchronous movement being controlled by a control valve communicating with the advancing cylinder via a control line, wherein:

• (a) a measuring cylinder having a space of variable volume extends substantially parallel to the extensible length of the advancing cylinder and is connected to an annular space in the auxiliary roof-support cylinder by a control line;
• and
• (b) a 2-position, 3-port, directional control valve which supplies pressurised fluid from a high pressure line to a pressure space of the auxiliary roof-support cylinder has a closure element spring-loaded in the direction of closure and is controllable by the pressure in the control line in such a way that the 2-position, 3-port, directional control valve is opened when the pressure falls below a pre-set threshold value and is closed when the pressure rises above the threshold value.