After setting, when the roof converges, the fluid pressure in the piston chamber will be increased. As the roof convergence [53] increases further, so does the fluid pressure in the piston chamber. To protect the hydraulic cylinder and piston from damage, a maximum allowable pressure [54], called yield pressure, is preset [55] for each support. This is usually provided by installation of a yield valve. When the fluid pressure in the piston chamber exceeds the yield pressure, the yield valve opens and fluid leaks out [56], which causes the fluid pressure in the piston chamber to drop rapidly below the yield pressure. This process repeats again and again if the roof continues to converge. The force exerted by each leg under yield pressure is called yield load.

9-5 Hydraulic Control Systems

High-pressure hydraulic fluid is used in each support to raise and lower the legs, advance the support, push the conveyor forward, set extension canopy and �� or face guard (sprag) [57]. Leg raising and lowering and support advance generally follow each other in order to advance and reset the support. But the other functions could be performed either before or after or simultaneously [58] with the support advance and reset. All of these functions are performed by the unit control valves. Depending on the designs of the control valves, there are various types of control systems. The types of hydraulic control systems are as follows:

Unit manual control [59].

Simple adjacent control [60].

Bi-directional adjacent control [61].

Batch or bank control [62].

Sequential control [63].

Electrohydraulic on-face control [64].

Electrohydraulic on-entry remote control [65].

NOTES TO THE TEXT

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