The chock shield combines the features of the chock and the shield. As such it possesses the advantages of both.

If all of the four or six legs are installed between the canopy and the base, it is called a chock shield. There is regular four-or six leg chock shield in which all legs are vertical and parallel. Others form V or X shapes. Some canopies are a single piece and some are two pieces with a hydraulic ram at the hinge joint. The chock shield has the highest supporting efficiency [34]. It is suitable for hard roof.

9-3 Common Elements of Powered Supports

The modern powered supports, regardless of the type, consist of the following common components:

Load-bearing unit [35]. These include the hydraulic legs,

canopy, base plate, caving shield, lemniscate bars [36], and joint pins.

Hydraulic rams. These include hydraulic rams for (a) pushing

the chain conveyor and advancing the powered supports, (b) operating the front canopy or face guards [37], (c) balancing or limiting the position of the canopy, and (d) operating other auxiliary equipment such as leg recovering devices [38] and side shields [39].

Control and operating unit [40]. These include internal control

valves such as check and yield valves [41] in the hydraulic legs, unit control valves [42], and high pressure hydraulic tubings [43].

D. Auxiliary devices. These include leg recovering, gob windows, face guards, base-lifting [44], lighting [45], and so on.

E. Hydraulic fluid [46]. This is the emulsion [47] for operating the powered supports. Most popular hydraulic fluid is the water emulsion with 5% oil and 95% water.

9-4 Setting Load [48] and Yield Load [49]

During setting the support, the control valve is firstly turned to ��setting�� position, which allows the pressurized fluid [50] to enter into the front chamber of the leg piston. As the pressurized fluid continues to flow, the leg continues to rise until the canopy touches the roof. After that, the pressurized fluid in the front chamber of the piston rapidly increases to the working pressure of the hydraulic pump [51]. At that time, the control v

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