The six-leg chock is designed for thin seams with two legs in the front and four legs in the rear, separated by a walkway. For the six-leg chock, the canopy is generally hinge-jointed above the walkway [19]. Most chocks are also equipped with a gob window [20] hanging at the rear end of the canopy. The gob window consists of several rectangular steel plates connected horizontally at both ends.
In most chock supports, there are hinge joint connections between the legs and the canopy and between the legs and the base. But in order to increase the longitudinal stability [21], it is reinforced mostly with a box-shaped steel frame [22] between the base and each leg. A leg-restoring device [23] is installed around each leg at the top of the box-shaped steel frame.
The chock is suitable for medium to hard roof. When the roof over hangs well into the gob and requires induced caving [24], the chock can provide access to the gob [25].
C. Shield
Shield, a new entry in the early seventies, is characterized by the addition of a caving shield at the rear end between the base and the canopy. The caving shield, which in general is inclined, is hinge-jointed to the canopy and the base making the shield a kinematically stable support [26], a major advantage over the frame and the chock. It also completely seals off the gob [27] and prevents rock debris [28] from getting into the face side of the support. Thus the shield-supported face is generally clean.
The hydraulic legs in the shield are generally inclined to provide more open space for traffic, because the canopy, caving shield, and base are interconnected, it can well resist the horizontal force without bending the legs. Thus, unlike the solid constraint in the frame/chock supports, the pin connections between the legs and the canopy, and between the legs and the base in a shield support make it possible that the angle of inclination of the hydraulic legs varies with the mining heights. Since only the vertical component of the hydraulic leg pressure is available for supporting the roof, the actual loading capacity [29] of the shield also varies with the mining heights.
There are many variations of the shield supports. In the following, six items are used to classify the shields, which enables a unified terminology [30] to be developed for all kinds of shields. The types of motional traces [31] of the canopy tip, leg positions and orientations [32], number of legs, canopy geometry [33], and other optional designs and devices can be clearly specified by the terminology.
D. Choc
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