ic props conventionally used underground. Thus it is the first type developed in modern self-advancing hydraulic powered supports. It involves setting up [9] to hydraulic props or legs vertically in tandem that are connected at the top by a single or two segmented canopies [10]. The two-segmented canopies can be hinge-jointed [11] at any point between the legs or in front of the front leg. The base of the two hydraulic legs may be a circular steel shoe welded at the bottom of each leg or a solid base [12] connecting both legs. If the steel shoes are used, spring plates [13] connecting the legs are used to increase the stability.
Generally a frame support consists of two or three sets of hydraulic legs. The set that moves first is the secondary set; the set that moves later is the primary set. There is a double-acting ram [14] installed between each set. The piston [15] of ram is connected to the secondary set and the cylinder [16] to the primary set. During support advance, the primary set is set against the roof while the secondary set is lowered and pushed forward by the piston. Having reached the new position, the secondary set is set against the roof while the primary set is lowered and pulled forward by the cylinder. The distance of each advance ranges from 20 to
36 in
. (0.50
-0.91m
).
Thus the frame support is very simple, but more flexible or less stable structurally. There is a considerable uncovered space between the two pieces of canopy, which allows broken roof rock to fall through. Consequently the frame support is not suitable for a weak roof. Frames have become seldom used because they are less stable and require frequent maintenance.
B. Chock
In a chock support, the canopy is a solid piece and the base may be either a solid piece or two separate parts connected by steel bars [17] at the rear and/or the front ends. In both cases a large open space is left at the center for locating the double- acting hydraulic ram, which is used to push and pull the chain conveyor and the chock in a whole unit, respectively, a distinctive difference from the frame support. This set up is also used in the shield and chock shield.
Again, all hydraulic legs are installed vertically between the base and the canopy [18]. The number of legs ranges from three to six, but the four-leg chock is by far the most popular one.
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