【文章內(nèi)容簡(jiǎn)介】 ee to six , but the fourleg chocks are by far the most popular ones . The sixleg chocks are y (Fig. ) . For the sixleg chocks , the canopy is generally hingejointed above the walkway . Most chock are also equipped with a gob window hanging at the rear end of the canopy . The gob window consists of several rectangular steel plates connected horizontally at both ends. A B C Fig . Schematics of various chock support 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 , it is reinforced mostly with a boxshaped steel frame between the base and each leg . A leg restoring device is installed around each leg at the top of the boxshaped steel frame . The chocks are suitable for medium to hard roof . When the roof overhangs well into the gob and requires induced caving , the chocks can provide access to the gob . SHIELD Shields , a new entry in the early seventies , are characterized by the addition of a caving shield at the rear end between the base and the canopy . The caving shields , which in general are inclined , are hingejointed to the canopy and the base making the shield a kinematically stable support , a major advantage over the frames and the chocks . It also pletely seals off the gob and prevents rock debris from getting into the face side of the support . Thus the shieldsupported face is generally clean . The hydraulic legs in the shields 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 ponent of hydraulic leg pressure is available for supporting the roof ,the actual loading capacity 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 to be developed for all kinds of shields . The types of motional traces of the canopy tip , leg positions and orientation , number of legs , canopy geometry , and other optional designs and devices can be clearly specified by the terminology . TYPES OF MOTIONAL TRACES FOR THE LEADING EDGE OF THE CANOPY. This is the most monly recognized way of classifying the shield . Based on this criterion , there are three types , lemniscate , caliper , and ellipse (Fig. 8. 11) . A . B . C . Fig . Three types of motional traces for leading edge of the shield canopy A .