煤炭英语的翻译,急寻高手!Face Equipment: Powered SupportsThe application

煤炭英语的翻译,急寻高手!
Face Equipment: Powered Supports
The application of modern powered supports can be traced back to the early 1950s. Since then, following its adoption in every part of the world, there have been countless models designed and manufactured in various countries.①But unfortunately there still is no uniform system of classification.
There are four types of powered supports, that is, the frame, chock, shield and chock shield, in order of evolution of their development. However, it must be noted that the trend of development in each type is such that it becomes less distinguishable in terms of application.②
Frame
The frame support is an extension of the single hydraulic props conventionally used underground. Thus it is the first type developed in modern self-advancing hydraulic powered supports. It involves setting up two hydraulic props or legs vertically in tandem that are connected at the top by a single or two segmented canopies. The two-segmented canopies can be hinge-jointed 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 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 installed between each set. The pistion of the ram is connected to the secondary set and the cylinder 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 pistion. 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. ③The distance of each advance ranges from 20 to36 in.(0.05～0.91m).
Thus the frame support is very simple, but more flexible or less stable structurally. There are considerable uncovered spaces 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.