英文原文Powered supportsPowered roof supports are used to support the roof at the face.They work in several steps mostly in the immediate forward support (IFS) type in order to support the newly exposed roof as soon as possible. In normal supporting condition the canopy of the support is set tightly against the roof strata,by the supporting resistances of the hydraulic legs. When the cutting machine cuts and passes several support units beyond the support in question, the support legs are lowered and pulled forward for a distance equal to the width of cut (web) by retracting the hydraulic advancing ram. The advancing ram acts against the face conveyor panline, whose position is held unchanged by the advancing rams, forces of the supports that are set on both sides of the support to be advanced. As soon as the support has been advanced to the designed position, the support is immediately reset against the roof. Finally, the advancing ram of the support in question is extended to push the conveyor forward and becomes ready for the next cut. The sequential steps described are for the advance of an individual support in the direction perpendicular to the faceline.But along the faceline direction, the conveyor cannot be advanced in a sharp step immediately after the shearer has passed it, due to the rigidity of the panline. Thus the fourth step is usually sometime after the shearer,s pass, resulting in a curved or snaked section.During panel development the panel entries are roof bolted as usual. But in the tailentry one or two rows of cribs are erected to supplement the roof bolting. During retreating mining, the roof at the headentry T-junction area (up to 500 ft (152m) outby the face) is generally reinforced with supports of some type to increase support density to cope with the moving front and side abutment pressures. Modern longwall mining employs hydraulic powered supports at the face area. The supports not only holds up the roof, pushes the face chain conveyor(AFC), and advances itself, but also provides a safe environment for all associated mining activities. Therefore its successful selection and application are the prerequisite for successful longwall mining. Furthermore, due to the large number of units required, the capital invested for the power support usually accounts for more than half of the initial capital for a longwall face. Therefore both from technical and economic points of view, the powered support is a very important piece of equipment in a longwall face. Classification of powered supportsThe application of modern powered supports can be traced back to early 1950s. Since then, following its adoption in every part of the world, there have been countless models design and manufactured in various countries. But unfortunately, there still is no uniform system of classification.In this section a simplified classification is used. Since a powered support consists of four major components (i. e , canopy, caving shield, hydraulic legs and props, and base plate), the ways by which they are interrelated are used for classification. In this respect, two factors are most important: (a) presence or absence of caving shield- if a caving shield is included, the support is a “shield” type, otherwise, a frame or a chock; (b) number and type of arranging the hydraulic legs- since support capacity is generally proportional to number of hydraulic legs, it is important to specify the number of hydraulic legs that a support has. Furthermore, the way the hydraulic legs are installed is important; for example, a vertical installation between the base and the canopy has the caving shield has the least efficiency in supporting the roof.Based on this concept, there are four types of powered support, 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.The four types of roof supports can be obtained for either longwall retreating or advancing systems, and they are available in standard, one-web-back, and immediate forward support (IFS) versions.With the standard system, the wining machine takes a cut or a slice, and the armored face conveyor is pushed over by the hydraulic rams that are fixed to the support units. The support units then are advanced sequentially to the conveyor. With the one-web-back system, a support is set back from the conveyor by a device that automatically keeps the leading edge of the support at a fix distance from the conveyor. This allows easy access though the face and employs the standard method of advancing; i. e , pushing the conveyor first, and then advancing the support.With the IFS system, the support units is advanced to the conveyor immediately after the cutting machine has passed, and the forward canopy of the support units is long enough to the support both the recently an