1、英文原文 Powered supports Powered 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 a
2、gainst 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 advanc
3、ing 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 re
4、set 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 d
5、irection, 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 b
6、olted 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 wi
7、th 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. T
8、herefore 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 fro
9、m technical and economic points of view, the powered support is a very important piece of equipment in a longwall face. Classification of powered supports The application of modern powered supports can be traced back to early 1950s. Since then, following its adoption in every part of the world, ther
10、e 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 le
11、gs 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
12、 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 be
13、tween 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 tren
14、d 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. Wit
15、h 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 con
16、veyor 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 un
17、its 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 and newly exposed roof sections. After the supports have been advanced, the conveyor is pushed over. Frame The frame support i
18、s 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 ca
19、nopies. 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 bottom of each leg or solid base connecting both legs. Generally, a frame support consists of two or three set
20、s of hydraulic legs. The set moving first is the secondary set, the set moving later is the primary set. There is a double-acting ram installed between each set. The piston of the ram is connected to the secondary set and the cylinder to the primary set. During support advance, the primary set is se
21、t against the roof while the secondary set is lowered and pushed forward by the piston. Having reached the new position, the secondary set is 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) The fr
22、ame 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. Frames have become seldom used because t
23、hey are less stable and require frequent maintenance. Chock In a chock support, the canopy is a solid piece and the base may be either a solid or piece or two separate parts connected by steel bars at the rear and/or the front ends. In both cases a large open space is left at the center for locating
24、 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 setup is also used in the shields and chock shields. Again, all hydraulic legs are installed vertically between the ba
25、se and the canopy. The number of legs ranges from three to six, but the four-leg chocks are by far the most popular ones. The six-leg chocks are designed for thin seams with two legs in the front and four legs in the rear, separated by a walkway. For the six-leg chocks, the canopy is generally hinge
26、-jointed 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. In most chock supports, there are hinge joint connections between the legs and the canopy
27、 and between the legs and the base. But in order to increase the longitudinal stability, it is reinforced mostly with a box-shaped steel frame between the base and each leg. A leg restoring device is installed around each leg at the top of the box-shaped 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.
