翻译部分英文原文A STUDY OF THE INTERACTIONBETWEEN THE 2-LEG SHIELD SUPPORTAND THE ROOF STRATAINTRODUCTIONThe 2-leg shield powered support is shown in Fig.1.It is known that in order to asses the adaptability of a powered support normally there are two principles to be considered:Fig.1 2-leg shield support EFFECTIVENESS OF ROOF CONTROL Obviously, shield support is much easier to prevent the broken rocks from falling into the working space, but it is much harder to prevent the broken rocks from falling into the face-to-canopy area. On the basis of the statistical data obtained from the Collieries Yang-Quan and Zhai-Li, the down-time leads to stop production due to falling roof in the face-to-canopy area is about 40-60% of the total down-time in the working face. Collapse of roof strata along the faceline is shown in Fig.2. That is to say, in a face installed with 2-leg shield powered support much more attention must be paid to the problem of immediate roof control, especially in the face-to-canopy area.EFFECT ON SUPPORT STRUCTURE UNDER THE ACTION OF ROOF PRESSURE Recent reports from some collieries reveal that 2-leg shield support has been broken under the action of roof pressure, especially at the joint of the canopy and the stabilizing cylinder as shown in Fig.3. It is evident that the supporting capacity of this type of support could not be considered as adequate to some such kind of roof conditions and must be improved.Fig.2 Collapse of a longwall face at the facelineFig.3 Damage at the joint of the stabilizing cylinder and the canopy ANALYSIS OF LOADING CONDITION OF 2-LEG SHIELD SUPPOIRT The forces acting on the canopy of 2-leg shield support are: the roof pressure, the forces from the support legs, ram, hinge pin of the canopy and the caving shield, the surface friction between the canopy and the roof strata.Assuming that the surface friction and the force acting on the caving shield are not taken into account, the following formula can be obtained:The meanings of all the symbols used in this formula are illustrated in Fig.4a.Assuming that then we can obtain the following formula.It can be seen that When P is increased to the yield load P+, the force thus in the ram would be distributed as shown in curve Z in the Fig.4b. In fact the ram has a yield load in push and pull. For example, for the shield support W.S.1.7,the yield load in push is equal to 67.7t and in pull 62.4t. So the curve of the force from the ram would be redistributed in the face as curve Z+, and the curve of force for the support legs would be redistributed as carve P shown in Fig.4b. Then the total load Ps for the whole support can be given as follows:, Assuming that W=0, then:Thus, according to the position where the roof pressure acts on the canopy and refer the support performance to the load of the ram Z is equal to +Z, (the yield load of the leg ) and -CD zone, on which the load of ram is equal to Z (the yield load of the ram in pull ).The load bearing characteristics of the support legs and the each zone of the canopy are shown as follows:Fig.4 Three working zones of support canopy zone Z=Z+. zone P=P+. zone Z=-Z-Obviously, the resistances of zone and zone on the canopy are produced by the yield load of the ram. For example, if Z is equal to zero, the resistance of the support itself in zones and would loss and the resistance can be produced only when there exists some additional forces from the corresponding zones. In zone or . There exists a balance force produced by the roof strata. If the yield load of the ram is increased, obviously, the interval of the zone would become much wider, and the resistance on the zones and will be increased accordingly. There are shown in Fig.5.Fig.5 Resistance Curve of different yield load of ramINTERACTION BETWEEN ROOF PRESSURE AND SUPPORT RESISTANCEIt is well- known that the roof pressure acting on the canopy of the support can be divided into two components, they are: Q1 produced by the immediate roof and Q2 by the main roof, as shown in Fig.6.As a general rule, the immediate roof can be considered as a discontinuous media (like a loose body) and there is a free face along the caving line. Load Q1 acts steadily on the supports. Load distribution on the canopy may be considered as uniform. Load Q2 from the main roof may be considered as a concentratedload which acts on the immediate roof and then acts on the canopy of the support. Based on the displacement measurement of roof strata it has been found that the main roof of the overlying strata can be considered as a structure formed by layers of rock blocks interlocking with one another, when the coal face advances, each block becomes to move forming a turning block. The displacement of the main roof is shown in Fig.7.Obviously, the acting position of