A STUDY OF THE INTERACTION BETWEEN THE 2-LEG SHIELD SUPPORT AND THE ROOF STRATA
INTRODUCTION
The 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 faceline
Fig.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
