be produced in the rear part of the canopy must be kept intact or must not cave equal to the resistance force (Ps) of the support. In the opposite condition the balance force Q3 would be produced in zone Ⅰ.
From this we can see that the resistance of this type support can thus be formed in the condition when the balance force Q3 occurs on the canopy. That is to say, the immediate roof must not cave at all.
According to the analysis mentioned above, now consider that is under the different conditions: The roof is unbroken and the resistance of the support Ps is equal to P+ (the yield load of the legs). Then the resistance of the support can be expressed as follows:
Q＋Q3＝Ps
Assume Ps＝P+, so that
(Agz?Bgp)?1
(z?Bgx)Then the acting position where the roof pressure Q acts would become x＝P＋（1－A）·z/B
Assume that the acting position where the roof pressure Q acts is at x1, and the balance force Q3 is x3 (the origin is in the hinge pin point), then the following formula is obtained:
Q·x1＋Q3·x3＝（Q＋Q3）·（p＋（1－A）·z/B） The roof pressure Q which the support can resist is equal to:
Q3?(x1?p?(1?A)gz/B)gQ
(p?(1?A)gz/B?x3)The roof pressure Q which the support can resist is equal to:
Q?(p?(1?A)gz/B?x3)P?
(x1?x3)Take Q/P+ to stand for the efficiency of the support, obviously, this has relation with the following factors: the geometrical parameters of the support, i.e. parameters of the balance force (reaction) of the immediate roof x3. It is obvious that the nearer the value x1 approaches to zone Ⅱ, the higher the efficiency of the support would be. Something the value x3 can be
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represented as an index to stand for the interactive relation between the canopy and the immediate roof. When Q acts in the position (p?(1-A)z/B), the balance force Q3 is equal to zero, and the efficiency of support Q/P+, is equal to 1.
Fig.8 shows that when a variable roof pressure (Q) acts in three different positions (x1) in the zone Ⅰ of the canopy and with different index x3 in zone Ⅲ, in order to resist the roof pressure (Q), a corresponding balance reaction force Q3 with different values must be given in zone Ⅲ. For example, when the roof pressure is acting on the tip of the canopy and is equal to 80t if x3>37cm. then there would be no such balance force formed in the rear part of the canopy.
Because roof fall occurs in the face-to-canopy area where the roof would become irregular, thus the canopy would have three kinds of operating condition for the canopy to swing: downwards (<0○) upwards (>10○) and at an angle from 0○ to 10○. According to statistical data collected from Zhai-Li Colliery, the percentage of the operating of the operating conditions of the canopy swinging canopy in <0○ accounts for 3.5% and that of in >15○, for 11%.
Due to the fact that the acting position of the roof pressure on the canopy is different, the angle between the canopy and the caving shield may be variable. Table1 shows the variation accounts for 44.8%, which means that the canopy and the caving shield may be variable. Table1 shows the variation of this angle in each operation cycle.
From Table1, we can see that the percentage of positive variation accounts for 44.8%, which means that the roof pressure (Q) firstly acts on zone Ⅰ and than the balance force (reaction) (Q3) is formed on zone Ⅲ;finally, the acting position of the combined force (Q+Q3) would move towards zone Ⅱ. In Table1 the percentage of negative variation accounts for 19.4%.
Similar results have also been obtained from field measurements in working face No.332 of Zhai-Li Colliery as shown in Table2 and Fig.9.
Obviously, whether the roof pressure acts on zone Ⅰ or Ⅲ of the canopy, if the acting position of the combined force (Q+Q3) moves towards zone Ⅱ,the operating condition of the support would be normal. But if the acting position of the combined force moves over zone Ⅱ and continuously moves forwards or backwards, the support would then work in abnormal conditions.
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Fig.8 Balance force Curves for different index X3 in zone Ⅲ
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