Powered roof supports [20] are used to support the roof at the face. They work in several steps, mostly, in the immediate forward support (IFS)[21] type in order to support the newly exposed roof as soon as possible. In normal supporting condition, the canopy [22] of the support is set tightly against the roof strata by the supporting resistances [23] 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 [24] for a distance equal to the width of cut (web) by retracting the hydraulic advancing ram [25]. 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 [26] of the panline. Thus the fourth step is usually sometime after the shearer��s pass, resulting in a curved or snaked section [27].

During panel development [28], the panel entries are roof bolted as usual. But in the tailentry one or two rows of cribs [29] are erected to supplement the roof bolting[30]. 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 [30] to cope with the moving front and side abutment pressures.

NOTES TO THE TEXT

[1] Longwall mining techniques�����ڲ�ú����

[2] production capacities����������

[3] the drilling and blasting mining�����۷��ڲ�ú

[4] millisecond blasting�����뱬��

[5] individual hydraulic props������Һѹ֧��

[6] mechanized mining����е����ú

[7] plow����ú��

[8] fully mechanized mining���ۺϻ�е����ú

[9] armored face conveyor (AFC)����������װ���ͻ�

[10] stage loader��ת�ػ�

[11] belt conveyor���������ͻ�

[12] panline�����ͻ����

[13] faceline����������

[14] self-contained motorized winch����װ�Ĵ��綯���ʳ�

[15] cutting force���ظ���

[16] guide channel�������

[17] T-junction��T������

[18] unloading end��ж�ض�

[19] overlap���ص������

[20] Powered roof supports��Һѹ

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