1.Introduction

In longwall coal mining, the coal cut down by the shearer or the plow is transported by the armored face chain conveyor (AFC), which is laid across the full face width, to the headentry T-junction, where it is transferred to the entry belt conveyor through the mobile stage loader. The armored face chain conveyor has large carrying capacity [1]. It is structurally very strong, bendable like a snake [2], and low in body height. It serves not only as a guide rail [3] for the shearer or the plow, but also as the pivot for support advance [4]. If the conveyor is equipped with a ramp plate [5], it can also load left on the mine floor.

Although based on the carrying capacities and roof control requirements,t here are various types of face chain conveyors designed for various seams and roof conditions. The basic structures and major components are similar. AFC consists of a drive head, various pans (i.e., line pans [6],ramp pans, connecting pans [7], and adjusting pans [8]), link chains [9], scraper(or flight) bars, and a tail end(either driven or no driven). In addition, most AFCs are equipped with auxiliary devices such as face-side ramp plate, gob-side spill plate [10] and cable handling channel [11], and chain tensioning and conveyor anchorage devices. Furthermore, a double-acing ram is used for both the support and conveyor advance.

The chain links are connected to form a continuous chain between the drive end at the headentry T-junction and the tail end at the tailentry T-junction. The flight bars are tied to the link chains at fixed intervals. Duiring operation, the link chains, driven by the driven-head motors, move from the tail end toward the head end. As they move, the fight bars, being tied to the link chain, also move and carry the coal toward the head end. After discharging the coal, the link chains and flight bars return to the tail end through the underside of the pans. Frequently the tail end is also equipped with drive units to increase its capacity.

2. Major Components of Armored Face Conveyors

2-1 Drive Head

The drive head consists mainly of the drive frames; drive units, including the electric motor, gearbox, fluid coupling [12], and chain sprockets [13]. The electric motors range from 100 to 170hp (75-125kw) with rotational speeds from 1450to 1480 rmp. The gearboxes are basically of the same type but vary in design. Fluid coupling is always used between the electric motor and the gearbox.

The primary objective of the fluid coupling is to reduce loads and shocks on the motor and transmission, particularly during starting. It can raise the starting capability and improve the starting characteristics. Most importantly it protects the conveyor. The fluid coupling contains a limiting fluid port [14] through which a fixed amount of working fluid is introduced to ensure that the output from the fluid coupling will not exceed the maximum value allowable

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