In coal and other kinds of mining by the Longwell technique, it is conventional for minerals to be removed by a single or double ended ranging shearer drum, which traverses the mineral face with a rotary cutting head carried by the, or each, ranging arm to follow the seam. Typically, each drum is provided with 50 or more locations where a cutting tool is required. A pick holder is welded in place at each location. Each pick holder supports a replaceable pick designed to engage the ground. In some constructions, each pick holder also contains a water sprayer to the rear of the pick for spraying the working end (i.e., the head) of the pick and the coal with water. In general, each pick comprises a pick shank, a securement mechanism to maintain the pick in the pick holder, a head, and a transition area between the head and the shank. The transition area often consists of a rear heel and a forward toe or shoulder.
In use, the shearer drum is rotated about its central axis. As the drum rotates the pick holders spin around with the drum so that picks engage the ground. The water sprayer within the pick holder sprays water on the pick and the coal to minimize dust and the risk of frictional ignitions.
When the pick contacts the wall while the shearer drum rotates, the picks experience forces F as the pick breaks up the material to be excavated. The force F will at times be normal N to the tip 24 with respect to the material face such as along a line 1a normal and orthogonal to the frontal direction. Line 1a goes through the forward most impact point of the pick 10a to the center of rotation of the pick assembly 8a around the excavating equipment 4. In this application, a force that is along line 1a and only has a normal component N is referred to as a normal (or inward) force and a force F that is collinear or tangent T to the cutting path (i.e., orthogonal to line 1a and only has a tangential component) is referred to as a tangential (or rearward) force.
As a pick rotates around with the drum 6, the pick will experience a force F that will at times be primarily tangential (i.e., a force that extends perpendicular to a force that extends normally through the strike point of the tip to the center of rotation of the pick assembly around the drum and has an angle α of 0 degrees). Other times the pick will experience a force at an angle α from tangential T that has a tangential component and a normal component (i.e., a force between tangential T and normal N). As the pick continues to rotate the pick will experience a force that is primarily normal (i.e., a force that is primarily inward on the pick and has an angle α of 90 degrees from tangential T and that extends normally through the strike point of the tip 24a to the center of rotation of the pick assembly around the drum 6). The transition of forces between those that are predominantly inward or normal and those that are predominantly rearward or tangential causes the pick to rock within the pick holder. The cyclic rocking causes the pick to wear the pick holder prematurely as can be seen in FIG. 4. Premature wear on the pick holder causes the pick locking system to become ineffective and leads to picks being ejected from the pick holder during use. Typically, when a pick breaks or is ejected the pick will also break the water sprayer.
Damaged holders must be cut from the drum and new holders welded in their place. Because of the risk of frictional ignitions and tight dark working areas, typically, shearer drums must be removed from the longwall (i.e., removed from service) and moved to a safe location for refurbishment, for example to the surface. Moving the shearer drum, cutting the welds between the shearer drums and the pick holder, and welding new pick holders in place is time consuming. Such refurbishment can be lengthy and expensive.