19th U.S. Symposium on Rock Mechanics (USRMS),
Permission to Distribute - American Rock Mechanics Association
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Two of these modes are the natural longitudinal and torsional vibrations; the third mode, a forced vibration, is excited as a result of variations in the cutting force. The periods for the natural vibrations vary as the square root of the length of the drill string whereas the period for the forced vibration varies as the length of the drill string and inversely with the speed of rotation. High stresses induced in the rock at depth cause spalling and enlargement of both the pilot and the reamed holes. This permits lateral displacement of the reamer head in the hole and calculations show that stresses, induced in the drill string of a raise boring machine as a result of bending forces acting together with axial and torsional loading, are sufficiently high that failure of the drill string adjacent to the head is likely. Experimental results indicate that when boring operations are conducted in hard rocks, any tendency for the cutters to skid results in damage to the cutters and reduces their life substantially.
Premature failure of cutters when boring by machine in hard rock at depth is a significant problem and one of the main impediments to the widespread use of boring machines under these conditions. Lack of stability of the machine cutter head during the cutting process is thought to be a major cause of this problem. When the head is not supported adequately, vibrations are induced which result in the application of impulsive loads to the cutters. In deep mines stability of the machine cutter head often is reduced, because high stresses in the rock at depth frequently cause failure of the rock surrounding a hole and, in general, the cutter head is guided by reacting the cutting forces against this rock.
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