| 1. |
- Beccu, R., et al.
(författare)
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Transmission and dissipation of stress wave energy at a percussive drill rod joint
- 1987
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 6:3, s. 157-173
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Tidskriftsartikel (refereegranskat)abstract
- Transmission and dissipation of stress wave energy at a percussive drill rod joint is determined for three joint types, three joint preload levels, three hammer lengths ( incident wave lengths) and a range of impact velocities ( incident wave amplitudes). The energies of the incident, reflected and transmitted waves are evaluated from measured strains, and then the dissipated energy is determined as the deficit of energy in the two latter waves compared with the first. The accuracy needed is obtained by introducing a compensation factor and determining its value from the requirement of momentum conservation. The experimental results for threaded standard joints are compared with experimental results for threadless dummy joints, made in one piece, and also with theoretical results. The latter are based on one model which represents the joint as a well on a one- dimensional elastic rod (characteristics impedance or CI model) and on another model which represents the joint as a rigid mass between one-dimensional elastic rods (rigid mass or RM model). For the joints 70-100% of the incident wave energy is transmitted and 0-20% is dissipated. The corresponding figures for the dummies are 90-100% and (±)1%, repectively. The latter figure indicates the inaccuracy in the measurement and evaluation procedures. The two models agree fairly well with the dummies. Under certain conditions they also give reasonably accurate predictions for the energy transmitted through a joint. As they sometimes give quite inaccurate results for energy transmission and furthermore fail to predict energy dissipation they need be refined.
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| 2. |
- Lundberg, Bengt
(författare)
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Efficiency of percussive drilling with extension rods
- 1987
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Ingår i: International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts. - : Elsevier BV. - 0148-9062 .- 1879-2073. ; 24:4, s. 213-222
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Tidskriftsartikel (refereegranskat)abstract
- Extension drilling of rock is simulated numerically using a Victor desk-top computer. The hammer, adapter and extension rods are uniform and have the same characteristics impedances. The same is true for the joints and the bit. Two efficiencies are determined as functions of six dimensionless parameters which represent the number of joints, the joint-to-hammer length ratio, the joint-to-rod characteristic impedance ratio, the initial gap between bit and rock and the bit/rock interaction (two parameters). The first efficiency is defined as the ratio of the work performed on the rock to the impact kinetic energy of the hammer. The second is based on the sum of the work performed on the rock and the rebound kinetic energy of the hammer, which is assumed to be fully retrieved. Both efficiencies generally decrease with the number of joints and the joint-to-hammer length ratio. Exceptionally, however, the efficiencies may increase slightly with the number of joints. The difference between the second and first efficiency, which is due to hammer rebound, generally increases when the rock becomes increasingly hard. This dependence on rock hardness is strongest when the number of joints and the joint-to-hammer length ratio are low. In the absence of joints the efficiencies have an oscillatory dependence on a small initial gap. When one or several joints are used, however, the efficiencies essentially decrease with the initial gap.
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