GB699265A - Improvements in or relating to earth boring apparatus - Google Patents

Abstract

699,265. Percussive drilling assembly. SOUND RILL CORPORATION. May 16, 1951, No. 11424/51. Class 85 To prevent loss of energy from a percussive rock drilling assembly due to the setting up of vibrations in the surrounding drill mud fluid, a damping means or an acoustic decoupler is mounted on the drill tool in communication with the mud fluid and spaced a distance from the source of wave generation of the tool which is substantially less than a quarter wave length, measured in the surrounding mud fluid of the sound wave generated in said mud fluid. The drilling assembly comprises a bit secured to a heavy column of substantial cross-section which is vibrated at its resonant frequency by a wave generator coupled thereto. The longitudinal impulses applied to the end of the column result from either the rotation of out of balance rotors 161 so arranged that all the forces and moments set up, except the forces acting longitudinally of the column, are neutralized, or from the longitudinal reciprocation of a mass, the rotors 161 or mass being operated by a high-speed low torque mud fluid driven turbine or an electric motor. The purpose of the wave generator is to transform the quantity of energy developed by the high speed low torque motor into a similar quantity of energy to be delivered into the ground by the bit as a large oscillatory force of small amplitude so sending . alternating waves of tension and compression through the soil to cause it to fail by stress failure. The frequency of the pulses applied to the column is such that velocity anti-nodes occur at the ends of the column and a velocity node at the centre thereof. To prevent loss of energy and vibration of the motor, it may be coupled to the drive of the vibrator unit by a splined shaft 121 and is situated at a quarter wave length or at a velocity node from the end of the column, i.e. a distance of half the length of the column. To prevent loss of energy due to vibraticns set up in the drill pipe supporting the drilling assembly, there is substantial reduction in the cross-section at the joint between the casings of the vibratory unit and the case of motor. In another form the joint between the drilling assembly and the drill pipe is a rubber sleeve bonded to each of them. The acoustic decoupler may comprise a downwardly directed bell 311 secured to the bit shank, there being entrapped in said bell a quantity of air which is compressed and expanded by the mud fluid to damp out vibrations started therein. Alternatively the end of the bit shank is recessed and contains trapped air or gas filled grease. To prevent vibrations being set up by the drilling assembly, it is separated from the mud fluid in the bore by a sleeve or pneumatic tubes may be fitted in recesses formed in the sides of the drill shank. The bit may have pivoted spring-biased blades or an axially spring-biased blade, and the drill assembly may be rotated.