Vibration isolator

Abstract

A vibration isolator includes telescoping tubular mandrel and barrel members between which torque is transmitted by an internally splined urethane bushing affixed to the cylindrical interior of the steel barrel and a urethane layer over the splined outer surface of the steel mandrel. Axial loads are transmitted in both directions between shoulders on the mandrel and barrel through two annular urethane members, one for each direction, disposed in annular pockets between the mandrel and barrel, which have greater radial thickness than do the rings, allowing room for deformation of the rings by axial loading. A replaceable sliding seal between the upper end of the mandrel and the barrel retains drilling fluid passing through the isolator. A sliding bearing between the mandrel and the lower end of the barrel cooperate with the seal at the upper end of the mandrel to take bending moments. The urethane annuluses are shaped, e.g. tapered at their ends, to give a desired load-displacement curve, providing a soft cushion initially and gradually increasing stiffness with increased displacement. The urethane annuluses are preloaded in an amount at least equal to the expected set they will take after use, thereby to eliminate any axial play in the isolator. The material and geometry of the axial load transmitting annuluses and their pockets is such that over a wide range of loads the resonant frequency of the isolator for axial vibration is nearly constant and about equal to the lowest expected impact frequency of a three cone rock bit running at typical speed of sixty revolutions per minute, i.e. three cycles per second.