Rattling Loose of Thread Connected Steels in Percussive Rock Drilling

Abstract

A common practice to uncouple drill steels in percussive rock drilling is by rattling. Rattling is primarily a continuation of a drilling process without the rotation of the drill steels. By examining the forces which exist at the contacting threads between two drill steels and a coupling, it is shown that normal thread loads in tightly coupled steels and coupling systems always produce a loosening torque. The loosening torque is opposed by circumferential friction torques developed at the thread interface and at the drill steels’ interface. Using a theory developed by Goodier for loosening of threaded fasteners by vibration, it is shown how tensile forces generated by rattling can overcome the friction torques to provide a mechanism for uncoupling the drill steels. A sequence of events during rattling loose of multiple steels is postulated, and conditions which facilitate the loosening process are discussed. Repeated pushing and pulling tests of manually torqued-up steels and couplings in a tensile testing machine for four commonly used steels confirms that tensile forces generated during rattling are indeed responsible for rattling loose. Complete loosening was achieved for all four steels tested under various combinations of an initial tightening torque and the cyclic tensile load, and the total number of load cycles required for a complete loosening were determined. Test results also show that loosening due to compressive forces generated during rattling should be very small. The dependence of rattling loose on the helix angle and the static thread efficiency of the thread form is discussed.