Investigation of the effects of thrust and hardened washers on the installed tension of a roof bolt

The effects of thrust and hardened washers on the installed tension in a roof bolt were studied. Three sets of laboratory tests were run using 30-inch-long bolts with flat, bell, and donut plates, anchored with bail-type expansion shells in a specimen of Indiana limestone by a modified roof-bolting machine. Each type assembly, with and without hardened washers, was randomly installed at a fixed output torque for various levels of applied thrust from zero to 4000 lb. Two salient findings of the investigation were: (1) the application of thrust severely reduced the tension at which the bolts were installed, and (2) the use of hardened washers increased considerably the consistency of the installation of the bolts.