Dynamic measurements were made on an impact wrench tightening several bolted joints. Using hammer energy and bolted joint characteristics, energy absorbed by the joint and other parts of the system was calculated. The impact process is found to be consistent with an energy model in which kinetic energy of the tool hammer is primarily absorbed by the bolted joint. Hammer energy was also absorbed by elastic deformation of the tool output shaft and by friction-type losses in the components being assembled. Bolt torque is closely correlated with hammer energy delivered by the tool as was hammer rebound angle.