A New Approach to Check the Heath of Engine Mounting & Suspension Bolted Joints

The torque required to tighten any threaded joint is different from the necessary torque to untighten threaded bolt or nut, and it is not observed or widely known since this is a regular and straightforward operation. Typically the torque needed to untighten a newly tightened clamp is around 10% to 30% less than the torque to stretch it further. During tightening a threaded bolt, a significant amount of torque required to overcome friction in the threads and under the nut face. The proportion of the torque used to overcome frictional resistance depends upon the friction value. When we tighten a joint with a coefficient of friction of 0.12, only about approximately 14% of the torque required to stretch the fastener producing the clamp load with 86% of the torque is lost overcoming friction. The torque needed to pull the bolt always acts in the untightening direction, resulted in untightening torque lags behind the tightening torque. Sufficient preload has to be there in the bolted joint to meet service requirements. In this approach tightening torque, reading is Ton, and untightening torque is as Toff and then based on Ton & Toff reading preload is calculated in the bolt and checked with the requirement. If preload is not sufficient enough to serve in all load cases, then frictional parameters are to be changed to achieve necessary preload in a bolted joint. A case study of engine mounting bolt presented with this methodology.