This paper describes development of and the procedures and equipment described in API RP 7A1, Recommended Practice for Testing of Thread Compounds for Rotary Shouldered Connections, published in 1991.
Trouble-free rotary-shouldered-connection performance depends on proper joint makeup. Joints must be tight enough to prevent shoulder separation under bending and tensile loads but not so tight that their tensile capacity decreases or the pin or box is damaged. The preload in a connection from tightening depends on the makeup torque and frictional properties of the thread compound.
Besides acting as a gall preventive and sealing aid, thread compound must provide a consistent, predictable friction coefficient. When a rotary-shouldered connection is bucked up, the frictional forces acting on the makeup shoulders and thread flanks create a resistance to tightening. With the same makeup torque, a connection with low friction resistance will be made up tighter than a connection with high friction resistance.
The induced axial stresses that preload a rotary-shouldered connection are proportional to the makeup torque. API RP 7G2 recommends that the makeup torque produce a tensile stress of 62,500 psi in a drill-collar connection and 72,000 psi in a tool joint.