K-Factor Testing of PTFE-Coated Studs

Vendors offer PTFE-coated studs to provide corrosion resistance in harshly corrosive environments. Vendors advertise PTFE-coated studs as having reduced K-Factor compared to uncoated studs, and differences of opinion exist on whether or not lubrication is required when assembling joints with PTFE-coated studs. In fact, some end users have adopted the practice of assembling joints with PTFE-coated studs without lubrication when the stud is used for the first time and then applying lubrication when studs are reused. This study tested the K-Factor of a set of studs in different sizes from seven vendors to determine what K-Factor should be expected when using PTFE-coated studs, how much variation in K-Factor one should expect between vendors and sizes, and the effect lubrication has on variability in K-Factor. K-Factors were derived using an automated test fixture that plotted K-Factor vs bolt load across a range of loads from 30 ksi to 80 ksi. This study found there is significant variation in K-Factor from vendor to vendor, from stud size to stud size, and even within individual lots of a single size from a single vendor. The study found that without lubrication, the average K-Factor increased significantly with bolt load, making prediction of gasket stress difficult. The study concluded that properly lubricating PTFE-coated studs during assembly significantly reduces the variability seen in unlubricated studs.