Reliability’s Sexy Side: Threaded Fasteners

By Drew D. Troyer CRE, CMRP, Contributing Editor

So the title is misleading. Threaded fasteners aren’t very sexy. But I wanted to quickly draw your attention to what is one of the most important, albeit often overlooked, aspects of machine reliability. Threaded fasteners are, literally, the nuts and bolts of putting reliability on the plant floor. They hold things together and hold them down. When they’re managed correctly, looseness-related vibration, which leads to mechanical wear, leaks, and poor electrical connections, is kept to a minimum. When approached casually and with a lack of precision, threaded fasteners are a primary root cause of mechanical and electrical failure.

Effective fastening is achieved by applying the proper amount of clamping force between two fastened components. Clamping force is achieved by stretching the threaded fastener, which acts like a spring to create the tension required to secure the connection. The amount of tension applied is very important. The threads on bolts are elastic, which enables them to stretch—the greater the stretch, the greater the tension, up to a point.

Hooke’s Law defines the elastic limit of fastener materials. Much like a spring, if a bolt is stretched to a point within the elastic region for the material, the metal returns to its pre-tension state undamaged and can be used again. If, however, the threads are stretched beyond the elastic limit, the material fatigues, is permanently distorted, and can’t maintain the tension required to achieve the desired clamping force. The trick is to tension the fastener to a reasonable level within the material’s elastic limit to achieve the desired tension without exceeding what is called the yield, or plastic deformation point. Doing so destroys the threads, which brings us to tension versus torque.

Torque simply measures the twisting force applied to a fastener, which is a combination of tension resistance, which creates the desired elastic stretch on the bolt, and friction, which is a wild card. Studies on test stands suggest that 50% or more of the resistance measured on a torque wrench is due to friction. Thus, it’s imperative to lubricate fasteners prior to tightening. Machine oil is good, but a purpose-formulated fastener oil or grease works better and minimizes the friction-related error between torque-wrench readings and actual achieved tension.

Remember, when it comes to fasteners, an ounce of prevention is worth a ton of cure. With a little investment of time and money, diligence in fastener practices pays big dividends in increased plant reliability and reduced maintenance costs. For more details on managing these critical components, see my Efficient Plant May e-newsletter article “Threaded Fastener Best Practices” at efficientplantmag.com/bolts. In the meantime, read about some real-world fastening mysteries in this month’s feature article, “What’s Wrong with My Bolts?” EP

Based in Tulsa, OK, industry veteran Drew Troyer is principal with Sigma Reliability Solutions. Email Drew.Troyer@sigma-reliability.com.