Threads are a Perfect Fit

It’s not surprising that today’s engineers and designers need optimum performance from every component in a product design, and this includes an often overlooked but essential component: fasteners. While development has improved the strength of nuts and bolts, most of the research and design improvements have been to increase the size and shape of the bolt head or add flanges to the nut body.

Room for Improvement
While these approaches do increase fastener strength, a critical feature still remains less explored: the strength of the male and female screw thread. An in-depth look at the screw thread profile reveals several areas for improvement that are commonly used and a few lesser-known improvements in regard to strength.

Keeping in mind the basic 60º included angle profile or “V”-style design of the screw thread, the male portion of the thread is simple to manufacture. The prevailing method of making male threads in bolts and screws is the process of rolling threads. Born mostly out of necessity to increase bolt/screw production, the process of rolling or forming threads has increased bolt/screw production exponentially over the thread cutting method. Also, by forming the threads, grain flow is not interrupted and the base material is work hardened to provide additional strength and toughness.

A similar scenario for female screw threads exists: the most common manufacturing method for female threads is to cut the thread profile. By using a cutting tool with a matching thread profile, known as a tap, the tool is inserted and turned simultaneously into a pre-drilled hole to create the proper thread profile and helix angle.

Another opportunity for increasing thread strength lies in the actual profile of the thread. The most common fractional thread form is the Unified (UN) style for coarse and fine threads (Figure 1). This style is specified as UNC and UNF respectively. However, this common thread form has a drawback; the form features a flat-bottomed root, which is a source for high stress concentrations under load. An offshoot of this thread is the Unified “J” style (Figure 2), which has an increased root radius for added strength and reduction in stress concentrations at the base of the thread profile.

Even with processing and shape improvements to the UN thread, it still has its weakness when a load is applied to the mating threads. The “V” shape of the thread creates a cantilevered load on each loaded thread (Figure 3). Besides the high stress concentration at the base of the thread, variability in tolerances and imperfections in the threads can cause 85 percent of the loading to be concentrated on the first two to three threads. Uneven thread loading has been researched for years by the fastener industry and can cause threads to deform and deformation certainly affects reusability.

Fastener expert J. Shigley in Mechanical Design Textbook 5th Edition states that when using UN threads: “During tightening, the first thread of the nut tends to take the entire load; but yielding occurs, with some strengthening due to coldwork that takes place, and the load is eventually divided over three nut threads.” More importantly, he continues: “For this reason you should never reuse nuts; in fact, it can be dangerous to do so.”