Written by Derek Dandy
Strategic Market Manager – Fasteners – S3P
Although the majority of threaded fasteners are used to create clamp load in a bolted joint, others — such as set screws — are used to create a compressive force to perform their function. This difference in functionality can bring a new set of design considerations when selecting the ideal set screw for an application.
One such design consideration is the hardness of the set screw relative to the mating component, so as to effectively create holding force. This can be less of a challenge when dealing with materials that harden, such as alloy steels, compared to working with materials that don’t, such as austenitic stainless steel.
In this article, we consider how Kolsterising, a surface-hardening process for austenitic stainless steels, can greatly improve the holding power of set screws.
Kolsterising is a low-temperature, diffusion-based surface hardening process for austenitic stainless steels, as well as nickel and cobalt-based alloys. This hardened layer is able to maintain surface integrity to provide more consistent fastener performance.
Although the surface is extremely hard compared to the base material, the layer remains ductile and tough with no risks of embrittlement. Due to the fact that this process occurs at a low temperature (< 500° C), the corrosion resistance of the base material is preserved. It can also be done after any standard heat-treating processes without impacting a material’s bulk properties.
Independent testing was performed using 5/16″ -18 x 1⁄4″ 316 S/S set screws, shaft collars, and shaft material. The holding power was evaluated for both the untreated cup point set screws and the cup point set screws treated with the Kolsterising process.
The theory behind the testing is that the hardened surface would improve the nesting action of the set screw into the shaft material. The holding force was measured after a single tightening and after tightening five times. As can be seen in Figure 1, the treated cup point set screws showed a significant increase in holding power — 42% for the single tightening and 49% for the fifth tightening of the cup point set screws.
We also wanted to evaluate another point design so we tested the untreated knurled point set screws and compared their performance to the treated knurled point set screws. Figure 1 shows a 25% increase in holding power for the single tightening of the knurled point.
One of the factors that led to the increased performance after the tightening five times was the ability to maintain the surface integrity of the threads. This leads to maintaining the coefficient of friction of the set screw/shaft collar interface. As repeated tightening led to surface wear of the threads, the amount of input torque that translate into holding power is reduced.
To maintain the required holding power when using the same tightening torque, the coefficient of friction must remain consistent — just as with creating the clamp load in a bolted joint. In addition to being able to maintain the holding power over subsequent tightening, another factor to consider is obtaining more holding power from a particular size of set screw by being able to apply more tightening torque without the risk of galling.
As has been shown in previous testing, Kolsterising can maintain the surface integrity of a threaded fastener up to the yield point of the base material. This can allow the use of smaller set screws and/or the use of less set screws to achieve the desired holding power.