Rules of Thumb for Thread Engagement

What is Thread Engagement?

Thread engagement has two factors, length of thread engagement and percentage of thread engagement.

Thread Engagement

Length of thread engagement is measured by the length of interaction between the fastener and nut member. (i.e. nut or mating material for screw)  For example a standard thread forming screw applied in 10mm of material will have more length of thread engagement than the same thread forming screw in 8mm of material.

Percent of thread engagement is based size of the mating hole compared to the fastener and how much the height of threads of the screw are engaged.  For example a #10-24 thread forming screw will have around 80% thread engagement in a .1683” hole, while around 70% thread engagement in a .1710” hole.

Why is Thread Engagement important?

Thread engagement is important because it directly relates to the integrity of the joint. With too little thread engagement (combination of length and percentage) the mode of failure may be the nut member stripping out and lowering the drive/fail ratio. Breaking the externally threaded fastener (bolt or screw) is the desired mode of failure.  On the other hand, with too much thread engagement, the torque needed to drive the screw will tend to be high. This can lead to inadequate clamp load or even breaking fasteners during assembly.

Percent and length engagement can also be traded for one another; i.e. If the nut member material isn’t optimally thick, a smaller hole can be used to get a more optimal amount of thread engagement and in effect a better joint.

How does Thread Engagement Effect the Tensile Strength of a Bolt/Joint Combination?

Simply put, more thread engagement can result in higher tensile strength a joint.  Tensile strength is the force required to pull something until it breaks or the capacity of that material to withstand that load.  If a bolt is longer than needed to develop full tensile strength in a nut member, that excess material is wasted. On the flip side, if there is not enough bolt length engaged in a nut member, the bolt has a higher probability of stripping out before full tensile strength capability is achieved. Depending on the strength of the nut material, you need at least 1-1.5 bolt diameter engaged in the nut member to achieve optimum joint strength with a thread forming fastener.  One factor that affects this is the nut material. Steel is a close to a 1-1.5 relationship, while softer material, like plastic will need more thread length engagement to achieve optimum joint strength.

Author: Dan Derry