What is Thread Engagement?
Thread engagement has two factors, length of thread engagement and percentage of thread engagement.
Length of thread engagement is measured by the length of interaction between the fastener and mating 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.
Importance of Thread Engagement Length
Selecting the optimum thread engagement length is important for following reasons:
If you choose to keep the thread engagement length shorter than the screw or the bolted joint may not be strong enough, on the other hand if you keep the engagement length too long then it will not only call for higher overall cost and the torque needed to drive the screw will tend to be high.
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.
What is the recommended thread engagement to make a strong connection for a component with a tapped hole?
Basic design rules require the bolted joints to be designed so that the screw breaks before the threads strip.
For components with tapped holes the length engagement (i.e. the number of threads which are engaged between the screw and the tapped hole) has to be adapted to respect this criteria.
Thumb Rule
Thread engagement distance calculation thumb rule
In case of the threads for steel – Min. thread engagement distance = 1.0 to 1.5 x base diameter of screw or bolt
In case of the threads for cast iron, brass – Min. thread engagement distance = 1.5 to 2.0 x base diameter of screw or bolt
In case of the threads for Aluminium – Min. thread engagement distance = 2.0 to 2.5 x base diameter of screw or bolt
In case of the threads for softer materials like plastic – Min. thread engagement distance will be higher and require trials to establish the optimum joint strength.
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Back in my day, in steel, we assumed that there should be 1x the diameter of the bolt or screw. Never had an issue using that rudimental formula. Aluminum was always 2x, plastic was…meh.