Selecting the optimum thread engagement length is important for following reasons: if you choose to keep the thread engagement length shorter then 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 machining cost but also will increase the chance of the perpendicularity error, i.e, the slight inclination of the tapping tool will cause significant offset of the tap hole from the desired location.
So, by now, you must have convinced that a minimum recommended thread engagement length must be carefully calculated either using bolt thread engagement formulas or using the thumb rules. First i will discuss the calculation method and then show the rule of thumbs for thread engagement.
Bolt or screw thread engagement length calculation by equations based on thread shear area
Le = (D-0.938194)2 / (D – 0.64952 x p ) …………………eq.1
J = Tensile strength of bolt or screw / Tensile strength of nut ……………….eq.2
Le1 = Le x J …………………………….eq.3
In case of the nut and bolt (or the male and female thread parts) has the same material property then the minimum thread engagement length (Le) can be found out by using the eq.1.
In case of the nut and bolt (or screw) has the different materials and the value of J from the eq.2 is more than 1, then we have to make use of the eq.3 for calculating the minimum thread engagement length (Le1).
Le , Le1 – required minimum thread engagement length
D – Basic diameter for screw or bolt (e.g., 10 for a M10 bolt etc.)
p – Pitch distance for the thread (e.g., 1.5 for a M10 x 1.5 bolt etc.)
Thread engagement distance calculation thumb rule
In case of the threads for steel – Min. thread engagement distance = 1.5 x base diameter of screw or bolt
In case of the threads for cast iron, brass – Min. thread engement distance = 2 x base diameter of screw or bolt