Effect of Thread Angle

Before moving on to the dynamic models of lead screw systems, the effect of thread geometry on the contact forces is considered here. The force interaction shown in Fig. 2.7 is essentially correct for the square threads where the normal force is parallel to the lead screw axis. For Acme or other types of threads, a slight modification is needed to take into account the thread angle. 

Since N is the normal force, using (2.4) the friction force for trapezoid threads is calculated by Ff = /t x sgn(0), where jl is the true coefficient of friction. One can define the apparent coefficient of friction as 

---