# Set screws stress

1. Apr 13, 2010

### walitat

Hi all, I am having problem doing this analysis now and I am new here. :D hi~
ok, so the problem is:
I have a smaller diameter shaft and a bigger diameter shaft, I drew a hole on the bigger diameter one, and put the smaller one in, and use a set screw to hold them in place, one of the shaft will have a torque of 50 in-lb, and I need to know what size of set screw I can use. Is this a direct shear problem?

2. Apr 14, 2010

### Skrambles

If you were using a pin to connect the two shafts it would be a shear problem, but a set screw uses friction to hold a shaft in place.

According to Machinery's Handbook, 28th Ed. you can calculate the amount of power or torque that can be safely transmitted by a set screw using these two equations:

P = (DNd^2.3)/50 T = 1250Dd^2.3

P is horsepower
T is torque in in.-lbs.
D is shaft diameter in inches
N is speed of the shaft in rpm's
d is the diameter of your set screw in inches

These values will only be an approximation because it still depends on the shape of the point on your set screw and the materials it and the shaft are made from.