Torsion Shaft Calculations

[metootoo]

We had the output shaft of a torque wrench fail. The torque wrench is rated for 3200 ftlbs. The square output shaft is practically 1.08 inches square. According to the formula I used [ Max Torque = (2/9)x shear stress x(width of square)^3, where shear is .42*yield], the shaft would have to be made of 417ksi material. Hardness testing a new shaft resulted in HRC between 42-46 on the outside, suggesting a yield of around 217ksi at best.

I'm I looking at this wrong, or is the torque wrench over-rated? We found the same problem with another top brand as well.

 

[btrueblood]

Max shear stress in a square section (from Roark) is in the center of the flat side,

Max. [Τ] = 0.601 T / a³

where 2a is the length of the square side.

so, for a 1.08 square, at 3200 in-lb, the max shear stress would be 12 ksi and a bit; well within the allowable shear for even a mild steel tool (but not much margin there). At 3200 ft-lb, the max. shear is 146 ksi and a bit; this would require 300 ksi material not to yield, and no margin of safety.

Are you sure about the tool's rating? Most common 1" drive socket tools have a max. torque rating closer to 1000 to 1200 ft-lb., though I've seen one rated a little over 2000 ft-lb.

 

[csd72]

Possible strain hardening?