Spring Load

Made from a 0.25 inch diameter wire, a helical torsion spring has a mean spring diameter of 5 inches. If the spring’s bending stress is 5,000 lb/in^2 due to a load applied 2.5 inches away from the coil’s center, what is the applied load in lb?

Expand Hint
For a helical torsion spring, the bending stress is:
$$$\sigma=K_i(\frac{32Fr}{\pi d^3})$$$
where $$K_i$$ is the correction factor, $$F$$ is the applied load, $$r$$ is the radius from the coil’s center to the load, and $$d$$ is the wire diameter.
Hint 2
Correction factor:
$$$K_i=\frac{(4C^2-C-1)}{[4C(C-1)]}$$$
where $$C=D/d$$ , $$D$$ is the mean spring diameter, and $$d$$ is the wire diameter.
Correction factor:
$$$K_i=\frac{(4C^2-C-1)}{[4C(C-1)]}$$$
where $$C=D/d$$ , $$D$$ is the mean spring diameter, and $$d$$ is the wire diameter.

First, let’s solve for $$C$$ :
$$$C=\frac{5in}{0.25in}=20$$$
Next, solve for the correction factor:
$$$K_i=\frac{(4(20)^2-(20)-1)}{[4(20)(20-1)]}=\frac{(1,600-20-1)}{[80(19)]}=\frac{1,579}{1,520}=1.0388$$$
For a helical torsion spring, the bending stress is:
$$$\sigma=K_i(\frac{32Fr}{\pi d^3})$$$
where $$K_i$$ is the correction factor, $$F$$ is the applied load, $$r$$ is the radius from the coil’s center to the load, and $$d$$ is the wire diameter.
$$$5,000\frac{lb}{in^2}=1.0388\times \frac{32F(2.5in)}{\pi (0.25in)^3}$$$
$$$F=62.5\frac{lb}{in^3}\times \frac{\pi (0.25in)^3}{1.0388}=2.95\:lb$$$
2.95 lb
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