Torquing a nut beyond a known value using degree of rotation?

Question

I want to torque a nut to 320 ft lbs, but my torque wrench measures only to 250 ft lbs. Since I know I can get it to 250 ft lbs, is it possible to use that value to calculate the additional degree of rotation it would take to get it to approximately 320 ft lbs? For example... torque to 250, switch to another ratchet to avoid damaging the torque wrench, then rotate the nut an additional 45 degrees.

Answer 1

The key to solving your problem is to understand what the units of torque are and how to apply them to your particular situation. Torque is defined as force per unit length. In every torque problem there is an arm that rotates around a fixed point and force that acts on the end of that arm. The equation that describes the resultant torque is

$\tau = r * F * cos\theta$

Here is a graphic that discribes the equation.

This general equation takes into account that the force acting on the arm may not be perpendicular to it.

Now to asnwer your question.

is it possible to use that value to calculate the additional degree of rotation it would take to get it to approximately 320 ft lbs?

No. It is much easier to calculate torque directly. You can do this by placing your wrench on the nut so that is parallel to the ground and then placing a weight on the end. The resultant torque is the length($r$) of the arm times the weight on the end($F$). Since the arm is parallel to the earth and the force of gravity is perpendicular to the earth, $\theta$ is at $90\unicode{xb0}$ and thus $cos\theta$ equals one and does not change the outcome of the equation.

In summary, divide 320 by the length of your bar in feet and place that many pounds of force on the end of your bar at a 90 degree angle and the resultant torque will be 320 foot pounds.