Things You'll Need
Flat double-ended box wrench
Tape measure
Spring balance calibrated in pounds
Incorrectly tightened nuts and bolts can cause serious accidents. Designers calculate the force that is required to clamp parts together safely, reducing the risk of premature failure. In many industries -- including aerospace, automotive and structural engineering -- a torque wrench, which can be calibrated in foot-pounds, is a common means of tightening threaded fasteners. Measuring torque without a torque wrench requires good access to the joint.
Step 1
Measure the length of the box wrench in inches from ring-center to ring-center. Divide this figure by 12 to convert it to feet. For example, if the length of the wrench is 9 inches, the answer is 0.75 foot.
Step 2
Divide 18 foot-pounds by the length of the wrench in feet and note the answer. For example, if the length of the wrench is 0.75 feet, the answer is 24 pounds; this is the force that you will need to apply to the end of the wrench to achieve 18 foot-pounds.
Step 3
Tighten the bolt or nut by hand until it is finger-tight and then fit the wrench onto it. If possible, position the work-piece so that the bolt or stud is vertical and the wrench is horizontal. If this is not possible, position the wrench at either top-center or bottom-center, pointing straight up or straight down.
Step 4
Hook the spring balance into the free end of the wrench. Hold the wrench in position on the bolt head and then pull the spring balance gently to start tightening the nut. Keep the balance in the same plane as the wrench.
Step 5
Position the balance so that you are pulling at right angles -- or perpendicular -- to the body of the wrench. As the nut tightens, the spring balance will indicate an increasing force. Check this force regularly. Do not exceed the value that you calculated.
Step 6
Note when the indicated force approaches the calculated value. If the bolt or stud is not vertical, reposition the wrench so that it is again at top center or bottom center. For example, if the calculated value is 24 pounds, remove the spring balance and reposition the wrench when the indicated force exceeds 20 pounds.
Step 7
Replace the wrench and the spring balance and reapply the tightening force. Keep the balance in the plane of the wrench and perpendicular to the body of the wrench. Increase the force until the balance indicates the calculated value.
Step 8
Remove the balance and wrench. The bolt is now tightened to a torque of 18 foot-pounds.
Tip
If you do not have good access to the bolt head, use an offset wrench or a socket wrench. Find a secure way to fasten the balance to the end of the wrench. In this case, your measurement should be the perpendicular distance between the ring- or socket-center and the fixing point for the balance.
Warning
Positioning the wrench at top center or bottom center ensures that its own weight does not affect the torque that you apply. If the bolt or stud is vertical, then the weight of the wrench does not affect the applied torque.
The torque that you apply will be dependent on the accuracy of your measurement and the calibration of the spring balance. Do not rely on this method for joints that are safety-critical.