Steel Solutions Center

6.9. Bolt Tension Calibration

6.9.1. The RCSC Specification discusses a “calibration device capable of indicating bolt tension.” What is an example of such a bolt tension calibration device?

One such device is the Skidmore-Wilhelm Bolt Tension Calibrator, manufactured by the Skidmore-Wilhelm Manufacturing Company. When a sample bolt is installed in the “Skidmore,” the tension is measured on a dial gauge. The appropriate torque for use in the calibrated wrench installation method may be determined, the proper tension resulting from the turn-of-nut, alternative design bolt, or the direct tension indicator methods may be verified. It is not intended that this discussion excludes the use of other similar devices.

6.9.2. When short bolts will not fit in the bolt tension calibration device how can they be tested?

Because devices such as the Skidmore have a minimum bolt length, testing of shorter bolts can be accomplished in any convenient steel plate by the use of a washer-type direct tension indicator (DTI). A similar DTI must first be tested using a longer bolt in the bolt tension calibration device to verify that they are neither under nor over strength. Alternatively, a calibrated torque may be determined using a bolt tension calibration device and a longer bolt with a hardened washer under the turned element. This torque may then be used for testing shorter bolts with a hardened washer under the turned element in a steel plate, provided lubrication and condition of threads for the long and short bolts are similar.