A torque is a force exerted at a distance from the axis of rotation. Change either the distance from the axis of rotation or change the amount of force that is exerted and the torque changes.
Add an extension on the end of a torque wrench and you have changed the distance from the rotational axis. That same effect is created when changing the length of the head you are using. That too changes the distance from the rotational axis.
The image to the left illustrates the concept posed above. Here are two preset click wrenches that are set to the same value. When they click they will provide a different torque value at the fastener because their common centerline is not identical.
Oddly enough, not all interchangeable head manufacturers adhere to the concept of a common centerline.
Sturtevant Richmont has standardized common centerline for our interchangeable heads. There are two common centerlines and it is based on fastener size.
Our smaller heads come with a 1 7/16" (1.4375 inches or ) common center line. Our larger heads come with a common centerline of 3 7/8" (3.8750 inches or 98.5 mm).
Our tools that have a larger capacity, (example: 400 ft. lbs.) is typically calibrated for a common centerline of 3 7/8".
We often see this take another form in two similar situations.
In either case, that tool is taken out on the floor and then fitted with a head that has a different centerline than the one that was used during calibration or certification. The tool is put to work and at some point, someone discovers that the torque application is not accurate. At this point finger pointing begins. The blame is placed on either the manufacturer or the calibration lab.
The truth is there is no need for blame. The need is for a system that ensures head length is properly accounted for and calculated.
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Sturtevant Richmont tools are proudly made by highly capable union hands.