What is the Common Centerline and Why is it Important?
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 a 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 centerline. 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.
- A tool is sent to the calibration lab and is certified.
- A new tool is put into service after the calibration certificate is viewed.
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, the 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.
Sturtevant Richmont has support options in this area:
- Our Exacta 2 Digital Torque Wrenches have an internal common centerline compensation calculator built into it. If you change the common centerline access the head length function, select the new head length, and the wrench recalculates for the differing head length.
- We have a torque slide rule calculator that determines the change in output when the common centerline is changed. You can go to our website and request a torque slide ruler and we will send you one. If you need more than one, please see your Sturtevant Richmont sales professional.
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Sturtevant Richmont tools are proudly made in Carol Steam Illinois by highly capable hands.
