The main mechanism in a torque wrench is a spring. Springs can lose their elasticity if they are stretched or compressed too much. So the best practice of relieving spring pressure on unused micrometer adjustable torque wrenches makes perfect sense.
So why could it be quietly killing your quality and you aren’t aware of it?
The design of some wrenches includes a torque block or a pawl. (Terms vary by manufacturer.) That pawl or torque block can be made of steel, plastic, or other materials.
When the wrench reaches the target torque, the tilt block tilts, and the tang hits the side of the wrench. You hear and feel the "click." The image to the right shows the wrench in the clicked position.
When taking pressure off the spring, the design may allow the wrench to relieve pressure on the torque block. This can allow the torque block to disorient, shift. Some wrenches can be set to zero pressure on the spring or have the handle further away from the spring. Not only can the torque block disorient, but it can also even fall out of place in the wrench.
If the torque block disorients, calibration is lost. Repeatability becomes an issue because the block no longer functions as designed. Typically in the tilt block design, the block itself is not square. That means when the tilt block is reoriented and is on its side, the torque value has changed without your knowledge. The scale still reads the same torque value, but what wrench says, and what it does, are two entirely different things. The difference in torque could be 20% or it could be 50%. It depends on the dimensions of the torque block.
The rounded case works against accuracy, repeatability, and durability.
The functioning parts have no support from ball bearings or the sides of the wrench.
That play allows the parts to move, wear in areas where it was not designed to wear. Unless you disassemble the wrench, there is no way to know that this is the source of the problem.
Calibrating the wrench won’t solve the problem. Even if you do get it to read within tolerance on the torque tester, the unsupported parts will move and change output and repeatability. At $60 to calibrate a wrench, you are simply throwing good money after bad.
Once that torque block is disoriented, every time you change the spring pressure the block may disorient in another variable.
When you are buying adjustable torque wrenches be sure to ask about the interior design.
Here is a short list of questions to ask that will help avoid buying the wrong adjustable torque wrenches?
1. Ask for the parts drawing so you can see the interior of the wrench. If they have a cutaway of the wrench all the better.
2. Ask about what the pawl or torque block is made from and how it is made. (Not all torque blocks are made of steel, some are plastic.)
3. Ask if the wrench can be taken down to zero pressure on the spring.
4. Ask if the wrench can be taken down below zero pressure on the spring.
5. If it has a rounded case, ask how that rounded design is an advantage. (Hint: It isn't) The rounded case makes it easy to "side-load" the wrench. Side-loading diverts force away from 90 degrees from fastener principle of torque application and torque measurement. If a wrench is lever sensitive, side-loading dramatically changes the torque application.
If you still have questions or are uncertain, you can take their wrench into the cal lab and put it through its paces. Or, you can call your local Sturtevant Richmont sales professional.