Selecting the Proper Torque Wrench
There are a wide variety of preset and adjustable torque wrenches and selecting the shape and size for your application can be easy. However, understanding the variety of torque delivery mechanisms of torque wrenches can be confusing.
A preset torque wrench is similar to a person setting an alarm clock to signal the achievement of a selected time. The wrench is preset to the required torque value of the application and then the tool signals the user when torque is achieved. A preset torque wrench must be preset using a torque analyzer. An adjustable torque wrench features a torque scale that allows you to see and adjust the torque setting.
There are three styles in which a mechanical torque wrench can signal achieving torque, either by a "click," "break-over," or "cam-over." Each of the three wrench styles has a specific purpose and utility. When you decide to spend the money for one, it's important that you select the one that will do the job properly and not generically.
"Click" wrenches are the most widely used torque product in the world. When the set torque is reached, the tool typically emits a loud audible "click." The operator can feel the impulse from the tool and most break about 3 degrees after the set torque is reached and then become positive. This positive action can cause over-torque conditions. Proper use and training are required so that operators stop pulling the moment the click sound is heard or felt. Resetting of the tool takes place when the hand pressure is released. Work can then immediately continue on the next fastener.
"Break-Over" wrenches are essential to limiting the amount of torque applied to a fastening application. Upon reaching the preset torque value, the tool "breaks" at a specific point along the tool's length - usually at a pivot point near the tool's head. It typically deflects 20 degree or 90 degree on torque delivery; thus, indicating torque has been reached. Continuing force after the break will increase the torque applied above the preset value. Many break-over wrenches require manual resetting, while others have an automatic resetting feature.
"Cam-Over" wrenches utilize a ball and lobe design that allows the tool to slip free when torque is reached. Even if the application of force is repeated, the preset torque value won't be exceeded, eliminating the possibility of over-torque. These tools are perfect for maintenance and production applications where over-torque conditions are not tolerated. The use of cam-over wrenches takes the operator influence out of the torque equation and offers more accurate and repeatable results than a standard " click' type wrench.