"E" as a plus dimension "E" as a minus dimension
Care of your torque wrench
As mentioned earlier, torque wrenches should be calibrated at least once a year. However, torque wrenches may have to be calibrated more often depending on usage and user needs.
When storing torque wrenches, try to store them in their original boxes. Also avoid storing them in areas of extreme hot or cold temperatures, or in areas of high humidity.
On clicker type torque wrenches, always turn them down to the lowest setting when storing them. Storing it at any settings other than the lowest one will cause main spring wear and failure. But be careful not to go past the lowest setting or it may come apart in your hand, and you'll have to ship a box of springs and parts to the torque wrench repair shop.
Keep torque wrenches clean. They should be kept free of grease, moisture, and debris. All of these can be destructive to a torque wrench and throw off calibration.
Don't abuse torque wrenches. They are precision measurement equipment, and dropping them or using extensions on them can cause serious damage.
Watch those calibration records
When your torque wrench comes back from calibration, it is a good practice to look at the calibration records. The shop will include the readings indicated on the torque wrench versus the actual torque applied. Even though the torque wrench comes back to you within limits, it may have been significantly off before the lab adjusted it. Normally, the errors are not far off, but if a gross error exists, such as the torque wrench applying 70 in./lbs. of torque when it was set at 40 in./lbs., it could warrant going back and re-checking work that was done using that torque wrench before it was sent in.
Keep in mind that external factors can affect the accuracy of torque. As mentioned earlier, torque specifications take into account the friction on the given bolt or nut. If lubricant is called out to be used, it is important to use it. Not using lubricant will result in more friction during the torquing sequence, and even though the torque is achieved on the torque wrench, the actual amount of preload on the part will be less than called for. Along the same line, if lubricant is not called for but is used on the application, then when the specified torque is reached the actual preload will be greater than spec, and failure could result. Needless to say, cross threaded bolts or nuts will result in erroneous torque values as well.
So in the end it's not so hard after all. With a little knowledge on torquing procedures, you don't have to feel like you have that torture device tied around your neck.
Always Preload Your Torque Wrench
Preloading a torque wrench is an important process in overall accuracy of these tools. It must be performed each time the torque wrench is used after periods of non-use or whenever torque direction is changed. There are several reasons for preloading your torque wrench. First, it will set internal components so that when force is applied, torque begins immediately with no internal settling. Second, it distributes lubrication to moving internal parts. The final reason is the hysteresis characteristic of the steel, initial stress should be applied to the steel in order to moderate the hysteresis presence.
How to do it:
2. Mount torque drive in a stationary fixture (i.e. socket welded to bench, vise).
3. Exercise the torque wrench three to five times in the direction you will be verifying.
4. Perform torque measurement.
Remember to store torque wrenches (click-type) in the low setting. Otherwise calibration will be needed at shorter intervals.
Duncan Intelligence, Fall '99 (For a complete listing of Duncan Intelligence Newsletters, you can visit them online at www.duncanaviation.com).
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