August 2014- Interestingly enough, most of the times when we use our torque wrenches, it’s not so much the torque we’re interested in. It’s the amount of tension or crushing force we’re exerting on the assembly through the tightening of the fastener.
Because the threads, materials and finishes in high-quality nuts and bolts are standardized, a given amount of torque (or twisting force) on a given threaded fastener will produce a fairly consistent amount of tension in the fastener. Because it’s difficult if not impossible to directly measure the tension in the fastener, we do the next-best thing: we check the torque applied to the nut.
Only with correct tension can any fastener deliver its optimum strength. Thus it follows that the best-maintained structures may be built with fasteners that are as light as possible. The trade-off is that significant care must be used in assembly and maintenance. Since aircraft are designed closer to “optimal” (as opposed to, say, your wheelbarrow), aircraft fasteners need more attention.
A good torque wrench will deliver a predictable amount of torque to a fastener, time after time. A simple “bar” torque wrench uses a stiff handle and an indicator with a free end. As more torque is applied to the work, the free end of the indicator shows the amount of bend in the handle, giving a reasonable indication of the torque being applied.
A more sophisticated device called a “snap” torque wrench uses a compression spring to hold the drive head of the wrench in position inside a hollow tube handle. When sufficient torque is applied to the head, the compression spring can no longer hold all the internal parts in alignment, and the parts snap out of line, indicating that a preset torque has been reached.
Because the parts are more consistently made and quality-controlled, and because the design is more protective of the critical parts, the snap torque wrench and its derivatives are favored by professionals.
How do you know it’s working?
When you use any style of wrench, you have a feel for how tightly you’re twisting the nut. If you do a lot of work with a particular size and type of fastener under controlled conditions, your hands will eventually become extremely accurate in judging torque.
A good mechanic will often apply an adequate torque, but the more critical the assembly—and aircraft are critical!—the more important it is to get the right torque on the assembly. A torque wrench is essential.
When we work on experimental or homebuilt airplanes, we are not required to use calibrated torque wrenches, or to ever calibrate them. But it’s a good idea to know what we should do to ensure our wrenches’ accuracy and protect their precision.
The only way to know your torque wrench is working properly is to have the torque wrench calibrated—professionally checked. A metrology lab is the place to send it.
While visiting the Dayton, Ohio airport (KDAY), I was invited to the nearby Tool Testing Lab (TTL), a broad-service metrology lab that does 60 percent of its business testing torque wrenches and transducers up to 2,500 foot-pounds (the twisting force exerted by a 2,500-pound mass at the end of a foot-long lever).
Robb Thomas, TTL’s lab manager, said his most common repairs have to do with tools that are misused or dropped; some lived in difficult environmental situations; and some just had stripped ratchets (“You get what you pay for,” he noted) or suffered other damage. Most wrenches, though, are sent in just to be sure they were working properly.
For all the sophistication of the test equipment, the basic torque wrench is a pretty simple device, so its calibration is straightforward.
Allowing variances in manufacture means that whatever inevitable errors are built into the wrench are in the calibration phase placed as near the middle of the range as practical so that, overall, errors are minimized. For the wrench to be certified, no error throughout the tool’s range may exceed manufacturers’ tolerances. (See the sidebar on page XX for more about types and tolerances. —Ed.)
TTL sells professional grade torque wrenches and can recondition many brands. They also sell and service tensiometers, so you’ll know that your cables are under proper tension.
Tool Testing Lab’s other services range from mechanical gauge size verification (down to a millionth of an inch) and electronics instrument testing; cable and thermal (thermocouple) testing and verification; some weights and measures… well, you get the idea. The obsessive-compulsive television detective Adrian Monk would be in heaven in a place like this!
For reference, torque wrench calibration fees run from $32 (for a 1/4-inch drive) to $118 (for a 2,500 foot-pound, one-inch drive unit). Your typical half-inch ratchet drive wrench will be tested, calibrated and certified accurate for $42.
Tim Kern, CAM, MBA, has authored features in over 40 aviation publications. He writes technical, publicity and expository pieces for several companies in the aviation industry. Kern is a private pilot with a seaplane rating, and is listed as the manufacturer (“with a lot of help!”) of an experimental aircraft. Send questions or comments to editor@www.piperflyer.com.
RESOURCES
Tool Testing Lab, Inc.
11601 North Dixie Dr.
Tipp City, OH 45371
(937) 898-5696
ttlcal.com
US Industrial Tool & Supply Co.
www.ustool.com
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TTL was kind enough to let us into their lab, but they stress that there are many metrology labs than can help you, if you know where to look.
Here’s where to look:
The Thomas Register
Thomasnet.com is a favorite industrial source for metrology labs. At the main page, type “Metrology Services” into the search bar on the home page for a listing of companies.
