Torque Talk

There was a lot of talk about torque at the 2012 Aircraft Maintenance Engineers Association of Ontario annual conference. Tongues were wagging about a presentation by John Nigro, national sales manager for Sturtevant Richmont, who was at the show to talk about error-proofing processes for hand torque tools.

Nigro’s presentation got the attention of Ontario aircraft maintenance engineers (AMEs) for two reasons: what he had to say can greatly reduce errors associated with the use of hand torque tools; and, it can also save aircraft maintenance organizations (AMOs) a significant amount of money.

Today, most Canadian AMOs send their torque wrenches and other hand torque tools out for annual testing and recalibration. Depending on how many pieces of equipment must be sent out, this can be an expensive endeavour, with yearly costs running between $100 and $150 per wrench. Plus, there is the added inconvenience of equipment being out of the shop.

But aside from the cost and inconvenience, there’s another issue that strikes at the heart of aviation safety. “The problem with sending wrenches out once a year is that when the report comes back saying it’s out of calibration, you don’t know when it went out, and what you used it on since then,” explained Nigro. “Calibration can go out any time due to poor tool design or misuse [the human factor].”

If the calibration of a wrench is off, it can lead to over- or under-tightening of bolts and fasteners, and either scenario is not good. “When it’s over-torqued, the bolt may be stretched beyond its limit,” Nigro told Canadian Skies “Over time, the rattle and vibration from the engine will crack that bolt, because it’s gone past its yield point. It may not fail right away, but it will fail down the road. On the other hand, if you under-torque, the bolt may not appear to be loose, but it becomes loose over time.”

He added that the first thing investigators look at after an aviation accident is torqueing issues. “They look at what was put together; the last maintenance on it. You’re looking at human factors there.”

Nigro’s company, Sturtevant Richmont, specializes in advanced torque calibration and audit systems. Established 70 years ago, the company’s founder, P.A. Sturtevant, introduced the original deflecting beam torque wrench. Today, Sturtevant Richmont focuses solely on torque, taking a “systems-based” approach that includes selling torque tools and then managing them with error-proofing products and processes.

For a small investment of about US $3,500, maintenance shops can purchase two digital torque testers from Sturtevant Richmont that will check and calibrate large and small clicker and camover-type torque wrenches and screwdrivers. The equipment eliminates the need for annual certification because it checks and corrects the calibration of a wrench in-house, right on the shop floor. If an AMO has 10 or more torque wrenches, which are calibrated at an annual cost of $100 each, the testers will pay for themselves fairly quickly.

“To have this torque tester onsite does two things,” said Nigro. “One, you verify the wrench is right before you start the job. People don’t always understand that torque wrenches need to be exercised before you use them, especially in a maintenance application. If it hasn’t been used in a while, the first few clicks may be out of tolerance due to lubrication setting up inside the wrench. Once you pick it up to use it, you have to exercise the wrench to move the lubrication. After a few clicks, it will get back into tolerance. The reasons to have a torque verifier are to make sure that your wrench is calibrated, and second is to exercise the wrench. It’s critical that these things are checked.”

Micrometer adjustable click wrenches are plus or minus four per cent accurate. “But on the first couple clicks on a click wrench, you can have double that accuracy, up to eight per cent on the first click or two,” explained Nigro. “If the spec on your job is five per cent and you don’t check, you could be out of spec for what you’re working on. That’s the danger.”

Another benefit of testing calibration on site is the ability to double check manual calculations. For example, torque output changes when an extension tool is added to a wrench. An engineer must perform a calculation to determine the new output, but they can also walk over to the verifier to double check their math.

Nigro said torque-testing technology has been employed in the automotive and agricultural manufacturing industries for decades, but it’s more prevalent in aviation now. Sturtevant Richmont has developed a “torque management pyramid” that describes how maintenance shops can go from the basic periodic calibration of torque tools all the way up to the pinnacle of torque management, an advanced assembly and audit system that ensures torque is properly applied, captures the data to prove it, and then gathers the necessary info to audit the torque application process.

“This technology hasn’t been used in the aerospace manufacturing industry much, but it’s coming,” said Nigro. “We’ve always talked to OEMs about this type of technology, but they didn’t have the systems in place to collect the information. But they are studying it now; it’s going to become more accepted. Some manufacturers are now accepting ZigBee radio protocol, and that’s the first step.”

ZigBee radio is a low-power communications protocol that is best suited for periodic data transmissions from a sensor device, such as a torque wrench that is used on a job and then transmits torque application data back to a main data storage device. This type of system would be employed as maintenance shops work their way up the torque management pyramid, using more complex and expensive tools.

“The advanced systems feed through Ethernet and the customer can put it in a database with all the info for that wrench,” explained Nigro. “You can program it by the tail number of the airplane, call a bolt by a specific name, or name the joint you pulled it from; it’s that detailed. It will even reject a click if a wrench is used improperly, which addresses human error.”

But for smaller shops that can’t afford to purchase the Cadillac of auditing systems, basic torque verifier units are still available and make financial sense. Canadian regulations say that AMOs don’t necessarily have to send their wrenches out for calibration, as long as they have a documented process in place to ensure torque tools are checked and adjusted. A torque verifier would qualify as an acceptable process, and that’s why AMEs were excited about Nigro’s presentation.

“The reason why it’s such an innovative idea is that people don’t realize they can check their own wrenches,” he said. “We’re talking about how to error-proof their processes, starting with calibrating their tools, all the way to actually assembling whatever they’re working on. We want to error-proof each step. We’re talking about how to reduce the human factor.”