What could be simpler than putting a nut on a bolt and tightening it down? Ask Jeff Drumheller, Project Testing Engineer at RS Technologies’ fastener laboratory, and he’ll tell you the answer is “lots of things.”
Drumheller sees the full range of threaded fastener applications in his work, which includes everything from one-inch diameter studs and lug nuts used on truck wheels, to tiny 090 screws used to assemble cell phones. Most of the lab’s work is devoted to specification development and evaluating impact factors, such as how a change in a coating or surface finish will affect joint integrity.
The fastener lab is accredited by the American Association for Laboratory Accreditation (A2LA), a nonprofit, non-governmental, public service membership society that provides comprehensive services in laboratory accreditation and laboratory-related training.
“The accreditation is more stringent than a quality registration,” Drumheller said. “It’s not just about documenting our processes and procedures, but also demonstrating competence and knowledge about the accredited processes to the satisfaction of the auditor.”
All Types of Customers
The lab’s largest group of customers are auto companies and their suppliers, followed by firms that apply coatings to fasteners and anyone else who uses threaded fasteners to assemble their product. Most of the work is literally nuts and bolts, but the lab can test any kind of threaded fastener that a customer may be using.
“Just now, we are seeing some work related to the European Community’s Restrictions On Hazardous Substances (RoHS) requirement that seeks to eliminate a range of toxic chemicals from the electronics industry,” Drumheller said. “Two of the substances RoHS is concerned about are hexavalent chromium and cadmium which are used on a lot of fasteners. So, when other materials are substituted to meet RoHS requirements, we have to audit and re-qualify joints to make sure they still meet the original performance specifications, and it isn’t just electronic parts.”
How Testing is Done
A typical test involves the use of calibrated torque-angle transducers, clamp force load cells, powerful DC nutrunners to tighten the fasteners, and closed-loop data acquisition systems. Recorded data includes input torque, fastener rotation, tension load, and thread torque. The data can then be used to determine proper assembly methods and specifications, analyze yield characteristics, calculate friction coefficients and many other factors that are important to the fastener engineer.
“We have fixtures for a lot of the more common parts,” Drumheller said, “but if we don’t we’ll build one. That can be a problem with really small components, like the small screws used in cell phones, but we have enough experience that we can handle them efficiently too. Normal turnaround is in the range of a few days to a few weeks, depending on what we have to fabricate to support the testing.”
When an on-site response is required, Drumheller and his team will sometimes pack up the necessary equipment to perform testing in the customer’s plant.
“Testing on the customer’s site is not covered under our accreditation like the lab tests are,” Drumheller noted. “But often we can help identify the cause of the problem so that our customer can get their line back into production while we conduct a more complete series of tests back in the lab. We often will run torque to failure tests, where we take the fastener into yield or failure, to determine the cause of the problem. Despite the destruction of otherwise good parts, we often find that such tests yield very useful information.”
According to Drumheller, the problem can usually be solved by simply slowing down the nutrunner. “Many times assembly specifications such as rpm are set by a ‘whatever the tool can do’ mentality,” Drumheller said. “But when you over-speed the operation you run the risk of creating a lot of friction and heat that can adversely affect joint performance. Slow the tool down to a more manageable speed and results become more predictable and reliable, although that’s not always an easy sell to customers with tight production timetables.”
“Selling” Test Reslts
Helping to “sell” a test result can also be a part of Drumheller’s job. He and his associates often participate in international conference calls to explain the reasons behind their test results and the recommendations that develop from them.
“We’re just here to supply the numbers,” Drumheller said, “but sometimes we have to explain what we’ve found in the lab. I suppose it’s just human nature, but it seems that the further away the customer is, the less likely they are to accept test results at face value.”
RS Technologies also does computer joint simulations using SR1, a modified VDI 2230 method for calculating bolt tension in a joint. The software lets the user model a joint and determines loads and safety factors. Often this modeling will point the way to the leading cause of the problem.
“It’s very useful where we can’t test a joint,” Drumheller said. “So, we do a software simulation on a joint instead. Results aren’t certified, but they’re good enough to make some preliminary decisions during the design process.”
Whether it’s a giant lug nut and stud for a truck wheel, or a tiny titanium screw for a medical appliance, there is a lot more to threaded fasteners and the joints they hold together than meets the eye.
RS Technologies/CAESAR Technologies