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AIA: Locking Nuts With Ease

Hendrickson Trailer Suspension Systems (Canton, OH) is a division of Hendrickson International, the world's largest supplier of air-ride trailer suspensions. Like many manufacturers, engineers at the Canton plant are concerned with fastener performance, especially where the trailer subframes are attached to each suspension's air springs. On the one hand, the fasteners need to be secure. On the other, they need to be easy to install.

"In our industry, prevailing torque nuts are standard, but problematic in certain applications," says Hendrickson senior project engineer Greg Copeland. "Since air spring studs are relatively long and use fine threads, it took significant time and effort to run them down the stud."

Resistance-heavy prevailing torque nuts also caused problems due to the fact that the material in the studs is softer than typical bolt-grade steel, thanks to the high temperatures used to furnace-braze the studs into the top plate of the air spring. As a result, all-metal prevailing torque nuts, because of their relative hardness, tended to gall or damage the stud threads. They would also damage the plating on some of the stud threads, allowing rust to form that made it difficult to remove the nut later on during maintenance.

According to Copeland, one alternative was to use prevailing torque nuts with an inside polymer patch to prevent galling. These types of fasteners, however, still required extra labor to run down the air spring studs.

Ultimately, Hendrickson went with nuts manufactured by Spiralock Corp. (Madison Heights, MI), which use a proprietary thread form combining locking performance with easy installation. Specifically, the female thread form incorporates a 30-degree "wedge ramp" that eliminates the sideways motion that causes vibrational loosening. Compatible with standard, male-threaded components, the configuration helps distribute a joint's load throughout all the engaged threads and is free-spinning until clamped to a final torque-retaining position.

"We'd heard of the Spiralock thread form and were intrigued by it, but were resistant until we finally justified it by cost and function," says Copeland. "In the end, it was an elegant solution."

According to Copeland, his company especially benefited from the new nuts' ability to distribute clamping forces along their entire length. This property minimizes the chances of stripping the soft threads on the stud, even if the nuts are inadvertently overtorqued upon installation or reinstallation. This represents an important safety feature, because the studs are not replaceable or repairable, meaning a stripped stud requires replacing the entire air spring, an expensive component.

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"We went to the free-spinning Spiralock nuts over prevailing torque nuts for simplified installation and maintenance with equivalent or better locking ability," says Copeland. "The Spiralock nuts were interchangeable and backward compatible with what we'd used previously, and we realized overall cost savings by making the change. The change was well worth it both immediately and in the long haul."

For more on threaded fasteners, call 800-521-2688 or visit www.spiralock.com.