Controller impacts torque wrench

Impact wrenches can squeeze plenty of torque out of a hand-held tool. But because they provide only a weak link between applied torque and what the operator feels, they can have a problem with accuracy. "In automotive tire shops, where impact wrenches are used, it's not uncommon for lug nuts to be over- or under-torqued, creating a safety issue," says David Cripe, a research engineer at Magna-Lastic Devices Inc. (Carthage, IL). Cripe has invented a new way to stop an impact wrench when it tightens a fastener to a pre-set torque.

His patent-pending MDI Impact Tool Torque Controller consists of a magneto-elastic sensor on the impact wrench's output shaft and a control algorithm to analyze the resulting torque signal. Upon reaching the predetermined torque value, the controller opens a solenoid valve on the impact wrench's air line, stopping the tool.

Controlling torque is one of those things that isn't as easy as it sounds, and Cripe's invention forced him to address two problems. First he had to design a rugged sensor that would stand up to the harsh environment of a tire shop. The answer came in the form of a magnetostrictive sensor built into the wrench's output shaft. Comprised of a magnetostrictive alloy and a corresponding pick-up coil, this torque sensor detects changes in the alloy's magnetic field based on torque-related shear stress in the output shaft. "A voltage proportional to the rate-of-change of the torque is induced upon the coil," he explains.

Cripe next defined control algorithms that infer the actual torque from the information provided by the sensor. They are not one and the same due to tightening dynamics. "The mass of the socket and fastener results in a rotational moment of inertia that limits the ability of the tool to instantaneously rotate the fastener," Cripe explains. "So, the torque within the fastener does not follow the torque of the impact tool shaft." What's more, the non-linear effects of static friction on the fastener further muddy the relationship between measured and actual torque. Cripe took these and other tightening effects into account too.

While the MDI system targets automotive lug nuts, Cripe believes it can serve in a variety of industrial applications that use expensive motorized torque tools for the sake of accuracy. "An impact wrench with torque control costs just a fraction of your typical nut-runner," he says.