Piezo-based system controls spring diameters

St. Paul, MN--When engineers employ a spring in a machine design, they want assurances that the spring's diameter and pitch are right for the application. A diameter that's too large or a pitch that's too wide could affect the spring's rate and its force capabilities. As a result, a spring-loaded valve or clutch might open too quickly or too slowly.

Until recently, there's been no easy way to precisely control a spring's diameter and pitch. Spring makers typically make them, measure them, then discard the rejects.

But now an engineer at Vadnais Technologies has found a better way. Using a patented piezo-based design, engineer Mark Kempf created a wire-bending system that corrects the diameter of a spring as it winds it. This technique enables the system to coil springs with accuracies of 0.0005 inch. Spring-winding machinery that uses conventional technology, Kemp says, typically achieved accuracies of 0.005 inch--about one-tenth as good as Vadnais' system--and even then required sorting afterward.

Key to the new system is an electronic position-control loop that enables the machine to monitor and correct spring diameter on the fly. During operation, the machine's feed rollers push the wire out through a "quill." As the wire emerges from the quill, it contacts a coiling tool, which bends it to create the helical spring shape. The distance between the quill and the coiling tool determines the spring's diameter. By moving the tool closer to the feed source, the machine creates a smaller spring; moving it farther away causes the spring to become larger.

After the wire is deflected at the coiling point, a linear variable displacement transducer (LVDT) touches the spring's diameter. The LVDT acts as a sensor, sending an input signal to a computer that controls the process. By reading the signal at a frequency of 400 Hz, the computer can monitor spring diameter. Using a proportional integral derivative (PID) loop in its software, the computer compares the measured value to a desired value, then sends out a correction signal.

The low-voltage correction signal passes through an amplifier, then goes to a piezo-translator embedded in the coiling tool.

Applying a voltage to the piezo crystals in the tool causes the six-inch-long tool to shrink or grow as much as 0.004 inch. As it does so, it moves closer to or farther from the feeding quill. As a result, the tool makes the spring diameter smaller or larger. Tool deflection is proportional to the voltage applied. "Once we get the tool to where we want it, we let this micro-positioning action take over," notes Kempf, president of the company and inventor of the system. "The piezo action is very, very fast. You couldn't achieve the same speed mechanically."

Vadnais Technologies initially designed the system as a means of controlling the production of wrap spring clutches for its parent company, Reell Precision Manufacturing. Using the process, Kempf was able to accurately control spring diameter and pitch--the distance between spring wraps. His technique also enabled Reell to control the quality of its own product, thus freeing them from reliance on outside vendors. "We weren't spring makers, but we needed to hold our spring diameters to tight tolerances," Kempf says. "That was what drove us."

By reaching those tight tolerances on the fly, Kempf's machine eliminates the need for sorting, dramatically reduces the scrap rate, and creates a more consistent product. "We found that if you control the spring's diameter and pitch, you can more consistently control its force and its spring rate," Kempf says. "And when it comes to compression springs, that's what users want."

Additional details...Contact Mark Kempf, Vadnais Technologies, 1375 Wolters Blvd., St. Paul, MN 55110, (612) 484-2627