Latch holds key to clean air 22414

Concordville, PA--McDonald's covers the world. But what it doesn't want to do is cover the world with dirty air. For this reason, the giant fast-food chain called on United Air Specialists (UAS; Cincinnati, OH) to develop a custom air cleaner for its restaurants.

Called the Universal SMOG-HOG(reg), the all-stainless-steel, hood-mounted cleaning system fits into McDonald's standard kitchen equipment above a fryer or grill. It uses electrostatic precipitators to remove grease and other particles from the cooking process before they enter the store's ductwork and the outside air.

But the story doesn't end here. UAS called on its long-time partner Southco Inc. for help in designing a key element of the system: the latch.

Access doors on the front of the unit allow workers to clean internal components of grease and others debris, a process that can occur daily in McDonald's high-volume stores. The Southco latch had to withstand this frequent use, secure and seal the doors, and meet stringent UL fire and safety requirements. And it had to accomplish this at a competitive cost.

Taking the first step. Southco's line of E3 Vise-Action(reg) compression latches seemed perfect for the project. The latch's head design features special-tool access and requires a key for entry. The key prevents individuals not qualified to service the equipment from gaining access. And it can't be easily replaced by a screwdriver or common tool.

The latch also can apply up to 6.4 mm of consistent pressure to compress EMI or environmental gasketing. "The latch can pull down a gasket up to 1/4 inch, which gives us a good tight seal," says Neal Clevenger, product development engineer at UAS.

Moreover, the device allows quick and easy access, and provides perpendicular compression that helps eliminate rattle and won't scratch the cleaning system's frame. How? The latch first rotates 90 degrees, then pulls directly in, so there is no forcing the gasket closed, explains Clevenger. "The person closing the latch doesn't have to push the door and overcome the compression. Instead, the latch does all the work," he says.

Cleaning up the act. Even with all these features going for it, the E3 Vise-Action latch didn't quite fit the bill. In fact, Southco and UAS engineers determined the cleaning-system application would push the latch to its limits.

"The shaft, which serves as the main load-bearing member, was a little too light," notes Clevenger. "It would work fine as long as people didn't abuse the system. But we saw a potential problem down the road."

Another issue, Clevenger admits, was that the latch was a little more expensive than anticipated. Plus, manufacturing found that assembling the latch to the cleaning unit was difficult because the pawl didn't always locate properly.

Southco engineers immediately went to work to modify the existing latch. "By lengthening the shaft slightly and using a standard pawl, we achieved the optimum grip range and maintained the requested 'envelope' dimension," says Kevin LaValley, engineering technician, Product Specifications, at Southco. From concept drawings to prototypes, Southco turned the project around in about four weeks.

The new version features a larger, more robust shaft that easily withstands daily abuses. Southco also added a self-fixturing assembly, with double-D flats on the shaft that ensure the pawl fits correctly. That translated into less work in manufacturing.

Better yet, the new design delivered an unexpected cost savings. Minor modifications to the original E3 latch to obtain a UAS-specified dimension had required custom manufacturing, significantly elevating the cost. With the new version, made entirely of standard components, UAS realized a cost savings of 12%. "I expected the cost to go up slightly since I asked for heavier components," Clevenger recalls. "We thought it would end up being more of a specialized latch.

"In the end, we got a latch with heavier components for a reasonable cost savings," Clevenger adds. "And it's easier to assemble, saving time in the final assembly."

Latch criteria

- Incorporate a 1/4-inch grip range to ensure a tight seal.

- Withstand abuse of high-volume fast-food store.

- Include a head design that accomodates special-tool access.

- Meet both UL fire and safety regulations.

- Look good.

- Be cost competitive.

Original latch complications

- Too light a shaft that might succumb to abuse.

- High cost due to use of custom component.

- Difficult assembly caused by problems with pawl location.

Design solutions

- Larger, more robust shaft.

- Self-fixturing assembly that automatically locates pawl correctly, easing assembly.

- Use of all standard components, reducing cost by 12%.