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Sprag clutch applications vary widely depending on the aircraft. Rotational velocity may be extremely high (15,000 revolutions per minute or more). Functional temperature may dip to forty degrees below zero and reach as high as 120 degrees Centigrade (200 Fahrenheit). Lubrication under these conditions will depend greatly on available oil flow paths, aircraft or gearbox orientation, and the g-forces involved. Contact stress between mating components, inertia of rotating masses, angular accelerations, drive-train natural frequencies, and centripetal forces on clutch components all need to be understood, controlled and kept within design guidelines during the most demanding drive and flight conditions. In the end, the clutch must simply function under all possible conditions. Sprag clutch design is an empirical science – knowledge only comes from experience. Yet, to succeed the design must start with a good idea or several good ideas coming from insight into what the application will demand, coupled with supporting calculations.Walt Riley, Chief of Drive Systems Analysis for the Bell Helicopter Drive Systems Group in Ft. Worth, TX, has worked with Formsprag for over 20 years and describes the design process in somewhat open ended terms, especially for an engineer.

"The design of a sprag clutch is kind of esoteric," Riley says. “I used to call it 'intuitive design', but you can work out the physics. You just have to start with an intuitive feel that it’s going to work." Riley says he learned that from Ted Zlotek.

Chuck Duello, manager of Sikorsky’s Transmission Engineering Group in Ft. Worth, worked on clutch designs for Bell Helicopter with both Walt and Ted Zlotek for 25 years…and he speaks a similar language.

"Ted’s knowledge comes from experience and just a willingness to try. The work he’s in – well, heck there isn’t a book. He wrote the book. It’s intuition and judgment and gut feel that gets you started on a path, and of course it all has to be borne out through calculation, analysis and testing," Duello says.

"But it seems to work best when you start with a gut feel, followed with an empirical demonstration. You have to see it work first, and then you come back with analysis and theory and provide an explanation as to why. It’s more like being a garage tinkerer than a rocket scientist, but it gets you there. And maybe it gets you there sooner or better than the rocket scientist can. I don’t know. I really can’t explain it. But it goes back to intuition and instinct, and experience."

Nobody has counted, but the best guess is there are less than a dozen people in the world who design sprag clutches for helicopters.

"There's really nowhere to go to learn how to do this,” said principal engineer Jim Shimon who has been with Formsprag since 1976. "It's really a matter of total experience with the application of the product, and exposure to the helicopter field."

Overrunning clutches are also designed into applications where power sharing between multiple gas turbine engines is required. Ted Zlotek was 68 years old when he and the Formsprag team were asked to design a clutch for a twinning application in the Bell Boeing V-22 Osprey. It was one of the toughest challenges ever presented in the design of sprag overrunning clutches for a helicopter application. Where the clutch resides in the V-22 drive-train, the high-speed engine generates incredibly high surface velocities. In addition to the speed, high centrifugal forces and pressures, the clutch needs to transmit thousands of horse power, which means a lot of torque at those speeds.

On top of that is the problem of orientation.A helicopter drive-train clutch often spins on a horizontal shaft submerged in oil in a dammed up oil annulus. But because the V-22 nacelle can be tilted vertically, the clutch needs to operate in both a horizontal and vertical orientation, fully lubricated with no loss of function.

Beginning in 1987, the clutch design effort ended three years later with Chuck Duello, then at Bell, flying home from Michigan with clutches in his luggage, ready to start the ground testing of the Osprey.

"I spent some time and a few late hours up there in Warren, Michigan working with the hardware and the test stands," Duello says. "At the time, it was one of the toughest applications to-date in terms of PV (pressure velocity) – the contact pressure of the sprag and the sliding velocity of the sprag – due to the high torque requirement and engine speed.

"I know it was advancing the state-of-the-art at Bell and possibly the entire rotorcraft industry. We worked hand-in-hand with Ted and Jim and others, and ultimately we came up with a successful design that is flying now." – And instantly recognizable!

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