How rolling-ring actuators work
Rolling-ring bearings are basically standard ball bearings but with a ridge machined into the inner race. Just the ridge apex makes contact with a smooth, threadless shaft inserted in the bearing bore. The resulting clearance permits angling the bearing left or right while maintaining continuous point contact with the shaft surface.
Changing the bearing angle relative to the shaft centerline determines bearing carrier travel direction and pitch, the distance traveled per shaft revolution. Angle changes, and hence speed and direction changes of the bearing carrier, can be done on the fly while the shaft rotates at constant speed. Rolling-ring actuators give backlash-free linear motion resolvable to 0.0004 in. for reciprocating, positioning, and indexing applications.
Jennings International Corp., Norristown, Pa. (www.jenningsinternational.com), builds ram extruders that make Teflon hose, tube, tape, or PTFE-insulated wire. Finished product exiting an extruder is wound onto a motor-driven take-up reel while a traverse mechanism evenly distributes the material on the reel.
"The traverse mechanism is a minor yet key part of the machine. Breakdowns or frequent maintenance here can halt production," says Jennings Vice President, John Porta. "This is especially troublesome because these machines run batch processes. Successful production requires complete consumption of any given batch and a failed traverse mechanism could prevent that."
Providing take-up-reel drives used to be the end-user's responsibility. No more. Jennings now offers take-up reels as an option with the machines. It chose for the traversing portion a rolling-ring linear actuator from Amacoil Inc., Aston, Pa. (www.amacoil.com). The rolling-ring design allows the use of a single, unidirectional motor to drive a simple pulley system.
The pulleys synchronize reel rotation to linear actuator traversing motion without clutches, cams, or gears. This eliminates a separate variable-speed, reversible motor and the attendant encoder and controls otherwise needed for a conventional screw-driven traverse mechanism. The approach saves about a $1,000 per machine, says Jennings. And, because the rolling-ring actuators run on smooth, unthreaded shafts, thread clogging isn't an issue so protective bellows aren't necessary.