It's usually safe to assume that steel is stronger and more wear resistant than plastic.
But plastic gears have been shown to outperform metal ones in high-speed feeders that insert postcards inside magazines.
Rugged operating conditions tend to play havoc with the card feeders, often leading to costly breakdowns in time-sensitive production runs. Thus, improving durability in their feeders has been a long-standing challenge at QuadTech, Sussex, Wis. (www.quadtechworld.com), a manufacturer of automated control systems and handling equipment for the print industry.
The culprit was premature failure of two small metal gears a 4-in.-pitch-diameter gear segment (a portion of a 360 gear) and a 1.75-in. pinion. Mounted in a rotating drum, the cam-driven gear rotates the pinion back and forth 150 times/min. Grippers mounted to the pinion shaft open and close as the pinion oscillates, transferring cards from a stack to a conveyer belt for insertion in magazines.
The problem is that the metal drive gears must be lubricated. But mixing paper dust with grease creates an abrasive paste that quickly wears them out. The effects of worn gears, in turn, damage cams, bearings, and other components.
In the hunt for a better design, the company considered oil-bath gear lubrication, according to Dale Peterson, QuadTech's project leader. But sealing the rotating gearbox proved too costly. Another idea was to make the relatively accessible pinion a sacrificial component out of soft steel. It would wear faster than other components and provide a warning to replace the pinion and thoroughly clean all parts. But it wouldn't solve the underlying problem.
Peterson finally turned to Power-Core self-lubricating gears from Intech Corp., Closter, N.J. (www.intechpower.com). The nylon-12 gears are cast in a proprietary process that ensures a homogeneous crystalline structure, according to Tody Mihov, Intech's engineering manager. They are said to transmit higher torque than nylon-6 or Delrin gears and weigh less than metal ones.
Calculations showed the nylon-12 gears could handle the card-feeder drive loads without lubrication. Mihov optimized the gear mesh using Intech's gear-life software, which analyzes tooth-root stresses and flank wear. Based on the results, these additional design changes helped maximize life:
- Widen the gear to reduce tooth stresses.
- Remove unnecessary material to lower inertia in the large, metal-segment gear. This decreased shock loads during rapid oscillations.
- A metal core on the plastic gear permits more durable shaft attachment. And by adding a second keyway, at the first sign of wear, the gear can simply be flipped over to the unused portion, effectively doubling gear life.
- Modify the tooth root to reduce wear on the tooth flank.
- Add a special coating on the mating metal gear to further lessen mesh friction.
The lube-free gears met all expectations, eliminating unscheduled maintenance and extending pinion and bearing life, according to QuadTech. An added bonus was substantially quieter operation. The company has since replaced the feeder's metal cam followers with roller bearings and Power-Core tires because the originals wore out too quickly due to unlubricated metal-on-metal contact.