Anti-cage creep mechanism helps bring Lasik surgery equipment into focus

Motion control is essential for the digitization and automation of equipment, and bearings are at the core of frictionless movement. In 1956, just distributing bearings in California was viable, even on a small scale. For many small distributors, with growing globalization through the end of the 20th century, it was change or die. This is where the entrance of strong engineering skills came to the rescue of such distributors.

Bearing Engineers, one such distributor, recently changed its name to Motion Solutions to reflect their evolution into custom designers of motion solutions. How this distributor became a manufacturer is illustrated by the development of their newest product.

A company who needed a better linear motor table for positioning laser surgery equipment brought this challenge to the engineering team. According to Shawn Hakim, mechanical engineering supervisor at Motion Solutions, “The client came to us with the complaint that their Lasik eye surgery equipment was not performing smoothly enough and its running parallelism was not up to their standards.” The problem was caused by linear bearing cage slippage. Though positioned horizontally, at full operating speed, the bearing retainer would creep as the momentum of the bearing movement was transferred to the cage (also termed the retainer). An anti-cage creep mechanism was the logical solution.

The term “anti-creep” describes the method of eliminating any slippage of the retainer holding the crossed rollers between the two V-grooved rails of the slideway. In addition to maintaining precise movement, without creepage, downtime is reduced lowering the cost of maintenance.

An anti-creep device eliminates this “creeping” of the retainer, so the slideway can be used in any mounting direction and with lower momentum motors such as linear motors. Several complex anti-creep devices have been developed. To prevent cage creep/slippage, manufacturers have used a few different approaches, such as a rack and pinion mechanism; an external attachment made of plastic gears outside of the rail; and a metal gear inside of the rail. Some of these devices are quite expensive.

After comparison tests, Hakim settled on one mechanism that does not use a gear. It is an anti-cage creep mechanism that uses a roller with round balls studded around its surface. It has the smoothest tracking motion and therefore is quieter than an externally attached toothed-gear type anti-creep device. In this Studroller mechanism, creepage is prevented because the raceway has depressions that track the studs or nodules, preventing slippage in any position.

By placing studs in the center roller and machining a path along the rail, this retainer will not slip. It is suitable for high acceleration, vertical or horizontal mounting and uneven load distribution.

The engineering team saw that the improved linear motor table held great promise as a proprietary product. However, they had commercial concerns; cost was their number one concern. Costs vary for crossed roller bearings with anti-creep mechanisms—depending on the complexity of their design and whether the application has to be custom designed to accommodate them. The Studroller, being the simplest non-slip design, uses a studded roller, as opposed to a gear or exterior control, so its cost is almost the same as a standard slideway.

“We were able to make the product more cost competitive. Our product lowered the client’s cost by 17%. We gave them an equivalent product that is lower cost, has better running parallelism, better smoothness and with about 30% more force,” said Hakim.

“Secondly, long lead times—say, six to eight weeks—could doom the product. We established processes that would allow us to shorten the lead time to somewhere between two and three weeks,” he added.

The linear motor table, which initially was incorporated into Lasik eye surgery systems—focusing optics and lasers on the eye—has a level of precision that is also good for other applications.

That challenge also led to the development of a line of precision positioning tables that meet the requirements of liquid-crystal related equipment, measuring instruments, assembling systems and material transfer equipment.

The design team’s Linear Motor Table is powered by an ironless core linear motor. This smooth, non-contact drive system prevents force ripple (attraction force or cogging), unlike an iron core motor. Designed for high speed positioning, the linear motor tables allow speeds up to 2,550 mm/sec. A compact, lightweight aluminum alloy enclosure houses a linear encoder that precisely positions down to 10 nm resolution.

NB Corp.
www.nbcorporation.com