Machine Tool Components And Accessories - Smith-Renaud - Brief Article

HOLD-ROL bearings (right) are used on a Center-less grinder rebuilt with an upgraded regulating wheel and drive unit mounted en it.

Improving Centerless Grinders

One way to improve machine tool performance is to replace older equipment with newer equipment. However, a less costly alternative is to upgrade or remanufacture existing machines. This approach often allows companies to obtain features that would be prohibitively expensive on new machines.

Smith-Renaud (Cheshire, Connecticut) rebuilds Cincinnati Milacron (now Cincinnati Machine) centerless grinders, as well as machines from other manufacturers. These machines are commonly used to finish grind round parts (such as bearings), fasteners, taps and other tough-to-grind parts where machine stability improves the process.

"We have been rebuilding centerless grinders for 20 years," says Tim Emerson of Smith-Renaud. The company has learned that a key to upgrading centerless grinder performance is replacing the regulating wheel spindle bearings. The company used to replace the bearings with duplicate OEM bearings. But then Smith-Renaud started looking at other solutions. What it found were HOLO-ROL cylindrical roller bearings from Kaydon Corporation (Muskegon, Michigan). "We first learned about the bearings at a technology exchange seminar and decided to give them a try," Mr. Emerson says. Due to their performance characteristics, HOLO-ROL bearings offer advantages over conventional bearings in this application. The OEM bearings would require periodic adjustments by an experienced technician, but the HOLO-ROL bearings do not.

HOLO-ROL bearings use hollow cylindrical rollers instead of solid rollers. These rollers are radially preloaded, giving them more radial stiffness than conventional ball bearings. Two performance characteristics create this greater stiffness. All of the hollow rollers are in tight contact with the bearing races, and the rollers all work together to distribute the load. Preloading ensures that roller contact deflection against the bearing races is taken up before a load is applied to the bearing.

Bearing lubricant can flow through the centers of the hollow rollers, dissipating heat more effectively than solid rollers. The bearings therefore run cooler, improving precision, permitting higher operating speeds and extending lubricant life. The hollow rollers also weigh less than solid rollers, so they reduce centrifugal forces at high operating speeds. As a result, contact stresses are lower, and operation is smoother.

These bearings have half the radial runout of an ABEC 9 precision class bearing, so they reduce the radial runout of a regulating wheel spindle. This improved performance is due to three factors:

* Preloading eliminates all internal clearance in the bearing;

* The hollow cylindrical rollers absorb variations in the bearing raceway surfaces and maintain a constant rotating center; and

* The hollow rollers are compressed to the same radial height, and they rotate as if they were all exactly the same diameter.

These bearings can be used for regulating wheel and grinding wheel spindles, but the most significant performance improvements are achieved by upgrading the regulating wheel spindle bearings. The improvements achieved by using HOLO-ROL bearings on regulating wheel spindles are such that Smith-Renaud offers upgrade kits for those wishing to install the converted assembly.

HOLO-ROL bearings improve centerless grinder productivity in numerous ways. First, their lower radial runout permits greater stock removal rates, which reduces cycle time. Second, fewer wheel dressings are needed due to the greater stiffness and decreased vibration of the bearings. Plus, the reduction in frequency of dress means longer wheel and diamond life. Finally, these bearings are lubed for life and sealed when they are assembled. No grease fittings are used, and no adjusting is needed.

Decreased radial runout with these bearings equates to less movement of the regulating wheel shaft and spindle. This reduced movement results in improved part geometry and roundness. Additionally, the surface finish on the part surpasses that of a grinder using conventional bearings.

"HOLO-ROL bearings are more expensive than the OEM replacement bearings that we were using, but the performance benefits are worth the extra cost," Mr. Emerson says. "The bearings reduce maintenance and improve the grinder's performance. From a performance perspective, not only do you reduce scrap with these bearings, but you can also grind more aggressively, so you can make parts faster.