Conventional wisdom has it that grease lubrication is fine for grinding spindles so long as speeds stay fairly low. As speed approaches 10,000 to 15,000 rpm, however, then heat becomes a major problem, leaving little choice but to go to an oil-and-air mist system.
Or so the thinking goes. But according to GTI Spindle Technology, Inc. (Manchester, New Hampshire), the combination of synthetic lubricants, hybrid ceramic bearings and other innovations are enabling grease lubed-for-life bearings to operate at much higher speeds than ever before. The company has achieved a great deal of success in replacing existing oil/air-mist grinding spindles with new grease-lubricated designs, and with no sacrifice in performance. Moreover, the cost of such a retrofit is generally only marginally higher than a standard repair, they say, and offers substantial overall savings down the road.
GTI claims that through the use of synthetic greases and hybrid bearings, speed factors up to 1.5 million nDm are obtainable. (nDm is a grease speed capability metric that equals bearing bore diameter in millimeters plus OD diameter, divided by two, times rpm.) In practice, they have ID grinding spindles in the field that are routinely operating at 35,000 rpm, and believe that grease-lubricated units are feasible in applications up to 50,000 rpm.
The spindles employ a labyrinth sealing system with thirteen 90-degree angles and constant air purge (about 4 to 7 psi) to prevent contamination, even in very hostile environments. To date, the spindle maker has used a variety of unshielded bearings from Barden Precision Bearings (Danbury, Connecticut) with good success, and is currently testing a new Barden shielded ball bearing which has the potential to eliminate the need for air purge altogether.
GTI has applied grease lubricated spindles in a variety of high speed ID grinding applications. Some are operating around the clock in extremely hostile environments at high production facilities. GTI also validates designs on an in-house test station that allows the builder to monitor temperature, vibration and noise levels while the spindle is subjected to a range of performance tests under a variety of operating conditions.
The goal of GTI's retrofit program, they say, is to eliminate the need for oil mist lubrication systems in order to reap a number of benefits that include the enhancement of speed capabilities, part quality and spindle life. They argue that replacing oil mist with grease will reduce overall system costs by eliminating the need for oil, filters, lines and fittings, as well as the maintenance required to keep an oil mist system in good working order. And that's not to mention the cost of catastrophic failures that can be caused when some component of a more complex lubrication system fails. Moreover, moving to grease will eliminate the introduction of oil mist into the general shop environment, a source of increasing environmental concern to many manufacturers.