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Bearings for high-speed spindles.

Your demands for higher speed machining and greater precision are being met by many people, from the major builders of machine tools to a growing number of much smaller companies supplying special spindles and retrofits for your existing menagerie of machine tools. Some are doing a much better job than others, and a key evaluating factor is the choice of bearings in those spindles. Do these people really know their bearings? Do you? Here, based on an interview with Ted Silver, director of marketing, Barden Corp, Danbury, CT, is a look at what the precision-bearing people are doing to respond to today's spindle requirements. Barden specializes in the higher bearing grade levels-ABEC Grades 7 or better-in sizes up to 12" dia. A third of their bearings are used in machine tools, the remainder primarily in aerospace, with some use in medical and computer applications.

According to Silver, the users challenging machine-tool bearing producers today are those doing high-speed milling, particularly in the aircraft industry. "These people either want to mill exceptionally fast, or conversely, are doing precision work that demands really low noise and vibration levels-two very difficult problems to solve."

Temperature trauma

At high speeds, the bearing is limited by its own heat generation and/or the heat conducted from nearby machine elements. Normal operating temperatures inside the bearing are 120 to 130 F. Over time, temperatures higher than this will destroy bearing lubricant. Every 20 F temperature rise cuts bearing life in half.)

"Grease is great," says Silver, what everyone would like to use. Just pack it in the bearing and seal it up, and forget it. But when temperatures reach the point where they aggravate grease life, you must try something different.

"Some people have tried water-cooled spindles or slingers to get the heat out of the shaft, and you can water-jacket the housing to carry heat out of the external parts of the spindle, but its tough to get heat out of the shaft." Some space-age scientists have even considered sodium-filled shafts-a passive heat-pump approach.

Reducing ball size is one way to stick with grease and reduce heat generation in the bearing. Surprisingly, a larger number of smaller balls generate less torque, energy, and heat inside the bearing than standard-size balls. For the same stiffness (spindle rigidity), you can use lighter preloads, and the torque and energy consumed in the bearing will be 15% to 20% less.

On the negative side, you sacrifice some dynamic capacity-fatigue life will be less than for a bearing with larger-diameter balls. But spindles don't fail in fatigue, Silver points out. "They ultimately wear, you get chatter, the spindle is loose, and bearings need to be replaced. It's very rare that you get catastrophic spalling, fatigue-type failure, where the mettle of the metal gives out. The usual spindle problems are lubrication and wear." Thus, a given spindle will run cooler with a switch to smaller bearings, offering longer grease life; or, if you want to increase speed, you can do so and get the same life.

Contamination consciousness

Barden offers greased bearings either open or shielded. Most spindle-building shops are not clean-room environments, notes Silver, and some still don't consider the shielding of bearings against contamination a necessity; but it is. He is encouraged, however, by a growing consciousness of bearing contamination. Seminars addressing the effects of contaminant particle size, even those as fine as smoke particles, are helping people become more aware of the need to maintain a clean operating environment.

"A lot of our customers like to grease their own bearings, but we shudder at that," he worries. "Most don't take proper care to filter the grease, which is very important because it doesn't come from the major oil companies as clean as it should be. They also don't handle grease properly, or get the proper amount in the bearings. You can't just spatula it in, assuming that more is better. Standard grease fill is 25% of the open area in the bearing, and for high speeds, that's reduced to 15% to avoid churning and overheating."

He points out the similarity to boat-trailer wheel bearings. "You deliberately pack them full to keep the salt water out, but if you forget to reduce that grease content, and take that trailer out on the highway, those bearings will quickly get blazing hot.

"Inserting the right amount of grease and making sure it's clean are things a good bearing manufacturer pays a lot of attention to, and once this is done, adding shields makes a lot of sense to us."

Ceramic solutions

If doing all these things right-the right amount of high-temperature grease, the right preloads, using a bearing with smaller balls, and protecting the bearing with external seals-doesn't solve your problem, then what?

A simple switch from steel to ceramic balls can achieve an effect similar to using smaller steel balls. Because ceramic balls are much lighter, the resulting centrifugal ball loads are a lot less at super high speeds. Silver admits that price premiums are presently steep for ceramic, considering you can get the same speed and temperature benefits with small-ball designs. However, because of the dissimilar contacting materials-ball and race-noise and vibration characteristics of ceramic bearings are better.

Unfortunately, the price premiums for ceramic ball bearings border on the incredible-most are $5 and $10 per ball-potentially adding $2000 or more to the price of a spindle. Fortunately, these price premiums are dropping with wider use (in one case, as much as 40% overnight).

"Ceramic balls are going to extend bearing performance significantly," Silver predicts, "and in time, when the prices get to more economical levels, their use will take off significantly. If I were building high-performance machine tools, I would definitely offer ceramic-bearing spindles as options-and strongly urge users to evaluate them."

Noise

Improving on bearing noise and vibration is becoming increasingly important to both bearing users in the automotive industry as well as spindle builders pushing machining to higher speeds. Vibration is caused by displacements due to imperfections in ball roundness, coplane error, part misalignment, and-at high frequencies-surface finish.

That's right. Depending on the application," Silver explains, "some users are concerned not only about imperfections, mounting, and trueness of running, but also high-frequency effects of surface finish."

So what's Barden's response?

"We're more selective. In addition to ABEC 7 grades, we've added incremental steps of perfection-a tightening of tolerances that we can offer at increasing price premiums. Otherwise, you'd have to jump from an ABEC 7 to an ABEC 9, and double the price.

"This involves tightening up on the geometry, keeping the bearings and lubricant super clean, and doing things like superfinishing the ball grooves. Even ball geometry-normally the most precise thing in the bearing (0.000 003" to 0.000 005" out-of-round)-is becoming more of a concern although at first, this sounded ridiculous to our engineers.

With oil thickness of something like 0.000 015" between the race and balls, ball out-of-roundness of a few millionths is pumping oil up and down with each revolution, creating noise and vibration. "At today's high operating pressures and speeds," Silver points out, "oil acts like a solid, not liquid, so further improvements in ball geometry can make a significant difference."
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Sprow, Eugene
Publication:Tooling & Production
Date:Mar 1, 1991
Words:1202
Previous Article:High-speed spindles for milling?
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