Superprecision High Speed Spindle Bearings Improve Machining Productivity

Machine operators understand better than anyone how the smallest vibration grows as it travels. If the machine spindle is not held rigidly in place, tools wear prematurely; part surface finishes are adversely effected; machine speeds are reduced as is productivity; and machine maintenance increases while overall equipment life is diminished.

Advanced manufacturing applications are placing even greater demands on machine tooling and components, with equipment expected to operate faster, more reliably and with increasing efficiency. The Torrington Company (Torrington, Connecticut), a leading developer of advanced bearing technology, has been working closely with equipment manufacturers in offering a new series of high speed Fafnir brand ceramic and steel bearings specifically to achieve the level of performance required in this manufacturing environment. This high speed bearing design is offered with either steel or ceramic balls.

According to the company, trends in machining call for components that combine high precision and increased speed, together with reduced friction and vibration. For example, the ideal bearings must be designed for both high speed, increased stiffness and moderate loads, even under the most marginal of lubrication conditions.

"In the past, there's always been somewhat of a tradeoff between load capability and speed," said Matt Boylan, Machine Tool Industry Marketing Manager. "What Torrington has done is create a fine balance between the two, with our 99100 series of bearings that closely fit the product to the marketplace."

Whitnon Spindle, for example, specializes in the design and manufacture of highly engineered precision spindles. With tolerances measured down to one millionth of an inch, Whitnon requires the highest levels of quality and consistency in their spindle components. "The bearing is certainly one of the critical components making up the spindle head," says Lawrence Hermanowski, Sales Manager at Whitnon in Farmington, Connecticut, "Rigidity must be maintained in order for the process to be vibration free." He reports that with Torrington high speed bearings, it takes 4.5 million pounds of force to move the spindle head less than half the thickness of a human hair.

From an engineering standpoint, the internal geometry used to develop the design of these bearings while maintaining critical operational rigidity provide important benefits to machine tool applications. Their smaller ball size means less resistance to rotation and a lower centrifugal force. More importantly, testing has shown that the improved, more open race curvatures have had a positive effect on operating speeds through lower rotational resistance, reduced ball skidding and therefore lower heat generation. And the lightweight ceramic balls also have reduced gyroscopic slip, further contributing to reduced heat generation.

With a combination of ceramic rolling elements and standard steel races, this hybrid technology has helped bring about significantly reduced bearing wear and lubrication breakdown, resulting in increased production efficiency.

Ceramic rolling elements are made up of silicon nitride. The high strength of the silicon-nitrogen bond is what accounts for the high level of elasticity, hardness and temperature capabilities of components. The ceramic material is chemically inert, providing corrosion resistance for the rolling elements. Also, unique tribological (lubricating) features enhance operation under low lubrication conditions and extend both the life and speed capabilities of lubricants. Furthermore, it may be possible for the machine tool to be converted from oil to grease lubrication when changing from conventional all-steel bearings to a ceramic hybrid. The reduced lubrication requirements may eliminate expensive oil recirculation systems or environmentally unfriendly processes in machine tool applications. When properly used in machine tool spindles, for example, these benefits can provide up to five times extended life, with an increase in accuracy by up to 50 percent, developers claim.

Bryant Grinder Corporation of Springfield, Vermont, is another important user of hybrid bearings. Bryant has been building precision grinding machinery since 1909, and pioneered development of a motorized spindle for grinding back in the 1930s. This technology was transferred to milling applications in the late 1970s, and later enhanced with the use of hybrid ceramic bearings. Today, Bryant offers a wide range of internal and external CNC grinding equipment and high speed milling spindles, and has developed internal standards more stringent than ABEC industry standards.

Dr. Russell Kulas, Manager of Spindle Technology, has noted that in determining when to go with the hybrids, Bryant used a DN value to rate bearing speed--calculating inner ring bore in millimeters × rpm--and also considered bearing size, load, capacity and stiffness. "The ceramic bearings provided a marked improvement in stiffness and speed and enabled us to use a larger bearing; this also allowed us to increase load capacity," said Dr. Kulas. "The hybrid reduced noise and vibration, while the lighter balls significantly reduced stress on the bearing." Bryant is now moving into the small ball, hybrid bearing products. Dr. Kulas said this offers the double advantage of higher speeds (the lighter and smaller ball) as well as greater stiffness.

Whitnon Spindles also promotes the advantages hybrids bring to their high speed machine tools. "The reduced heat from the ceramic balls allows for better control of expansion of the material, and that means better accuracy," says Mr. Hermanowski. "Also, the lubricant doesn't thin out, and therefore remains effective longer."

Mr. Hermanowski points out that a significant portion of Whitnon's work is in the aerospace business where oil can easily stain composites. "Using permanently pre-greased ceramic bearings not only allows speeds up to two-to-three times faster," says Mr. Hermanowski, "but reduces the time-consuming need for oil mist monitoring and eliminates this kind of expensive oil contamination."