New spin on spindle bearings

Better materials and sealing technologies meet the challenges of modern machine tools and coolants.

Bigger, faster machine tools obviously put greater demands on spindle bearings. Most spindles use a combination of angular-contact ball bearings and cylindrical roller bearings. The angular-contact types install at a spindle work end because they handle both axial and radial loads. The other end houses a cylindrical roller bearing which, by design, floats or slips axially to compensate for spindle thermal expansion yet operates with zero radial clearance for rigidity.

Within these two bearing types exist multiple variations in design and materials. A basic understanding of available options can help design engineers select the right bearing for the job.

Seals
Most spindle bearing failures can be attributed to coolant or cutting fluid leaking past bearing seals. Liquid contaminants wash lubricant away and allow the bearing to rust or wear prematurely and possibly seize.

Labyrinth seals are probably the most common way to keep contaminants out. Spindle labyrinth seals may include an external splashguard followed by a narrow gap, a large drainage chamber, and, space permitting, more gaps and drainage chambers. Splashguards mostly prevent fluid from entering the first gap while subsequent gaps keep it from going further. Drainage chambers, as the name implies, catch fluid being flung off the spinning spindle shaft and return it to the work area.

But labyrinth seals alone may not work in some cases. Another option is angular-contact ball bearings with integral seals or shields such as those from SKF Machine Tool Precision Technology, Bethlehem, Pa. (www.linearmotion.skf.com). These shields consist of molded rubber formed onto a steel backing that acts as an added barrier against contaminants.

Sealed angular-contact ball bearings share the same basic design as conventional nonsealed types and can be supplied alone or in sets. Bearing sets are typically specified when loads exceed ratings of single bearings or when bidirectional axial loads are anticipated.

Bearing materials
Spindle bearings have traditionally been made from high-carbon chromium steels such as 100Cr6. These bearings work well in most cases, though higher spindle speeds (>100,000 rpm) and even longer lifetimes at moderate speeds may require alternative bearing materials.

Silicon-nitride ceramic balls, for example, weigh less and are stronger and stiffer than steel balls, resolving dynamics issues that plague the latter at high speeds. And ceramic balls better withstand intrusion of water-based coolants, important because such coolants are steadily replacing less environmentally friendly cutting oils.

Ceramic balls run on steel races (hybrid) also eliminate metal-to-metal welding that can happen with marginal lubrication from air-oil lubrication systems used in high-performance spindles. For these reasons, more and more spindle makers these days are going to hybrid bearings as prices continue to drop.

High-nitrogen-content stainless steels such as Nitralloy are yet another choice for angular-contact ball bearings. Nitralloy bearings, either open or sealed, can better resist corrosive environments and live longer than conventional bearings, especially those run at high spindle speeds.

Lubrication
Ceramic and stainless-steel bearings require less lubrication than their all-steel counterparts, though some is still needed to control heat and minimize wear. Lubricant choice and delivery method depends primarily on conditions such as operating temperatures and speeds, but may also be dictated by lubrication requirements of adjacent components such as gear wheels.

Applying a thin oil film (mist) is the most common method for lubricating bearings in high-speed spindles. But a recent ball-bearing design improves upon this method. Here, a hole in the outer ring of open-type angular-contact ball bearings or cylindrical roller bearings delivers lubricant where it's needed with minimal waste or chance of contamination.