The lowdown on high-performance bearings

Thomas J. Wenck
Machine Tool Segment Manager
SKF Machine Tool Precision Technology
Bethlehem, Pa.

Many of the latest advances in the design and development of machine tools have been driven by a need to reduce costs and improve productivity. Compared with units of only a few years ago, today's machine tools offer greater accuracy, higher speed, and greater stability, and with it moreefficient production. For example, CNC lathes now feature four linear axes enabling two tools to work simultaneously. Modularly designed and computer-controlled grinding machines adapt easily to perform different and multiple tasks. And spindles are frequently designed as separate units to allow for quick interchangeability and less downtime.

As these and similar machine tools have become more complex, demands on system components have likewise increased. Superior performance is expected system-wide, in particular from the bearings. Rigorous application demands have spurred development of high-precision bearings and bearing arrangements specially engineered to satisfy a wide range of exacting requirements.

Spindle-support bearings, for instance, must exhibit a range of characteristics typically beyond the reach of standard bearings used in general-purpose applications. Spindle applications often require one set of bearings at the tool side and another set at the drive side. In general, high-precision bearings for machine tools are characterized by:

• Tight dimensional tolerances.
• A large number of rolling elements with small diameters.
• Low sectional height.
• Light and strong cages.
• Preloadable designs.

These features enable bearings to operate with minimal runouts, greater stiffness, high reliability, and remain cool at high speeds. Most bearing arrangements for precision machine tools are customized to meet the most important of these parameters.

High-precision bearings
An overview of the basic types of highprecision bearings serves as a useful first step in selecting the proper system for machinetool applications.

Angular-contact ball bearings are nonseparable, essentially single-row bearings featuring raceways in the inner and outer rings. Loads transmit from one raceway to another at an angle to the bearing axis. These bearings, therefore, can carry axial loads acting in one direction, as well as radial loads. Bearings subject to a radial load produce an axial force. An externally applied opposing force must counteract the axial force, so these bearings typically adjust against a second bearing.

Angular-contact ball bearings are supplied as single units or in sets, and as full steel or hybrids — bearings having steel rings and ceramic rolling elements. Engineers most often specify bearing sets when the loadcarrying capacity of a single bearing is inadequate, or the bearing must accommodate axial loads in both directions.

Cylindrical roller bearings are available in many designs, dimension series, and sizes. These high-precision, double and single-row bearings feature low cross-sectional height and high load-carrying and speed capabilities. They permit spindle-bearing arrangements designed for heavy radial loads, high stiffness, relatively high speed, and axial compliance.

Manufacturers typically recommend doublerow cylindrical roller bearings to carry high loads. Spindles that require high speeds and a more-compact design generally use single-row bearings. Hybrid cylindrical roller bearings incorporate specially designed ceramic rollers for superior rigidity, speed, and service life.

Angularcontact thrust ball bearings are well suited for machine-tool work spindles when applications demand accuracy and rigidity.

Single-direction thrust ball bearings consist of a shaft washer, housing washer, and a balland-cage thrust assembly. As the name suggests, these ball bearings can accommodate axial loads in one direction and locate a shaft axially in one direction.

Double-direction thrust ball bearings consist of one shaft washer, two housing washers, and two ball-and-cage thrust assemblies. These types can axially locate a shaft in both directions.

Hybrid bearings feature rings and cages manufactured from bearing steel and balls made from silicon nitride — which weigh only 40% of their steel equivalents. This translates to low centrifugal force and low load on the raceway at high speeds. Hybrid bearings feature high-speed capability, long service life, and high wear resistance, and can also provide electrical insulation.

As an example of the advantages, a leading machine manufacturer designed a high-speed angle grinder, using hybrid bearings, with the pneumatic turbine rotating at 60,000 rpm. This level of performance would have been impossible with steel bearings. Standard dimensions of hybrid and steel bearings are the same, so a switch to hybrid bearings requires no physical design changes.

Bearing characteristics
Perhaps the most common operating requirements for spindles in today's machine tools are:

• High running accuracy.
• High-speed capability.
• Wide speed range.
• High stiffness.
• Low and even running temperature.
• High reliability.

Bearings for spindle applications are typically combined and customized to meet a machine's specific operating requirements. For example, grinding spindles operate in a very narrow speed range, so bearing designs should be optimized for that particular speed. Machining and lathe spindles, in contrast, often operate over a wide speed range, and load and stiffness requirements differ at various speeds. A customized bearing system for these applications likely represents a compromise between several performance characteristics.

To match a bearing with the application, designers typically evaluate a range of properties in the context of a machine tool's operating and performance parameters. Here are some important bearing characteristics.

Load-carrying capacity and life. In general machinery applications, load-carrying capacity usually determines bearing size. However, desired life and operational reliability also play a role.

The selection process for machine-tool spindles is a bit different. Criteria such as stiffness of the system, fixed dimensions for the toolholder, or the spindle bore almost always determine bearing size. Bearings selected according to such criteria tend to exhibit extremely long life.

For precision bearings, determining the load can be particularly complex, since it involves many influencing factors. Some bearing manufacturers have developed special computer programs to calculate load-carrying capacity and life.

Rigidity. Bearing stiffness, the relationship between bearing load and deformation, affects the rigidity of a bearing system. Stiffness depends on bearing type and size. Important factors that influence stiffness include the types of rolling element (rollers or balls), number and size of rolling elements, contact angle, applied load, and preload.

Because rollers have a much larger contact area between raceways, compared with balls, roller-bearing stiffness greatly exceeds ballbearing stiffness. In addition, as the contact area between rolling elements and raceways is smaller under light loads, stiffness will be lower than with an identical system under heavy load.

As a result, follow these rules of thumb. Where designs require high radial stiffness, use bearings with the smallest possible contact angle. Conversely, designs that demand high axial stiffness should use the largest possible contact angle.

The number, not size, of rolling elements has the greatest influence on bearing stiffness. Increasing the number of balls or rollers will increase bearing stiffness by a greater amount than an equivalent increase in rolling element size. In addition, using two or more bearings at one position will further increase the stiffness of a bearing arrangement. Angular-contact ball bearings supplied in matched sets are best suited in these cases. Preload can also enhance bearing stiffness.

Speed. Temperature largely governs the maximum speed at which rolling bearings can safely operate. Low friction — and therefore low heat generation — bearings are most appropriate for high-speed operation. In general, that means ball bearings.

However, the overriding parameter that limits operating speed in bearing systems is the maximum permissible temperature for safeguarding lubricant life and the complete system's thermal stability. Operating temperature depends on many factors, including ambient temperature, heat generated by motors and electrical losses, and bearing friction.

Bearings generate heat due to internal design, the ring and rolling-element materials, type of lubrication, and loads acting on the bearings, including preload.

Preload. Machine-tool spindles are almost always fitted with preloaded bearings or bearing sets for two key reasons: Preloading increases both bearing stiffness and running accuracy.

Single-row angular contact ball bearings generally adjust against each other by axial displacement of the inner or outer rings until the bearing arrangement attains a certain preload (or clearance). Single-row angular contact ball bearings that mount in sets can be matched in production so that they produce predetermined preloads when installed immediately adjacent to each other.

Cylindrical ball bearings with tapered bores are preloaded by driving the inner ring up onto its tapered seat. For double-direction, angular-contact thrust ball bearings, manufacturers size the spacer sleeve between the shaft washers so that mounting the bearing produces a suitable preload. In high-speed arrangements with angularcontact ball bearings, springs typically generate axial preload. This maintains a constant preload throughout a wide range of operating conditions.

In practice, there are practical limits to the amount of preload that can be applied to a rolling bearing. Frictional losses and operating temperature increase with preload and reduce bearing life. An optimum preload yields the greatest possible bearing stiffness for the smallest increase in friction.

Tolerances. International standards govern tolerance classes for precision bearings, and hybrid bearings are made to the same tolerances as corresponding all-steel bearings. For applications that demand extreme precision, most major manufacturers can supply bearings with even greater accuracy than found in standard classes.

However, using high-precision bearings does not guarantee maximum running accuracy, high speed capacity, and low operating temperatures. These objectives can only be met if the mating parts and other associated components are made with equal precision.

In part, this is because bearing rings are relatively thin-walled and tend to adapt to the form of the mating shaft or housing. Any deformation or variance from specs of the shaft and housing bore seating, therefore, will transmit to the bearing-ring raceways. Resulting angular misalignment of one bearing ring in relation to the other may then cause high operating temperatures, especially at high speeds. Avoid this by precisely machining all parts, including axial support surfaces for the bearing ring faces.

Solve potential problems at the outset by taking advantage of the design and engineering expertise that experienced bearing manufacturers offer. Such professional support can prove invaluable in selecting bearing types and arrangements, and in designing customized solutions to fit the application. Whether the goal is accuracy, stiffness, speed, load capacity, service life, or a combination of these, expertise sought at the beginning of the design stage can make all the difference in ultimately satisfying machinetool requirements.