Machinery designers want smaller, faster and quieter motion. So which type of bearing can help achieve these goals?

The design trends for machinery aim for “smaller, faster, quieter and longer life.” Every component ultimately will come into play as designers strive to meet these goals. Along the way, their choices among rolling bearing solutions have multiplied due to significant advances in technology and in response to evolving application parameters.

Bearings serve wherever rotating equipment is turning. Rolling element bearings are used for their load-carrying ability and friction characteristics. Applications include motors, gearboxes, fans, pumps and compressors; larger equipment used in the paper, metalworking, mining and power generation industries; and tools and robotics used in the medical, semiconductor and pharmaceutical industries.

They traditionally have functioned to reduce friction with rolling elements; support shaft loads in rotating equipment and provide system rigidity and shaft location. But far from becoming “commodities,” new purposes abound in line with their becoming smaller, “smarter” and highly “application-specific.”

As evidence, miniature ball bearings fit needs where a design envelope may be small without sacrificing performance requirements; “intelligent” units can ascertain the precise motion status of rotating or axially traveling components; “smart housings” for bearings can reliably track machine performance; lighter and more durable hybrids incorporating ceramic rolling elements will protect against electric currents; and specialized coatings for bearings can insulate and resist wear.

All of these underscore that advances in bearing technology have allowed design engineers to derive more than ever before from proper bearing selection

Miniature Ball Bearings

With bore diameters as small as 3 mm and thin cross-section profiles, miniature ball bearings have flourished, especially in compact tool designs offering limited real estate for components. Typical applications include drills, handheld tools, spindles, syringe pumps and automated pipettes.

Despite their smaller size, these ball bearings deliver big benefits. They can perform quietly and smoothly at high speeds; run with low friction; and generate low heat for optimized, healthier and longer service.

Designers have at their disposal a wide range of miniature bearing types, designs, and materials (usually 52100 steel or stainless steel). ABMA (American Bearing Manufacturers Association) and ABEC 1 class tolerances tend to be standard and higher-precision ABEC 5 class bearings will handle more demanding high-speed applications. Seals or shields and various pre-greased lubrication options will further help satisfy particular application requirements.

Sensor-Bearing Units

Precise information on the motion status of rotating or axially traveling machine parts is essential to delivering reliable open-loop and closed-loop control of movements. Sensor-bearing units (integrating a sensor, an impulse ring and a ball bearing) present noteworthy enablers.

These “intelligent” bearings are designed to record number of revolutions, speed, direction of rotation, relative position/counting and acceleration or deceleration. They are characterized by sensors with uniform and high signal quality, minimal liability to interference, and insensitivity to vibration or high temperatures.

Typical applications include electric motors, linear actuators, steering systems, conveying and handling systems and automation equipment, and they can be specified as viable alternatives to incremental encoders. Their compact dimensions appeal to designers striving to save space and reduce overall component weight in control-engineering equipment.

Here’s how they work: The bearing unit’s impulse ring moves past the stationary sensor ring when the inner ring rotates, generating a magnetic field of changing polarity. The sensor outputs a pulse, based on the number of polarity changes per second. The sensor output signal then is transmitted via connecting cable and used to generate the required application-specific information. Speeds down to zero can be recorded.

‘Smart Housings’ for Bearings

For the most part the process of monitoring machines has traditionally been handled by installing external instrumentation and sensors around bearing housings. The “downside” of this approach has included susceptibility to vibration and a need for additional protection or shielding in order to operate effectively. Bearing housings with built-in sensors offer an unconventional and innovative solution.

In a “smart housing” design, the parameters that can be monitored include machinery rotational speed, temperature and vibration. Signals from the sensors are processed in a PC board to provide a robust output signal resistant to interference. The PC board also will have power surge protection circuitry to shield the sensors from anomalies in the power supply signal.

Hybrid Bearings

These types of bearings combine bearing grade silicon nitride (ceramic) rolling elements with steel rings to exhibit demonstrable advantages compared with conventional all-steel bearing counterparts. They are lighter, harder and more durable; can run at higher speeds and with lower operating temperatures; and are highly resistant to wear, even under poor lubrication conditions.

Their ceramic balls are roughly 40 percent less dense than steel balls, contributing to reduced centrifugal force and friction and enables them to run faster and cooler. The relatively greater hardness yields enhanced wear resistance against hard particles, contaminants and vibration; and their highly inelastic nature means increased bearing stiffness and reduced deflection under load to promote reliable performance.

Natural insulating properties of hybrid bearings prove particularly advantageous in applications that will involve variable speed motors and generators, where electric arcing problems may arise and resulting bearing damage may occur from the stray shaft currents. Hybrid bearings offer designers a viable option for insulating “from the inside.”

Specialized Coatings

Newly formulated coatings have helped to resolve insulation issues, too, and boost service life in severe operating conditions. Most bearing types can be coated and, from a designer’s perspective, coated bearings represent a perfect “drop-in fit” within existing design envelopes, eliminating any need for drastic design changes or reconfigurations.

For insulation properties, a thin aluminum oxide layer can be applied to form a superior barrier against electric arcing and the problems that can follow. Other coatings have been engineered to address a wide range of additional operating challenges.

As one example, specialized wear-resistant coatings can withstand severe operating conditions due to sudden load variations, high operating temperatures, poor lubrication, vibration, smearing and/or contamination. These coated bearings can optimize bearing speed and life in countless demanding applications, including paper machinery, compressors, fans, hydraulic pumps and motors and equipment for material handling, mining and construction.

Compared with standard uncoated types, such bearings are harder, generate less friction (and resulting heat) and can better tolerate potential damage from contamination and marginal lubrication. Users gain bearings able to resist wear, operate at higher speeds, accommodate higher loads and perform even during periods of insufficient lubrication.

These are just a few examples demonstrating how today’s (and tomorrow’s) bearings are bringing more to the designer’s table. But knowledge is essential.

When specifying, designers should understand the potential operating conditions that a bearing will encounter in each and every distinct application. A particular bearing may perform quite well in one application, but not in another. By partnering with an experienced manufacturer early in the design process, potential pitfalls can be avoided and problems can be solved.

Daniel R. Snyder, PE, is director of applications engineering for SKF Industrial Division, SKF USA Inc., 1510 Gehman Rd., Kulpsville, PA 19443. He can be reached at daniel.r.snyder@skf.com. More information is available by calling 215-513-4680 or at www.skfusa.com.