Bearing Basics: Extra Precision Bearings

Posted on February 14, 2018

In some industrial jobs, standard bearings simply can’t provide the precision needed for the application. This is where extra precision bearings come into play. These bearings allow for extra fine tolerance and precision, and are ideal for applications with stringent requirements for speed (RPM) and guidance accuracy. There are two primary types of extra precision bearings: deep groove bearings and angular contact bearings.

Deep groove bearings include full shoulders on each side of the inner and outer ring raceways, allowing for the easy addition of seals or shields. These bearings can handle radial loads, thrust loads, or a combination of both. They are offered in many sizes, including miniature precision bearings, and are available various cage types. Extremely versatile, deep groove bearings are among the most widely used types of bearings.

Angular contact bearings have one complete ring shoulder; the other ring shoulder is partially or completely removed. This allows them to have a larger ball, which offers greater load capacity and speed capability. Various cage designs are also available to meet specific high-speed requirements. Contact bearings can support thrust loads or a combination of thrust and radial loads, but they can’t support radial loads alone. When implemented back-to-back or face-to-face, they can support thrust loads in both directions.

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Angular contact bearings are available in two types: separable or non-separable. These classifications denote whether the outer ring assembly can be removed from the inner ring. Separable angular contact bearings are especially useful when installing in a blind hole and in applications requiring press fits.

Closures and Cages for Extra Precision Bearings

Bearing closures serve to protect the bearings from internal damage caused by handling. For extra precision bearings, these closures are typically made from Teflon, stainless steel, or other high-performance materials. There are two major closure types available for extra precision bearings: shields and seals. Both are designed to keep out contamination and keep lubrication in place.

Shields are applied to the inner ring of the bearing, seals to the outer ring. Choosing between these two types of closures means choosing priorities — whether an application requires sealing efficiency, speed capability, or bearing torque. Shields have a lower sealing efficiency, but they don’t increase bearing torque or limit speeds. Seals allow for stronger bearing protection, but they restrict operating speed while increasing torque and temperature.

Most bearing assemblies make use of bearing cages to create and maintain uniform ball spacing, reduce torque, and cut down on heat buildup. Bearing cages come in a number of designs that can be applied to both deep groove bearings and angular bearings. Cages are usually selected based on the specific needs and purpose of the bearing at hand.

Standard precision bearings generally use stamped steel cages. Extra precision bearings, on the other hand, use cages made from stainless steel, bronze, or phenolic— Cages made from these materials offer higher speeds and better loading capabilities. Cages may be used to serve general-purpose needs, as well as low-torque, high-speed, lubrication-free, and sound-level requirements.

ABEC Parameters for Precision Bearings

All precision ball bearings must be manufactured to Annular Bearing Engineers Committee (ABEC) standards, which conform to the standards outlined by the International Organization for Standardization (ISO). ABEC standards define tolerances for key bearing characteristics and dimensions, allowing for consistency across the industry.

Parameters covered in these guidelines include bore roundness and taper, width variation, bore and race runout, and radial runout. Ascending ABEC numbers indicate tighter tolerances — the higher the number, the higher-performing the bearing. ABEC 1 through 3 bearings are typically intended for general use, while ABEC 5 through 7 bearings are used for applications requiring higher precision and higher speeds. Bearings from higher ABEC precision classes, intended for use in stationary housings, have rounder balls and smoother raceways.

However, customers should not automatically opt for the higher classes. Higher-end bearings can cost three to five times as much as lower-end bearings, so it’s important to understand the exact needs of the application in order to make a proper selection. In many cases, higher-standard ABEC bearings are not necessary.

Ensuring Peak Performance of Extra Precision Bearings

Before putting extra precision bearings into play, users should make sure their systems are set up for success. There are several key factors to consider, but preloading and radial internal clearance are two of the most important. Users should closely examine these elements to ensure bearings are able to operate efficiently in favorable conditions.

The bearings must have the right amount of radial internal clearance — the total maximum movement possible for the inner ring in relation to the outer ring. This is also referred to as “radial play.” Proper radial internal clearance is critical, as metal will inevitably expand; selecting the correct clearance prevents metal ball bearings from seizing in their housings when expansion does occur. Bearings without enough radial play will fail due to these interference and binding effects.

Preloading, on the other hand, involves the removal of internal clearance in a bearing. Applying permanent thrust load eliminates radial and axial play, making the setup very rigid. Preloading allows for the removal of endplay and runout. This can be accomplished through spring preloading, axial adjustment, or the application of duplex bearings.

Duplex bearings are matched pairs of bearings designed to provide built-in preloading. The bearings are clamped together at installation in face-to-face (DB), back-to-back (DF), or tandem (DT) configurations. Since each of the bearings has a matching preload offset, they counterbalance one another. This increases axial and radial rigidity and allows them to withstand bidirectional or heavy unidirectional thrust loads.

Shaft and housing fits are also important factors to keep in mind when employing extra precision bearings. Users should evaluate the bearing’s intended application before deciding how tight a fit is appropriate; looser fits are necessary when axial movement must accommodate spring loading and thermal expansion, or when axial clamping forces are at play. Assembly is also much simpler with looser fits. Tighter fits are ideal when the application requires allowance for heat transfer, elimination of vibration impact, or support of heavy radial loading, to name a few common scenarios.

Finally, users should determine whether extra precision bearings will perform better with grease or oil lubrication. Grease is the standard choice in most scenarios. However, oil should be considered for applications involving higher speeds as these scenarios provide a higher degree of cooling and create minimal torque. Oil must be replaced more frequently than grease, which lasts longer without any needed upkeep.

Extra Precision Bearings from Emerson Bearing

Emerson Bearing is proud to be a leading distributor for many renowned bearing manufacturers. We provide extra precision bearings from several top brands, including Barden, GMN, Nachi, NTN, and NSK.

Our team of experts can also help you find the right bearing for your specific application and requirements. We can assist in evaluating the various factors discussed above — including navigating ABEC standards — in order to equip you with the knowledge you need to avoid costly pitfalls and downtime.

Reach out to us today to get started with the bearing selection process. 

Topics: bearings, ABEC Classes, ABEC Parameters, extra precision bearings

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