Jim Bryan
EASA Technical Support Specialist (retired)

Rolling-element bearing construction has become a very precise and exacting process. Studies have shown that more than one-half of motors that come to service centers are because of worn out or failed bearings. This is understandable since this component is subject to wear and sometimes abuse. Bearing manufacturers are called upon to improve the quality and reliability of their product to increase the time in service before it becomes necessary to replace the bearings. Proper application and maintenance of the bearing is also a key to improved reliability. We will discuss in this article some of the components used to better understand what applications can be accommodated.

Figure 1 shows the basic components of rolling-element bearings. The type of rolling element (ball or roller) and the metallurgy determine the load capacity of the bearing. Generally, the higher the load capacity of the bearing, the longer the expected life will be at a given load point. Following is a discussion of the purpose and design features of the common elements of these bearings: the inner and outer races, the rolling elements, cages, and shields and seals.

Races
The races provide a track for the rolling elements and confine them to a specific location. This helps position the parts of the equipment while the component that is in motion is free to move. In the case of a motor, the rotor is allowed to spin in the stator while maintaining a small air gap and limiting axial movement.

The steels used for both the races and rolling elements must have high fatigue strength and wear resistance. The steel used may be through hardened (carbon chromium steel) or case hardened (chromium-nickel or manganese-chromium steel). Both are used with practically no difference in fatigue life. These steels provide a stable performance to about 121ºC (250ºF). Higher temperatures can be achieved with special steels up to about 232ºC (450ºF). These higher temperatures will require special lubricants as well.

Rolling elements
Rolling elements can be configured in several different geometries. The basic breakdown is balls or rollers. Rollers have increased load capacity over balls since the rolling element has more contact area than a ball. The ball contacts the raceways at a relatively small point, while that contact surface of a roller extends along the length of the roller. Various types of rollers are used including cylindrical, tapered, spherical and toroidal.

The cylindrical rollers have a line-of-contact but the spherical rollers extend that line further and increase the capacity. Tapered rollers add a capacity for axial loading as well as the radial load of the other configurations. The toroidal roller is a relatively new innovation that has a lower minimum redial load requirement than other types of rollers. These bearings can be used in relatively heavy radial loading or with very low loading such as a direct-coupled application. A motor with these bearings then becomes an attractive spare with flexibility in where it is used.

Another relatively new innovation is the use of ceramic materials such as silicon nitride for the rolling elements. This material provides excellent insulation to prevent damaging currents from flowing through the bearing. Additionally, they are lighter and smoother than steel and have less friction. They can be used at higher speeds and higher temperatures. Price and availability have been an issue but as they are used more widely that issue is diminishing.

Cages
The purpose of the cage is to maintain the proper spacing of the rolling elements around the race and to keep them apart. The cage can be land riding or ball riding (see Figure 2). Land riding cages will be guided by either the inner or outer race while ball riding cages are centered on the balls.  Experience has shown that the land riding cages will impede the oil flow in oil lubricated systems. In this case, the ball riding cage is preferred.

Cages can be made using a variety of materials. Among these are pressed steel, molded polyamide, machined bronze, pressed or machined brass and phenolic cages. By far the most common is pressed steel. They are high strength, light weight and provide good value. They are used in the most common sizes and applications where it provides good service life.

Some smaller bearings may use molded polyamide cages. These have the advantage of smooth self-lubricating surfaces that have less friction. They have low inertia that provides superior dynamic balance and quieter operation.

Machined bronze cages are used in large heavy-duty bearings and provide a stable location of the rolling elements.  Machined bronze bearings with ball riding cages are generally preferred in vertical thrust bearings, especially if they are oil bath lubricated.  Pressed brass cages are used in some small to medium bearings. Machined brass is similar in application to machined bronze but should be avoided where ammonia is present due to the corrosive effects.

Phenolic (also known as Bakelite) cages are used in angular contact bearings primarily. Care should be extended when using these bearings, especially in high speed (greater than 2500 rpm) motors because they are not as stable as the other cages.

Note: The cage has much more surface contact with the rolling elements than the races. They are more reliant on good lubrication so that a damaged cage is a good indicator that there is an issue with the lubrication.

Shields and seals
Shields and seals offer protection to the bearing from contamination damage by reducing or eliminating the possibility of infiltration of foreign materials.  Seals are a more aggressive approach than shields but they also produce more heat from the rubber seal riding on the race. Low-friction seals reduce this additional heat.  The seal is also a wear item that will eventually fail to keep contaminates out.  For these reasons seals should only be used where necessary based on the application. See Figure 3.

Shielded bearings have a sheet steel barrier attached to one of the races with a small clearance, typically about 0.005” (0.13 mm), to the opposite race. The question often arises whether the shielded bearing can be re-lubricated. After considerable debate, we have resolved that if any oil or grease can migrate through the small opening, it will not be enough to appreciably increase the service life of the bearing. For this reason, shielded bearings should be used where the expected life of the lubricant exceeds that of the bearing, thus they are “lubed for life.”

Conclusion
Understanding the purpose of the components of the rolling element bearing will help to ensure they are properly applied. Also, when trying to determine the cause of a failure, the interaction and purpose of these components will help explain what happened and what corrective action is required. Sometimes this may mean using a different configuration of bearing.

Bibliography 

• Engineering Handbook. Kulpsville, PA: MRC Bearing Services, 2005. Print.
• SKF Bearings and Mounted Products. Kulpsville, PA: SKF Bearing, 2008. Print. 

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Categories: Bearings, Design/theory/application

Tags: Bearings

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