Most people are familiar with roller bearings at least in concept if not in detail. As a quick refresher, here’s a look at a common roller bearing, the most popular type, see Figure 1. Firstly, the whole purpose of a roller bearing is to allow parts of a machine to rotate independently while minimizing friction and reducing wear.

A roller bearing is the most common way to do this. All metal parts are usually made of specially hardened steel and machined to very tight tolerances to maintain smoothness. The inner ring (or race) is a precise diameter to accept a standard dimensional shaft; exactly 12.0 mm for example with a typical tolerance on critical dimensions of +/- 0.005 mm. It is also referred to as the “bore size”. The outer race is designed to mount to a rigid surface with a hole drilled to a precise diameter. The width and the number of ball bearings that will be used is determined by the load that the bearing needs to carry. Finally, there is a metal spacer that fits between the inner and outer race and keeps the balls of the bearing evenly spaced. It also helps to keep lubricant, usually a grease, contained between the two races thereby minimizing wear while maintaining performance. The inner part of the outer race and the outer part of the inner race each have a precise rounded groove machined completely around the circumference to help keep the balls contained and to give them a clean “roadway” to ride in.

Thrust bearings

Thrust bearings are designed purely to accommodate an axial load while allowing rotation. There is an upper and lower raceway for the bearings to ride in and a cage to maintain the ball spacing and hold lubricant. These parts are similar to a roller bearing. However, one subtle difference is that since the top and bottom portions of a thrust bearing need to each turn independently, one of the bearing races needs to have a slightly larger internal diameter than the other to allow clearance for rotation. In most cases, the design geometry ensures capture of the thrust bearing assembly and will maintain a load due to the weight of the load pressing down.

Typical applications might include a ball screw or a linear motor drive. In some cases, where the axis is horizontal during operation, for example, It may be necessary to include a spring on one of the faces of the bearing to ensure continuous, even contact while in operation. With these few differences, the operation of a thrust bearing is similar to a roller bearing.

Bearing design and variants

Roller bearings are designed with the assumption that the majority of the load will be in the radial direction (along a line that is perpendicular to the rotational axis). In reality, bearings often will have some attachment on the end of the shaft that creates an eccentric load. Most roller bearings can tolerate a slight side load of about 10% of the rated load, and specialty variations can be made that will take even more side load. Nonetheless, if a significant side load is anticipated, it is best to use a bearing which is designed for this condition to begin with.

A perfect example is the wheel of a car. When the car is driving straight ahead, most of the load is axial, and directly in line with the bearing. But when the wheel turns, it must accommodate a large axial load. The conical shape and the tapered rollers in place of balls help to accommodate this variation in loads and the tapered bearings are self-aligning and can take a higher load.

Other common variations on most bearing types include greater thickness and races with larger diameters to accommodate larger loads. Sometimes the outer race will have a protruding flange that acts as a stop when inserting the bearing into the bearing mounting. Since bearing races are often held in place using epoxies, this also provides a little more area for the epoxy to grip the bearing housing. Bearings can also be made with cylindrical or even barrel shaped rollers with corresponding changes in the raceways to accommodate the different geometry. These types or bearings are well suited to high radial loads for roller bearings while keeping the overall size compact. For environmental protection against dust and some splashing, it is possible to include flexible side seals to provide protection. The tradeoff is that friction between the seals and the housing generate heat so this may limit the operation of the bearing to a lower RPM.

Role of lubricants

It is important to pay attention to lubricants as well. This is its own special science, but unless specified otherwise typical applications indoors will use some sort of lithium grease as a general-purpose lubricant. For high temperatures a dry molybdenum – based lubricant is appropriate and for operation in cold weather, mineral oils or synthetic greases can take temperatures as low as -60˚ C. One specialty application that requires a light lubricant is when the bearing is used for oscillatory movement. Since the shaft is not rotating completely around, but back and forth like a pendulum, the grease does not circulate and fully lubricate the bearing. This is especially a concern for roller bearings. In those very few cases, a light oil will always migrate to the low point. Just make sure that the design has bearing movement in this location so it can spread the oil out from the low spot.

Consult with the bearing manufacturer if you have any of this special conditions.


The science behind the design and operation of bearings is highly sophisticated. There are a range of high-quality manufacturers and experts ready to help you find the solution that suits your application best.

About the author

Scott Orlosky has an MS in Manufacturing and Control Theory from the University of California at Berkeley and has worked over 30 years designing, developing, marketing and selling sensors and actuators for industrial and commercial industries. He has written numerous articles and application notes for speed and position sensors used in industrial and hazardous area environments including an author credit in “Encoders for Dummies.” Scott authored an industrial newsletter for nearly 15 years and is also co-inventor on a number of patents involving design and manufacturing of inertial sensors.