There are many different types of bearings available today with very little information on the differences between them. Maybe you’ve asked yourself “which bearing will be best for your application?” Or “how do I choose a bearing?” This bearing selection guide will help you answer those questions.
First off, you need to know that most bearings with a rolling element fall into two broad groups:
Within these groups, there are sub-categories of bearings that have unique features or optimized designs to enhance performance.
In this bearing selection guide, we’ll cover the four things you need to know about your application in order to choose the right type of bearing.
Bearing loads are generally defined as the reaction force a component places on a bearing when in use.
When choosing the right bearing for your application, first you should find the bearing’s load capacity. The load capacity is the amount of load a bearing can handle and is one of the most important factors when choosing a bearing.
Bearing loads can either be axial (thrust), radial or a combination.
An axial (or thrust) bearing load is when force is parallel to the axis of the shaft.
A radial bearing load is when force is perpendicular to the shaft. Then a combination bearing load is when parallel and perpendicular forces produce an angular force relative to the shaft.
To learn more about axial and radial ball bearings, contact our team of engineers!
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Ball bearings are designed with spherical balls and can distribute loads over a medium-sized surface area. They tend to work better for small-to-medium-sized loads, spreading loads via a single point of contact.
Below is a quick reference for the type of bearing load and the best ball bearing for the job:
Roller bearings are designed with cylindrical rollers that can distribute loads over a larger surface area than ball bearings. They tend to work better for heavy load applications.
Below is a quick reference for the type of bearing load and the best roller bearing for the job:
Bearing runout is the amount a shaft orbits from its geometric center as it rotates. Some applications, like cutting tool spindles, will only allow a small deviation to occur on its rotating components.
If you are engineering an application like this, then choose a high precision bearing because it will produce smaller system runouts due to the tight tolerances the bearing was manufactured to.
Bearing rigidity is the resistance to the force that causes the shaft to deviate from its axis and plays a key role in minimizing shaft runout. Bearing rigidity comes from the interaction of the rolling element with the raceway. The more the rolling element is pressed into the raceway, causing elastic deformation, the higher the rigidity.
Bearing rigidity is usually categorized by:
The higher the bearing rigidity, the more force needed to move the shaft when in use.
Let’s look at how this works with precision angular contact bearings. These bearings typically come with a manufactured offset between the inner and outer raceway. When the angular contact bearings are installed, the offset is removed which causes the balls to press into the raceway without any outside application force. This is called preloading and the process increases bearing rigidity even before the bearing sees any application forces.
Knowing your bearing lubrication needs is important for choosing the right bearings and needs to be considered early in an application design. Improper lubrication is one of the most common causes for bearing failure.
Lubrication creates a film of oil between the rolling element and the bearing raceway that helps prevent friction and overheating.
The most common type of lubrication is grease, which consists of an oil with a thickening agent. The thickening agent keeps the oil in place, so it won’t leave the bearing. As the ball (ball bearing) or roller (roller bearing) rolls over the grease, the thickening agent separates leaving just the film of oil between the rolling element and the bearing raceway. After the rolling element passes by, the oil and thickening agent join back together.
For high-speed applications, knowing the speed at which the oil and thickener can separate and rejoin is important. This is called the application or bearing n*dm value.
Before you select a grease, you need to find your applications ndm value. To do this multiply your applications RPMs by the diameter of the center of the balls in the bearing (dm). Compare your ndm value to the grease’s max speed value, located on the datasheet.
If your n*dm value is higher than the grease max speed value on the datasheet, then the grease won’t be able to provide sufficient lubrication and premature failure will occur.
Another lubrication option for high-speed applications are oil mist systems which mix oil with compressed air and then inject it into the bearing raceway at metered intervals. This option is more costly than grease lubrication because it requires an external mixing and metering system and filtered compressed air. However, oil mist systems allow bearings to operate at higher speeds while generating a lower amount of heat than greased bearings.
For lower speed applications an oil bath is common. An oil bath is when a portion of the bearing is submerged in oil. For bearings that will operate in extreme environments, a dry lubricant can be used instead of a petroleum-based lubricant, but the lifespan of the bearing is typically shortened due to the nature of the lubricant’s film breaking down over time.
There are a couple of other factors that need to be considered when selecting a lubricant for your application, see our in-depth article “How to Choose the Correct Ball Bearing Lubricant".