One of the most important determining factors of bearing performance and life is the type and amount of load that will be applied to a bearing in a system. Engineers should consider system bearing loads when designing a new application, tweaking an existing application, and especially after a bearing failure or malfunction.
Bearing load is defined as the force that’s transferred from one bearing ring, through some or all the rolling elements, to the other bearing ring.
Typically, application loads transfer to the system’s shaft, then to the bearing’s inner ring and continue to extend to the outer ring.
There are many different combinations of loads that a bearing system encounters but most can be boiled down to these four main groups:
- Radial Loads
- Axial Loads
- Preloads
- Centrifugal Loads
This article will discuss each type of system load, how it effects the bearing and what kinds of modifications you should make to your bearing selection.
Bearing Radial Load
Bearing radial loads are forces that are perpendicular to the axis of the shaft, parallel to the bearing’s radius.
A radial load is produced in many ways like the weight of a horizontal shaft assembly, gears, pullies or cutting tools, are a few examples.
When in operation, the shaft assembly pushes radially on the inner ring of the bearing transferring a load through the rolling elements to the outer ring.
Radial loads don’t transfer force in an equal and uniform way onto the rolling elements. Typically, you’ll see the distribution of force shaped like a bell curve with the rolling element directly under the application load receiving the most force. Then each successive rolling element, in both directions, transfers less and less load to the other.
If your application will place a radial load on the bearing, then either a radial ball bearing or an angular contact bearing with a low contact angle will be a good bearing choice.
Bearing Axial Load
A bearing axial load is a force that acts parallel to the axis of the shaft, sometimes referred to as a thrust load. Axial loads have a more uniform load distribution on the bearing’s rolling elements, compared to a radial load.
Typically, axial loads are applied directly in line with the shaft, like a drill, or it can be a reactive load offset from the shaft axis, like a bevel gear.
When in operation, the load pushes the shaft axially through the bearing which causes the inner race to want to also move axially and thus equally push each rolling element axially into the outer race. Thrust loads that are offset from the center of the axis, like the bevel gear, have a moment force applied to the inner race. When a moment force is applied, the load distribution on the bearing’s rolling elements isn’t as uniform.
If your application will place an axial load on the bearing, then an angular contact bearing with a higher contact angle around 25° will be a good bearing choice.
Bearing Preload
One special type of bearing axial load (or thrust load) is called preload.
This is a predetermined load that is applied to a bearing and is separate from system loads. Adding a preload establishes an optimal synergy between the rolling elements and the bearing races.
Preload benefits:
- Protects from excessive skidding
- Increases the system’s rigidity, decreases application vibrations and sliding friction
- High running accuracy – even if load conditions keep changing
- Increase in load capacity
Consider using a preloaded bearing in applications that require high speeds and/or rigidity.
Bearing Centrifugal Loads
Centrifugal loads come from the rotational speed (RPMs) of an application. High-speed applications can produce a powerful centrifugal load which can sometimes be the reason for the max speed of an application.
Centrifugal force is the apparent force that is felt by an object moving in a curved path that acts outwardly away from the center of rotation.
In a rotating bearing, the interaction between the rolling elements and the outer ring produce a centrifugal radial load, here’s how:
- The inner ring rotates the rolling elements
- The rolling elements, following the motion, want to continue straight along the path tangential to the arc of rotation
- The outer ring must force the rolling elements to continue along the circular arc of the bearing
Centrifugal loads are important in bearing selection because of the effect they have on bearing life. If an application requires high speeds, consider using an angular contact bearing with smaller balls, like our KH series, or switch stainless steel balls to a ceramic ball material. The smaller or lighter balls reduce rotational mass and therefore reduce the centrifugal load of the application.
Bonus: Hertzian Contact Pressure
So far, all the bearing loads we’ve discussed are from a contact force that is transferred from one bearing ring to the other through the rolling elements. This contact force creates pressure where the rolling element is pushing on the raceway. This pressure is known as Hertzian contact pressure or Hertzian stress.
The Hertzian contact pressure is an important value in finding out how much and what type of load a bearing can handle. A bearing’s ability to withstand a load is determined by how close the Hertzian contact pressure is to the stress limit of the bearing. The closer it is to the bearing’s stress limit, the less time it will take for the bearing to experience plastic deformation.
For steel AISI 52100 ball bearings, the stress limit is generally accepted to be 4,200 MPa. GMN considers Hertzian contact pressures above 1,500 MPa for steel balls and 1,800 MPa for ceramic balls to be close enough to the stress limit to have a significant impact on the overall life of the application.
If the application has high Hertzian pressure, then changes may need to be made to the application to reduce the pressure. A few solutions could be changing the bearing size, using ceramic balls, or adding more bearings to the system to help divide up the loads.
Summary: Bearing Load Types
We hope you found this article helpful. If you’d like further information on bearing loads, our on-site engineers would love to discuss it h you. Fill out our contact form or give us a call at 281.858.7000.
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