A bearing contact angle is the spot where the bearing balls meet the inner and outer bearing raceway corresponding to the bearing axis that’s perpendicular to the shaft.
Angular contact bearings are usually manufactured with a 15° to 25° contact angle. These contact angles are sometimes called “free static contact angles” because they have no outside factors affecting the degree of the angle.
The Bearing Contact Angle Changes When in Operation
Static & Dynamic Contact Angle Calculations
Static and dynamic contact angle calculations will help you estimate the changes that will happen to the contact angle when it’s installed and operating.
These calculations are important because if the contact angle gets too high, then there is a risk that the ball may extend past the raceway shoulder and cause premature bearing failure.
On the flip side, if the contact angle gets too low, the balls could get pinched in the raceway causing the bearing to lock up. It could also run the risk of having all loading effects, including preload, get removed which causes bad ball action and premature failure.
To prevent this from happening, there are two main contact angle calculations you need to perform:
- Effective static contact angle calculation
- Dynamic contact angle calculation
The Effective Static Contact Angle Calculation
The effective static contact angle calculates the impact a bearing press-fit and thermal expansion will have on the bearing’s contact angle.
- Bearing press-fit: A press fit causes the bearing’s inner ring to expand and fit snugly onto the shaft, thereby putting extra pressure on the balls and raceway altering the contact angle.
- Thermal expansion: Thermal expansion: Moderate to large temperature differences between the shaft and housing can cause the inner ring of the bearing to expand and increase the pressure on the balls and raceway, which changes the contact angle.
- Keep in mind that the material of the bearing can also increase thermal expansion, so make sure the ball and ring material is a friendly combination.
The effective static contact angle is important to know because If the change in contact angle is too drastic, the bearing could fail when you add in regular application operating conditions.
To reverse and counteract these influences on your contact angle change the press fit or operating temperature of your application.
The Dynamic Contact Angle Calculation
Application loads and centrifugal force from RPMs also change a bearing’s contact angle and are the two factors used to calculate the dynamic contact angle of a bearing.
It’s important to calculate the dynamic contact angle of a bearing so you can make sure the angle is within a safe range for operation. If the contact angle gets too high or too low, it can cause the bearing to fail, so changes will need to be made in the system to prevent that from happening.
Application Loads Change the Bearing’s Contact Angle
For a bearing receiving axial loads (force parallel to the shaft), the balls push up on both the inner and outer bearing raceway, increasing the bearing’s contact angle. The contact angle change is consistent for each ball in the bearing since each ball receives the same amount of force.
Radial loads have the opposite effect.
Radial loads (force perpendicular to the shaft) cause the balls to move down both bearing raceways, decreasing the contact angle. Since only a few of the balls will carry the radial load, each ball receives a different amount of force. This means that the contact angle for each ball could be different.
That’s why it is important to calculate the min and max dynamic contact angle for the inner and outer bearing ring. Doing so will account for this non-uniform contact angle.
Centrifugal Forces Change the Bearing’s Contact Angle
In rotating high-speed applications centrifugal forces change the contact angle too but not in the same way as application loads.
As the balls rotate, they produce a centrifugal force that causes them to push radially outward and move up the bearing raceway. This decreases the contact angle of the outer race because the ball moves in the direction of the radial axis but increases the inner race contact angle as the ball moves up and away from the radial axis.
For this reason, centrifugal forces need to be a factor when calculating the dynamic contact angle.
Summary of Bearing Contact Angles
As you can see, changes to a contact angle can happen very easily during operation. Be aware of these outside factors when engineering a new application or redesigning an existing application so you can avoid a complete failure of the bearing.
Our no cost bearing analysis will calculate the effective and dynamic contact angles of your specific application. If you would like more information about that, our onsite engineers would love to help! Contact us via our online contact form or give us a call at 800.323.5725.