A bearing contact angle is measured from the plane perpendicular to the ball bearing axis; The angle is formed from the contact points between the ball and the inner and outer raceway.
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.
A Ball Bearing Contact Angle Changes When in Operation
Avoid Contact Angles That Become Too High or Too Low
Static and dynamic contact angle calculations help estimate the contact angle changes that will happen when the ball bearing is 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 remove the bearing preload, which could cause 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 what impact the bearing press-fit and thermal expansion 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 contact angle changes too drastically, the bearing could fail when you operate it in regular conditions.
Change the press fit or operating temperature of your application to reverse and counteract these two influences on your contact angle.
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, 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 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.