In profiled rail linear guides that use balls or rollers, the geometry and arrangement of the bearing raceways play a significant role in the bearing&#;s load capacity, friction, rigidity, and ability to withstand errors in mounting. But there&#;s another aspect of a bearing&#;s design that also plays a role in its load capacity and rigidity &#; the contact angle.
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The ISO standard for linear motion rolling bearings defines the nominal contact angle as:
The angle between the direction of load on the linear bearing and the nominal line of action of the resultant of the forces transmitted by a bearing raceway member to a rolling element
Linear guides that use Gothic arch geometry &#; including miniature profiled rails and most roller bearing guides &#; have four-points of contact between the ball and the raceway, which results in a contact angle of 45 degrees.
The benefit of the 45 degree angle is that it provides the bearing with equal load capacity in all four primary directions &#; radial (downward and lift-off) and lateral (side) loading. This means the guide can be used in any orientation without the need to de-rate the load capacity.
Linear guides that use circular arc or offset Gothic arch geometry, on the other hand, can be designed with varying contact angles to produce higher load ratings in one direction, although at the sacrifice of load capacities in the other direction.
For example, one circular arc design uses a contact angle of 90 degrees on the top rows of balls, with a smaller angle of 30 degrees on the lower rows of balls. This gives the bearing extremely high load capacity for radial (downward) loads &#; which is the primary loading direction in many applications &#; since the load is being transferred from the top rows of balls directly downward into their raceways. It also gives the bearing very high rigidity (low deflection) when radial loads are applied. The tradeoff for higher load capacity and rigidity in the radial direction is lower load capacity and rigidity in the reverse radial and lateral directions.
Another design, based on the offset Gothic arch geometry, uses a 50 degree contact angle for all four rows of balls. This provides higher load capacities in both the radial and reverse radial directions, but lower load capacity in the lateral direction.
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.
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:
The effective static contact angle calculates what impact the bearing press-fit and thermal expansion have on the bearing&#;s contact angle.
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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. To learn more, contact our team of in-house engineers at GMN Bearing USA.
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.
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.
For assistance with calculating the ball bearing contact angle for your application, contact our team at GMN Bearing USA.
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.
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..
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