Bearings are a critical component that connects what’s rotating to what’s stationary and keeps the overall machine functioning properly.
In a perfect world, bearings would barely be thought about because they’d perform so well.
But outside factors can have a dramatic effect on bearing performance. This article will explain x# common bearing problems and how to prevent them.
Bearing Lubrication Problems
Lubrication can either be a blessing or a thorn in your side. It’s probably no surprise that problems 1-3 are caused by under or over lubrication of a bearing.
Lubrication is a leading cause of bearing problems. Most people understand that the balls inside a bearing should never ride directly against the bearing raceway. The tricky part is knowing how much or how little lubrication a bearing needs.
First, let’s quickly explain the science of bearing lubrication. A ball bearing has a curved raceway, so, the relative motion between the ball and the raceway will always produce skidding. For this reason, there needs to be a film of lubrication between the balls and raceway that allows the skidding to occur.
All that being said, here are the top three common issues you could run into with bearing lubrication, and what you can do to prevent them.
1. Not Enough Bearing Lubrication
If there isn’t enough lubrication a direct metal-to-metal contact will occur and cause friction that produces destructive heat.
You can usually identify this problem by looking at the bearing and seeing discoloration on the ball and raceway. It can also appear as an overloading failure because the heat causes the bearing ring to expand, increasing the resultant load to the bearing.
2. Your Bearing is Rotating Faster Than the Grease Rating
If you’re using grease lubrication make sure to check the speed rating for the grease.
If your bearing is rotating faster than the grease rating, the thickener won’t have time to re-adhere after the ball (or another rolling element) passes over it.
Make sure to also check the grease life. If the grease life is expired then the grease will start to break down, preventing it from creating that repeated film as the balls roll over it.
3. Your System is Too Hot or Too Cold
If the system temperature is too high, the viscosity of the grease can decrease drastically which prevents the grease from staying where it’s needed in the bearing.
On the flip side, if the system temperature is too low, the viscosity can become too thick making the bearing work extra hard to push through and separate the oil and thickener as the balls pass over the grease.
Protect the bearing from poor lubrication issues by choosing a lubricant that maintains an adequate, thick film under the temperature conditions. To maximize the life of the lubricant, consider the system life, temperatures, viscosity, and speed when selecting the lubrication.
4. Bearing Contamination
Another common cause of bearing failure is contamination. Contamination happens when particles enter the bearing raceway during operation and becomes pressed between the balls and the bearing raceway.
As the balls keep rolling over the particles, craters are formed on the raceway leading to machine vibration and excessive heat generation.
To identify bearing contamination, take a look at the raceways and see if you find a crater. Hard contamination tends to leave sharp edges around the crater, and soft contamination tends to leave smoother edges.
You may also see a “comet tail” on one side of the crater. This is from the ball hitting the edge of the crater and then going back out smoothing that edge along the way.
Prevent Bearing Contamination
Implement these best practices to keep contamination away:
- Purchase bearings with seals or shields
- Add a labyrinth seal to your application design. Labyrinth seals are added in front of the bearing to protect it from all types of contamination.
- If clean pressurized air is available in your system, then pressurize the bearing compartment to prevent contamination.
5. Overloaded Bearings
Overloaded bearings occur when the pressure between the balls and bearing raceway becomes too high resulting in plastic deformation. Overloaded bearings are usually caused by excessive loads.
Here is how to spot where the excessive load is coming from:
- Too much axial load results in uniform deformation on both the inner and outer bearing raceway.
- In systems with a rotating shaft and too much radial load, the inner raceway will show uniform deformation.
- This is from the shaft rotating through the pinch point of the bearing, but the outer raceway remaining stationary.
- You should be able to spot a “load zone” where the deformation is the most noticeable, directly at the pinch point.
The most common reason for overloaded bearings is excessive loads like we mentioned above, but there are other less common sources of overloading, like:
- Improper bearing installation, causing plastic deformation
- Centrifugal forces from the rolling element (if rotating at very high speeds)
- Shaft imbalances
- Shaft imbalances have similar characteristics to a radial load but flipped. The inner race will have the load effected zone since the radial load is stationary relative to the inner race. The outer race will have uniform damage since the radial load is rotating around it.
Overloading can have similar symptoms as contamination, above. Both issues create craters on the surface of the raceway which will cause excessive vibration and/or heat.
If you think you have an overloaded bearing issue, then here are some helpful tips to correct the problem, depending on what the cause is:
- Overloading from excessive axial or radial loads: add bearings or change the bearing size to reduce the pressure on each individual bearing.
- Adjust the contact angle on an angular contact bearing to optimize for the specific type of load.
- If the damage is from high rotational speeds, switch to ceramic balls because they have a lower ball weight. Also purchase a bearing with smaller balls, like our KH series of angular contact ball bearings. These options will decrease the centrifugal forces at high speeds.
6. Underloaded Bearings
Underloading damage occurs when there is not enough force between the rolling elements and raceways. This can cause the rolling element to slip and skid against the raceway which results in excessive heat. This type of damage is more common in high-speed applications.
To find out if your bearing failure is from underloading, take a quick peek at the balls in the bearing. If you can spot several distinct circumferential lines (also called “saturn rings”) then you may have an issue of underloaded bearings. This type of damage is more common in high speed applications.
Protect against underloaded bearing damage by properly preloading your bearings. Preloading a bearing means applying an axial force (independent of the application force) against the raceway to prevent skidding. This can be done using bearing offsets or a wave spring, among other methods.
7. Bearing Cage Damage
Bearing cage damage is usually the result of something else going on in the application. For example, underloading and rapid acceleration can cause the rolling elements to bang against the sides of the cage pockets. If the issue is underloading, increasing the preload can help limit the amount of relative movement of the rolling element in the cage pocket. For rapid accelerations, a stronger cage material like brass or bronze can be used.
For certain cages, like ones with phenolic material, high temperatures can cause the cages to dry out and become brittle. Force from a ball (or another rolling element) can then bang against the cage and cause it to become damaged and even fail.
Cage damage can also occur from any of the above issues we’ve already covered like contamination and excessive loading. These problems can cause the balls to bounce around in the ball pocket of the cage and result in catastrophic damage to the cage. So, finding and analyzing the source of the cage damage is a necessary first step to preventing further cage damage.
8. Bearing Damage From Micro-Fretting
Micro-fretting is caused by the rubbing of two materials against each other. It can look like small rust spots on the surface but is the result of brown oxidation forming from the friction generated by the rubbing. It occurs in bearings when the fit between the outer ring and the housing or the inner ring and the shaft is too large. This can be corrected by decreasing the slip fit or going to a press-fit between the two mating components. An axial clamping force can also be used to make sure the ring does not slip.
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