Bearing failures can occur for many reasons. Problems such as misalignment, imbalance, play and friction are all caused by bearings, which can sometimes lead to failure. And because bearings are often the most critical part of a machine, failure can result in unplanned and costly downtime, damage to neighboring components, and significant repair costs. However, the good news is that each failure leaves its own mark on the bearing. In most cases, signs of damage can help you identify the root cause of the failure, determine corrective actions, and prevent the failure from recurring.
Types of bearing failures
In general, when bearings are used correctly, they last much longer than their estimated lifespan. However, bearings often fail prematurely due to avoidable errors. The causes of such premature failures are incorrect assembly, handling errors, insufficient lubrication, ingress of foreign particles or even abnormal heat generation.
How to Identify Failure in a Bearing
Bearing failure is a serious problem because the rotating parts of the machine can suddenly stop and cause significant damage. However, there are common symptoms that can help detect bearing deterioration before it's too late. The first sign of bearing damage is excessive vibration. Eventually, the bearing will begin to heat up beyond acceptable levels and produce excessive noise, whether it is a high-pitched noise or a grinding sound. There are 5 main symptoms of bearing failure:
- Temperature : During operation, the temperature exceeds the acceptable level.
- Vibration : Excessive vibration occurs during operation.
- Lubrication : Incorrect bearing lubrication, i.e. no more lubricant needed, too much lubricant present, or wrong lubricant used.
- Contamination : The lubricant has been contaminated by particles or foreign elements.The
- noise : There is excessive noise during operation, usually a grinding noise or high-pitched noise.
Bearings become hot during operation. For many bearings, acceptable temperatures are 180°F (82°C) or lower. Since bearings reduce friction between rotating parts of the machine, a damaged or faulty bearing will result in increased friction and higher operating temperatures. This is why it is necessary to regularly check the temperature using a temperature sensor, such as an infrared temperature sensor. If unusually high temperatures are detected, this is a sign that the bearing needs to be replaced before it fails.
Excessive vibration from a faulty bearing can be caused by a number of reasons. For example, abrasion can damage the raceway and cause rolling elements to bounce. Slight differences between individual rolling elements, due to machining errors during bearing manufacturing, can also cause vibration. There are different tools and methods designed to test bearing vibrations. If you notice abnormal vibrations that increase over time, this is usually a good indicator that the bearing needs to be replaced.
The most common cause of bearing failure is lack of lubrication. Bearings need precise lubrication to function properly. Bearing failure can be caused by the following:
- Lack of adequate lubrication can increase frictional resistance, which can cause premature wear and overheating.
- Excessive lubrication can lead to increased pressure, seal failure, lubricant leaks and excessive heat.
- Using the wrong type of lubricant can increase friction, cause wear and excessive heat generation
- Contaminated lubricants can increase friction, cause wear and excessive heat
Always check the bearing manufacturer's instructions before performing any lubrication-related maintenance activities. Portable grease analysis kits are available for lubrication testing, even in remote locations.
Bearing contamination can occur in several ways. One of the most common is relubrication maintenance. Poor quality seals can allow foreign contaminants to mix with the lubricant. Additionally, high-pressure washing of the bearing can emulsify the lubricant. Ultimately, contaminants lead to wear that significantly reduces bearing life. If contaminants are visible when inspecting the bearing, the bearing must be replaced.
Defective bearings may make unusual noises, such as screeching or high-pitched noises. The best method for using noise as an indicator of bearing damage is subjective. You need to pay attention to the noise of the bearing operating normally to know if there is a problem.
Failure of the ball bearing cage
Many ball bearing designs use a cage surrounding the individual rollers to keep them aligned. Damage to the cage causes cage failure and quickly leads to overall bearing failure. Failure of ball bearing cages generally occurs in four stages:
- Step 1 : There is no noticeable increase in temperature or noise, but the bearing vibrates a little more than usual. There is no cause for concern at this stage.
- Step 2 : The temperature remains normal, but the rolling noise during operation increases slightly. The vibrations continue to increase in amplitude.
- Step 3 : The temperature has risen somewhat and excessive noises are easily perceptible to the ear. At this point, the vibrations increase significantly.
- Step 4 : Temperature and noise increase considerably and vibrations are significant. At this point, the system may be damaged.
The most common causes of bearing failures
The main purpose of a bearing is to allow one part to rotate relative to another part along a specified axis of rotation. Although bearings, once their type and dimensions are carefully evaluated, have a long life, their failure can cause serious damage to the equipment in which they are installed.
According to studies, insufficient lubrication is the most significant cause of bearing failure. Bearings must be lubricated with the appropriate grease for proper application. The speed of rotation, the ambient temperature and the load applied to the bearing determine the choice of grease to use.
Keeping grease clean is extremely important. This is because bearings containing grease contaminated with dust or even metal fragments will not last long. These particles cause premature wear, dents and eventually flaking. This results in an increase in noise and vibrations, leading to a change in significant dimensions. Fractures then multiply rapidly, leading to catastrophic failure.
When the bearing is exposed to electric current, an arc is formed across the thin film of oil at the contact points between the raceway and the rollers. Burn lines in the case of bearings or pitting in the case of ball bearings then appear on the rings. Since the bearing is in continuous motion, the contact point changes and therefore the entire bearing will be marked with these lines.
Fatigue is a natural cause of bearing failure. Over time, it is normal for a bearing to wear out and reach a level of fatigue, which means it is time to replace it. Cracking is a sign of premature bearing fatigue. The cause is usually that the external load is too high for normal use. Cracks may occur in the inner or outer ring. This phenomenon leads to a sharp increase in vibrations. At this time, the replacement of the bearing is very urgent.
Never directly strike the inner or outer race of the bearing during assembly. The bearing must be installed on the shaft by applying force to the inner ring and to the housing by applying force to the outer ring.
Before beginning assembly, the technician must ensure that his work area and tools are clean. He should also get into the habit of taking the bearing out of its packaging just before using it!
Recommendations for reducing bearing failures
How long do bearings last ?
The life of a bearing is defined as the number of revolutions it can complete under a given load before the first signs of cracking appear. The force transmitted by the rollers causes compressive and shear stresses inside the rings. As the bearing rotates, these stresses change cyclically at each point, creating fatigue stresses that limit the life of the bearing.
The fatigue process of bearing steel is characterized by long-term deformation of its crystal structure, followed by cracks which are usually located in the sublayer (where the shear stress is maximum) and which will reach the surface causing bursts. Thus, the fatigue resistance of bearing steel depends on the cohesion of the crystal structure and its cleanliness, but also on the speed of crack propagation which is not only influenced by the same factors but also by the orientation of the metal structure obtained during manufacturing.
However, the problem posed by the phenomenon of fatigue is its random aspect. Indeed, two identical bearings, of the same brand and manufactured from the same batch, can fail after very different operating times. Life expectancy is not an exact data but a statistical concept.
In fact, the lifespan of the bearings can be reduced. Indeed, it can deteriorate quickly and especially prematurely due to numerous factors such as poor lubrication of the bearing, liquid or particulate contamination (water, dust, rain, sand, etc.) in the bearing due to the possibility of poor sealing of the joints. , incorrect mounting of bearings, poor adjustment, excessive load on unsuitable bearings, etc.
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