Bearing Failure: Causes and Preventative Measures
Bearing failure is a critical and very real problem faced by a wide range of operators within many different industries. It can occur unexpectedly and cause serious problems, financial or otherwise. In a factory setting, bearing failure can cost as much as $50,000 per hour through lost productivity, unplanned outages, and downtime. In an aircraft, bearing failure can be a detriment to safety and airworthiness. For these reasons and many others, preventing bearing failure is pivotal. This blog will discuss the seven most common reasons why bearings fail, as well as some ways to prevent them from happening.
- Lubrication Failure
When troubleshooting a bearing failure, lubrication should be one of the first areas to be inspected. Because there are so many lubricant options available, they are often misused. In rolling element bearings, adequate viscosity at operating temperature is required for propeller lubrication. Highly polished or discolored bearing raceways and rolling elements are a good indication that working surfaces do not have adequate viscosity at operating temperature. Another indication of metal-to-metal wear conditions is surface roughening in contact, which results from insufficient oil film.
Over-lubricating can be just as problematic as under-lubricating. Under-lubrication risks metal-to-metal contact, but over-lubrication causes heat build-up and friction as the rolling elements continuously try to push the excess lubricant out of the way. To prevent lubrication failure, be sure to maintain the correct oil levels and check for faulty bearing seals. Faulty bearing seals allow oil to escape, resulting in advanced wear.
- Contamination and Corrosion
The most common contaminants are dirt, sand, and water, but chemicals and other corrosives will also adversely affect a bearing. These can dilute the oil film, reducing viscosity, or corrode the bearing surfaces, disrupting the oil film and causing erosion and leading to the formation of thousands of abrasive particles. To reduce the chances of contamination and corrosion, keeping your work areas, tools, fixtures, and hands clean can be a big help.
- Misalignment
Certain types of bearings are able to withstand minor misalignments between a shaft and bearing housing. Despite this, in some cases, misalignment can be seen on the raceway of the non-rotating ring by a ball wear path that is not parallel to the raceway edges. Bent shafts, burrs or dirt on shaft or housing shoulders, shaft threads that are not square with shaft seats, and locking nuts with faces that are not square to the thread axis are the most commonly-found causes of misalignment. Significant misalignment can cause excessive vibration and loads on the bearing. To prevent this, the shafts and housings should be inspected regularly. When necessary, shim the housings using precision grade lock-nuts.
- Distorted Components
When a housing bore becomes distorted, it will result in excessive loads and wear. This problem frequently occurs when the housing is mounted on a non-flat pedestal. If a distorted housing bore is suspected, the mounting surface should be corrected before installing a new bearing. This also applies when aligning pillow block bearings. When adjusting height, even by just a fraction of an inch, the pillow block base must be fully supported by shims. If the center of the block is unsupported, the housing bore can distort and cause failure in the future.
Seals or shields can also become distorted during the mounting process if they are pushed in and damaged. These can interfere with the functioning of the bearing cage or rolling elements and ultimately allow contaminants to enter the bearing.
- Poor Fitting
In order to work, a bearing must be properly mounted to an accurately-sized shaft. If the shaft is oversized or expands, causing an overly-tight fit, the internal clearance is reduced, leading to wear from friction and heat. Adversely, if the shaft is undersized, causing a loose fit, the bearing creeps on the shaft. If there is an adapter sleeve mounting or tapered bore bearing on a tapered shaft, looseness likely indicated that the bearing was improperly mounted. Therefore, the bearing should not automatically be re-tightened. To prevent this, ensure you are using matching, properly-mounted bearings, taking into consideration the differences in materials and operating temperatures.
- Fatigue
Failure resulting from fatigue, also known as spalling, is the fracture of the running surfaces and subsequent removal of small, discrete particles of material. It can occur on the inner ring, outer ring, or balls. Fatigue is progressive, meaning once it begins, if you continue to use the bearing, fatigue will spread. It will generally be accompanied by a significant increase in vibration, the first sign of a problem. If premature fatigue is being caused by an excessive load, it can be prevented by reducing the load or replacing the bearing with one that has a greater capacity. Otherwise, consider using bearings with longer calculated service lives.
- Inadequate Internal Clearance
The internal clearance affects the friction, load zone, and fatigue life of a bearing. An inadequate internal clearance will cause heat to build up, which will in turn affect the viscosity of the lubricant, causing lubrication problems. This problem can become so severe that the internal friction causes the bearing to lock up. The operating clearance required for a bearing to perform satisfactorily is application dependent. You must ensure that the bearing has a minimum initial internal clearance of a size that, when it is reduced due to the effects of mounting and other influences, is equal to or greater than the required minimum operating clearance.
Bearing failure is a serious issue that all factory and aircraft operators should take seriously. If you need to replace your bearings, ensure you are getting them from a trusted source. For all types of aircraft bearings and much more, look no further than Fulfillment by ASAP. We are an online distributor of aircraft parts as well as parts pertaining to the aerospace, civil aviation, defense, industrial, electronics, and IT hardware markets. We’re always available and ready to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, call us at +1(920) 785-6790 or email us at sales@fulfillmentbyasap.com.
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