As with any working piece of equipment, however high its quality and well maintained it may be, a bearing will inevitably fail at some point. Its longevity will generally be determined either by a period of time or a total number of rotations. The causes of bearing failure are many and having knowledge of these reasons means it is possible to maximise working life. Central to this is knowledge of the product, its ultimate purpose and conditions of use. Roy Thomason, Managing Director for Industria Bearings and Transmissions, looks at some of the main causes of bearing failure and offers his advice on how to avoid the inconvenience and cost implications when things go wrong.
There are many reasons why rolling bearings fail and these could be down to incorrect fit, improper lubrication, faulty adjustment, mishandling and misalignment, corrosion, impact load and pitting.
When a bearing failure occurs, however insignificant it may appear to be at the outset, it is important to investigate the situation fully. Key to this will often be the timing of the failure, particularly when it has been identified as an actual fracture. In addition, the shaft housing and lubricant used with the bearing should also be examined.
Those carrying out inspections should have sufficient knowledge and experience in bearings and lubricants and a good understanding of the overall equipment, installation conditions and operating processes.
If bearing fracture happens shortly after mounting, there are a number of explanations as to what could have brought this about. It may be an inappropriate use of the bearing, meaning the problem began at the specification stage or that the installation itself was flawed.
Abnormal operation will inevitably lead to an increase in temperature, and this can be caused by excessively tight internal clearance on the bearing or ‘creep’ on the bearing ring. Using an over heavy load and improper centring of the mounting are other potential causes.
Another cause is failure of the oil seals due to the wrong type, excessive interference, shortage of lubricant or inappropriate contact with other seals or parts within the system. Correct installation of new seals or changing the seal type or method will usually be the best remedy.
Incorrect use of lubricants can lead to a range of problems – from too much or too little, the wrong lubricant being used for the application, or an incorrect method of use. In addition, contamination to the fluid from water, dust or other substance will also have a detrimental effect.
Excessive noise is a symptom of approaching failure. When this occurs at regular intervals it is usually due to flaws or ‘pitting’ or cracking of the inner or outer rings and a complete replacement with a new bearing is usually the best solution.
If the noise occurs on an irregular pattern, then it could be down to contamination from foreign matter, contact with another rolling part or general wear and tear. Changing the lubricant or modifying parts may correct the situation, but again a new bearing may be the most effective route to solve the problem.
Excessive vibration can also be caused by contamination from foreign matter or flaws in the raceway or rolling contact surface. Changing lubricant or remounting the bearing should be tried before a complete replacement is considered.
Large rotational torque is usually down to an improper mounting, sealing device or lubricant. Widening the internal clearance, while taking extra care with centring, reduction or interference of the oil seal and reducing the volume of lubricant can usually solve this problem.
With most manufacturing and processing equipment, regular maintenance is important to ensure continued, trouble free operation. While in recent years some may have saved a few pounds on trimming the maintenance budget, inevitably this will be a false economy. In addition to the repair bill, lost production and downtime can have serious implications for any manufacturer.
There are other areas to consider and steps that can be taken to avoid problems arising from bearing failure.
Bearings that are thoroughly tested will ensure customers’ confidence in what they are buying. For example, quality distributors will supply products from recognised manufacturers which have their own metallurgy departments and research facilities where products are extensively tested, so that customers can have absolute confidence in the bearings they are purchasing. It is always advisable to select bearing equipment from those which carry ISO 9001 accreditation.
Probably most important is to carefully assess the application. Bearings can be used for something as simple as a skateboard to a highly complicated aerospace application and, while the product principle will be the same, the actual demands are worlds apart. Care needs to be taken to ‘design in’ a number of elements such as the overall size, speed and load it will need to handle as well as the direction of load.
When working with bearings, knowledge of the product and its application is vital so that any problems can be identified and dealt with early to safeguard continued operation. Working with experienced manufacturers and suppliers will go a long way to ensuring that any failure problems are kept to a minimum and if possible, avoided altogether.