Avoiding the perils of rolling bearing failure
Published: 07 March, 2017
Roy Thomason, managing director for Industria Bearings and Transmissions, examines some of the main causes of bearing failure and gives advice on what to look out for to avoid the inconvenience and cost implications when things go wrong.
As with any working piece of equipment, even if it is high quality and well maintained, bearings will inevitably fail at some point during their lifetime. The longevity of a bearing will generally be determined either by a period of time or a total number of rotations before failure occurs. However, there are a number of reasons they fail, and as such there are ways of maximising the working life of a bearing. Key to this is knowledge of the product, its purpose and its conditions of use.
There are a number of reasons why rolling bearings fail and the more common causes would be down to incorrect fit, improper lubrication, faulty adjustment, mishandling and misalignment, corrosion, impact load and pitting.
To begin with when bearing failure ensues, however insignificant it may appear to be initially, it is important to investigate the situation thoroughly to find out why the problem occurred in the first place. Key to this will often be the timing of the failure, particularly when the failure has been identified as an actual fracture of the bearing. In addition to checking the bearing, the shaft housing and lubricant used with the bearing should also be thoroughly examined.
It is important that those carrying out the inspection have sufficient knowledge and experience in bearings and lubricants as well as a good understanding of the overall equipment, installation conditions and the operating processes involved. If it is felt that the level of knowledge is insufficient, then turning to the bearing manufacturer or supplier for help might be an option worth considering.
Where the fracture has appeared shortly after the bearing has been mounted, 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 in the bearing, and this in turn can be caused by excessively light internal clearance on the bearing or ‘creep’ on the bearing ring. Using an excessively 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.
In many cases, incorrect use of lubricants can lead to a range of problems. This can be too much or too little lubricant, the wrong lubricant being used for the application concerned, or an incorrect method of using the lubricant. In addition, contamination to the fluid from water, dust or other substance will also inevitably have a detrimental effect on operation.
Excessive noise is another 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, in such circumstances, a complete replacement with a new bearing is usually the best course of action.
If the noise tends to occur on an irregular pattern, then it could be down to contamination from foreign matter, contact with another rolling part or simply down to general wear and tear. Changing the lubricant or modifying some of the parts may correct the situation, but again a new bearing may be the most effective route to solve the problem in the long run.
Excessive vibration can be caused by contamination from foreign matter or objects 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 solve this problem without resorting to replacement.
As with most items of manufacturing and processing equipment, regular service and 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, in the long run this will always be a false economy. In addition to the potential repair bill, the cost of 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 in-house 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 try and select bearing equipment from major brand manufacturers and distributors which carry ISO 9001 accreditation. This will demonstrate the supplier is fully committed to high quality as well as providing good value for money.
Probably most important when selecting bearings is to carefully assess the application they are to be used for. 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.
The environment that a bearing has to run in will ultimately affect its performance. As mentioned, dirty and dusty applications in particular can have a detrimental effect on seals. This is particularly relevant to those operating in the food industry who may require stainless steel bearings that cope better with aggressive wash down procedures. In addition, high temperatures create their own special challenges and these need to be factored into the specification process.
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 distributors will go a long way to ensuring that any failure problems are kept to a minimum and if possible, avoided altogether.
For further information please visit: www.industria.co.uk