Bearing damage: catch it sooner
Published: 04 September, 2018
If damage to bearings can be detected early, the potentially extensive downtime and repair costs arising from failure in service can be avoided. Phil Burge, marketing and communications manager at SKF looks at the tell-tale indicators of bearing wear and puts forward the argument for condition monitoring.
When a bearing is damaged, it performs less well. Left in place, its continued deterioration threatens secondary damage to your machinery and its components. Eventually it may fail catastrophically, resulting in major expense and loss of productivity.
Around 90% of bearings actually outlive the equipment to which they are fitted, and a further 9.5% are replaced through routine preventative maintenance. Although 0.5% failing in use might not sound like a big problem, 0.5% of the 10 billion manufactured annually, worldwide, is still 50 million failed bearings. One of them could easily be yours.
By identifying problems at the earliest possible stage, bearing repair or replacement can be conveniently worked into your condition-based maintenance programme. This is much more economical than reacting to crises and it also improves the scope for diagnosing faults and rectifying their root causes.
It makes sense to check bearings, seals, seal counterfaces, housings and lubricants during scheduled shutdowns, but damage to bearing surfaces and raceways is initially invisible. Condition monitoring techniques, by contrast, are applied while the equipment is operating, with the aim of detecting small-scale wear and measuring its rate.
Condition monitoring technology
The classic indicators of bearing damage are increases in vibration, noise and heat. With experience and vigilance, maintenance technicians can feel and hear when something is wrong. However, by the time changes become detectable by human senses the damage may already be substantial.
To be effective, condition monitoring must be routinely carried out using objective instrument-based technology. Today, monitoring instruments and analytical tools can alert you to much smaller changes, much earlier, so pre-warning time is longer and corrective actions are easier to plan.
Sensing instruments can be used, for instance, to identify slight abnormalities in a machine’s vibration pattern, or signature. The latest devices benefit from advanced technologies like enveloped acceleration. This filters out extraneous background machine noise, so accurate real-time vibration signals can be produced.
Vibration and temperature can be read at set intervals by hand-held devices fitted with probes. The associated software enables functions such as time-stamping and storage of recorded data, as well as analysis of operating conditions. Where continuous measurement is needed, it is now possible to incorporate hard-wired or wireless vibration sensors into modified bearing housings.
Once the normal pattern for a parameter has been established, through standardised timing and methodology of measurement, any change becomes obvious. In the longer term, information from analysis of observed trends is helpful in root cause diagnosis.
The key parameters you should monitor will now be examined in turn:
Vibration and noise
Damage resulting in increased vibration and accompanying noise can be caused by a variety of individual or combined factors. Examples include:
• Contaminants in lubricants – scratching or degrading bearing surfaces or raceways
• Moisture ingress through damaged seals – leading to micro-corrosion
• Incorrect fitting of bearings or shafts – allowing excess movement
• Insufficient lubrication – increasing metal-to-metal contact
Once surface damage begins, the rate of frictional wear accelerates. In addition, the vibration itself has powerfully damaging impacts on the bearing’s rolling elements and cage. These increase as underlying problems develop and the vibration intensity grows.
Most mechanical problems produce raised levels of vibration, which makes this an especially useful indicator of machine health. If the normal ‘purr’ has become a grind, a squeak or some other unpleasant sound, significant damage to your machine’s bearings may already have happened.
Thankfully, modern vibration monitoring instruments are now so sensitive that you can spot any increase well before it becomes audible. What’s more, you can analyse changes in your machine’s vibration signature to help identify what is causing them.
Temperature
Factors resulting in excess heat in bearings include:
• Too much or too little lubricant
• Lubricant choice unsuited to the operating conditions
• Seals too tight, incorrectly orientated, wrong size or otherwise defective
• Insufficient clearance
• Improper loading
At its extreme, overheating may cause deformation, cracking or other damage in bearing materials. At lower levels, raised temperature can harm bearings indirectly by adversely affecting the lubricant.
Temperature is relatively simple to monitor, and if it increases while all other operating conditions are stable you should take it as a warning. Something to bear in mind, however, is that after relubrication with grease the temperature will naturally be raised for a day or two.
Lubricant condition
Correct lubrication is one of the basics of maximising bearing life and performance, and changes in the lubricant’s condition can be a sign of trouble. Lubrication-related causes of bearing damage include:
• Insufficient lubricant to protect surfaces against corrosion and metal-to-metal contact
• Poor protection due to wrong lubricant specification
• Contamination reducing lubricant effectiveness
• Particulate contaminants in lubricant causing friction
• Wrong amount or type of lubricant resulting in overheating
The appearance of a lubricant may tell you something about its condition. For instance, fresh grease is brown, but with use it gradually turns grey. If you want a much more accurate assessment, you can take lubricant samples periodically and have them analysed by a laboratory.
Other signs
Here are a couple of other indicators of bearing issues, with examples of their causes:
• Excessive shaft movement – due to loose assembly, incorrect internal bearing clearance or surface damage
• Increase in frictional moment of shaft rotation – due to sealing drag, surface damage, poor shaft design or pre-loaded bearing problems
These factors can result in damage through vibration and friction. You should look out for them specifically in your routine inspections. Also check the bearings, and all components with which they interact, for any other defects or changes.
What to do when damage is detected
The rate at which a minor initial defect progresses to serious damage varies enormously. In a high-speed application, the bearing may become unserviceable within seconds. In a large machine with slow rotation, it may still have months of life.
It therefore makes sense, in some cases, to leave the bearing for a while rather than immediately halt your equipment to deal with it. Some bearings are repairable, but often replacement is the only option. Crucially, your intervention should be timed to minimise disruption while avoiding risk of the bearing failing in service.
One final point is that the repaired or replaced bearing should be carefully inspected to find out what caused the damage. This diagnosis, on which a bearing specialist can advise, is essential if the underlying problem is to be permanently fixed.
