General
Bearings deserve respect

IN THE 21st century industry is more reliant on bearings than ever before. Machine builders are putting increasingly higher demands on bearing manufacturers – requiring bearings that are smaller, carry greater loads at higher speeds, and last longer.
“Bearing manufacturers, particularly the world leaders, have responded by producing bearings with performance levels that could only have been dreamt of 50 years ago,” says Phil Williams, training manager at SKF New Zealand. “For example, a current E design SKF spherical roller with a bore of 75mm and a weight of 5.25 kilos can carry the same load for the same hours as a 50 year old bearing that had a bore of 110mm and weighed 18 kilos.”
Applications for bearings exist today that were not even thought of 50 years ago, he adds, particularly in the area of miniaturization, such as cameras and laptops.
“The Hasseblad camera that went with the astronauts to the moon was equipped with SKF bearings. At the other extreme, today’s mining and construction machinery and the world’s tunnel boring giants require bearings that are metres in diameter and weigh in excess of 40 tonnes.”
Unfortunately when it comes to the maintenance of bearings, there are a number of issues that need to be addressed – taking shortcuts is one of them.
“There will always be a conflict between production and maintenance in many plants and this is where perhaps one of the most common shortcuts takes place,” says Williams. “And that is not rectifying the problem when it occurs but delaying maintenance until a window opens in the production cycle.
“For instance, a fan gets out of balance and, rather than fix it, maintenance is delayed. This creates an artificial, out of balance mass in the fan which, in turn, puts an undue load on the bearings and shortens their useful life.”
Simon Hurricks, a machine dynamics engineer with Genesis Energy, has seen shortcuts when it comes to the actual bearing replacement. “One of the common ones we see is for the defective bearing to be replaced with an identical bearing – that is, a 6302 will be replaced with a 6302 – but the shaft and housing will not be measured to ensure that the fit is still correct. Ninety percent of the time you’ll get away with this, but if the housing and/or shaft is worn, or the fit incorrect to start with, then you’ll have premature failure on your hands.”
Bearing transportation and storage is another big issue, according to Jason Tranter, founder and MD of the Mobius Institute in Australia and a specialist trainer in vibration analysis.
“Transportation and storage is a pretty big issue because of false brinelling. This is where the surfaces are being damaged because the balls/rollers are vibrating against the raceways. “Bearing lubrication is also a big issue. Industry does not appear to value the importance of correctly lubricating bearings and reducing contamination.”
Bearing installation and removal is yet another huge issue, he adds. “And running machines smoothly so there’s no unbalance, misalignment or resonance is critically important.”
Tranter believes there’s a need to study the ‘forensic science’ of bearings and determine why they fail. “Rather than cutting bearings off and scrapping them, their correct removal and analysis of the patterns on the bearing surface should provide clear indication of why the bearing failed. If you know why it failed you can take action to avoid the failure in the future.
“Being proactive in the first place to give the bearing the best possible chance of surviving, and then determining why it failed and correcting the root cause has to be the best formula for success,” he says.
Larry Wiechern, who heads the Maintenance and Reliability Centre at the Manukau Institute of Technology, backs up Tranter’s comments regarding lubrication.
“Not providing the right grade and type of lubrication to get the best possible expected and designed life out of the bearing is a real issue.
“Often engineers have no idea the damage moisture within the oil has on the life of the bearing either. Any amount greater than 200ppm will greatly reduce the life of rolling element bearings,” he says.
“Contamination of lubricant is also of major concern. It is a proven fact that lubrication related plant failures can be drastically reduced by improving the control of lubricants.”
Wiechern says training courses are invaluable for passing on expert advice on best practice.
That includes the correct use of tools and the best methods for removing and fitting bearings.

Worst case failure
Some bearing failures cause catastrophic damage. Phil Williams recalls one “significantly large coupling” that was thrown across a machinery room due to the shaft bending. “Heat had been generated to such a degree that the steel shaft had become plastic-like. Lubrication-related issues could have played a part in this failure, as could poor fit and tolerances.”
Williams believes bearings are often mistakenly regarded as just a commodity that fit in a hole and go around. “Bearings are a very precise product made to exacting standards with tolerances and clearances in the order of microns. They must be treated with all due respect.”
He says training is absolutely paramount if plant operators are going to get anywhere near the theoretical life of their bearings.

“Tradesman must understand that shaft and housing tolerances are critically important.
The condition of the shafts and sealing devices are also important if lubrication is going to be retained correctly and contaminants excluded.
“One of the most misunderstood aspects of maintenance I have come across is the way that bearing clearances are adjusted to attain the correct running clearances in spherical roller bearings. This is a highly skilled operation. There are measurements and calculations required to ensure that the correct clearance is arrived at. Quite often arbitrary ‘hit and miss’ methods are used, resulting in very poor bearing life.”

Training focus
Bearings fail for a variety of reasons. SKF research shows that 14 percent fail due to contamination; poor installation practices cause 16 percent to fail; 34 percent of failures are due to premature fatigue; and the biggest cause of failure, at 36 percent, is lubrication issues. Williams says understanding all these areas is critical to get the best out of bearings – but if there has to be a single focus, his money’s on understanding lubrication.
“Too often the role of machine lubrication is left to a person who may not really understand that different applications running at different speeds and different temperatures require greases that are significantly different in their makeup. Greases are complex chemical mixes with a wide range of bases and oils, which are not necessarily compatible with one another. Mixing greases incorrectly can lead to absolute disaster when the resulting product fails to perform as it should.
“Correct lubrication selection for a given application and correct re-lubrication intervals are absolutely essential if ultimate bearing life is to be achieved.”

Counterfeit bearings
One final issue on bearings which many people may not be aware of, is that of counterfeit product. Williams says he has personally seen counterfeit bearings in this country.
“There are methods to determine quite quickly whether a bearing is a fake or not. Some of the copies on the surface appear to be genuine, but detailed examination usually determines whether the item is a fake. Should the copy be really well made then metallurgical analysis can be used as final proof.
“In Europe the problem is significant and has got to the stage where companies that are traditional rivals are co-operating to drive counterfeiters out of business. There are many stories documented on the Internet detailing some of the significant cases over the past few years, including a Czechoslovakian distributor who went to jail for having 30 tonnes of counterfeit bearings in his possession.
“The old saying ‘if the price seems to be too good to be true then it generally is’ certainly applies in these cases.”
Note: While SKF is quoted in this article, the opinions and thoughts of Phil Williams are his own and not necessarily endorsed by SKF.

 

 

 

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