Michelle Knott discovers that it’s vital to put bearings on the agenda in any discussion about the maintenance and protection of rotating equipment.
When it comes to protecting the performance and longevity of rotating equipment it’s all about the bearings. Specifying the right bearings is obviously essential, but proper handling from storage through to installation, operation and maintenance will also have a huge impact on ongoing reliability and costs.
Specifying the correct bearings for your load and speed is the obvious starting point, but that’s not quite as trivial as it sounds, according to Chris Johnson, managing director with SMB Bearings: “As well as their size and material, bearings can be categorised under their dynamic radial, static radial and axial load rating. However, these ratings can often be deceptive.
“For example, a dynamic load rating indicates the capacity that a bearing can endure for one million inner ring rotations before the first signs of fatigue or damage develop. While one million rotations sound like a lot, for a bearing operating at around 5,000 revolutions per minute (rpm), with the maximum dynamic load applied to it, one million ring rotations would be completed in just over three hours.”
Bearings may experience higher loads and/or speeds only briefly, which often does not dramatically shorten life
Chris Johnson, managing director, SMB Bearings
Johnson therefore recommends limiting actual loads to between six and 12% of a bearing’s dynamic load rating. “When you think about it, the margin is not surprising. Imagine a car whose engine would be expected to last for 150,000 miles or more. If you drove that car everywhere at its maximum speed at maximum rpm, most people wouldn’t be surprised if the car broke down after 1,500 miles due to load and speed stress on all engine components,” he says.
Even down at lower loads the difference in theoretical life between running at 6% and 12% is enormous. Running at 12% of the dynamic load rating reduces the theoretical life by a factor of almost 80 to one compared with running at 6%. Similarly, running at 50% slashes the theoretical life to approximately 0.2% of running at 6% load, while 75% brings that down still further to a paltry 0.06%.
Johnson also notes that there’s a corresponding impact on long-term performance from pushing the usual running speed too high. However, he adds that an occasional ‘blip’ in the load or speed needn’t be terminal: “Bearings may experience higher loads and/or speeds only briefly, which often does not dramatically shorten life.”
Materials are another key factor when specifying bearings to minimise the total cost of ownership (TCO). While standard, low-cost bearings will suit a range of applications, it may be worth bringing in the experts if it’s a more challenging application, according to Sally Sillis [pictured], technology centre manager at Schaeffler (UK): “Schaeffler’s application engineers take time to understand the specific customer application and to develop technology to suit…
“For example, Schaeffler offers rolling bearings made from Mancrodur carbonitrided steel. Carbonitriding involves subjecting the bearings to a special heating process in which the bearing surface is enriched with carbon and nitrogen. This increases the surface hardness and wear resistance of the bearing.”
Specific design features can also help minimise maintenance costs in the future. For example, Sillis highlights how split spherical roller bearings are especially helpful in tough applications and heavy industries: “Replacing damaged or worn parts on heavy machinery that requires regular servicing or maintenance can be a costly problem, particularly for companies in heavy industries such as mining, quarrying, oil and gas, paper, and steel production, where the value of capital equipment and production machinery is high.
“The use of split spherical roller bearings reduces the time it takes to replace bearings, particularly in areas that are difficult to access such as shafts with multiple supports. In a split bearing, the inner ring, outer ring and roller-and-cage assembly are split into two halves that are clamped together using screws. A cylindrical bore provides direct mounting onto the shaft, which means the bearings typically support high thrust loads and dynamically compensate for any misalignment.
“Also, by using a split version of the bearing, many stages of the mounting and dismounting process are omitted. This means that downtimes – and any associated costs – are reduced significantly.”
Long or short of it
Even so, opting for the ‘cheap and cheerful’ end of bearing technology in terms of technology and design can sometimes be the right answer in non-critical applications. It might also dictate the future condition monitoring and maintenance strategy, as Sillis explains: “For non-critical machines and equipment, the decision may be made by an end user to run the low cost bearings to failure, rather than to condition monitor the bearings to detect early any signs of wear or failure.”
Even correctly specified bearings need careful handling, according to suppliers. That starts with storage in a cool, clean, low-humidity environment where they won’t be subjected to dust, shocks and vibrations. Some will also have a ‘best before date’, since their lubrication deteriorates over time.
IoT and Industry 4.0 digital technologies are making it possible to monitor equipment that wasn’t previously seen as cost effective to monitor
Sally Sillis, technology centre manager, Schaeffler (UK)
Then it’s essential to mount and operate them correctly, paying particular attention to the lubrication regime, according to Phil Burge, marketing and communications manager with SKF: “Ensuring the bearing is correctly mounted, using the appropriate fitting tools and method, also means that it will perform at optimum levels.
“Lubrication is key to longevity too. The bearing must be properly lubricated throughout its life, with the appropriate grade of oil, or with grease that has the correct base oil viscosity. This enables full elastohydrodynamic lubrication, which separates the rolling surfaces within the bearing.
“Many bearing types are available pre-lubricated and with integral seals or shields, which can minimise maintenance requirements. However, for bearings which require frequent lubrication, or are in hard-to-access areas of the plant, automatic lubrication systems are a worthwhile investment.
“Good sealing is also essential to prevent ingress of solid or liquid contamination, and to retain the lubricant. Even soft fibres can cause damage under the very high contact pressures that occur between bearing rolling elements and raceways.”
Monitor wizard
Finally, it’s worth mentioning the role of condition monitoring in bearing health, which is increasingly impacting on rotating machinery as the Industrial Internet of Things (IIoT) continues to gain traction.
“The ability to be able to monitor bearings and rotating machinery to enable planned maintenance, rather than unplanned stoppages, is essential,” says Burge. “With condition monitoring, this is now possible. One of the main challenges with condition monitoring is the process of converting data to asset knowledge. By using web-based reporting tools and machine health learning algorithms, a confident prognosis for bearing and machine health can be made. With this in mind, SKF is developing a new generation of smart wireless sensors. These will interact with data processing algorithms, with the aim of accelerating the data analysis process still further.”
“IoT and Industry 4.0 digital technologies are making it possible to monitor equipment that wasn’t previously seen as cost effective to monitor,” says Sillis. “To date, most condition monitoring systems have predominantly been used as local systems, collecting vibration data from machines and using analysis algorithms and a rolling bearing database to check for wear, defects or unusual behaviour.
“Systems are now available that offer the added value of being able to share and compare local machine condition data, via the Cloud. “Companies become more aware of the need for an insurance policy against failure or breakdown of critical assets and machinery. Condition monitoring iis essential for reliable operation, to maximise process life and minimise production downtime.”
