Bearings - 2018-06-08
The two direct-drive overhung fans, which are supported by spherical roller bearings in plummer blocks, operate 24/7. As primary total air fans, their role is to feed air to the boiler at a rate of 8 m3/second with an output pressure of 13 kPa. However, the bearings in each unit were failing every three months on average, a situation that would take three engineers 10 hours to remedy each time, incurring high costs per breakdown.
The long time and high cost of replacement was largely due to the size of the task. Each fan is around 3.5 m in diameter and attached to the end of a 2 m long, 135 mm diameter shaft. Shaft rotation is 1487 rpm, powered by a directly coupled 350 kW electric motor.
“Condition monitoring was being performed by the plant on both bearing units for each fan,” explains NSK Sales Engineer Dennis Briggs-Price. “Temperature would be monitored continuously by a magnetic probe, with weekly vibration checks performed on each bearing, measuring velocity in a radial position [vertical]. Normal condition readings would be around 70°C, with a velocity of 5 mm/s. Every time these values were significantly exceeded, the bearings would be replaced by the plant as a precautionary measure as imminent failure was perceived. Typical values achieved before replacement would be 90-100°C and 20-33 mm/s velocity. Indeed, a recent bearing failure on one unit broke the steel frame/mounting brackets, with excessive temperatures and vibration figures recorded. In the past, the plant had even tried balancing the fans using a third-party resource, but the bearing failures continued.”
NSK’s comprehensive application review included a failed bearing examination from the drive and non-drive end bearings. A number of issues, including fretting corrosion, were noted to the external diameter of the outer ring. It was also clear that both bearings exhibited a raceway which had supported an axial load; evident from a wider and more pronounced running band. This is unusual as the fan configuration had a fixed and free-end arrangement – to allow for shaft expansion from operating temperatures – thus the free end should only support a radial load.
As the free end bearing at the energy plant exhibited axial loading, NSK was able to attribute the root cause of the ongoing failures to incorrect bearing arrangement. High-capacity Spherical Roller Bearings within SNN Plummer Blocks were therefore recommended, along with correcting the fixing arrangement. NSK’s Spherical Roller Bearings use state-of-the-art materials that make them suitable for high speeds and loads. Offering special ring tolerances to withstand vibration, shock loads and misalignment, the bearings are heat stabilised up to 200°C. Furthermore, high-capacity Spherical Roller Bearings were the preferred choice as the internal design is more reliable for direct-coupled overhung industrial fan units, thanks largely to inner ring ribs that provide better roller guidance.
“A trial was conducted with our engineering team overseeing the installation and implementation of our recommendations,” explains Mr Briggs-Price. “The result was zero bearing failures over a 12-month period. Furthermore, our solution offered increased productivity with reduced maintenance and downtime costs, with four times greater bearing lifetime.”
As can be seen in this example, using the wrong bearings or fitting them incorrectly can reduce the output of machines or industrial units, and even cause a whole plant to grind to a halt. NSK’s AIP is designed to make operations and maintenance processes more efficient and more profitable by saving money at each value-added step.
Experienced NSK specialists use a tried and tested method to develop the ideal solution for each specific application, working with the customer and taking every requirement detail into account. At the UK energy plant, the AIP process encompassed services from initial survey and product selection, through to customer training and final inspection. The results speak for themselves. Due to the new solution, savings of €134,040 are being achieved annually based on reductions in bearing costs, fitting costs and downtime costs.
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