Bearing the load

Due to its inherent characteristics, cement production is one of the most severe processing environments and the bearings employed must be reliable, tough and robust to withstand such harsh operating conditions. By NSK Deutschland GmbH, Germany.

Bearings employed in cement production must be resilient and able to withstand the impacts of water, dust, grit, extreme temperatures, vibration and high loads. Advances in bearing technology ensure the cement industry continues to make gains from longer operating life and higher limiting speeds, thus delivering the toughness required. In turn, this maximises uptime and minimises maintenance costs, both of which have a direct impact on bottom-line profitability. After all, the failure of a single bearing can quite easily impact the entire cement manufacturing process.

Bearing contamination

Bearing contamination is the biggest threat to reliability and hitting production targets at cement plants. Bearing specialist NSK suggests that 80 per cent of premature bearing failures are still the result of poor or low lubrication, and high contamination levels. This startling statistic is why through its four core technologies (tribology, numerical simulation, materials and mechatronics) NSK continues to drive innovation in all areas of bearing development, including lubrication and sealing. The aim is to provide the cement industry with bearings that resist common defects such as spalling, flaking, wear and seizure, all of which are caused by damage from contamination.

Dust

Cement is manufactured using both wet and dry processes, so both moisture and dust are an issue for bearing users. Of the two, cement dust is the primary issue due to its high levels of abrasion. Cement dust is hygroscopic, which means that it absorbs lubricating oil from the bearing grease. In addition, the ‘blotting paper’ effect of dust ingested into a bearing has the effect of drawing oil away, leaving the bearing with minimal lubrication at the same time as it is struggling to cope with increased friction resulting from the circulation of dust debris. Severe internal wear of cage pockets and raceways can be expected, and shafts may no longer be located rigidly as a result. Therefore, the life expectancy of other components in the powertrain is compromised.

Moisture

Moisture is also an issue because it can be drawn into rolling bearings by the vacuum created as internal bearing air pockets cool after machines are shut down. With bearings at a standstill, moisture saturates the grease lubricant, congregating around the points of contact between balls and raceways, thereby exposing these areas to developing corrosion. The result is noise and rough running, eventually leading to premature bearing failure.

Myriad of applications

Bearing applications at cement plants are wide-reaching and include kilns, grinding mills, ball mills, pulverised-coal mills, crushers, separators, vibrating screens, pumps and gearboxes, hydraulic roll presses, fans and air handling units, motors and conveyors.

In terms of conveyors, the primary failure point in these plant-critical systems is the pulley bearings. Failures also often occur in the motor and gearbox of the conveyor, even though the bearings themselves are not always the root cause of the issue. Ultimately, extending the life of the bearing is vital to extending the life of the conveyor belt. NSK’s high-performance spherical roller bearings (NSKHPS) were designed to do exactly this: reduce failure rates and provide longer operating life. 

Bearings produced to the NSKHPS standard offer innovations that include the use of ultra-clean steel and advanced surface processing of the raceways and rollers. This results in considerably-higher dynamic load factors, which in turn promotes longer service life, reduced maintenance requirements and lower total cost of ownership. Further characteristics of NSKHPS bearings include increased limiting speed and very high running precision.

Another specially-developed range of NSK bearings is the patent-pending SPACEA series of SJ bearings for high-temperature applications in the cement industry. Resembling a peapod, the cage pocket of SJ bearings contains a solid lubricant spacer joint positioned between two specially-treated balls. This ground-breaking design overcomes the issues of high-temperature environments and achieves extended, maintenance-free performance.

The material factor

Material is clearly a key focus area in bearing development. Recent NSK breakthroughs in this area include Tough Steel (TF), which delivers an optimised combination of hardness and resilience, therefore reducing wear from contaminants and extending the production run in arduous operating environments. In combination with a special heat treatment, the high purity of TF enables the resistance of the material’s surface to be increased. This property ensures that no cracks develop from indentations caused by rolling over particles of contamination. Instead, the surface is simply ‘smoothed out’ by the roll-over pressure.

NSK can also offer Super-Tough (STF) bearing steel, a proprietary material for very high load applications. One of the major factors enhancing the performance of STF is its proven ability to reduce surface-induced wear from contaminated lubricant. In addition, the material prevents or retards the appearance of white etching cracks, which form below the surface and can lead to premature bearing failure.

Compared against bearings made of ordinary carburised steel, STF offers up to 10 times the expected bearing life with contaminated lubricant. Where service life is reduced by insufficient lubrication, STF can still provide around 5.5 times more life.

Savings on offer

There are many examples of NSK bearings being successfully deployed in cement industry applications. A case in point involves bearings for a vibrating screen. In fact, for this very application NSK can offer its dedicated Long-Life Vibrating Screen series of spherical roller bearings, which are purpose-designed to withstand commonly encountered issues such as high loads, misalignment and vibration.

By switching to these bearings, a vibrating screen manufacturer based in the UK managed to achieve annual cost savings of €117,000. The company was experiencing difficulties with the consistency of the bearing mounting in its shaker box assembly. At the same time, the plant was looking for opportunities to save manufacturing costs and improve the overall performance of its machines.

NSK engineers worked closely with the customer and proposed that a complete bearing assembly be designed such that the resultant unit could simply be bolted to the machine frame without any complex assembly of bearings, seals and lubrication.

Part of a value proposal, NSK created a bespoke design with full CAD drawings for approval by the customer. NSK also took over the complete housing, seals and bearing assembly operation. As a result, the product could be supplied to the customer as a built unit – packed and pre-greased, ready to run. Not only were bearing performance and reliability improved, but a significant reduction in manufacturing costs was documented.

Value-added services

As a process industry, the cement sector can benefit from many NSK services, not least the company’s AIP added value programme. Research by NSK shows that the costs associated with bearing replacement as a result of premature failure due to contamination can be considerable. In addition to the replacement cost of the bearing itself, and the time and effort involved in sourcing and physically replacing it, there is the cost of unplanned plant and equipment downtime, and the negative effect on production schedules.

The key to determining bearing failure modes is through individual bearing examinations, combined with consideration of bearing replacement history and applied loads. With this data, an assessment can be made of the relative costs to a business. Moreover, once the issue is determined, it is usually straightforward to overcome by using bearings that are optimised for use in their respective operating environment.

Bearing optimisation can be achieved as part of AIP, which delivers tangible savings and improves the working knowledge of maintenance and engineering personnel. AIP is not complex, and usually starts with a site survey. The overall aim is to provide added value through the provision of engineering solutions that, when documented, provide substantial savings against operating and maintenance budgets.
AIP’s effectiveness was demonstrated recently in a screw pumping conveying system at a cement plant. The pumping system was experiencing regular failures due to large angular contact bearings falling foul of dust contamination.

Hygroscopic dust was wearing the raceways and increasing clearance to the point that, to avoid catastrophic failures, the bearings had to be changed prematurely and on a regular basis. NSK assessed the application under AIP and recommended a material change in the bearing specification to Hi-TF Steel. Since installation, bearing consumption at the cement plant has reduced by more than 50 per cent.
Similar to the cement industry, NSK bearings also find favour in the quarrying and mining sector. The company’s solutions offer both equipment manufacturers and quarry and mine operators longer service life under arduous operating conditions.

Among recent NSK innovations in this area is the first detachable seal for spherical roller bearings used on conveyor belt drive pulleys. Until now there were two types of design from which to select. One choice was the use of open bearings with no seals, which, although could be installed easily, were prone to early failure due to the penetration of foreign particles. The alternative has been to use sealed spherical roller bearings, but these have the disadvantage that bearing play cannot be measured during fitting, which increases the risk of installation errors. The detachable seals of the latest NSK spherical roller bearings provide an industry-first solution and endorses the message that new product development is the best way to achieve competitive gains.  

This article was first published in International Cement Review, February 2019.