When bearings wear

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    2016 August 24, 10:00 CEST

    It’s a fact of life that all bearings, even the most highly engineered, will eventually wear and require either repair or replacement. This article explores the options for engineers working in the paper and pulp industry.

    Bearings are critical components that are used heavily in both pulping and paper production machines. Buried deep in the heart of each machine, they ensure that rotating parts move smoothly, efficiently and with minimal friction.

    In many instances, bearing designs are highly specialised to meet demanding conditions; for example, operation in areas such as suction rolls where there is constant exposure to moisture, or in dryer sections with high levels of humidity and heat. Installed and maintained correctly, and protected by appropriate lubrication systems, bearings should provide a long and trouble free operating life.

    Unfortunately, it is not always possible to maintain bearings under ideal conditions, as bearings expert and SKF Business Unit Manager, Rudolf Groissmayr, explains. “Bearings can wear prematurely and fail unexpectedly for many different reasons. The most common causes include poor or incorrect lubrication, failed seals, misalignment of shafts, and changes in machine operating conditions. These often arise if attempts are made to increase line speeds or steam temperatures in dryers as a means of improving output; this can, however, move the bearing performance envelope outside its original specification”.

    Although it’s unusual for a bearing to fail unexpectedly – the latest condition monitoring and oil analysis systems should provide sufficient advance warning to prevent such an occurrence – it is common to find bearings suffering from indentations and micro-fissures in rolling surfaces and raceways that, over time, affect the performance and efficiency of the bearings and thus of the shafts or cylinders that they support.

    Ultimately, regardless of how carefully engineered, installed and maintained they are, bearings that are in constant use will eventually reach a point where they require either repair or replacement. Although there are arguments in favour of each approach, in the current economic climate, where mills face a combination of intense global competition, rising input prices, there is a strong impetus wherever possible to repair rather than replace bearings.

    Rudolf Groissmayr manages one of SKF’s Industrial Service Centres, specialising in the remanufacture of bearings for the pulp and paper sector. He notes that, “One of the biggest challenges for production or maintenance engineers is minimising machine downtime. The problem with bearing replacement is that it’s often impossible to determine how damaged a bearing has become until it’s dismounted and removed from the machine, by which time of course the line has stopped. If a new bearing is required then this can be costly and, as few suppliers keep such specialised or expensive components in stock, may require a special factory order, which can take weeks or in some cases months to fulfil. The alternative is to remanufacture the bearing.”

    “Remanufacture is possible in over fifty percent of applications and can normally be carried out within days and at a considerably lower cost than purchasing a new product. It is also possible to remanufacture a bearing – especially older bearings – to a higher standard of quality and performance than the original part.”

    Besides productivity gains, Rudolf Groissmayr sees some real environmental benefits of remanufacturing bearings. “Not only are there real commercial and technical benefits for mill operators, there is also a powerful argument in favour of environmental sustainability, as remanufacturing uses up to 90 percent less energy than that required to produce a new component.”

    The purpose of remanufacturing, however, is not generally to produce a bearing better than the original but to increase its service life.

    It should be recognised that remanufacturing is an extremely demanding process that requires specialised knowledge and equipment to ensure that the bearing properties are maintained and guarantee continued reliability once the product is back in operation. “Working with a specialised supplier is essential”, says Rudolf Groissmayr. “Not only will they have the capabilities to carry out the work quickly to the highest standards, they will also be able to help a customer understand why the bearing was damaged in the first place and to assist with subsequent machine optimisation to minimise the risk of subsequent failures.”

    Not all bearings are suitable for remanufacture. Those with heavy damage or fractures are generally only fit for recycling. The remanufacturing process therefore begins with an expert assessment of bearing condition, to determine both suitability for remanufacture and the type and extent of work required. An important aspect that is often overlooked is to assess bearing condition in the context of its application, taking into account the bearing load, lubrication conditions and time in operation; this enables the nature of the problem that has caused the damage to be fully understood.

    A clear distinction has to be made between problems of subsurface-initiated fatigue and surface-initiated fatigue. The former describes the shear stresses that appear cyclically immediately below the load carrying surface of the rings and rolling elements. These stresses cause microscopic fissures that gradually extend to the surface and, as the rolling elements pass over these fissures, fragments of the surface material spalls or breaks away. Bearing raceways with damage caused by subsurface-initiated fatigue are not normally suitable for remanufacture, while those suffering from surface initiated fatigue can generally be restored by honing or grinding.

    When a bearing arrives at an SKF remanufacturing centre, it is visually inspected and parameters such as residual magnetism and clearance are checked. The bearing is then disassembled and cleaned before the component parts are carefully inspected and their dimensions measured. This includes standard measurement of ring wall thickness and ovality, with the option of ultrasonic testing to detect subsurface micro-cracks.  Additionally, measurement of hardness, roller diameter set variation and outer dimensions can be added depending on the condition of the bearing and the criticality of the application.

    This initial assessment phase is then followed by the submission of a customer report and a recommendation for further actions. The subsequent remanufacturing process is undertaken in a dedicated production facility, combining advanced automation and control systems with the engineering knowledge of experienced technicians.

    The remanufacturing process is effectively divided into four categories: service level 1 (SL1) covers inspection and analysis of failures; service level 2 (SL2) covers the process of restoring bearings that have not been used but may have degraded due to lengthy or incorrect storage; service level 3 (SL3) covers the remanufacture of bearings, primarily by polishing processes, with the reuse of existing components; service level 4 (SL4) is for the extensive remanufacture of bearings requiring the replacement of components and grinding of raceways. In each case, remanufactured bearings are reassembled, quality inspected and marked for traceability before being packed and returned to the customer.

    Rudolf Groissmayr believes that bearing remanufacture offers considerable advantages. “Our experience has shown that remanufacturing can help paper mills reduce their annual bearing replacement costs. This can vary, depending on the business model, but can typically be between ten and twelve percent. Just as importantly, the relatively short lead times mean that, with careful planning, bearings can be remanufactured during normal line shutdown, thereby minimising any loss of productivity. Finally, the potential energy savings also make remanufacturing an attractive option from an environmental perspective”.

    Aktiebolaget SKF
          (publ)

    For further information, please contact:
    Press Relations: Nia Kihlström, +46 31-337 2897; +46 706 67 28 97; nia.kihlstrom@skf.com

    SKF is a leading global supplier of bearings, seals, mechatronics, lubrication systems, and services which include technical support, maintenance and reliability services, engineering consulting and training. SKF is represented in more than 130 countries and has around 17,000 distributor locations worldwide. Annual sales in 2015 were SEK 75 997 million and the number of employees was 46 635. www.skf.com

    ® SKF is a registered trademark of the SKF Group.

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