SKF’s Revolutionary Products & Global Experience in The Papermaking Industry



Global bearing specialist, SKF, with over 100 years experience in rotating equipment, secured the project for the upgrade of drying cylinders on the paper machine at Mondi Packaging South Africa in Piet Retief. This long-term project covers the upgrade of the bearing arrangements on the 50 drying cylinders with the ultimate goal of a higher overall machine speed for increased paper production.

According to SKF Engineering Manager, Warren Walker, SKF was approached by Mondi in 1998 to review the design of the drying cylinder bearing assemblies. "This large paper manufacturer recognises that SKF is the preferred OEM and aftermarket bearing supplier to the papermaking industry and we have also shared a successful long term business relationship with them", says Walker.

The local SKF engineering team conducted an on-site assessment and thorough design review of the existing drying cylinder bearing arrangements at the Piet Retief plant to assess current problem areas and obtain the necessary operating data in order to arrive at an upgrade solution that is cost effective, easy to implement and offers increased reliability. The paper machine at the plant operates with 50 drying cylinders, each approximately 5m long, for the production of brown kraft paper which is used in packaging material. The operating conditions for the bearings used in the drying cylinders are extremely arduous. As the drying cylinders are a critical part of the papermaking process, poor bearing reliability, apart from adversely affecting paper quality, can wreak havoc with the equipment, resulting in costly unplanned downtime and production losses.

High temperature steam passes though hollow bearing journals into the cylinders for the drying process resulting in typical bearing operating temperatures of between 90º and 125º C.

"Due to the abnormally high bearing temperatures, unfavourable lubrication conditions are common and short start-up periods can cause high thermal stresses in the bearings. Consequently, correct bearing and lubricant selection is crucial for optimum functioning in these extreme conditions", explains Walker.

The characteristics of the white metal bearings that were originally specified for the drying cylinders are such that they limit performance and speed upgrade potential for a modern paper making machine. One of the problems highlighted by Mondi personnel at the start of the project was that during start up, the high viscous friction associated with the colder oil between the white metal bearings and the shafts was increasing the load on the drive motors.
In some cases manual barring was required to start the machine after an extended duration shut. Apart from posing major safety concerns, the increased friction was placing high demands on the drive motors and energy efficiency was adversely affected. One of the main reasons why rolling bearings have almost universally replaced white metal bearings in modern paper making machines is that the internal rolling friction generated within the bearings is significantly lower than the sliding friction that occurs in plain bearings. A second problem expressed by Mondi at the time was the costly downtime associated with the repair of a damaged white metal bearings due to the specialised knowledge required and time consuming nature of the scraping process.

Mondi Packaging South Africa provided SKF with all necessary dimensional and process information for re-design purposes. "What made this a particularly interesting challenge was the fact that Mondi specified that no modifications were to be made to the journals, gear casing bores and machine frame. So, apart from designing for an application with extreme operating conditions, we were required to work around the geometry of the existing journal ends and the dimensional constraints imposed by the existing gear train drive components and machine frame. This was essential to ensure that the cylinder conversions could be carried in-situ during the limited duration annual plant shut without any machining of the cylinder journals", comments Walker.

After a thorough review of the existing arrangement and the process data provided, SKF presented Mondi with a solution which included the replacement of the white metal bearings with custom designed rolling bearing assemblies. The Compact Aligning Roller Bearing (CARB) invented by SKF in 1995, was proposed as the non-located bearing on the front-side of the machine.
This unique bearing addressed all previous bearing-related problems such as friction, expansion, high axial loads, vibration, misalignment, etc. The CARB bearing is a single row roller bearing with relatively long, slightly crowned rollers. The inner and outer raceways are correspondingly concave and symmetrical. Due to its unique internal geometry, the CARB bearing is an ideal non-locating bearing as it combines the self-aligning ability of an SRB with the axial displacement ability of a CRB. Benefits specific to drying cylinder bearing arrangements can be summarised as follows:

Displacement between the inner and outer rings of the CARB bearing occurs to accommodate the considerable thermal elongation of the cylinderThe self-aligning ability of the CARB bearings enables it to accommodate misalignment caused by roll deflection, alignment inaccuracy and settling foundations without any detrimental effect on bearing service life.As both bearings can be mounted in fixed housings, the high vibration levels often associated with non-located housings mounted on rockers are avoided.

The spherical roller bearings (SRB's) recommended as the located bearings on the drive end of the machine were installed in custom designed cartridge housings that fitted into existing recesses in the gear casing frame. The self aligning ability of these bearings, together with their high radial and axial load carrying capacity made them the optimum choice for the located side of the cylinders.

"We gave our customer an ideal end-to-end solution with both a global and local touch by combining international know-how in bearing technology with local design experience / manufacturing capability and installation via our Mechanical Field Service department. The housing assemblies, adapter sleeves for the bearings and associated components were all designed and manufactured locally in South Africa. Our customer wanted and was given lower energy consumption, shorter bearing change times, easier condition monitoring, lower replacement costs and minimum down time.

Once the upgrade is completed, the rolling bearings selected offer the necessary capacity for significant paper speed increases", states Walker. Walker continues, "We also advised Mondi on correct lubrication methods including oil viscosity selection and recommended oil flows to the bearings.

Mondi purchased the necessary SKF Maintenance Product tools and specialised equipment for easy mounting and dismount of bearings and their maintenance staff attended SKF bearing related training so that they could have a good understanding of the systems.

"We have given every consideration to finding the right solution and we are proud that Mondi has indicated complete satisfaction with SKF product quality, the ease of monitoring, bearing mounting and dismounting and greatly reduced downtime. The fact that our conversions significantly reduce energy consumption is of particular importance to Mondi as this is a company that is committed to sustainable environmental management", concludes Walker.

This is a long-term upgrade project and the first phase of the conversion was completed with the supply and installation of rolling bearing assemblies on eight drying cylinders from section 7 of the paper machine in May 2001. In June 2002, SKF supplied and installed assemblies on a further 16 drying cylinders from section 2 and 6 of the paper machine to complete phase two of the project. Phases 3 and 4 of the project each involving the conversion of 8 drying cylinders were completed in 2006 and February 2007 respectively. Conversion of the final 10 cylinders is scheduled for completion within the next 2 years.

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