Usually the choice between ball bearings and roller bearings is straightforward, with each type having its own distinct advantages and disadvantages for particular applications, depending on factors such as static and dynamic load bearing capacity, speed, and whether any shaft deflection is anticipated.
Sometimes spherical roller bearings provide a good compromise, yet there are inevitably applications for which all conventional types have their limitations – one example being the calender rollers in papermaking machinery.
Calenders are used to smooth the rough surface of paper web and eliminate variations in the web thickness. Two rollers are pressed together and the paper web is guided through the narrow gap.
However, the natural flexing of the rolls must be overcome, otherwise the calendered web will tend to be thicker at the centre than the outer edges. This is often achieved by means of a Nipco roll.
A Nipco roll has a rotating roll sleeve that is guided by a series of rolling-element bearings supported on hydrostatic adjusting elements mounted on a solid beam. Together, these ensure that the correct line pressure is exerted along the roller, thereby compensating for the natural tendency of the rollers to flex.
In full operation, the bearings are subjected to only light loads and there is therefore a risk of bearing slippage. But a different situation exists when the thickness calender is opened – for example, during maintenance or after an unplanned stoppage such as tearing of the paper web. In this case, the bearings are more heavily loaded by the weight of the roll sleeve itself.
In normal operation, should bearing slippage occur, the lubricant film can be broken and the rolling elements therefore come into direct contact with the raceways. This can lead to bearing damage and even cold welding.
Given the process-critical nature of this application, any resultant bearing failure and unplanned machine downtime is extremely costly.
An elegant solution to this problem is now available in the form of the tailor-made ASSR (anti-slippage spherical roller) bearing type FAG 808242A, available from INA FAG, part of Schaeffler Group Industrial. Unusually, this hybrid design uses both balls and spherical rollers.
The bearing consists primarily of the inner and outer rings of a standard spherical roller bearing; however, within each row of rolling elements, balls alternate with spherical rollers.
In the low-load (normal) operation, the ball set ensures slippage-free operation; but in the high-load phase, the balls undergo elastic deformation and become subordinate to the rollers, which take on the load-bearing task. The elastic deformation of the balls and rollers is calculated precisely by computer in order to match the particular operating conditions.
Free from slippage
Compared with conventional bearings in this application, the ASSR bearings are free from slippage when lightly loaded, yet they are still capable of withstanding angular misalignment caused by roller deflection, have low frictional torque, are suitable for high speeds, and can carry high loads for short periods.
And for the papermaker, the main benefits are reduced downtime, quiet running at high speed, plus the ASSR is interchangeable with conventional designs, which makes it a relatively easy upgrade.
INA quotes the example of the Papierfabrik Palm paper mill in Eltmann, Germany, that produces newsprint and telephone directory paper from 100 per cent recycled paper with an ash content of up to 20 per cent. On its Voith PM 3 production facility, which was commissioned in 1999, newsprint is manufactured with a working width of 8350mm at a line speed of 1730 to 2000 m/min.
In 2004 the calender between the dryer section and the reel was converted to a soft calender, with a Nipco roll to ensure a uniform web thickness. This roller is 965 mm in diameter and the line force exerted is 15 to 75 N/mm. During normal operation the bearing load is 17 kN, but this rises to 65 kN during a stoppage.
By replacing the conventional bearings with ASSR type 808242A bearings, Papierfabrik Palm has eliminated slippage and, most importantly, the problems caused by slippage.
Hybrid ball-roller bearings are not the only type of bearing that have been developed specifically for demanding applications within papermaking machinery. Triple ring bearings are used almost exclusively in papermaking machines for the support of variable crowned press rolls at the drive side. These have the advantage that they can be used to mount a ‘shaft within a shaf’ while simplifying both the surrounding structure and the installation.
SKF produces three alternative types of triple ring bearings with different inner/outer bearing combinations: cylindrical/spherical roller bearing, spherical/cylindrical roller bearing and spherical/spherical roller bearing (Fig. 2).
Meanwhile, NSK’s TL series spherical roller bearings are said to provide outstanding performance under the extreme conditions found in the dryer sections of papermaking machinery, where temperatures can reach 200 degrees C or more (Fig. 3).
A common cause of bearing failures under these conditions is fracture of the inner ring. Particular features of the TL series bearings include a special grade of steel and heat treatment process that strengthens the inner ring against increasing hoop stresses caused by rising shaft temperatures, and increased hardness of the raceway surface to provide longer life in the presence of contamination.
As a result, the TL series bearings exhibit superior resistance to inner ring fractures while providing extended life and excellent dimensional stability. For end users, this equates to improved productivity and lower costs.
Papermaking machinery is not the only niche market for specialist bearings. Both the automotive and aerospace industries, for example, routinely call for specially developed rolling element bearings to solve particular problems.
Nonetheless, the three examples outlined above clearly demonstrate what can be achieved when bearing companies are faced with out-of-the-ordinary challenges.