Advances in machine technology have made it possible to run equipment faster than ever before. The Pulp & Paper industry is one that has seen speeds increase with each new major design. Unfortunately, the same increased speeds often contribute to higher vibration levels, which can cause shaft and bearing damage and jeopardize the quality of the output. So, the potential for increased production is diminished because the equipment must be kept running and the high cost of maintenance minimized. One paper mill, however, recently installed SKF’s CARB toroidal roller bearing, resulting in a 663 percent return on investment through reduced downtime and increased productivity.
In past decades, rocker housings were a solution. These housings are designed to move with the shaft to accommodate thermal expansion in the shaft. However, the increased running speeds possible today result in vibration levels so high that they can cause wear on the rockers. In some cases, vibration causes the housing to jump off the rockers, stopping the machine.
At one paper mill where machine capabilities offered the potential for gains in productivity, the high speeds needed to reach the production goals caused similarly high vibration levels. To prevent vibration-related bearing damage and downtime, it was necessary to reduce speeds by 4 percent, from 750 meters per minute to 720 meters per minute, a slowdown that decreased production by 4,000 tons annually.
In the existing arrangement, the bearings and housing were equipped with a conventional self-aligning bearing assembly and rocker housing, both designed to accommodate shaft elongation. Even so, the bearings and housing could not accommodate the significant changes in shaft behavior caused by increasingly higher machine speeds.
An SKF paper specialist recommended rebuilding the housing to incorporate an SKF CARB toroidal roller bearing to accommodate misalignment and axial displacement within the bearing. The CARB toroidal roller bearing was developed to accommodate linear expansion within the bearing, up to 10 percent of the bearing width, as well as misalignment. This means the bearing can be mounted with a tight fit in the housing as well as on the shaft.
The rocker housing was also modified so that it could be fixed to the frame of the machine to reduce cylinder vibration levels and improve paper quality. Typical benefits of the CARB arrangement include: increased speeds, higher reliability, maximization of bearing life, reduced downtime, reduced vibration and accommodation of thermal expansion within the system – virtually without friction.
Saving per machine by avoiding speed loss: €528,000 ($ 769,211)
SKF rocker housing rebuild and CARB bearing investment: -€72,000 ($ 104,916)
Total savings: €456,000 ($ 664,295)
Total ROI: 633 percent
(All figures are rounded and based on customer’s estimates of labor and production costs.)
Other pulp and paper mill applications that might benefit from rocker housing rebuilds with CARB bearings include: all types of paper and board machines, coaters, dryer cylinders and Yankee/MG cylinders.
For more information, visit www.skf.com.