Anew rolling element bearing, dubbed the CARB (Compact Aligning Roller Bearing), combines the main features of conventional spherical, cylindrical, and needle roller bearings. Like a spherical roller bearing, it handles misalignment and heavy radial loads. But like cylindrical and needle roller bearings, it also accommodates axial displacement. It can be supplied with a low section height, similar to a needle roller bearing, to fit in small radial spaces. Developed by SKF in Sweden, the bearing is well-suited to applications such as paper mills, fans, metalworking, and gearboxes.
Closely-matched roller and race geometries in the new bearing give the rollers a self-guiding characteristic. As a result, the races require no axial guide flanges for the rollers, Figure 1.
With the guide flanges eliminated, designers lengthened the rollers to obtain higher radial load capacity. The rollers are longer than those of either a single or double-row spherical roller bearing of comparable size, Figure 2, so that they increase load capacity 5 to 10%. The increase is even more compared to cylindrical bearings. A full complement version of the bearing — where the cage is eliminated to make room for more rollers — boosts the capacity as much as 30%.
The bearing doesn’t accept axial loads, and both inner and outer races must be located in the machine assembly to allow axial displacement within the bearing.
The bearing accommodates angular misalignment up to 1.0 deg, which makes it especially suitable for drying cylinder applications in paper mills. These applications impose substantial misalignment because the bearings are mounted at opposite ends of a long cylinder.
Angular misalignment of most rollertype bearings causes uneven load distribution, which increases rolling-element stresses, thereby reducing the bearing life, Figure 3. With the new bearing, however, the radiused surfaces of its rollers and races maintain more even load distribution than other bearing types. Tests under misalignment conditions show that the CARB bearing maintains its effective load rating (load at which life is not reduced) as well as a spherical bearing and better than a cylindrical bearing, Figure 4.
Engineers say that the bearing rollers can adjust within the races to handle misalignment caused by factors such as outof- square mounting. This capability lets designers ease restrictive housing tolerances, but should not be overused, as it may degrade bearing performance.
Because the bearing races have no guide flanges, the rollers can displace axially within the bearing (between the inner and outer races).
Like misalignment, axial displacement also causes uneven load distribution for most bearing types. With a double-row spherical roller bearing, axial displacement can shift the load onto one set of rollers (unless the entire bearing moves axially within its housing), which again shortens life.
Calculations indicate that, even with misalignment, the new bearing displaces axially up to 10% (or more, depending on internal clearance) of its width without reducing its life, Figure 5. In fact, it accommodates both misalignment and axial displacement over a larger range than spherical or cylindrical roller bearings.
In January 1994, SKF applied prototypes of the new bearing at the Braviken paper mill in Sweden. At this mill, the drying cylinder section of a paper-making machine produces 29-ft wide rolls of paper at 3,940 fpm. These machines operate at high temperatures that cause shafts to expand in length up to about 0.4 in. Bearings are located 36-ft apart, which allows substantial misalignment. Additionally, the high-speed operation causes vibration.
Initially, the mill operators replaced two spherical bearings with CARB bearings in the drying section. The CARB bearing was installed on one end (front side) of each shaft as a floating bearing to accommodate axial displacement. A spherical bearing was still used on the other end (drive side) to handle axial loads.
The new bearing performed very well in these field trials. Because of its axial displacement capability, the bearing accommodated the shaft expansion without the rocker mounts used previously with spherical roller bearings (see box). Moreover, it reduced the vibration of the drying cylinders by up to 85%.
The company later replaced the remaining front side bearings on this machine with CARB bearings. In 1996, they installed an additional paper machine equipped with CARB bearings on the front side of the dryers. This machine recently operated at speeds up to 5,587 fpm — a world record for making newsprint.
Table 1 compares the capabilities of different bearing types commonly used in paper mills.
Applications and availability
Following the paper mill field trials, SKF introduced the CARB bearing at the 1995 Hannover Fair in Germany. Approximately 700 bearings were subsequently applied in 70 paper and pulp mills worldwide. Though most of these mills are in Europe, several U.S. applications were made in 1996 and early 1997.
Bearing sizes range from 200 to 260 mm diam (7.87 to 10.24 in), but the company plans an expanded range of 50 to 500 mm (1.97 to 19.7 in.). The new bearings now cost slightly more than other types, as expected for a relatively new product. But the processes used to manufacture the CARB bearing are the same as for other types. Therefore, SKF expects to bring the cost down to a comparable level in the near future.