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Backing bearings for cluster mills

Backing bearings have been specially developed for cold rolling mills of the cluster type, but can also be used for other applications, e.g. straightening or bending machines. Backing bearings are roller bearings having a particularly thick-walled outer ring and can be mounted on stationary support shafts to form back-up or straightening rolls. The forces from the work rolls are transmitted via the support shaft and a system of support saddles to the roll or machine stands. Fig 1 shows the arrangement of the backing bearings on the eight support shafts of a 20-roll mill.

SKF backing bearings for cluster mills are produced in many designs but are generally based on double or three-row cylindrical roller bearings. Single row needle roller bearing and double row tapered roller bearing designs are also available. The logarithmic contact profile between rollers and raceways produces a vastly improved stress distribution under all load conditions as well as when the bearing is misaligned under load. An optimised finish on all surfaces in contact serves to enhance lubrication conditions. As lubricant is supplied to the bearings via the support shaft, SKF backing bearings have lubrication holes and some have one or more annular grooves in the inner ring.

Cylindrical roller backing bearings

SKF backing bearings based on cylindrical roller bearings have two or three rows of rollers and are available as full complement bearings (without cage) or as caged bearings, with or without integral flanges or flange rings. The advantage of the bearings without flanges lies in their particularly simple geometrical form and their very high radial load carrying capacity.

Cylindrical roller backing bearings without flanges or flange rings cannot accommodate any axial load. It is therefore necessary to position thrust washers of bronze or plastic between the side faces of the outer ring and the support saddles (fig 2).

Thrust washers are not required with backing bearings with flanges (or flange rings) as the design of these bearings enables them to accommodate simultaneously acting radial and axial forces.

Cylindrical roller backing bearings are the most popular backing bearings. Consequently SKF produces many different designs which are listed below.

Backing bearings without axial guidance, without seals

BCZ.1 designDouble row cylindrical roller backing bearing with one integral flange on both inner and outer rings, cage-guided roller complement (fig 3).

BCZ.2 designDouble row cylindrical roller backing bearing with one integral flange on both inner and outer rings, cage-guided roller complement and annular lubrication groove in the inner ring bore (fig 4).

BCZ.3 designThree-row full complement cylindrical roller backing bearing with flangeless outer ring and two loose flange rings on the inner ring (fig 5).

BCZ.4 designThree-row full complement cylindrical roller backing bearing with flangeless outer ring and two loose flange rings on the inner ring and two annular lubrication grooves in the inner ring bore (fig 6).

BCZ.5 designThree-row caged cylindrical roller backing bearing with flangeless outer ring and two loose flange rings on the inner ring, cage-guided roller complement (fig 7).

BCZ.6 designThree-row caged cylindrical roller backing bearing with flangeless outer ring, two loose flange rings on the inner ring, cage-guided roller complement and two annular lubrication grooves in the inner ring bore (fig 8).

BCZ.7 designThree-row caged cylindrical roller backing bearing with flangeless outer ring, two loose flange rings on the inner ring, inner ring guided roller complement and two annular lubrication grooves in the inner ring bore (fig 9).

BCZ.8 designThree-row caged cylindrical roller backing bearing with two integral flanges in the outer ring, cage-guided roller complement and two annular lubrication grooves in the inner ring bore (fig 10).

Backing bearings with axial guidance without seals

BCZ.9 designDouble row caged cylindrical roller backing bearing with three integral flanges in the outer ring, two loose flange rings on the inner ring, cage-guided roller complement and annular lubrication groove in the inner ring bore (fig 11).

BCZ.10 designThree-row full complement cylindrical roller backing bearing with two integral flanges in the outer ring and two loose flange rings and two loose guide rings on the inner ring (fig 12).

BCZ.11 designThree-row caged cylindrical roller backing bearing with two integral flanges in the outer ring and two loose flange rings and two loose guide rings on the inner ring, cage-guided roller complement (fig 13).

BCZ.12 designThree-row full complement cylindrical roller backing bearing with four integral flanges in the outer ring and loose flange rings on the inner ring (fig 14).

BCZ.13 designThree-row caged cylindrical roller backing bearing with four integral flanges in the outer ring and loose flange rings on the inner ring, cage-guided roller complement (fig 15).

Sealed backing bearings with axial guidance

BCZ.14 designDouble row full complement cylindrical roller backing bearing with three integral flanges in the outer ring, two loose flange rings on the inner ring, radial shaft seals, no lubrication holes (fig 16).

BCZ.15 designDouble row caged cylindrical roller backing bearing with three integral flanges in the outer ring, one integral and one loose flange ring on the inner ring, cage-guided roller complement and radial shaft seals (fig 17).

BCZ.16 designDouble row caged cylindrical roller backing bearing with three integral flanges in the outer ring, two loose flange rings on the inner ring, cage-guided roller complement and "Fey-ring" seals (fig 18).

BCZ.17 designDouble row caged cylindrical roller backing bearing with three integral flanges in the outer ring, two loose flange rings on the inner ring, cage-guided roller complement and annular lubrication groove in the inner ring bore (fig 19).

BCZ.18 designThree-row caged cylindrical roller backing bearing with two integral flanges in the outer ring, two loose flange rings on the inner ring which form a gap-type seal with the outer ring raceway, cage-guided roller complement (fig 20).

BCZ.19 designThree-row full complement cylindrical roller backing bearing with two inserted flanges in the outer ring, which form a gap-type seal with the flanges on the inner ring (fig 21).

BCZ.20 designThree-row caged cylindrical roller backing bearing with four integral flanges in the outer ring, one integral flange and one loose flange ring on the inner ring, cage-guided roller complement (fig 22).

Needle roller backing bearings

The two smallest backing bearings in the SKF range are based on needle roller bearings (fig 23). They are single row full complement needle roller backing bearings with flangeless outer ring and two integral flanges on the inner ring.

Tapered roller backing bearings

Tapered roller backing bearings (fig 24) are basically double row tapered roller bearings of the TDO design (back-to-back arrangement) and are consequently able to accept the axial loads which occur in operation as well as heavy radial loads. They are sealed at both sides with pressed steel washers. Lubrication grooves in the inboard inner ring side faces facilitate lubricant supply to the bearing.

TolerancesBecause of the narrow tolerances required for the thickness of the rolled material, SKF backing bearings are produced with a running accuracy to P4 specifications, or better. In addition, the tolerances for the cross sectional height of the bearings are very tight, see table 1.

Depending on the tolerance range, the backing bearings are normally graded in three groups where the height difference is 5 Ám. The position of maximum wall thickness is marked by an arrow on the side faces of the inner and outer rings. The consecutive number of the bearing is marked to the left of this arrow and the cross sectional height group is marked to the right.

Load carrying capacityIn contrast to normal rolling bearings where the outer ring is supported over its whole circumference by a housing bore and retains its form even under load, the outer ring of a backing bearing only has a small contact area between the outer ring and the supporting roll. This limited support causes deformation of the outer ring which alters the load distribution in the bearing and thus influences the load carrying capacity. The degree of deformation depends on the wall thickness of the outer ring.

The basic load ratings given in the product table under the heading "Roller" take this influence into account. In addition to these ratings, maximum permissible dynamic and static radial forces are also given in the product table. These take into consideration the deformation and strength of the outer ring.

The dynamic load which can be applied to a backing bearing depends on the requisite life, but it is important that the maximum permissible radial force value Fr should not be exceeded. The permissible static load on a backing bearing is determined by either F0r or C0. The smaller of the two values represents the maximum static load.

LubricationIn most cluster mills the backing bearings are lubricated by the oil or water/oil emulsion used in the rolling process to remove the frictional heat as well as the heat produced by the reduction of the material thickness. This has the advantage that the rolled material is not contaminated by the bearing lubricant. The viscosity of the emulsion to be used for bearing lubrication should be at least 8 to 12 mm2/s at 40 ░C. This low viscosity is not normally sufficient to provide adequate bearing lubrication, so that the flow per backing bearing must be much higher than would be needed for conventional oil lubrication. The cleanliness of the rolling oil also has a decisive influence on the service life of the backing bearings. The flow rates given in table 2 are recommended.

Certain backing bearings, for example, the double row cylindrical roller backing bearings with integral "Fey-Ring" seals or the double row tapered roller backing bearings, must have oil mist or oil spot lubrication. For cylindrical roller backing bearings with integral radial shaft seals, circulating oil lubrication with a separate supply system should be used.

For oil-lubricated backing bearings, the use of a mineral oil with EP additives and a viscosity of 100 to 150 mm2/s at 40 ░C is recommended. Table 3 contains recommended oil flow rates for the individual lubrication methods. Higher flow rates are required under heavy loads and/or high roll speeds.

Mounting instructionsIt is normal to use backing bearings belonging to the same cross sectional height group on any one support shaft. After removing the locking devices used during transportation, the backing bearings should be pushed on to the shaft alternately with the saddles using suitable mounting equipment. When mounting backing bearings, care must be taken to see that, for example, the inner ring of cylindrical roller backing bearings is not pushed proud of the roller complement as this will cause the rollers of full complement bearings to fall out of the bearing.

To avoid a negative effect of the wall thickness variation of the inner rings on the quality of the rolled material, all inner rings on a support shaft should be turned so that the arrows indicating the position of the maximum wall thickness all point in the same direction.

Maintenance and reworkingThe backing bearings in the middle of the roll are subjected to a heavier load than the backing bearings at the ends of the roll. As the middle bearings wear more than the others, they should be replaced more often.

Because of the design, the outer shafts of cluster mills are much more heavily loaded than the inner ones. This also leads to heavier wear on the backing bearings on the outer shafts. It is therefore recommended that the backing bearings on a heavily loaded shaft should regularly be moved to a less heavily loaded shaft. This strategy pays off as the service life of all the backing bearings is considerably extended by this means.

As the inner rings are subjected to point load, it is also recommended that after each bearing inspection, the inner rings are turned so that another part of the raceway becomes the loaded zone. The arrows marked on the side faces of the rings provide a useful indication of the position of the loaded zone.

Unavoidable production disturbances (e.g. rupture of the rolled material), the overrolling of foreign matter or uneven wear, will sooner or later result in damage to the outside cylindrical surface of the backing bearings, including indentations, flattened areas and irregularities resulting from cold welding. To prevent such damage from causing premature backing bearing failure, the outside diameter must be reground. The SKF repair service will undertake this reworking. If required, instructions on how to regrind will be supplied.

After reworking, the outer rings must be cleaned, re-measured, marked and re-assembled with the appropriate roller complements or roller and cage assemblies and inner rings to form complete backing bearings. It is also necessary to regrade into cross sectional height groups.

Suitable intervals for bearing exchange, turning the inner rings and regrinding the outer ring outside diameters depend on the operating conditions and must be determined during the mill commissioning phase. Guideline values for appropriate intervals are given in table 4.

Supplementary designationsThe designation suffixes used to identify certain features of SKF backing bearings are explained in the following.

A, B, C Modifications to the original design. Combinations of these letters are also used, e.g. AC. The actual modification is specific to the individual backing bearing and reference must be made to the drawing

DesignationBearing Features
HB1Inner and outer rings bainite hardened
HB2Outer ring bainite hardened
VFull complement bearing (no cage)


Bearing terminology | Y-bearings | Deep groove ball bearings | Needle roller bearings | Combined cylindrical roller/taper roller bearings | Cylindrical roller bearings | Angular contact ball bearings | Combined needle roller bearings | Spherical roller bearings | Tapered roller bearings | Thrust ball bearings | CARB« toroidal roller bearings | Cylindrical roller thrust bearings | Angular contact thrust ball bearings | Tapered roller thrust bearings | Needle roller thrust bearings | Track runner bearings | Spherical roller thrust bearings | Indexing roller units | Backing bearings for cluster mills | Other SKF rolling bearings | Bearing accessories | Engineering products | Mechatronics

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