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Spherical Roller Bearing

Spherical Roller Bearing

Spherical roller bearings have two rows of rollers with a common sphered raceway in the outer ring and two inner ring raceways inclined at an angle to the bearing axis (fig). This gives them an attractive combination of design features making them irreplaceable in many demanding applications. They are self-aligning and consequently insensitive to misalignment of the shaft relative to the housing and to shaft deflection or bending.

SKF spherical roller bearings are leading in design and can, in addition to high radial loads, accommodate high axial loads acting in both directions.

The standard range of SKF spherical roller bearings comprises

In addition to the standard range, SKF offers a wide range of special spherical roller bearings adapted for specific applications.


Spherical roller bearing: Open bearings

SKF spherical roller bearings are produced in several designs, depending on bearing series and size. The differences are

  • the arrangement of the floating guide ring as well as
  • the design of the inner ring and/or cages,

as described in the following.

  • C(J), CC
  • Two window-type steel cages, flangeless inner ring and guide ring centred on the inner ring (fig).

  • EC(J), ECC(J)
  • Two window-type steel cages, flangeless inner ring, guide ring centred on the inner ring and reinforced roller complement (fig).

  • CA
  • One-piece machined brass cage, double-pronged, retaining flanges on the inner ring and guide ring centred on the inner ring (fig).

  • CAF
  • As CA, but with a steel cage.

  • ECA, ECAC
  • Reinforced roller complement, one-piece machined brass cage, double-pronged, retaining flanges on the inner ring, guide ring centred on the inner ring (fig).

  • ECAF
  • As ECA, but with a steel cage.

  • E
  • For bearings with a bore diameter d = 65 mm:

    Two pressed window-type steel cages, flangeless inner ring and guide ring centred on the inner ring (fig).

  • For bearings with a bore diameter d > 65 mm:
  • Two pressed window-type steel cages, flangeless inner ring and guide ring centred on the cages (fig).

  • CAFA
  • One-piece machined steel cage, double-pronged, centred on the outer ring raceway, retaining flanges on the inner ring and guide ring centred on the inner ring (fig).

  • CAMA
  • As CAFA, but with a brass cage (fig).

With some exceptions, all SKF spherical roller bearings are produced with a cylindrical bore as well as with a tapered bore. The tapered bore of bearings in the

  • 240, 241, 248 and 249 series have a taper of 1:30, designation suffix K30, and the
  • other series have a taper of 1:12, designation suffix K.

Annular groove and lubrication holes

To facilitate efficient bearing lubrication, SKF spherical roller bearings are provided with

  • an annular groove and three lubrication holes in the outer ring (fig), designation suffix W33, or
  • three lubrication holes in the outer ring (fig), designation suffix W20.

E-design spherical roller bearings have the annular groove and three lubrication holes feature as standard so that the designation suffix W33 is omitted from the bearing designation.


Sealed bearings

A selection of SKF spherical roller bearings is also produced in a sealed version with contact seals on both sides (fig). The seals are reinforced with sheet steel and made of an oil and wear-resistant

  • acrylonitrile-butadiene rubber (NBR), designation suffix 2CS
  • hydrogenated acrylonitrile butadiene rubber (HNBR), designation suffix 2CS5
  • fluoro rubber (FKM), designation suffix 2CS2.

The seals are inserted in recesses in the outer ring. For smaller bearing sizes, the seals are pressed into the recesses (fig) while the seals for the larger sizes are held in position by means of retaining rings (fig). The seals have two sealing lips contacting the lead-in at the sides of the inner ring raceway, to provide efficient sealing.

Sealed bearings are lubricated as standard with an extreme pressure bearing grease according to table 1. They should not be heated to temperatures above 80 °C during mounting, and should not be washed.

Sealed bearings do not need to be relubricated when the operating temperature does not exceed 70 °C and the rotational speed is below 50% of the limiting speed listed in the product table. When temperature and/or speeds are high, relubrication with a similar grease with lithium thickener is recommended (table 1). In this case the polymer band, which covers the lubrication holes in the outer ring must be removed before mounting (fig). Note that only a small amount of grease is needed to relubricate sealed bearings. The grease should be pressed in slowly through the lubrication holes in the outer ring while the bearing is rotating. Excessive pressure should be avoided so as not to damage the seals.

The internal design of a sealed bearing corresponds to that of an open bearing. The external dimensions are also the same except for bearings based on the 222 and 223 series. These bearings are slightly wider and carry the series designation BS2-22 and BS2-23 respectively.

Sealed bearings are available with a cylindrical bore as standard. However most bearings in the BS2-22 series are available with a tapered bore as well. Every sealed bearing can be supplied with a tapered bore to special order.

To prevent interference with the seal, the diameter of the shaft abutment should not exceed damax at least for the 1 to 2 mm closest to the bearing (fig).

If the bearings are secured axially on the shaft by a lock nut, SKF recommends using a KMFE lock nut (fig) or to position an intermediate ring between the bearing and the lock nut (fig 6c).

Warning

Seals made of fluoro rubber exposed to extreme temperatures above 300 °C give off hazardous fumes. Therefore the safety recommendations must be followed, see "WARNING! Safety precautions for fluoro rubber"


Bearings for vibratory applications

Vibratory applications, such as vibrating screens or exciters, induce accelerations of rollers and cages in the bearings. This puts extra demands on the bearing design. SKF spherical roller bearings for vibratory applications can withstand considerably higher accelerations than corresponding standard bearings. The permissible acceleration depends on the lubricant and the type of acceleration - rotating or linear acceleration.

Rotating acceleration

The bearing is subjected to a rotating outer ring load and a rotating acceleration field. This generates cyclic loads on the cages from the unloaded rollers. Typical examples are vibrating screens and planetary gears. Road rollers are subject to a mix of rotating and linear accelerations (fig).

Individual values for the permissible rotating accelerations are provided in the product table and are valid for oil lubricated bearings. The values are expressed in m/s2, where 28 g stands for 28 × 9,81 = 275 m/s2 for example.

Linear acceleration

The bearing is subjected to impact loads and thus linear accelerations. This causes hammering in the cage pockets by the unloaded rollers. A typical linear acceleration is generated when rail wheels are rolling over rail joints (fig). An analogous application using bearings for vibrating applications is a road roller where the roller is vibrating against a relatively hard surface.

Individual values for the permissible linear accelerations are provided in the product table and are valid for oil lubricated bearings. The values are expressed in m/s2, where 90 g stands for 90 × 9,81 = 883 m/s2 for example.

Bearing design

SKF spherical roller bearings for vibratory applications have the same dimensions and performance values as bearings in the 223 series but have a C4 radial internal clearance as standard. They are available with either a cylindrical or tapered bore. To facilitate efficient lubrication all bearings are provided with an annular groove and three lubrication holes in the outer ring.

SKF spherical roller bearings for vibratory applications are, depending on their size, available in one of the designs described below.

E/VA405 (bearings with d = 65 mm)

Two surface hardened window-type steel cages, flangeless inner ring and guide ring centred on the inner ring.

E/VA 405 (bearings with d > 65 mm)

Two surface hardened window-type steel cages, flangeless inner ring and guide ring centred on the cages (fig).

EJA/VA405 and CCJA/W33VA405

Two surface hardened window-type steel cages for bearings of EJA design (fig) or CCJA design (fig), flangeless inner ring and guide ring centred on the outer ring raceway. EJA/VA406 and CCJA/W33VA406

These bearings have a PTFE coated cylindrical bore and the same features as a VA405 design bearing. These bearings are intended for the non-locating bearing position in vibratory applications to prevent fretting corrosion between the shaft and the bore of the bearing. Shafts do not require special heat treatments or coatings.

System solutions for vibrating screens

In addition to single bearings for vibrating screens, SKF has developed fault detection and bearing systems that can extend performance, reduce maintenance and monitor machine condition in vibratory equipment. More information on this on "www.skf.com/copperhead".


SKF Explorer class bearings

High performance SKF Explorer spherical roller bearings are shown with an asterisk in the product tables. SKF Explorer bearings retain the designation of the earlier standard bearings, e.g. 22220 E. However, each bearing and its box are marked with the name "EXPLORER".


Special bearings

SKF produces a wide range of special spherical roller bearings to meet specific customer needs. These are, for example, bearings for

  • printing machines, paper mills or coaters in high precision execution
  • for very arduous operating conditions as for example in continuous casting machines
  • high temperature applications
  • mounting with loose fit on roll necks
  • railway vehicles

For detailed information on these spherical roller bearings please contact SKF.


Bearings on sleeves

Spherical roller bearings with a tapered bore can be mounted on smooth or stepped shafts using

  • an adapter sleeve (fig)
  • a withdrawal sleeve (fig).

The sleeves facilitate bearing mounting and dismounting and often simplify bearing arrangement design.

When sealed bearings are to be mounted on an adapter sleeve it is necessary to protect the sealing lips from being damaged. This can be done by

  • using an E-design adapter sleeve (see section "Adapter sleeves")
  • inserting an intermediate ring between the bearing and the locking washer (fig).


Appropriate bearing housings

The combination of a spherical roller bearing and an appropriate bearing housing constitutes an economic, interchangeable and reliable bearing arrangement that meets the demands for easy maintenance. SKF produces appropriate housings in a variety of designs and sizes to suit a wide range of applications. The designs include:

  • SNL plummer block housings in the 2,3, 5 and 6 series (fig).
  • Large SNL plummer block housings in the 30 and 31 series (fig).
  • SDG plummer block housings (fig).
  • SONL plummer block housings for oil lubrication (fig).
  • SAF pillow (plummer) block housings for inch shafts (fig).
  • SDAF pillow (plummer) block housings for inch shafts (fig).
  • SBD plummer block housings (fig).
  • TVN plummer block housings (fig).
  • FNL flanged housings (fig).
  • 7225(00) flanged housings (fig).
  • THD take-up housings (fig).

Detailed information on housings can be found in the catalogue "Bearing housings".


Dimensions

The boundary dimensions for spherical roller bearings are in accordance with ISO 15:1998. The dimensions of the adapter and withdrawal sleeves correspond to ISO 2982-1:1995.


Tolerances

SKF spherical roller bearings are manufactured as standard to Normal tolerances.

SKF Explorer spherical roller bearings up to and including 300 mm bore diameter are, however, produced to higher precision than the ISO Normal tolerances. For example

  • the width tolerance is considerable tighter than the ISO Normal tolerance,
  • the running accuracy is to tolerance class P5 as standard.

For larger bearing arrangements where running accuracy is a key operational parameter, SKF spherical roller bearings with P5 running accuracy are also available. These bearings are identified by the suffix C08. Their availability should be checked.

The tolerance for the bore and the outside diameter of SKF Explorer spherical roller bearings for vibratory applications have been reduced from Normal to P5 and P6 respectively.

The values of the tolerances are in accordance with ISO 492:2002.


Internal clearance

SKF spherical roller bearings are produced as standard with Normal radial internal clearance and most are also available with a greater C3 clearance. Many bearings can also be supplied with a smaller C2 clearance or the much greater C4 or C5 clearances.

SKF spherical roller bearings for vibratory applications are produced as standard with C4 clearance. The radial internal clearance limits are listed for bearings with

  • cylindrical bore in table 2 and with
  • tapered bore in table 3.

The clearance limits are in accordance with ISO 5753:1991 and are valid for bearings before mounting under zero measuring load.


Misalignment

The design of spherical roller bearings is such that they are inherently self-aligning, i.e. angular misalignment between the outer ring and inner ring can be accommodated without any effect on bearing performance (fig). Under normal loads and operating conditions (load ratios of C/P > 10) and when misalignment is constant in position with respect to the outer ring, the guideline values for permissible misalignment provided in table 4 apply. Whether these values can be fully exploited or not depends on the design of the bearing arrangement, the type of seals used etc.

Demo

When the position of the misalignment is not constant with respect to the bearing outer ring, e.g. in

  • vibrating screens with rotating imbalance and therefore rotating deflection of the shaft (fig)
  • deflection-compensating rolls of paper machines where the stationary shaft is bent,

additional sliding is caused in the bearing under the operating conditions. Therefore, with reference to bearing friction and associated heat generation, it is recommended that misalignment of the inner ring with respect to the outer ring should not exceed a few tenths of a degree.

Sealed bearings can accommodate angular misalignments of the shaft with respect to the housing of up to approximately 0,5°. Provided the guideline value is not exceeded, there will be no detrimental effect on the efficiency of the seals.


Influence of operating temperature on bearing material

All SKF spherical roller bearings undergo a special heat treatment so that they can be operated at higher temperatures for longer periods, without the occurrence of inadmissible dimensional changes. For example, a temperature of +200 °C for 2 500 h, or for short periods at even higher temperatures, is permitted.


Minimum load

To achieve satisfactory operation, spherical roller bearings, like all ball and roller bearings, must always be subjected to a given minimum load, particularly if they are to operate at high speeds or are subjected to high accelerations or rapid changes in the direction of load. Under such conditions the inertia forces of the rollers and cage(s), and the friction in the lubricant, can have a detrimental influence on the rolling conditions in the bearing arrangement and may cause damaging sliding movements to occur between the rollers and raceways.

The requisite minimum load to be applied to spherical roller bearings can be estimated using

Pm = 0,01 C0

where

Pm = equivalent minimum load [kN]

C0 = basic static load rating [kN] (see product data)

In some applications it is not possible to reach or exceed the requisite minimum load. However, if the bearing is oil lubricated lower minimum loads are permissible.

These loads can be calculated when n/nr = 0,3 from

Pm = 0,003 C0

and when 0,3 < n/nr = 2 from

where

Pm = equivalent minimum load [kN]

C0 = basic static load rating [kN] (see product data)

n = rotational speed [r/min]

nr = reference speed [r/min] (see product data)

When starting up at low temperatures or when the lubricant is highly viscous, even greater minimum loads than Pm = 0,01 C0 may be required. The weight of the components supported by the bearing, together with external forces, generally exceeds the requisite minimum load. If this is not the case, the spherical roller bearing must be subjected to an additional radial load.

NoWear spherical roller bearings have proven to give reliable operation at very low loads. They can with stand longer periods of insufficient lubrication, sudden variations in load and rapid speed changes.


Axial load carrying ability

Because of their special internal design, SKF spherical roller bearings are able to accommodate heavy axial loads and even purely axial loads.

Axial load carrying ability of bearings mounted on an adapter sleeve If spherical roller bearings with adapter sleeves are mounted on smooth shafts with no fixed abutment, the magnitude of the axial load that can be supported is determined by the friction between the shaft and sleeve. Provided the bearings are correctly mounted, the permissible axial load can be calculated from

Fap = 0,003 B d

where

Fap = maximum permissible axial load [kN]

B = bearing width [mm]

d = bearing bore diameter [mm]


Equivalent dynamic bearing load P = Fr + Y1Fa when Fa/Fr = e

P = 0,67 Fr + Y2Fa when Fa/Fr > e

The values of the calculation factors e, Y1 and Y2 can be found in the product tables.


Equivalent static bearing load

P0 = Fr + Y0Fa

The value of the calculation factor Y0 can be found in the product tables.


Supplementary designations

The designation suffixes used to identify certain features of SKF spherical roller bearings are explained in the following. The suffixes used to identify bearing (and cage) design, e.g. CC or E, are not included here as they are explained in the section "Open bearings".

C2 Radial internal clearance smaller than Normal C3 Radial internal clearance greater than Normal C4 Radial internal clearance greater than C3 C5 Radial internal clearance greater than C4 C08 Heightened running accuracy to ISO tolerance class 5 C083 C08 + C3 C084 C08 + C4 2CS Sheet steel reinforced contact seal of acrylonitrile-butadiene rubber (NBR) on both sides of the bearing. Annular groove and three lubrication holes in the outer ring covered with a polymer band. Lubricated with an extreme pressure bearing grease according to the table 2CS2 Sheet steel reinforced contact seal of fluoro rubber (FKM) on both sides of the bearing. Annular groove and three lubrication holes in the outer ring; covered with a polymer band. Lubricated with a polyurea high-temperature grease 2CS5 Sheet steel reinforced contact seal of hydrogenated acrylonitrile-butadiene rubber (HNBR) on both sides of the bearing. Otherwise as 2CS2 HA3 Case-hardened inner ring K Tapered bore, taper 1:12 K30 Tapered bore, taper 1:30 P5 Dimensional and running accuracy to ISO tolerance class 5 P6 Dimensional and running accuracy to ISO tolerance class 6 P62 P6 + C2 VA405 Bearings for vibratory applications with surface hardened cages VA406 VA405 and PTFE-coated bore VE552(E) Outer ring with three equally spaced threaded holes in one side face to accommodate hoisting tackle; the E indicates that appropriate eye bolts are supplied with the bearings VE553(E) As VE552 but with threaded holes in both side faces VG114 Surface hardened pressed steel cage VQ424 Running accuracy better than C08 VT143 Grease fill with an extreme pressure grease according to the table W Without annular groove and lubrication holes in outer ring W20 Three lubrication holes in outer ring W26 Six lubrication holes in inner ring W33 Annular groove and three lubrication holes in outer ring W64 Solid Oil filling W77 Plugged W33 lubrication holes W513 W26 + W33 235220 Case-hardened inner ring with helical groove in bore


Mounting bearings with a tapered bore

Bearings with a tapered bore are always mounted with an interference fit. The reduction in radial internal clearance, or the axial displacement of the inner ring on its tapered seat is used as a measure of the degree of interference.

Suitable methods for mounting spherical roller bearings with tapered bore are:

  • Measuring the clearance reduction.
  • Measuring the lock nut tightening angle.
  • Measuring the axial drive-up.
  • Measuring the inner ring expansion.

Small bearings with a bore diameter up to 100 mm can be properly mounted by measuring the lock nut tightening angle.

For larger bearings the SKF Drive-up Method is recommended. This method is more accurate and takes less time than the procedure based on clearance reduction or the lock nut tightening angle. Measuring the inner ring expansion, i.e. applying the SKF SensorMount® Method, enables large size bearings to be mounted simply, quickly and accurately, since a sensor is integrated into the bearing inner ring.

Detailed instructions for a selected bearing can be found at skf.com/mount.