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Paired Single Row Tapered Roller Bearings

For bearing arrangements where the load carrying capacity of a single tapered roller bearing is inadequate, or where the shaft has to be axially located in both directions with a given positive or negative axial play, the bearings listed in the section "Single row tapered roller bearings", can be supplied as matched pairs (fig 1) arranged

  • face-to-face (fig)
  • back-to-back (fig)
  • in tandem (fig).

Matched bearing sets provide an economic solution to many bearing arrangement problems and offer many advantages, including

  • simple mounting, since calibration of intermediate rings is not required, so that mounting errors are avoided
  • exact axial location of the shaft; the axial play is determined during manufacture
  • high radial and axial load carrying capacity
  • simple maintenance; the lubricant can be introduced via the annular groove and lubrication holes in the intermediate ring.

SKF can supply matched bearing sets in the arrangements described in the following. The bearing pairs shown in the product table are only part of the comprehensive SKF programme. Other bearing sets can be supplied to order.

Matched bearing sets

Face-to-face arrangementIn bearing pairs where the bearings are matched face-to-face, an intermediate ring is positioned between the two outer rings (fig) so that production is relatively simple. In face-to-face arrangements, the load lines converge towards the bearing axis. Axial loads acting in both directions can be accommodated by each bearing in one direction.

Back-to-back arrangementIn bearing pairs where the bearings are arranged back-to-back (fig) an intermediate ring is positioned both between the two inner rings and between the two outer rings. This is a more expensive production than required for the face-to-face sets. In back-to-back arrangements, the load lines diverge towards the bearing axis, thus providing relatively rigid bearing arrangements, which can also take up tilting moments. Axial loads acting in both directions can be accommodated by each bearing in one direction.

Tandem arrangementBearing pairs where the bearings are arranged in tandem are seldom used and also require an intermediate ring between both inner rings and both outer rings (fig). Because the load lines of the two bearings are in parallel, radial and axial loads will be equally distributed over the two bearings. The bearing pair can only accommodate axial loads acting in one direction and should be adjusted against a third bearing that can accommodate the axial loads acting in the opposite direction.

DimensionsThe boundary dimensions of the individual bearings with series designations of a set are in accordance with ISO 355:1977.

Bearings

  • paired face-to-face
  • paired back-to-back
  • paired in tandem

TolerancesThe matched bearing sets are made to Normal tolerances as for the single bearings. The values for the Normal tolerances conform to ISO 492:2002 and are listed in table T10. The tolerance for the total width of the set, although not standardized, can be found in table 1. In the table ? TsD designates the deviation of a single total abutment width of a bearing pair from the nominal.

Axial internal clearanceThe bearing sets of standard metric bearings are produced with the axial internal clearance provided in table 2 as standard. The values in the table apply to bearing pairs before they are mounted under measuring loads of

  • 0,1 kN for bearings with outside diameter D = 90 mm
  • 0,3 kN for bearings with outside diameter 90 < D = 240 mm
  • 0,5 kN for bearings with outside diameter D > 240 mm.

Matched bearing sets having a clearance other than the standard value are identified by the designation suffix C followed by a two or three-figure number which gives the mean axial internal clearance in Ám. The range of the special clearance is, however, the same as for the standard clearance, i.e. for the bearing set 32232 J2/DFC230, which has a mean axial internal clearance of 230 Ám, the clearance will lie in the range 200 to 260 Ám.

MisalignmentAny misalignment of the outer rings relative to the inner rings of matched bearing pairs can only be accommodated between the rollers and raceways by force. The increased stress in the bearing caused by misalignment should be avoided. If misalignment cannot be avoided, SKF recommends using the less rigid face-to-face arrangement.

CagesSKF single row tapered roller bearings that are matched in bearing sets, are fitted with

  • a pressed window-type steel cage, roller centred (fig), or
  • a steel pin-type cage (fig).

Minimum loadTo achieve satisfactory operation, paired tapered 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 to be subjected to high accelerations or rapid changes in the direction of load. Under such conditions the inertia forces of the rollers and cages, 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 radial load to be applied to matched pairs of SKF standard bearings can be estimated from

Frm = 0,02 C

and for matched pairs of SKF Explorer bearings from

Frm = 0,017 C

where

Frm = minimum radial load for a bearing pair [kN]

C = basic dynamic load rating of a bearing pair [kN] (see product data)

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

Equivalent dynamic bearing load

For bearing pairs arranged face-to-face or back-to-back

P = Fr + Y1Fa when Fa/Fr = e

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

and for bearing pairs arranged in tandem

P = Fr when Fa/Fr = e

P = 0,4 Fr + Y2Fa when Fa/Fr > e

Fr and Fa are the forces acting on the bearing pair. Values for the calculation factors e, Y1 and Y2 are provided in the product tables.

When determining the axial force for bearing pairs arranged in tandem reference should be made to the section "Single row tapered roller bearings - Determination of axial forces".

Equivalent static bearing load

For bearing pairs arranged face-to-face or back-to-back

P0 = Fr + Y0Fa

and for bearing pairs arranged in tandem

P0 = 0,5 Fr + Y0Fa

When P0 < Fr, P0 = Fr should be used. Fr and Fa are the forces acting on the bearing pair. Values of the calculation factor Y0 are provided in the product tables.

When determining the axial force for bearing pairs arranged in tandem reference should be made to the section "Single row tapered roller bearings - Determination of axial forces".

Supplementary designationsThe designation suffixes used to identify certain features of SKF paired single row tapered roller bearings are explained in the following.

DesignationBearing Features
CL7CHigh-performance design for pinion bearing arrangements
C...Special clearance. The two or three-figure number immediately following the C gives the mean axial internal clearance in Ám
DBMatched bearing pair arranged back-to-back. A figure combination immediately following the DB identifies the design of the intermediate rings
DFMatched bearing pair arranged face-to-face. A figure combination immediately following the DF identifies the design of the intermediate ring
DTMatched bearing pair arranged in tandem. A figure combination immediately following the DT identifies the design of the intermediate rings
HA1Case-hardened inner and outer rings
HA3Case-hardened inner ring
JPressed window-type steel cage. A figure following the J indicates a different cage design
QOptimized contact geometry and surface finish
TT, followed by a figure, identifies the total width of bearing pairs arranged back-to-back or in tandem
XBoundary dimensions changed to conform to ISO

Fits for bearing pairsThe values of axial internal clearance provided in table 2 are so dimensioned that if the bearings are mounted on shafts machined to

  • m5 for shaft diameters up to and including 140 mm
  • n6 for shaft diameters over 140 mm and up to and including 200 mm
  • p6 for shaft diameters above 200 mm

an appropriate operational clearance will be obtained. These shaft seat tolerances are recommended where loads are moderate to heavy and rotating loads apply for the inner ring. If tighter fits are selected, it is necessary to check that the bearings do not become pinched or clamped.

For stationary outer ring load, the recommended housing bore tolerance is J6 or H7.

Determining the load acting on bearing pairs

If matched pairs of tapered roller bearings arranged face-to-face or back-to-back are mounted together with a third bearing, the bearing arrangement is statically indeterminate. In these cases the size of the radial load Fr acting on the bearing pair must first be determined.

Bearing pairs arranged face-to-face

For bearing pairs where the two bearings are arranged face-to-face (fig) it can be assumed that the radial load will act at the geometric centre of the bearing set as the distance between the pressure centres of the two bearings is short compared with the distance between the geometric centres of the set and the other bearing (fig). In this case it can be assumed that the bearing arrangement is statically determined.

Bearing pairs arranged back-to-back

The distance between the pressure centres of two bearings arranged back-to-back in a matched set (fig) is large compared with the distance L between the geometric centres of the set and the other bearing (fig). It is therefore necessary to determine the magnitude of the load acting on the bearing pair and also the distance a1 at which the load acts. The magnitude of the radial load can be obtained from the equation

Fr = [L1/(L - a1)] Kr

where

Fr = radial load acting on bearing pair, kN

Kr = radial force acting on the shaft, kN

L = distance between the geometric centres of the two bearing positions, mm

L1 = distance between the centre of bearing position I and the point of action of the force Kr, mm

a = distance between bearing pressure centres, mm

a1 = distance between the geometric centre of the bearing set and the point of action of the radial load Fr, mm

The distance a1 can be determined using diagram 1. The distance of the pressure centres a and the calculation factor Y2 are given in the product table.


Tapered roller bearings | Single row tapered roller bearings | Paired single row tapered roller bearings | Double row tapered roller bearings | Four-row tapered roller bearings

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This page is still under construction. All bearing information remains the property and copyright of SKF Group.