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Double Row Cylindrical Roller Bearings

Double row cylindrical roller bearings have a low cross section, high load carrying capacity and high stiffness. Therefore they are mainly used in machine tools, rolling mill stands, plastic calenders, grinding mills and also large gearboxes.

SKF produces double row cylindrical roller bearings mainly in the NNU design and the NN design. The bearings are separable, i.e. the ring with integral flanges together with the roller and cage assemblies can be mounted separately from the other bearing ring or all bearing components can be mounted separately. This facilitates the mounting, inspection and maintenance of these bearings.

Double row cylindrical roller bearing, NNU design

Double row cylindrical roller bearing, NN design

Double row cylindrical roller bearings are produced with both cylindrical bore and tapered bore; bearings of the NN design are generally supplied with tapered bore. When mounting bearings with tapered bore it is possible to achieve a certain radial internal clearance or a given preload.

The bearings, except those of the NNUP design allow axial displacements between shaft and housing to take place inside the bearing itself within certain limits.

Double row cylindrical roller bearing, NNUP design

Double row cylindrical roller bearings: Design variants

SKF double row cylindrical roller bearings of the NNU, NN and NNUP designs differ mainly in the flange configuration but may also vary slightly in design for manufacturing, application or maintenance reasons, e.g. with an extended inner ring or with loose flange rings on the outer ring. The basic designs and their most important variants are described in the following.

For bearing arrangements where the bearings are to be mounted with a loose instead of an interference fit on the shaft or roll neck, e.g. to allow rapid roll changes, a special design of double row bearing is supplied by SKF. To alleviate the disadvantage of the loose fit - the inner ring turns on its seat and can cause damage to the bearing and the seat - these special bearings have a helical groove in the bore and/or lubrication grooves in the side faces of the bearing rings. These grooves enable efficient lubrication to be provided for the sliding contact surfaces.

Special designs of double row bearings

Bearings which exhibit one or more of the features named above are identified in the product table by the following letters:

DesignationFeatures
GHelical groove in inner ring bore
WLubrication grooves in the side faces of the bearing rings
WILubrication grooves in the side faces of the inner ring
WOLubrication grooves in the side faces of the outer ring
W33Annular groove and three lubrication holes in outer ring

The letters appear after the design code in the product table, e.g. NNU.2/W33GW.

NNU designOuter ring with three integral flanges. Roller guided machined double pronged cage of brass. With or without annular groove and/or lubrication holes in the outer ring.

NNU design Double row cylindrical roller bearing

NNU.1 designOuter ring with three integral flanges. Two roller guided machined brass window-type cages. With or without annular groove and/or lubrication holes in the outer ring.

NNU.1 design Double row cylindrical roller bearing

NNU.2 designOuter ring with integral central flange and two loose flange rings. Roller guided machined double pronged cage of brass. With or without annular groove and/or lubrication holes in the outer ring.

NNU.2 design Double row cylindrical roller bearing

NNU.3 designOuter ring with three integral flanges. Pierced rollers and two steel pin-type cages. With or without annular groove and/or lubrication holes in the outer ring.

NNU.3 design Double row cylindrical roller bearing

NNU.4 designOuter ring with integral central flange and two loose flange rings. Pierced rollers and two steel pin-type cages. With or without annular groove and/or lubrication holes in the outer ring.

NNU.4 design Double row cylindrical roller bearing

NNU.5 designOuter ring with two integral flanges. Double window-type cage of brass. With or without annular groove and/or lubrication holes in the outer ring.

NNU.5 design Double row cylindrical roller bearing

NNU.6 designOuter ring with two integral flanges. Pierced rollers and one double row pin-type cage of steel. With or without annular groove and/or lubrication holes in the outer ring.

NNU.6 design Double row cylindrical roller bearing

NNU.7 designOne integral flange on the outer ring and one loose flange ring. Machined brass cage (double row). With or without annular groove and/or lubrication holes in the outer ring.

NNU.7 design Double row cylindrical roller bearing

NNUB designOuter ring with three integral spananges. Extended inner ring. Roller guided double pronged cage of brass. With or without annular groove and/or lubrication holes in the outer ring.

NNUB design Double row cylindrical roller bearing

NNUB.1 designOuter ring with integral central flange and two loose flange rings. Extended inner ring. Roller guided double pronged cage of brass. With or without annular groove and/or lubrication holes in the outer ring.

NNUB.1 design Double row cylindrical roller bearing

NNUB.2 designOuter ring with two integral flanges. Extended inner ring. Double row window-type cage of brass. With or without annular groove and/or lubrication holes in the outer ring.

NNUB.2 design Double row cylindrical roller bearing

NNUB.3 designOuter ring with two integral flanges. Extended inner ring, Pierced rollers and one double-row pin-type cage of steel. With or without annular groove and/or lubrication holes in the outer ring.

NNUB.3 design Double row cylindrical roller bearing

NNUB.4 designOuter ring with three integral flanges. Extended inner ring. Two roller guided window-type cages of brass. With or without annular groove and/or lubrication holes in the outer ring.

NNUB.4 design Double row cylindrical roller bearing

NNUB.5 designOuter ring with central integral flange and two loose flange rings. Extended inner ring. Pierced rollers and two pin-type cages of steel. With or without annular groove and/or lubrication holes in the outer ring.

NNUB.5 design Double row cylindrical roller bearing

NNUP designOuter ring with three integral flanges. One integral flange and one loose flange ring on the inner ring. Roller guided double pronged cage of brass. Annular groove and lubrication holes in the outer ring.

NNUP design Double row cylindrical roller bearing

NNUP.1 designOuter ring with two integral flanges and a loose flange ring. One integral flange and one loose flange ring on the inner ring. Pierced rollers and two pin-type steel cages.

NNUP.1 design Double row cylindrical roller bearing

NN designInner ring with three integral flanges. Roller guided double pronged cages of brass. Annular groove and lubrication holes in the outer ring.

NN design Double row cylindrical roller bearing

NN.1 designInner ring with three integral flanges. Two roller guided cages of polyamide 66 or pressed steel. Annular groove and lubrication holes in the outer ring.

NN.1 design Double row cylindrical roller bearing


 

 

Other SKF double row cylindrical roller bearings

 

"Tailored" double row cylindrical roller bearings

If none of the bearings shown in the tables is suitable, it is recommended that reference be made to the SKF four-row cylindrical roller bearings to see whether it is possible to assemble a tailored double row bearing from the standard components of the four-row bearings. For example, a combination of the following components of the four-row bearing 319320

Four-row cylindrical roller bearing

1 inner ring IR-319320 (1)

1 outer ring OR-319320 (2)

2 roller and cage assemblies AR-319320 (3)

2 flange rings WG-319320 (4)

will give a double row bearing having the dimensions d D B = 480 680 210 mm. Such combinations may be advantageous in many cases from a price and availability point of view.

Bearing pairs of matched standard single row cylindrical roller bearings may also offer cost-favourable alternatives.With matched bearings, the bore and outside diameters as well as the radial internal clearance are matched in order to guarantee an even distribution of load. SKF matched pairs of single row bearings are identified by the designation suffix DR, e.g. NU 1076 MA/DR.

Bearing pair of matched standard single row cylindrical roller bearings

In case of doubt and before finalising bearing or bearing component selection it is advisable to consult the SKF application engineering service.

Double row cylindrical roller bearings: Dimensions

The boundary dimensions of the standard bearings conform to ISO 15:1998. The bore and outside diameters of most of the drawing number bearings are in accordance with Diameter Series 0 of this standard.

Double row cylindrical roller bearings: Tolerances

Standard SKF double row cylindrical roller bearings of series NN 30 and NNU 49 are usually produced with the high accuracy required for machine tools according to tolerance class SP. Suffix SP is included in the designation of these bearings in the tables. Some of these bearings are also available with even higher accuracy to tolerance class UP specifications. Details will be supplied on request.

The remaining standard bearings are produced, unless specially ordered, with Normal tolerances. Drawing number bearings are normally produced with dimensional accuracy to P6 and running accuracy to P5.

The actual values correspond to those specified in ISO 492:2002 for tolerance classes Normal, 6 and 5. Tolerance class SP specifies dimensional accuracy which corresponds approximately to tolerance class P5 and running accuracy to P4. Tolerance class UP is even more accurate, with a dimensional accuracy approximately to P4 and a running accuracy better than P4.

Double row cylindrical roller bearings: Internal clearance

Double row cylindrical roller bearings produced to SP tolerance class specifications have C1 radial internal clearance as standard although the clearance suffix is not shown in the bearing designation. The rings of similar bearings may not be mixed as otherwise the clearance may become inadmissibly large or small. For this reason, the components of a bearing are supplied as a packaged unit. If they are separately packed, the components of one bearing will carry the same serial number.

Bearings made to SP tolerance class specifications, particularly those of series NNU 49, can also be supplied with larger radial internal clearance. If bearings with greater radial internal clearance than C1 are required the bearing designation should include the clearance class identification: C2 for the special "SPC2" clearance, CN for class Normal or C3, when ordering. The radial internal clearance "SPC2" differs from the standardized C2 clearance; the range is narrower and displaced towards the smaller end of the C2 range.

The other standard bearings are produced with the radial internal clearance as stated. The radial internal clearance of the drawing number bearings is given in the product table together with the design following the bearing designation.

The values for the radial internal clearance conform to ISO 5753:1991. The values for bearings with cylindrical bore are given in table and for bearings with tapered bore in table. They are valid for bearings before mounting under zero measuring load.

Double row cylindrical roller bearings: Misalignment

Double row cylindrical roller bearings can only tolerate very slight angular misalignments of the inner ring with respect to the outer ring. For bearing arrangements, e.g. in rolling mill straightening machines, where misalignment of the inner ring in relation to the outer ring cannot be avoided, SKF can supply bearings having radial internal clearance which is adapted to the application and which is different for the two rows of rollers. In this way the load will be equally divided between the two roller rows under misaligned conditions.

Influence of operating temperature on bearing material

SKF double row cylindrical roller bearings are dimensionally stabilised by a special heat treatment so that they can be used at operating temperatures up to +150 C, without any inadmissible changes occurring in the dimensions. On request and against a surcharge, bearings stabilised for operating temperatures up to +200 C (designation suffix S1) or +250 C (suffix S2) can be supplied.

Double row cylindrical roller bearings: Cages

SKF double row cylindrical roller bearings are produced, depending on size and design, with the following cages:

  • Two cages of glass fibre reinforced polyamide 66.
  • Cage of glass fibre reinforced polyamide 66

  • Two pin-type cages of steel.
  • Pin-type cage of steel

  • Two window-type cages of brass.
  • Window-type cage of brass

  • One double pronged machined brass cage.
  • Double pronged machined brass cage

  • One double row window-type cage of brass.
  • Double row window-type cage of brass

  • One double row machined brass cage.
  • Double row machined brass cage

  • One double row pin-type cage of steel.
  • Double row pin-type cage of steel

Note

Double row cylindrical roller bearings with polyamide 66 cages can be used at operating temperatures up to +120 C. With the exception of a few synthetic oils and greases with a synthetic base oil, and lubricants containing a high proportion of EP additives when used at elevated temperatures, the lubricants generally used for rolling bearings do not have a detrimental effect on cage properties.

For further information regarding the temperature resistance and use of cages, please refer to the section Cage materials.

For bearing arrangements which have to be operated at continuously high temperatures or under arduous conditions, it is recommended that bearings incorporating metal cages be used.

Design of associated components

If bearings of the NN design, fitted with two cages of polyamide or brass or two pin-type steel cages, have to be able to accommodate axial displacements of the shaft relative to the housing, space must be provided at both sides of the bearing (fig 1). This prevents the cages from fouling adjacent components, for example, which would damage the bearing arrangement. The smallest value for the depth of this free space can be obtained from

Ca min = 1,3 s

where

Ca min= minimum depth of free space [mm]
s= permissible axial displacement from the central position (see product table) [mm]

Space at both sides of the bearing to accommodate axial displacements

To facilitate mounting and dismounting bearings with tapered bore having bore diameters of some 80 mm or larger, the use of the SKF oil injection method, where oil under high pressure is injected between the mating surfaces, is recommended. This considerably reduces the forces required and practically eliminates any risk of damaging the bearing or its seat on the shaft.

For bearings with cylindrical bore, the oil injection method is only used for dismounting.

In order to be able to apply the oil injection method it is necessary to provide the necessary oil ducts and distributor grooves.

Oil ducts and distributor grooves

Details of suitable dimensions for these as well as the thread for the connection will be found in the section Application of bearings - Designing associated components.

Minimum load

To achieve satisfactory operation, double row cylindrical 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, and the friction in the lubricant, can have a detrimental effect on the rolling conditions in the bearing arrangement and may cause damaging sliding movements to occur between the rollers and the raceways.

The requisite minimum load to be applied to double row cylindrical roller bearings can be estimated using

where

Frm= minimum radial load [kN]
kr= minimum load factor (see product data)
n= operating speed [r/min]
nr= speed rating [r/min] (see product data)
for open bearings nr = reference speed

for sealed bearings nr = 1,3 limiting speed

dm = mean diameter of bearing

= 0,5 (d + D) [mm]

When starting up at low temperatures or when the lubricant is highly viscous, even heavier loads may be required. The weights of the components supported by the bearing, together with the external forces, often exceed the requisite minimum load. If this is not the case, the double row cylindrical roller bearing must be subjected to an additional radial load.

Bearings to tolerance class SP specifications, when mounted, are under preload, so that it is normally not necessary to perform the above calculation.

Dynamic axial load carrying capacity

Double row cylindrical roller bearings with flanges on both the inner and outer rings can support axial loads in addition to radial loads. Their axial load carrying capacity is primarily determined by the ability of the sliding surfaces of the roller end / flange contact to support loads. Factors having the greatest effect on this ability are the lubricant, operating temperature and heat dissipation from the bearing.

Calculating the dynamic axial load carrying capacity

Under normal operating conditions the axial load carrying capacity can be estimated using the equations below. Conditions that are considered typical for normal bearing operation are:

  • a certain temperature
  • There is a difference of 60 C between the bearing operating temperature and the ambient temperature.

  • a specific heat loss from the bearing
  • There is a flow of 0,5 mW/mm2 C; with reference to the bearing outside diameter surface (A = p D B).

  • adequate lubrication
  • A viscosity ratio K ≥ 2 is required (see the section Lubrication conditions - the viscosity ratio K). For grease lubricated applications, the viscosity of the base oil in the grease may be used. If K is less than 2, friction and wear will increase. These effects can be reduced at low speeds, for example, by using lubricants with AW (anti-wear) or EP (extreme pressure) additives. In grease lubricated applications where axial loads act for long periods, the bearings should be relubricated frequently with a grease that has good oil bleeding properties at normal operating temperature (> 3% according to DIN 51 817).

  • sufficient radial load
  • The value of the radial load should be at least twice the value of the axial load. A lower ratio (axial versus radial load) is possible, but should be checked by the SKF application engineering service.

  • limited misalignment
  • Where misalignment between the inner and outer rings exceeds 1 minute of arc, the action of the load on the flange changes considerably. Therefore the safety factors included in the guideline values may be inadequate. In these cases, contact the SKF application engineering service.

For bearings with a heat emitting reference surface area Ar = 50 000 mm2, the permissible axial load can be calculated with sufficient accuracy from

Fap = k1 C0 104 / (n (d + D)) - k2 Fr

For bearings with a heat emitting reference surface area Ar > 50 000 mm2, the permissible axial load can be calculated with sufficient accuracy from

Fap = k1 C02/3 7,5 104 / (n (d + D)) - k2 Fr

When circulating oil lubrication provides efficient cooling, the permissible axial load can be raised by

ΔFap = k1 ΔTs Vs 15 104 / (n (d + D))

where

Ar= heat emitting reference surface area in accordance with ISO 15312:2003
=n B (D + d) [mm2]
Fap= permissible axial load [kN]
ΔFap= raise for permissible axial load due to cooling [kN]
C0= basic static load rating [kN]
Fr= actual radial bearing load [kN]
n= rotational speed [r/min]
d= bearing bore diameter [mm]
D= bearing outside diameter [mm]
B= bearing width [mm]
ΔTs= temperature difference between oil inlet and outlet [C]
Vsoil flow through the bearing [l/min]
k1= a factor
= 0,5 for oil lubrication
= 0,3 for grease lubrication
k2= a factor
= 0,05 for oil lubrication
= 0,03 for grease lubrication

The values for the permissible load Fap obtained from the equations are valid for a constant and continuous axial load provided there is an adequate supply of lubricant to the roller end / flange contacts. Where axial loads act only for short periods, the values can be multiplied by 2. For shock loads the values can be multiplied by 3, provided the following limits relative to flange strength are not exceeded. A short period can last anywhere from several seconds to a few minutes. It is characterized by a temperature spike of not more than 5 C after which time the bearing returns to normal operating temperature. As a rule of thumb, a short period is considered as the time it takes for the bearing to make 1 000 revolutions. Values calculated according to the above equations are not hard limits. If higher axial load carrying capacity than calculated is required, contact the SKF application engineering service for detailed analysis.

Axial load limit relative to flange strength

To avoid the risk of flange fracture, the constantly acting axial load applied to the bearings should never exceed

Famax = 0,0023 D1,7

where

Famax= maximum constantly acting axial load [kN]
D= bearing outside diameter [mm]

Where axial loads act only for short periods, the values for Famax can be multiplied by a factor of 2 while shock loads can be multiplied by a factor of 3.

Requirements for abutments

In applications where double row cylindrical roller bearings are subjected to heavy axial loads, axial runout and the size of the abutment surfaces of adjacent components can affect flange load and running accuracy. To obtain an even flange load and provide proper running accuracy, use the values provided in the table. For the diameter of the abutment surfaces, SKF recommends supporting the inner ring at a height corresponding to half the flange height.

Abutment diameter for proper flange support

das = 0,5 (d1 + F)

where

das= shaft abutment diameter [mm]
d1= inner ring flange diameter [mm]
F= inner ring raceway diameter [mm]

Equivalent dynamic bearing load

When double row cylindrical roller bearings are used as non-locating bearings

P = Fr

If cylindrical roller bearings of the NNUP design with flanges on both inner and outer rings are used to locate a shaft in one or both directions, the equivalent dynamic bearing load should be calculated using

P = Fr when Fa/Fr = 0,1

P = 0,92 Fr + 0,53 Fa when Fa/Fr > 0,1

Since axially loaded cylindrical roller bearings will only operate satisfactorily when they are subjected to a simultaneously acting radial load, the ratio Fa/Fr should never exceed 0,25.

Equivalent static bearing load

P0 = Fr

Supplementary designations

The designation suffixes used to identify certain features of SKF double row cylindrical roller bearings are explained in the following.

DesignationBearing Features
BPresent standard design with brass cage of double pronged type
BKB + K
CNNormal radial internal clearance; normally only used together with an additional letter that identifies a reduced or displaced clearance range

H Reduced clearance range corresponding to the upper half of the actual clearance range

L Reduced clearance range corresponding to the lower half of the actual clearance range

M Reduced clearance range corresponding to the two middle quarters of the actual clearance range

P Reduced clearance range corresponding to the upper half of the actual clearance range plus the lower half of the next greater clearance range

The above letters are also used together with the clearance class suffixes C2, C3, C4 and C5

C1Radial internal clearance smaller than C2
C2Radial internal clearance less than Normal
C3Radial internal clearance greater than Normal
C4Radial internal clearance greater than C3
C5Radial internal clearance greater than C4
FRoller-centred machined steel cage of double pronged type
FAOuter ring centred machined steel cage of double pronged type
FBInner ring centred machined steel cage of double pronged type
HA1Case hardened inner and outer rings
HA2Case hardened outer ring
HA4Case hardened rollers, inner and outer rings
HB1Bainite hardened inner and outer rings
HB2Bainite hardened outer ring
HB3Bainite hardened inner ring
KTapered bore, taper 1:12
KTN9K + TN9
K30Tapered bore, taper 1:30
MTwo-piece machined brass cage, roller centred
P4Dimensional and running accuracy to ISO tolerance class 4
P5Dimensional and running accuracy to ISO tolerance class 5
P6Dimensional and running accuracy to ISO tolerance class 6
SPSpecial precision class for machine tool bearings; dimensional accuracy approximately to P5, running accuracy approximately to P4
SPCNSP + CN
SPC2SP + C2
TN9Injection moulded cage of glass fibre reinforced polyamide 66
UPUltra precision class for machine tool bearings; dimensional accuracy approximately to P4, running accuracy better than P4
VE900Preliminary standard design
W33Annular groove and three lubrication holes in outer ring
W33XAnnular groove and six lubrication holes in outer ring

Mounting bearings with a tapered bore

See Mounting bearings with a tapered bore


Cylindrical roller bearings | Single row cylindrical roller bearings | Double row cylindrical roller bearings | Multi-row cylindrical roller bearings | Single row full complement cylindrical roller bearings | Double row full complement cylindrical roller bearings | Multi-row full complement cylindrical roller bearings | Split cylindrical roller bearings

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