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Double Row Full Complement Cylindrical Roller Bearings

Double row full complement cylindrical roller bearings (fig) incorporate a maximum number of rollers and are therefore suitable for very heavy radial loads. However, they cannot operate at the same high speeds as caged cylindrical roller bearings. SKF double row full complement cylindrical roller bearings are produced as standard in four designs, three open designs and one sealed. All the bearings are non-separable and have an annular groove and three lubrication holes in the outer ring to facilitate efficient lubrication.

NNCL designNNCL design bearings (fig) have an inner ring with three integral flanges and a flangeless outer ring. A retaining ring, inserted in the outer ring between the roller rows, keeps all bearing components together. Axial displacement of the shaft relative to the housing in both directions can be accommodated within the bearing. The bearings are therefore suitable for non-locating bearing positions.

NNCF designNNCF design bearings (fig) have three integral flanges on the inner ring and one integral flange on the outer ring enabling the bearing to provide axial location for a shaft in one direction. A retaining ring is inserted in the outer ring at the side opposite the integral flange and serves to hold the bearing together.

NNC designNNC design bearings (fig) are equipped with the same inner ring as bearings of the NNCL and NNCF design. The outer ring is split and held together by retaining elements, which should not be loaded axially. Both parts of the outer ring are identical and carry one integral flange, enabling the bearing to locate the shaft axially in both directions.

NNF designNNF design bearings (fig) in the NNF 50 and 3194(00) series are always sealed on both sides and filled with grease. The two-piece inner ring has three integral flanges and is held together by a retaining ring. The outer ring has an integral central flange. The bearings can be used to locate a shaft axially in both directions. Because of the large distance between the two rows of rollers, the bearings are also able to accommodate tilting moments.

The outer ring of an NNF bearing is 1 mm narrower than the inner ring and has two snap ring grooves in the outside diameter. Therefore it is possible to eliminate the need for spacer rings between the inner ring and adjacent components, for example, in rope sheaves (fig).

The bearings have a contact seal on both sides. The seals are retained on the inner ring shoulders, to provide efficient sealing at this position. The outer sealing lip exerts a slight pressure on the outer ring raceway. The operating temperature range for the seals is 40 to +80 C.

The bearings are filled with greases, which have good rust inhibiting properties (table).

Under certain conditions, sealed NNF design bearings are maintenance-free, but if they operate in a moist or contaminated environment, or if speeds are moderate to high, they must be relubricated. This can be done through the inner as well as the outer ring.

If bearings with one or no seals are required, the seals may be removed easily with a screwdriver. For applications where oil lubrication is to be used, the bearings can be delivered without seals and grease if economic quantities are involved. Otherwise the seals should be removed and the bearings washed before use. If oil lubrication is used, the limiting speed quoted in the product tables can be increased by approximately 30%.

Special NNB designThese bearings have three integral flanges on the inner ring and a wide flangeless outer ring (fig). They are special bearings designed for non-locating bearing arrangements where large axial displacements have to be accommodated within the bearing.

DimensionsThe boundary dimensions of SKF double row full complement cylindrical roller bearings are in accordance with ISO 15:1998, except for bearings in the NNF 50 and 3194(00) series. The outer rings of NNF bearings are 1 mm narrower than specified for the ISO Dimension Series 50. The dimensions of series 3194(00) bearings have been dictated by practical application requirements and are not covered by any international or national standard.

TolerancesDouble row full complement cylindrical roller bearings are manufactured to Normal tolerances as standard. The tolerances are in accordance with ISO 492:2002.

Internal clearanceDouble row full complement cylindrical roller bearings are manufactured with Normal radial internal clearance as standard. Bearings with the larger C3 or smaller C2 radial internal clearance can be supplied on request.

The clearance limits are in accordance with ISO 5753:1991. The clearance limits apply to unmounted bearings under zero measuring load.

The axial internal clearance of NNC and NNF design bearings, which can axially locate the shaft in both directions, is 0,1 to 0,2 mm for all sizes.

Axial displacementNNCL and NNCF design bearings can accommodate axial displacement of the shaft with respect to the housing as a result of thermal expansion of the shaft within certain limits (fig).

As the axial displacement is accommodated within the bearing and not between the ring and shaft or housing bore, there is practically no additional friction when the bearing rotates. Values for the permissible axial displacement from the normal position of one bearing ring in relation to the other are provided in the product table.

MisalignmentAny angular misalignment of the outer ring relative to the inner ring of double row full complement cylindrical roller bearings produces moment loads in the bearing. The resulting increased bearing load shortens bearing service life.

Influence of operating temperature on bearing material

SKF double row full complement cylindrical roller bearings undergo a special heat treatment. They can be used at temperatures of up to +150 C.

Minimum loadTo achieve satisfactory operation, double row full complement 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 relatively high speeds (n > 0,5 times the reference speed) or are subjected to high accelerations or rapid changes in the direction of load. Under such conditions, the inertia forces of the rollers 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 raceways.

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

where

Frm = minimum radial load [kN]

kr = minimum load factor (see product data)

n = rotational speed [r/min]

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

- for open bearings nr = reference speed

- for sealed bearings use 1,3 limiting speed

dm = bearing mean diameter

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

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, together with external forces, generally exceeds the requisite minimum load. If this is not the case, the double row full complement cylindrical roller bearing must be subjected to an additional radial load.

Dynamic axial load carrying capacityDouble row full compliment 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. When applying axial loads to full complement cylindrical roller bearings, lubrication by oil only is recommended.

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
  • Oil lubrication only is recommended. A viscosity ratio ? = 2 is required (see the section "Lubrication conditions - the viscosity ratio ?"). If ? 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.
  • 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 C0 2/3 7,5 104 / (n (d + D)) - k2 Fr

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

?Fap = 0,35 ?Ts Vs 15 104 / (n (d + D))

where

Ar = heat emitting reference surface area in accordance with ISO 15312:2003

= p B (D + d) [mm2]

Fap = permissible axial load [kN]

?Fap = 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]

?Ts = temperature difference between oil inlet and outlet [C]

Vs = oil flow through the bearing [l/min]

k1 = a factor

= 0,35

k2 = a factor

= 0,1

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 strengthTo 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 abutmentsIn applications where double row full compliment 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 (fig). The recommended shaft abutment diameter das can be obtained from the product table.

Equivalent dynamic bearing load

For non-locating bearings

P = Fr

If double row full complement cylindrical roller bearings 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 = 0,15

P = 0,92 Fr + 0,4 Fa when Fa/Fr > 0,15

Since axially loaded double row full complement cylindrical roller bearings only operate satisfactorily when they are subjected to a simultaneously acting radial load, the ratio Fa/Fr should not exceed 0,25.

Equivalent static bearing load

P0 = Fr

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

DesignationBearing Features
ADAModified snap ring grooves in the outer ring; two-piece inner ring held together by a retaining ring
ADBModified internal design, modified snap ring grooves in the outer ring; two-piece inner ring held together by a retaining ring
CVModified internal design, full complement roller set
C2Radial internal clearance smaller than Normal
C3Radial internal clearance greater than Normal
DAModified snap ring grooves in the outer ring; two-piece inner ring held together by a retaining ring
HB1Bainite hardened inner and outer rings
L4BBearing rings and rolling elements with special surface coating
L5BRolling elements with special surface coating
2LSContact seal of polyurethane (AU) on both sides of the bearing
VFull complement of rollers (without cage)


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.