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Combined Needle Roller / Cylindrical Roller Thrust Bearings

Needle roller / cylindrical roller thrust bearings combine a radial needle roller bearing and a cylindrical roller thrust bearing based on the 811 series. SKF supplies these bearings in the following two series:

  • NKXR series (fig), cylindrical roller thrust bearing is separable
  • NKXR .. Z series (fig), non-separable bearing
  • These bearings are supplied without an inner ring. For applications where it is not possible to harden and grind the shaft, the bearings can be combined with an inner ring (fig). Appropriate inner rings are listed in the product tables and can be ordered separately.

    Combined needle roller / cylindrical roller thrust bearings, NKXR series

    SKF needle roller / cylindrical roller thrust bearings in the NKXR series (fig) are separable. Both the cylindrical roller with the cage thrust assembly, and the shaft washer can be mounted separately from the rest of the bearing.

    Combined needle roller / cylindrical roller thrust bearings, NKXR .. Z series

    SKF needle roller / cylindrical roller thrust bearings in the NKXR .. Z series (fig) are non-separable. They have a stamped steel cover that extends over the shaft washer of the cylindrical roller thrust bearing. It is firmly attached to the housing washer that is integral to the needle roller bearing outer ring. The cover makes these bearings non-separable. SKF supplies the cylindrical roller thrust bearing in this series filled, as standard, with a high-quality grease that has good rust inhibiting properties. The cover, which has no lubrication holes, forms a gap-type seal with the shaft washer to retain the grease in the bearing.

    DimensionsThe boundary dimensions of the single direction needle roller / cylindrical roller thrust bearings are in accordance with DIN 5429-1:2005-08.

    TolerancesSKF needle roller / cylindrical roller thrust bearings are manufactured to the tolerances and tolerance classes listed in the table. The radial runout values are in accordance with ISO 492:2002 and the axial runout values are in accordance with Normal tolerance class of ISO 199:2005.

    The values for the deviations from the nominal diameter of the ISO tolerance classes F6 and E8 are listed in the table.

    Internal clearanceSKF needle roller / cylindrical roller thrust bearings combined with the appropriate inner ring, as listed in the product tables, have Normal radial internal clearance as standard. The clearance values are listed in the table and are in accordance with ISO 5753-1:2009. They are valid for unmounted bearings under zero measuring load.

    The radial internal clearance for SKF needle roller / cylindrical roller thrust bearings without an inner ring depends on the shaft tolerance. If the recommended k5 shaft tolerance class is applied, the tolerance for the inside diameter Fw provides an internal clearance that is slightly less than the Normal radial internal clearance.

    MisalignmentNeedle roller / cylindrical roller thrust bearings cannot accommodate any angular misalignment of the shaft relative to the housing, or between the axial support surfaces on the shaft or in the housing.

    CagesThe radial needle roller bearing is fitted as standard with a machined steel cage (fig) or a sheet steel cage (fig).

    The cage that retains the rollers of the cylindrical roller thrust bearing is made of glass fibre reinforced polyamide 66 (fig).

    NoteNeedle roller / cylindrical roller thrust bearings should not be operated at temperatures above 120 C.

    The lubricants generally used for rolling bearings do not have a detrimental effect on cage properties. Exceptions are a few synthetic oils and greases with a synthetic oil base and lubricants containing a high proportion of EP additives when used at high temperatures.

    For detailed information about the temperature resistance and the applicability of cages, refer to the section Cage materials.

    Design of associated componentsThe diameter of the support surface for the integral housing washer of needle roller / cylindrical roller thrust bearings must always be at least 0,5 mm larger than the dimension D1 or D2 (fig) to obtain a clearance fit. The values for the diameters D1 and D2 are listed in the product table.

    Shaft and housing tolerancesSKF recommends machining the shaft seat for needle roller / cylindrical roller thrust bearings to a k5 tolerance class for bearings with or without an inner ring.

    For the housing bore seat, SKF recommends a K6 tolerance class. For stiff bearing arrangements, SKF recommends a M6 housing bore tolerance class.

    In addition to their interference fit on the shaft and in the housing, the bearing rings must be axially located by a shaft shoulder or locating ring.

    Raceways on the shaftIf the load carrying capacity of a radial needle roller bearing in applications where the shaft is used as raceway is to be fully exploited, it should have the same hardness and accuracy as a bearing surface.

    The running accuracy of the bearing arrangement also depends on the accuracy of the raceways. For additional information about accuracy, suitable materials, surface roughness and hardness, refer to the section Raceways on shafts and in housings.

    SKF recommends machining the shaft seat to a k5 tolerance class for needle roller / cylindrical roller thrust bearings without an inner ring.

    LubricationTo promote efficient lubrication, SKF needle roller / cylindrical roller thrust bearings have an annular groove with lubrication holes in the outer ring of the needle roller bearing.

    The shielded thrust bearing of needle roller / cylindrical roller thrust bearings, in the NKXR .. Z series, is filled at the factory with a high-quality grease with good corrosion inhibiting properties. Technical specifications for this grease are listed in the table. SKF recommends SKF LGWA 2 grease for relubrication. Characteristics of this standard grease are listed in the table.

    Bearings in the NKXR series, without the designation suffix Z, should be oil lubricated whenever possible, as oil facilitates an adequate supply of lubricant to the bearing.

    Relubrication intervals for the needle roller bearing and cylindrical roller thrust bearing must be calculated separately. The lower of the two intervals should be used. A calculation is necessary for both oil and grease lubrication.

    For additional information, refer to the section Lubrication.

    Application recommendationsSKF needle roller / cylindrical roller thrust bearings can locate a shaft axially in one direction. For short shafts, where changes in length as a result of thermal expansion are relatively minor, two bearings can be used in a back-to-back (mirrored) arrangement (fig). In these cases, SKF recommends elastically preloading the cylindrical roller thrust bearings with cup springs (belleville washers). This elastic preload helps to avoid skidding of the rolling elements when one of the thrust bearing is unloaded. Elastic preload also reduces the noise level of the cylindrical roller thrust bearing.

    Minimum loadTo achieve satisfactory operation, needle roller / cylindrical roller thrust bearings, like all ball and roller bearings, must always be subjected to a requisite minimum load acting in both the radial and axial directions. Applying a requisite minimum load is especially important for bearings that operate at high speeds or are subjected to high accelerations or rapid changes in the direction of load. Under these conditions, not applying the correct minimum load can result in damaging sliding movement between the rollers and raceways caused by the inertial forces of the rolling elements and cages, and friction in the lubricant.

    The requisite minimum radial load to be applied to the radial needle roller bearing can be estimated using

    Frm = 0,02 C

    The requisite minimum axial load for the cylindrical roller thrust bearing can be obtained using

    where

    Frm = minimum radial load [kN]

    Fam = minimum axial load [kN]

    A = minimum load factor

    C = basic dynamic load rating [kN]

    n = rotational speed [r/min]

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

    Equivalent dynamic bearing load

    The equivalent dynamic bearing load is equal to the calculated bearing load for the radial as well as the axial part of the combined bearing.

    Therefore, for the radial needle roller bearing

    P = Fr

    and for the cylindrical roller thrust bearing

    P = Fa

    where

    P = equivalent dynamic bearing load [kN]

    Fr = actual radial load [kN]

    Fa = actual axial load [kN]

    Equivalent static bearing load

    The equivalent static bearing load is equal to the calculated bearing load for the radial as well as the axial part of the combined bearing.

    Therefore, for the radial needle roller bearing

    P0 = Fr

    and for the cylindrical roller thrust bearing

    P0 = Fa

    where

    P0 = equivalent static bearing load [kN]

    Fr = actual radial load [kN]

    Fa = actual axial load [kN]

    Load carrying capacity and lifeIt is necessary to calculate the life of the radial needle roller bearing and the cylindrical roller thrust bearing separately for needle roller / cylindrical roller thrust bearings.

    Supplementary designationThe designation suffixes used to identify certain features of SKF needle roller / cylindrical roller thrust bearings are explained in the following:

    Filled at the factory with high-quality lithium base NLGI 2 grease.
    DesignationBearing Features
    ZCylindrical roller thrust bearing with a cover over the outside diameter without lubrication holes.


    Combined needle roller bearings | Combined needle roller / angular contact ball bearings | Combined needle roller / thrust ball bearings | Combined needle roller / cylindrical roller thrust bearings

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