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Combined Needle Roller / Thrust Ball Bearings

Needle roller / thrust ball bearings combine a radial needle roller bearing and a thrust ball bearing.

SKF supplies these bearings in the following two series:

  • NX series (fig), a needle roller bearing with a full complement thrust ball bearing
  • NKX series (fig), a needle roller bearing with a thrust ball bearing with a cage

SKF supplies the bearings in both series without an inner ring. However, for applications where it is not possible to harden and grind the shaft, the needle roller bearing 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 / thrust ball bearings, NX series

SKF needle roller / full complement thrust ball bearings in the NX series are suitable for applications where there are moderate radial loads and lighter, single direction axial loads. Their extremely low sectional height enables shaft centrelines to be positioned close together as is the case for multi-spindle drilling machines.

For axial support, the bearings may be mounted against a shoulder or snap ring in the housing bore. The snap ring groove in the outer ring provides a cost-effective and axially space-saving arrangement (fig). The appropriate snap rings are listed in the product tables.

Needle roller / thrust ball bearings in the NX series have a stamped steel cover that extends over the shaft washer of the thrust bearing and is firmly attached to the radial needle roller bearing. These bearings are therefore non-separable. Bearings in the NX series have lubrication holes in the cover (fig) and are intended for oil lubrication.

Bearings in the NX .. Z series (fig) should be grease lubricated. SKF supplies the thrust ball bearing in this series filled, as standard, with a high-quality grease with 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.

Combined needle roller / thrust ball bearings, NKX series

SKF needle roller / thrust ball bearings in the NKX series combine a radial needle roller bearing and a thrust ball bearing based on the 511 series. They are able to support single-direction axial loads in addition to radial loads. They also permit relatively high-speed operation.

NKX series

NKX series needle roller / thrust ball bearings (fig) do not have a cover around the thrust ball bearing. As a result, both the ball and cage assembly and shaft washer can be mounted separately from the rest of the bearing.

NKX .. Z series

NKX .. Z series needle roller / thrust ball bearings (fig) are non-separable. They have a stamped steel cover that extends over the shaft washer of the thrust ball 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 thrust ball 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 needle roller / thrust ball bearings in the NKX series are in accordance with DIN 5429-1:2005-08.

The dimensions of bearings in the NX and NX .. Z series are not standardized, but are common in the market.

TolerancesSKF needle roller / thrust ball 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 Normal axial runout values are in accordance with 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 / thrust ball 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 / thrust ball 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.

MisalignmentSKF needle roller / thrust ball bearings in the NK and NKX series cannot tolerate any angular misalignment between the shaft and the housing or any misalignment between the shaft and the axial support surfaces in the housing.

CagesThe radial needle roller bearing is fitted as standard with a machined steel cage (fig) or a sheet steel cage (fig), no designation suffix. However, some small bearings are fitted as standard with an injection moulded cage of glass fibre reinforced polyamide 66 (fig), designation suffix TN.

The cage, that retains the balls of the thrust ball bearing (NKX series), is made of sheet steel (fig).

NoteNeedle roller / thrust ball bearings with a polyamide 66 cage 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 bearing arrangements that operate at continuously high temperatures or under arduous conditions, SKF recommends using needle roller / thrust ball bearings with steel cages.

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 a needle roller / thrust ball bearing 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 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 shoulder or snap ring.

Raceways on shaftsIf 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 raceway.

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 / thrust ball bearings without an inner ring.

Lubrication

To facilitate efficient lubrication, the needle roller bearing has an annular groove with one lubrication hole in the outer ring.

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

Bearings in the NKX and NX 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 axial and radial components must be calculated separately. The lower of the two intervals should be used. A calculation is needed for both oil and grease lubrication.

For additional information about greases, refer to the section Lubrication

Application recommendationsSKF needle roller / thrust ball bearings can locate a shaft axially in one direction only. 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 thrust ball bearings with cup springs (belleville washers). This elastic preload helps to avoid skidding of the balls when one of the thrust bearings is unloaded and facilitates proper and quiet performance of the thrust ball bearing.

Minimum loadTo achieve satisfactory operation, needle roller / thrust ball 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 thrust ball 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 / thrust ball 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 thrust ball 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 thrust ball 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 thrust ball bearing separately for needle roller / thrust ball bearings.

Supplementary designations

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

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


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