Koyo NK8/12 machined needle roller bearing, 8 x 15 x 12 mm

These Koyo NK machined needle roller bearings are heavy-duty bearings with a machined and ground outer ring. NK machined needle roller bearings have no inner ring, so the shaft acts as the inner raceway, or a separate inner ring is used where the shaft cannot be hardened and ground. Heavy-duty needle roller bearings have a machined and ground channel-shaped outer ring, a complement of needle rollers and a high-strength cage that retains and guides the rollers. The thick outer ring gives maximum load capacity and shock resistance with a relatively small radial cross section, and the design suits applications that need high speed capability and good lubricant retention.

Whenever the shaft can be used as the inner raceway, a bearing without an inner ring gives economy and close control of radial internal clearance in operation. A steel cage retains and guides the needle rollers, and the design gives roller stability and minimises friction between the cage and the rollers.

The range has 26 sizes with a roller complement bore diameter Fw of 8 to 110 mm. Koyo bearings are made by JTEKT Corporation.

Where this product is used

• Gear pumps
• Sheaves
• Automotive transmissions
• Two-cycle engines

Features

• Thick machined and ground outer ring: maximum load capacity and shock resistance in a relatively small radial cross section
• Cage retains and guides the needle rollers for optimum speed and lubricant retention capability
• No inner ring: the shaft acts as the inner raceway, with close control of radial internal clearance in operation
• Basic dynamic load rating C 4.57 to 118 kN and basic static load rating C0 4.89 to 254 kN
• Grease lubrication speed rating 2,500 to 26,000 min-1 and oil lubrication 3,800 to 41,000 min-1

Key specifications

Available sizes

Selecting the right option

Choose the size from the shaft or inner ring outside diameter (the roller complement bore diameter Fw), the housing bore (outside diameter D) and the width C.

Use the NKJ series, which includes an inner ring, where the shaft cannot be finished to raceway quality, or the NKS series where a lubrication hole in the outer ring is needed.

The Koyo catalogue lists most sizes in this range with an A suffix (modified internal design). Where the variant name differs from the catalogue designation, the variant specification shows the catalogue designation.

Needle roller bearings accommodate radial loads only. The maximum dynamic radial load P should be less than Cr/3, where Cr is the basic dynamic load rating, and the static radial load P0 is equal to the radial load Fr.

Fitting and use

Mount the bearing in the housing with a clearance fit if the load is stationary relative to the housing, or with a tight transition fit if the load rotates relative to the housing. For the caged bearings in this range the catalogue suggests housing bore tolerance H7 (J7) for a stationary load, K7 for general work needing larger clearance, and N7 where the load rotates relative to the housing.

For the shaft raceway, the catalogue suggests h6 (h5) where the load is stationary relative to the housing, g6 for general work with larger clearance, and f6 where the load rotates relative to the housing.

Select the bearing internal clearance to suit the operating conditions. The metric series follows the normal tolerance class of ISO 1206, with P6 and P5 classes available on request, and tolerance class F6 is the normal specification for the roller complement bore diameter of an unmounted bearing. Radial internal clearance is mostly the normal (C0) group of ISO 5753, and C2, C3 and C4 groups can be made available on request.

Locate the outer ring axially with housing shoulders or other positive means. The bearing rings should fit closely against the shaft and housing shoulders and must not contact the fillet radius, and the maximum shaft or housing fillet radius should be no greater than the minimum bearing chamfer rs min given in the variant specification.

To permit mounting and dismounting of the shaft, the shoulder diameter D1 must not exceed Fw minus 0.3 mm for Fw up to 20 mm, Fw minus 0.5 mm over 20 mm up to 55 mm, Fw minus 0.7 mm over 55 mm up to 100 mm, Fw minus 1.0 mm over 100 mm up to 250 mm, and Fw minus 1.5 mm above that.

Where the shaft cannot be finished to the raceway design requirements, use a Koyo JR series inner ring, which is ground after hardening and gives an inner raceway with the hardness, surface finish and roundness the bearing needs.

For special operating conditions such as extremely heavy radial loads, shock loads, a temperature gradient across the bearing, a housing material with a different heat expansion coefficient from the bearing, or oscillating motion, the mounting dimensions may need to differ from the standard suggestions.

Related products

Also available from Koyo: the Koyo NKJ range with inner ring, the Koyo NKS range with lube hole and Koyo JR inner rings.