Needle Roller Bearing Size Chart: Dimensions, Tolerances & Selection Guide

Needle roller bearings pack a high load capacity into a radial cross-section that can be 3–4 mm thin.

That combination of small envelope and strong performance makes them the default choice in gearboxes, automotive transmissions, two-stroke engines, and hydraulic pumps — anywhere a full-size roller bearing simply won't fit.

This guide covers the complete needle roller bearing size chart across the most common metric series — HK, NK, NA, and RNA — along with tolerance classes, load ratings, and a practical selection checklist.

All dimensional data aligns with ISO 15:2017.

What Is a Needle Roller Bearing?

A needle roller bearing is a radial rolling-element bearing whose rollers have a length-to-diameter ratio of at least 4:1 — typically between 4:1 and 10:1.

Standard needle diameters range from 1.5 mm to 5 mm, whereas the rollers in a conventional cylindrical roller bearing start at 6 mm and go up from there.

That slim geometry delivers three practical advantages:

Needle bearings are not suited to axial loads unless paired with a thrust washer or combined with a thrust bearing assembly. Pure radial or combined radial-axial configurations each require different series selections — covered in Section 5.

Needle Roller Bearing Size Chart

The tables below list standard metric dimensions for the four most widely stocked series.

All values in millimetres (mm); basic dynamic load ratings (C_r) and static load ratings (C_0r) in kilonewtons (kN).

Data follows ISO 15 and INA/Schaeffler catalog conventions.

HK Series — Drawn Cup, Closed End

HK bearings have a drawn steel outer cup with one closed end.

Bore range: 3–60 mm. Wall thickness: 0.6–1.0 mm.

The series suits blind-hole housings where a shaft must seat against the cup bottom.

NK Series — Machined Ring, No Inner Ring

NK bearings use a precision-machined steel outer ring with a caged roller complement.

They run directly on a hardened (58–64 HRC) ground shaft — no inner ring.

Bore range: 10–90 mm.

Typical applications: connecting rods, rocker arms, camshaft followers.

NA Series — With Inner Ring

NA bearings include a precision inner ring, which means the shaft does not need to be hardened or ground to bearing tolerances.

This is the right choice when retrofitting onto a standard shaft, or when shaft hardness cannot be guaranteed above 58 HRC.

Bore range: 4–160 mm.

RNA Series — Without Inner Ring

RNA bearings are the outer-ring-and-cage assembly only — no inner ring.

They fit into bored housings where the shaft acts as the inner raceway.

The shaft must be hardened and ground.

Outer dimensions are identical to NA; bore (d) is larger because there is no inner ring wall.

How to Read a Needle Roller Bearing Designation

Take NK 25/20 as an example. The letters identify the series; the numbers encode the bore and width.

For HK bearings, the convention differs slightly: HK 2016 means bore = 20 mm, width = 16 mm — both encoded directly in the four-digit suffix with no slash.

For NA/RNA, the suffix follows the bearing series number (4904, 4906, etc.), which corresponds to ISO dimension series 49.

Needle Roller Bearing Tolerances

Tolerance classes follow ISO 492 for radial bearings.

Most standard catalog needle bearings are supplied to Normal (PN) class.

Precision applications — machine tool spindles, high-speed servo axes — typically require class P6 or P5.

For NK and RNA bearings where the shaft is the inner raceway, shaft dimensional tolerance should be h5 or h6 for a free fit, or k5/m5 for a tighter interference fit.

Housing bore tolerance is typically H6 for standard applications and H5 for precision work.

How to Select the Right Needle Roller Bearing

Five parameters drive the selection decision. Work through them in order.

Lubrication Requirements

Needle roller bearings have a large roller-to-raceway contact area relative to their size, which makes adequate lubrication critical.

Under-lubrication is the primary cause of premature failure in this bearing type.

• Oil lubrication is preferred for speeds above 50% of the limiting speed. ISO VG 32–68 mineral oil covers most industrial applications. For food-processing or pharmaceutical equipment, use NSF H1-registered oils.

• Grease lubrication suits lower speeds and sealed or semi-sealed housings. Fill the bearing cavity to 30–50% with an NLGI 2 lithium-complex grease; over-packing generates heat and accelerates oxidation. Relubrication intervals depend on operating temperature: at 70°C, relubricate every 6–12 months; at 100°C, every 3 months.

Frequently Asked Questions

What is the standard bore range for needle roller bearings?

The bore range depends on the series. HK drawn cup bearings start at 3 mm. NK machined-ring bearings cover 10–90 mm. NA bearings with inner ring extend from 4 mm to 160 mm, making them the widest coverage series in the metric range.

What shaft hardness is required for NK and RNA needle roller bearings?

The shaft must reach 58–64 HRC and a surface roughness of Ra ≤ 0.4 µm (equivalent to N5 or N6 on the ISO surface finish scale). A shaft that is too soft — typically below 55 HRC — will develop brinelling and spalling at the roller contact zone, cutting service life significantly.

Can needle roller bearings handle axial loads?

No — not in isolation. Standard radial needle roller bearings have no axial load capacity. For combined loading, pair a radial needle bearing with an AXK thrust washer and needle roller assembly on each side, or specify a combined needle/thrust unit from the catalog.

What is the difference between NA and RNA needle roller bearings?

Both use the same outer ring. NA includes a precision inner ring, so a standard (unhardened) shaft can be used. RNA omits the inner ring — the shaft itself acts as the inner raceway. For an identical housing bore (D), RNA has a larger effective bore (d) because there is no inner ring wall, which also gives marginally higher load capacity per unit of radial space.

How do I calculate bearing life for a needle roller bearing?

Use the ISO 281 basic rating life formula: L₁₀ = (C_r / P_r)³ × 10⁶ revolutions, where C_r is the dynamic load rating from the size chart and P_r is your equivalent dynamic bearing load. Convert to hours by dividing by (60 × operating speed in rpm). For modified life calculations incorporating lubrication viscosity ratio and contamination level, apply the a_ISO factor per ISO 281:2007.

Are needle bearing sizes interchangeable between brands?

For standard metric series (HK, NK, NA, RNA) that conform to ISO 15 and DIN 617/618, bore, OD, and width are interchangeable between compliant manufacturers. Load ratings and limiting speeds may vary slightly due to internal design differences — always verify C_r, C_0r, and speed ratings against the replacement supplier's catalog rather than assuming equivalence.