Machined ring needle roller bearings use a turned, through-hardened outer ring that holds its shape under load far better than a thin drawn cup. The series covers NK and RNA49 without an inner ring, NKI and NA49 with one, and double-row NA69 for heavier radial loads. Because the ring wall is thicker, these bearings tolerate housing bores that are less than perfect and can be located axially in both directions. Choosing between them comes down to one question: can your shaft serve as the raceway? If the shaft is hardened and ground, NK saves radial space. If it is soft or cannot be reworked, NKI is the safer specification.

Machined Ring Needle Roller Bearings

Machined ring needle roller bearings use a turned, through-hardened outer ring instead of a thin drawn cup. That one difference gives them higher radial rigidity, positive axial location in both directions, and a higher load rating than a drawn cup bearing in a similar envelope. They are the right specification when the housing cannot act as a raceway, when the bearing has to be clamped between shoulders, or when the shaft carries high radial load at moderate speed. This page covers the NK, NKI, NA49 and NA69 variants, how the designations decode, and the four checks that decide whether a bearing survives its first year in service.

What Machined Ring Needle Roller Bearings Are

A machined ring bearing is built around an outer ring turned from bearing steel, through-hardened to 58–62 HRC, and then ground on both the raceway and the outside diameter. Inside sits a cage that retains a full set of cylindrical rollers, typically 2 to 5 mm in diameter and three to ten times longer than they are wide. The long rollers contact the raceway along a line rather than a point, which is why a bearing only 16 mm wide can carry the radial load of a much larger ball bearing.

The distinction from a drawn cup bearing is structural, not cosmetic. A drawn cup is stamped from strip steel and relies on the housing bore to keep its shape; press it into a soft or out-of-round housing and the raceway deforms with it. A machined ring keeps its own geometry. That matters in cast aluminium gearbox housings, in pump bodies, and anywhere the bore tolerance is H7 rather than a ground fit. The trade-off is cost and radial space: the thicker ring wall costs 2–4 mm of envelope and roughly 30–50% more per piece.

NK, NKI, NA49 and NA69: Which Variant Fits Your Shaft

Every variant in this family answers the same underlying question — whether the shaft itself can serve as the inner raceway. That single decision drives cost, mounting difficulty and service life more than any other specification on the drawing.

Without an inner ring: NK and RNA49

In an NK or RNA49 bearing the shaft is the inner raceway. The bearing is narrower for a given load, cheaper, and gives the highest possible radial capacity inside a fixed envelope. The price is a demanding shaft: 58–62 HRC through-hardened or case-hardened to an effective depth of at least 0.3 mm, ground to Ra ≤ 0.2 µm, with roundness and cylindricity held inside the bearing's own tolerance band. If the shaft cannot meet that, the raceway will brinell and the bearing will fail early — the bearing gets blamed, but the shaft caused it.

With an inner ring: NKI and NA49

An NKI or NA49 bearing ships with a hardened, ground inner ring, so the shaft only has to carry torque and locate the ring — a 45 steel shaft at 220–250 HB is usually sufficient. This is the safer specification for repair and retrofit work, for shafts that are already induction hardened to a softer level, and for assemblies where the shaft cannot be removed for grinding. The inner ring adds width and cost, and removes a small amount of radial capacity, which is rarely the limiting factor in practice.

Double row: NA69 and RNA69

Double-row variants place two rows of needles in the same ring, roughly doubling the radial load rating within a wider envelope. They are specified for gearbox intermediate shafts, axle pinions and other positions where a single row would need an uneconomically long bearing. Axial location is better than a single row, but permissible misalignment is tighter, so the housing and shaft shoulders have to be square within about 0.05 mm.

Heavy duty: NKS and NKIS

NKS and NKIS use thicker rings and larger needles to raise the rating within the same bore. They appear in agricultural PTO shafts, forklift masts and construction equipment, where shock loading rather than steady load governs the design. If your application involves repeated impact, start here rather than sizing a standard NK up.

Reading the Designation: Bore, Width and Series

The designation tells you almost everything, and misreading it is the most common cause of a wrong order. In the NK series, the number before the slash is the shaft or raceway diameter (Fw) in millimetres and the number after the slash is the ring width. So NK20/16 is a 20 mm raceway diameter, 16 mm wide. In the NKI series the first number is the bore diameter, so NKI20/20 is a 20 mm bore, 20 mm wide.

The NA and RNA series work differently: they follow a dimension series, and the trailing digits are a bore code in 5 mm steps. NA4904 is dimension series 49, bore code 04, which means a 20 mm bore. NA69 is the wider double-row equivalent. Suffixes carry the rest of the specification —-2RSfor contact seals on both sides,P6orP5for running accuracy,C3for increased internal clearance in high-temperature positions, andTNfor a moulded cage instead of steel. Always quote the full designation including suffixes on a purchase order; a suffix left off is a different part number, not a detail.

Load Capacity, Speed and Lubrication

Dynamic load ratings for this family scale with the number of rollers and the effective roller length, which is why a 16 mm wide NKI can rate higher than a 25 mm wide drawn cup in the same bore. When comparing catalogue figures between suppliers, check that both are quoted to ISO 281 with the same reliability basis — some catalogues quietly quote a rating at 90 million revolutions while others use 1 million, and the numbers are not interchangeable.

Speed limits are governed by the cage. A steel or brass cage tolerates higher speed and temperature than a moulded polyamide cage, but the polyamide cage runs quieter and is usually sufficient below 8,000 rpm for small bores. Grease lubrication with an NLGI 2 lithium soap grease covers the majority of applications between −30 °C and +120 °C. Above that, or where the speed index (bore in mm × rpm) exceeds roughly 200,000, move to oil circulation or oil-air lubrication and check the clearance class.

Shaft and Housing Requirements

Four measurements decide whether a machined ring bearing reaches its calculated life: shaft hardness, surface roughness, roundness and shoulder squareness. For NK and RNA49, the shaft raceway needs 58–62 HRC and Ra ≤ 0.2 µm; a softer or rougher shaft will indent the raceway within a few thousand revolutions under load. For NKI and NA49, the shaft only needs to locate the inner ring, typically h5 or k5.

Housing bores for this family are commonly machined to H6 or H7 depending on whether the outer ring rotates and whether the load direction is stationary or rotating relative to the ring. Where the outer ring is press-fitted into a split housing, add a shoulder on one side and a retaining ring on the other rather than relying on interference alone. Confirm the final values against the catalogue table for the exact series and size — tolerance recommendations shift between light and heavy series, and a housings table copied from a different product family is a common source of premature failure.

Typical Applications

These bearings concentrate in eight positions in practice. Gearbox intermediate and output shafts, where double-row NA69 carries the radial load and the shaft doubles as the raceway. Hydraulic pump and motor shafts, where NK keeps the envelope small. Compressors and blowers, where the cage has to survive continuous operation at temperature. Two-speed axle and final drive pinions, where axial location matters. Agricultural PTO and implement gearboxes, where NKS handles shock load. Forklift masts and lift chains, where space is fixed and loads are high. Power tool spindles, where small-bore NKI delivers high stiffness in a 10 mm package. And textile machinery, where low noise and low friction dominate the specification.

If your position is not on that list, size from the shaft rather than from the catalogue page. Measure the shaft diameter first, then the available radial space, then the load. Most wrong specifications we see come from picking a bearing that fits the housing while ignoring what the shaft can actually support — the housing is easy to rework, the shaft usually is not. In repair and aftermarket work, this is also why NKI outsells NK: the shaft is already in service and nobody is going to re-harden it.

Ordering, MOQ and Lead Time

Three inputs are enough for an accurate quote: the full designation including suffix, the annual or per-order quantity, and any deviation from catalogue specification (special clearance, non-standard width, alternative cage material, food-grade grease). Drawings are welcome but not mandatory for catalogue parts — sending a drawing of a standard NK20/16 usually slows the quote down rather than speeding it up.

Standard sizes ship from stock in small quantities; non-standard widths and special heat treatments are made to order. State the operating temperature and whether the bearing will be greased for life, because both change the grease selection and the shelf life quoted. Specify the packing as well — bulk cartons for production lines, individual boxes for distributors and repair shops — since repacking after the fact adds cost and handling damage to every order. For first orders, request a sample batch and check the raceway finish and cage riveting before releasing volume — those two details separate a bearing that meets its rating from one that merely matches the drawing.

Frequently Asked Questions

Can an NK bearing replace an RNA49 bearing?

Sometimes, but not by designation alone. NK and RNA49 are both inner-ring-free designs, yet their boundary dimensions come from different dimension series, so width and outer diameter rarely match exactly. Compare Fw, D and C from both catalogue tables, then check the load rating — a substitution that fits but rates 25% lower will fail earlier than the part it replaced.

What shaft hardness do I need for a bearing without an inner ring?

58–62 HRC at the raceway, with an effective hardened depth of at least 0.3 mm for case-hardened shafts. Below 55 HRC the raceway will deform permanently under rated load. If the shaft cannot be hardened, specify an NKI or NA49 with an inner ring instead of accepting a softer raceway.

Do you supply sealed and high-precision versions?

Yes. The-2RSsuffix adds contact seals on both sides for grease retention and dust exclusion, at the cost of roughly 15% of the speed rating.P6andP5running accuracy are available on request for machine tool and spindle applications.

What is the minimum order quantity and lead time?

Catalogue sizes are available from small quantities with short lead time. Non-standard widths, special cages and custom heat treatment are made to order, with lead time confirmed against the drawing. Send the designation and quantity for a firm quote.