Cylindrical Roller Bearings: NU, NJ and NUP

Cylindrical roller bearings carry load through line contact instead of the point contact of a ball bearing, and that one geometric difference explains why they exist. A roller resting on a raceway spreads the load across a narrow rectangle rather than a single spot, so an NU 205 fits the same 25 mm by 52 mm envelope as a 6205 and carries roughly twice the radial load. The trade is that the contact is built for radial load only, and the way the rings are ribbed decides whether the bearing can also locate the shaft in the axial direction.

This guide reads the code system that encodes that axial behaviour, explains how rib guidance and roller guidance differ at speed, shows how cage choice sets the temperature and speed ceiling, and works through the NU 205 to NU 212 sizes that cover most general duty. It is written for buyers and maintenance engineers who need to answer two questions with a number: will this carry the load, and can it float?

Line contact versus point contact

A ball touches its raceway at a mathematical point. As load rises the contact patch grows into a small ellipse and the local stress climbs quickly, because all the force passes through a tiny area. A cylindrical roller touches along a line, which in practice is a very narrow rectangle once elastic deflection is taken into account. The same load spread over a rectangle produces a much lower peak stress, so the bearing can either carry more load or carry the same load in a smaller radial section.

The visible consequence is the load rating. For a 25 mm bore in the light 2 series, a 6205 ball bearing is rated near 14 kN dynamic, while an NU 205 of almost identical envelope is rated in the region of 26 kN. That factor of roughly 1.9 is why cylindrical rollers appear wherever the radial load is heavy and the axial load is zero: electric motor shafts under belt tension, gearbox input and output positions, and the non-locating end of any shaft that grows with temperature and must be allowed to slide.

The axial-freedom code system

The letters NU, NJ, NUP, N, NF, RN and RNU are not quality grades and not size codes. They describe which ring carries ribs, and therefore how the bearing behaves when the shaft tries to move along its own axis. Reading them correctly is the single most important skill in selecting a cylindrical roller bearing, because a wrong letter means a shaft that either cannot float or cannot be located.

NU - the floating bearing

The outer ring carries two integral flanges that guide the roller and cage assembly, and the inner ring has no flanges at all. Nothing stops the shaft from sliding axially through the roller set, so an NU bearing makes an ideal non-locating position. Fit one NU at the free end of a shaft and a locating bearing at the other, and thermal expansion is absorbed without generating thrust. Almost every cylindrical roller position in a pump or fan follows this pattern.

NJ - locates in one direction

An NJ keeps the two integral flanges on the outer ring and adds a single integral flange on the inner ring. That shoulder catches the roller ends from one side, so the bearing takes axial load in one direction and floats in the other. Pair an NJ with a second NJ facing the opposite way, or with an NU, and the shaft is located without over-constraining it. The rib is designed to take light thrust only; it is a locator, not a thrust bearing.

NUP - locates in both directions

NUP keeps the single inner-ring flange of the NJ design and adds a loose flange ring - an angled collar that is not integral to the ring - on the inner ring. With a flange on each side of the rollers, the bearing locates the shaft in both axial directions and can be used as the fixed position of a shaft. The loose flange is a separate part, so it must be accounted for during mounting and it must not be swapped between bearings.

N, NF, RN and RNU variants

The N and NF designs mirror NU and NJ by moving the guiding flanges to the inner ring, which suits arrangements where the housing is easier to machine than the shaft. RNU is an NU design supplied without an inner ring, so the roller set runs directly on a hardened and ground shaft raceway; RN is an N design supplied without an outer ring, so the rollers run in a hardened housing bore. RNU lets the shaft be larger and stiffer, because no inner ring has to be fitted around it. All four follow the same axial logic: count the flanges and you know what the bearing will do.

Rib guidance versus roller guidance

A cylindrical roller has to be kept axial by something. In most NU and NJ designs the rollers are guided by their flat ends rubbing against the ribs of the inner ring. This is positive guidance: it keeps the roller square to the raceway under load and gives excellent stiffness, but the sliding contact between roller end and rib creates friction and heat as speed rises. Rib guidance is therefore the right choice for heavy load at moderate speed, which describes most gearbox and mill positions.

The alternative is roller guidance, where a machined cage guides each roller by its running surface. Friction is lower and the speed ceiling is higher, but the cage must be strong enough to control the roller on its own and the bearing costs more to make. When a catalogue lists an NU design with a machined brass cage and a high limiting speed, it is almost always a roller-guided design. If your duty is high speed and only moderate load, buy the roller-guided variant and pay for the cage; if the duty is heavy and slow, rib guidance is cheaper and stiffer.

Why a cylindrical roller out-carries a ball bearing

The capacity advantage is real but conditional. A single-row cylindrical roller carries more radial load than any ball bearing that fits the same envelope, and a full-complement design with no cage at all carries the most of all, because the maximum number of rollers shares the load. The conditions are equally clear: the load must be essentially radial, the two rings must stay parallel to a few minutes of arc, and the shaft and housing must be stiff enough that the roller does not edge-load.

Misalignment is the limiting factor. A deep groove ball bearing self-corrects a few minutes of arc through its raceway curvature, while a cylindrical roller has almost no capacity to do so. Permissible misalignment is typically only about 3 to 5 arc minutes, roughly 0.05 to 0.08 degrees. Exceed it and the roller contacts at one end only, stress concentrates there, and the fatigue life falls far faster than the small angle would suggest. That is why cylindrical rollers are usually paired with a self-aligning or a spherical roller bearing on a shaft that deflects.

Cage types and their trade-offs

The cage holds the rollers at even spacing and separates them. It is often the least expensive component in the bearing and the one that decides the temperature limit.

• Machined brass cages. The strongest and most temperature-tolerant option, good to about 250 °C in service and suitable for roller guidance. They are heavier and cost more, and they are the standard choice for heavy-duty and high-speed cylindrical rollers.
• Polyamide cages. Light, quiet and cheap, with a service ceiling around 120 °C. They are ideal for moderate load and clean lubrication, and they must not be used where the polyamide can be attacked by certain oils or excessive temperature.
• Stamped steel cages. The general-purpose default, offering a good balance of strength, cost and temperature resistance to about 200 °C, but limited to lighter duty than a machined cage.

Boundary dimensions: the NU and NJ 205 to 212 class

The boundary dimensions of a cylindrical roller bearing are fixed by ISO 15 in exactly the same way as a ball bearing. The 2 series shares the outside diameter of the 6200 ball series, which is why an NU 208 and a 6208 fit the same housing bore. The widths are likewise standard, so a cross-reference that matches d, D and B is dimensionally safe; only the internal geometry and the rib arrangement differ.

The NJ and NUP forms share the same envelope, so an NJ 210 and an NU 210 are interchangeable dimensionally while differing in axial behaviour. Typical dynamic ratings rise from about 26 kN at the NU 205 to roughly 96 kN at the NU 212, and static ratings rise in step. Confirm the figures against the intended brand, because capacity is not fixed by the standard. Our cylindrical roller range carries the full boundary dimensions and rib code on every listing, and the ISO 15 boundary dimension guide explains how the bore and series digits are read.

Speed, clearance and fit notes

Cylindrical rollers tolerate less misalignment and reach lower speed than ball bearings of the same size, so the surrounding design has to be a little more careful. Three parameters do most of the work.

• Speed. The limiting speed of an NU 205 is around 10,000 rpm with oil lubrication against roughly 18,000 rpm for a 6205, because the roller is heavier and the rib contact adds friction. Rib-guided designs lose more speed than roller-guided ones at the same load.
• Clearance. A press-fit inner ring expands and closes the internal clearance, and a cylindrical roller has less clearance to lose than a ball bearing. Heavy or hot duty generally calls for C3 rather than CN, with the numeric ranges defined by ISO 5753. The clearance class guide walks through C2, CN, C3 and C4.
• Fit. The rotating ring needs an interference fit to prevent creep, and the magnitude comes from the ISO 286 tables rather than from habit. The shaft and housing fit guide gives the selection logic, and the roller internal geometry article explains how the fit changes the running clearance.

How to choose cylindrical instead of deep groove

The comparison below is the one we use at the quotation desk. It assumes the same 25 mm bore and the same radial space, and it is deliberately blunt about where each design wins.

Work through the following sequence when a heavy radial duty arrives without a specified part number.

1. Confirm the radial load and speed. Calculate the equivalent load and check it against the basic dynamic rating to get the L10 life, working through the method in the L10 life and load rating guide and the bearing type selection guide.
2. Confirm there is no meaningful thrust. If the position must take axial load in both directions, an NUP or a paired arrangement is required, or a different bearing family.
3. Decide fixed or floating. Choose NU for a floating position and NJ or NUP for a location position, then check the other end of the shaft is compatible.
4. Check the misalignment budget. Estimate shaft deflection from the shaft deflection and the housing stiffness. If it exceeds a few minutes of arc, move to a self-aligning design.
5. Set the clearance and fit. Select C3 for heavy or hot duty, then pick the shaft and housing fits from the ISO 286 tables so the running clearance stays positive.

Common selection mistakes

These are the errors that reach us most often in enquiries, roughly in order of how much they cost.

• Fitting an NU where a locator is needed. The NU floats, so the shaft is free to move axially. If nothing else on the shaft locates it, the machine drifts and the shaft grinds its way through the housing.
• Using an NJ rib as a thrust bearing. The single outer rib takes light thrust only. Repeatedly loading it with the full axial load of the machine will spall the rib face.
• Ignoring misalignment. A cylindrical roller in a housing that deflects edge-loads, and the resulting spalling at one end of the roller is often misdiagnosed as a material fault.
• Reusing the wrong clearance class. Dropping a C3 bearing into a position designed for CN, or the reverse, changes the running clearance and with it the life, even though the number matches on the envelope.
• Over-tightening the shaft fit. Too much interference closes the clearance completely, and a cylindrical roller with zero running clearance seizes far sooner than a ball bearing in the same condition.
• Splitting a matched set. NUP loose flanges and matched roller-and-cage assemblies must stay with their own bearing; swapping them between units destroys the internal clearance.

If you already have an envelope and need a list of orderable numbers, the bearing selector filters the catalogue by bore, outside diameter and width, and the SKF and ANDGE listings show how the same envelope is offered across brands. Where the axial duty rules out a cylindrical roller, the tapered roller designation guide and the needle roller range cover the two neighbouring families.