Cylindrical Roller Bearings
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Cylindrical Roller Bearings
What is a cylindrical roller bearing?
A cylindrical roller bearing is a rolling bearing that uses cylinder-shaped rollers instead of balls. The rollers make line contact with the raceways rather than the point contact of a ball, which spreads the load over a larger area. This gives them a much higher radial load capacity than a ball bearing of the same size.
A standard bearing has four main parts:
- An inner ring, which usually fits onto a rotating shaft
- An outer ring, which usually sits in a fixed housing
- A row of cylindrical rollers running between the two raceways
- A cage that keeps the rollers evenly spaced and guided.
The rollers are guided by ribs (raised shoulders) on one or both rings.
- Where the ribs are placed decides the bearing type, for example NU, NJ or N, and whether the bearing can take any axial load.
- The roller ends are usually slightly crowned (profiled) to reduce stress at the edges of the contact and make the bearing less sensitive to small misalignment.
Most cylindrical roller bearings are separable, meaning the ring with the ribs, the rollers and the cage come apart from the other ring. This makes them easy to mount, as the inner and outer rings can be fitted separately. Rings and rollers are usually made from hardened chrome steel (grade 52100). Cages are most commonly pressed steel, glass fibre reinforced polyamide or machined brass.
Why are cylindrical roller bearings essential?
They are the go-to choice when a shaft carries heavy radial loads, particularly at moderate to high speeds. Line contact gives them high load capacity and stiffness, while their low friction lets them run faster than most other roller bearing types.
Types with ribs on only one ring, such as NU and N, let the shaft move axially within the bearing. This makes them ideal as the non-locating bearing at one end of a shaft, where they absorb thermal expansion without creating extra load. Their main limitation is that they carry little or no axial load, so they are usually paired with a deep groove ball bearing or another locating bearing. Their boundary dimensions are standardised internationally, so a bearing of a given reference has the same bore, outer diameter, and width, regardless of which reputable manufacturer makes it.
Typical applications include:
- Electric motors and generators
- Industrial gearboxes
- Pumps and compressors
- Rolling mills
- Machine tool spindles
- Railway axleboxes and traction motors
- Wind turbine gearboxes
- Vibrating screens and crushers
- Paper and printing machines
- Mining and construction equipment
The main types of cylindrical roller bearings
- NU - The outer ring has two ribs and the inner ring has none. The shaft can move axially in both directions, so the NU is the most common choice for a non-locating bearing.
- N - The inner ring has two ribs and the outer ring has none. Like the NU, it allows axial movement in both directions, but the rollers stay with the inner ring.
- NJ - The outer ring has two ribs and the inner ring has one. It can carry light axial loads in one direction and locate the shaft in that direction.
- NUP - An NJ design with a loose rib ring fitted on the inner ring. It can carry light axial loads in both directions and locate the shaft both ways.
- NF - The inner ring has two ribs and the outer ring has one. It works like an NJ, locating the shaft in one direction.
- NJ + HJ angle ring - An NJ bearing paired with a separate L-shaped angle ring (HJ). This also locates the shaft in both directions and is often used where an NUP cannot be fitted.
- Double-row cylindrical roller bearings (NN, NNU) - These have two rows of rollers for higher radial load capacity and stiffness. They are widely used in machine tool spindles, often in high precision grades and with a tapered bore.
- Full complement bearings - These have no cage, so more rollers can be fitted, giving the highest radial load capacity for the size. The trade-off is lower limiting speed and more friction. They are used in slow, heavily loaded applications such as cranes and rope sheaves.
- Multi-row (four-row) bearings - Used for the very heavy radial loads in rolling mill rolls. aterial and cage variants include:
- Pressed steel cages - Strong, cost-effective, and the standard choice in many sizes.
- Polyamide cages - Light, quiet, and good at running with little lubrication, but typically limited to around 120°C in continuous use.
- Machined brass cages - Very robust, suited to high temperatures, vibrati shock loads and high accelerations, such as in traction motors and vibrating screens.
- Electrically insulated and hybrid bearings - Used in inverter-driven motors and generators to prevent damage from stray electrical currents.
How to choose the right cylindrical roller bearing?
- Size and dimensions - The bore must suit the shaft; the outer diameter and width must fit the housing. For the same bore, a heavier dimension series has a larger outer diameter, larger rollers, and higher load capacity, but takes up more space.
- Load - Work out the radial and axial loads and whether they are steady or involve shocks. Compare them with the bearing’s basic dynamic load rating and basic static load rating, which are published in manufacturers’ catalogues. Cylindrical roller bearings handle very high radial loads, but if there is significant axial load, use a locating design such as NJ or NUP, or add a separate bearing to take it.
- Locating or non-locating - Decide whether the bearing needs to locate the shaft axially or allow it to move. NU and N types allow movement; NJ and NF locate in one direction; NUP and NJ + HJ locate in both.
- Internal clearance - C3 is greater than normal and is common in electric motors and wherever interference fits, or temperature differences reduce clearance in operation. C4 is larger still, for higher temperatures or heavier fits, while C2 is smaller than normal, used in applications needing minimal play.
- Cage - Pressed steel and polyamide cages suit most general applications. Machined brass cages are better where there is heavy vibration, shock or high temperatures.
- Alignment - Cylindrical roller bearings tolerate very little misalignment, so shafts and housings must be accurately machined and aligned. Where misalignment can’t be avoided, a spherical roller bearing may be a better choice.
Essential references and specialised ranges
How the numbering works:
The letters at the start identify the design: NU, NJ, NUP, N, NF, NN or NNU (see the types above).
The next digit or digits indicate the dimension series: 10 is extra light, 2 is light, 3 is medium, and 4 is heavy. 22 and 23 are wider versions of the 2 and 3 series, with higher load capacity.
The last two digits indicate the bore: 00 = 10mm, 01 = 12mm, 02 = 15mm, 03 = 17mm
From 04 upwards, multiply by five (for example, NU 205 = 25mm bore and NU 210 = 50mm bore)
For example, NU 2210 is an NU design in the 22 series with a 50mm bore.
Suffixes:
E = reinforced design with higher load capacity
EC = SKF reinforced design (often combined with a cage code, for example ECP, ECJ or ECM)
J = pressed steel cage
P, TN9, TVP2 = glass fibre reinforced polyamide cage
M, M1 = machined brass cage
MA, MB = machined brass cage guided on the outer ring (MA) or inner ring (MB)
K = tapered bore (taper 1:12)
C2, C3, C4 = internal clearance smaller than normal (C2), or greater than normal (C3, C4)
XL = FAG X-life series, with higher performance
Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.
FAQ - All you need to know about cylindrical roller bearings
What is the difference between NU and NJ?
An NU bearing has no ribs on the inner ring, so the shaft can move freely in both axial directions. It is used as the non-locating bearing. An NJ bearing has one rib on the inner ring, so it can carry a light axial load and locate the shaft in one direction.
Can cylindrical roller bearings take axial loads?
NU and N types carry no axial load at all. NJ and NF types can take light axial loads in one direction, and NUP and NJ + HJ combinations in both directions. For heavy or continuous axial loads, use a separate locating bearing such as a deep groove ball bearing, angular contact bearing or thrust bearing.
Can they tolerate misalignment?
Only to a very small degree, typically just a few minutes of arc, depending on the bearing series and load. Excessive misalignment overloads the edges of the rollers, causing heat, noise and early failure, so shafts and housings should be accurately aligned.
Do cylindrical roller bearings need lubrication?
Yes, lubrication separates the rollers from the raceways and ribs, reduces friction, and protects against corrosion. They can be grease or oil lubricated, depending on speed and temperature. Most cylindrical roller bearings are open, so lubrication and sealing are provided by the housing.
Click here to learn how to maintain a bearing.
Can I replace a cylindrical roller bearing myself?
In many cases, yes. Because most types are separable, the inner ring can be fitted to the shaft and the outer ring, rollers and cage to the housing, then the two are brought together. Take care to slide them together squarely, ideally while turning slowly, to avoid scratching the rollers and raceways. Use a bearing puller or induction heater for removal and fitting, never strike the bearing directly with a hammer, and don’t mix rings from different bearings.
How do I identify a bearing I need to replace?
The reference is usually marked on the side face of the rings. If the markings are worn away, measure the bore, outer diameter, and width with callipers and match them against a size chart. Also note where the ribs are on each ring, as this identifies the type (NU, NJ, N and so on), and the cage material.
Click here to learn how to measure a bearing.
How can I tell if a bearing is worn or failing?
Warning signs include:
- Grinding, rumbling, or squealing noises
- Increased vibration
- Rough or uneven rotation
- Play in the shaft
- Overheating
- Discoloured or contaminated lubricant
