Needle Roller Bearings
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Needle Roller Bearings
What is a needle roller bearing?
A needle roller bearing is a roller bearing that uses long, thin cylindrical rollers, called needles. The needles are much longer than their diameter, which gives the bearing a very small cross section while still providing a large contact area. This means a needle roller bearing can carry high radial loads in a much smaller space than a ball bearing or standard cylindrical roller bearing.
A standard bearing has four main parts:
- An outer ring, which may be machined or a thin drawn steel cup
- A set of needle rollers
- A cage that keeps the needles evenly spaced and parallel
- An inner ring, which is often left out so the needles run directly on the shaft.
Many needle roller bearings are designed to run without an inner ring.
- The needles run directly on a hardened and ground shaft, which acts as the inner raceway.
- This saves even more space and allows a larger shaft diameter for the same housing size.
Rings and needles are usually made from hardened chrome steel (grade 52100), while drawn cups are made from thin, hardened steel strip. Cages are most commonly steel, with glass fibre reinforced polyamide also available.
Why are needle roller bearings essential?
They give the highest radial load capacity for the smallest space of any rolling bearing, which makes them the go-to choice where a design is tight on room or weight. They are stiff, run smoothly, and handle oscillating movement well, which is why they are widely used in gearboxes, engines, and pivot points.
They are also low in cost, especially drawn cup types, which are simply pressed into the housing with no need for shoulders or retaining rings. 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:
- Automotive gearboxes and transmissions
- Engine connecting rods and rocker arms
- Universal joints and steering columns
- Two-stroke engines, such as in motorcycles and garden equipment
- Power tools
- Pumps and compressors
- Agricultural and construction machinery pivots
- Printing and textile machines
- Office equipment and household appliances
The main types of needle roller bearings
- Drawn cup needle roller bearings with open ends (HK) - These have a thin outer cup pressed from steel strip and are the most compact and low-cost type. They are simply pressed into the housing and need no other axial location.
- Drawn cup needle roller bearings with a closed end (BK) - These have one end closed, so they seal off the end of a shaft. They are used at shaft ends, such as in universal joints and gearbox covers.
- Full complement drawn cup bearings - These have no cage, so more needles can be fitted, giving higher load capacity for slow or oscillating movement.
- Machined needle roller bearings with an inner ring (NA, NKI) - These have a solid machined outer ring with flanges and come with an inner ring. They suit shafts that are not hardened and ground.
- Machined needle roller bearings without an inner ring (RNA, NK) - These are the same design without the inner ring, so the needles run directly on a hardened shaft.
- Needle roller and cage assemblies (K) - These are just the needles and cage, with no rings at all. They are used where both the shaft and the housing bore are hardened and ground to act as raceways, such as in gear and connecting rod bearings.
- Axial (thrust) needle roller and cage assemblies (AXK) - These carry axial loads in a very small space and are used with thrust washers or hardened faces.
- Combined needle roller bearings - These combine a radial needle bearing with a thrust bearing in one unit, so they carry radial and axial loads together.
- Sealed needle roller bearings - These have seals on one or both sides to keep grease in and dirt out.
How to choose the right needle roller bearing?
- Size and dimensions - Check the shaft diameter, housing bore, and width available. Needle roller bearings are often chosen because space is tight, so measure carefully.
- Shaft condition - If the shaft is hardened and ground, a bearing without an inner ring can be used. If not, choose a bearing with an inner ring, or fit a separate inner ring.
- Load - Work out the radial load and whether it is steady, oscillating, or involves shocks. Compare it with the bearing’s basic dynamic load rating and basic static load rating, which are published in manufacturers’ catalogues. Needle roller bearings carry no axial load, so add a thrust bearing if axial loads are present.
- Housing - Drawn cup bearings need an accurately machined housing bore, as the thin cup takes its shape and fit from the housing. Machined bearings are more tolerant and can be used in lighter housings.
- Speed - Caged bearings suit higher speeds. Full complement bearings suit slow speeds and oscillating movement.
- Environment - Use sealed bearings where dirt or moisture is present, or where the bearing must hold its grease.
Essential references and specialised ranges
How the numbering works:
The letters at the start identify the type:
HK = drawn cup, open ends
BK = drawn cup, closed end
NA = machined, with inner ring
RNA = machined, without inner ring
NK = machined, without inner ring
NKI = machined, with inner ring
K = needle roller and cage assembly
AXK = axial needle roller and cage assembly
The numbers work differently depending on the type:
HK and BK - the first two digits are the shaft diameter and the last two are the width in mm, so an HK 2016 fits a 20mm shaft and is 16mm wide.
NA and RNA - the digits follow the standard series and bore code, so an NA 4910 is a 49 series bearing with a 50mm bore. The RNA version is the same size without the inner ring.
NK and NKI - the shaft or bore diameter and width are shown with a slash, so an NK 12/16 fits a 12mm shaft and is 16mm wide.
K - the size is shown as shaft diameter x outer diameter x width, so a K 20x24x10 fits a 20mm shaft.
Suffixes:
RS = contact seal on one side
2RS = contact seals on both sides
ZW = double row
TV = glass fibre reinforced polyamide cage
TN = polyamide cage
M = machined brass cage
V = full complement, without a cage
AS1 = lubrication hole in the outer ring
IS1 = lubrication hole in the inner ring
C3, C4 = radial internal clearance greater than normal (C4 larger than C3)
Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.
FAQ - All you need to know about needle roller bearings
What is the difference between a needle roller bearing and a cylindrical roller bearing?
Both use cylindrical rollers, but needle rollers are much thinner and longer compared to their diameter. This gives needle roller bearings a much smaller cross section, so they fit into tighter spaces, while still carrying high radial loads.
Can needle roller bearings run directly on a shaft?
Yes, many types are designed to do this. The shaft must be hardened and ground to a smooth finish, so it can act as the inner raceway. If the shaft is not hardened, use a bearing with an inner ring or fit a separate inner ring.
Can they take axial loads?
No. Radial needle roller bearings are designed for radial loads only. Axial loads must be carried by a separate thrust bearing, such as an axial needle roller and cage assembly, or by a combined needle roller bearing.
Do needle roller bearings need lubrication?
Yes, lubrication separates the needles from the raceways, reduces friction, and protects against corrosion. Many drawn cup bearings are supplied lightly oiled or greased and rely on the surrounding oil or grease, such as in a gearbox. Sealed types are supplied pre-greased.
Click here to learn how to maintain a bearing.
Can I replace a needle roller bearing myself?
In many cases, yes. Drawn cup bearings must be pressed into the housing with a fitting mandrel that pushes on the marked end face of the cup, never with a hammer directly on the bearing, as the thin cup is easily distorted. Machined bearings can be pressed in or onto the shaft in the same way as other bearings. Always check the shaft for wear or marks if the needles run directly on it.
How do I identify a bearing I need to replace?
The reference is usually marked on the end face of the cup or the side face of the outer ring. If the markings are worn away, measure the shaft diameter, housing bore or outer diameter, and width with callipers and match them against a size chart. Also note whether the bearing has an inner ring, one or two open ends, and seals.
Click here to learn how to measure a bearing.
How can I tell if a bearing is worn or failing?
Warning signs include:
- Grinding, clicking, or rattling noises
- Increased vibration
- Rough or notchy rotation
- Play in the shaft
- Overheating
- Wear marks, pitting, or grooves on the shaft where the needles run
