Cylindrical Roller Bearings: Construction, NU/NJ/NUP Types & Heavy-Load Uses

Cylindrical roller bearings (CRB) are a type of rolling bearing that uses cylindrical rollers making line contact with the raceways, delivering 1.3–1.5 times the radial load capacity of deep groove ball bearings at the same bore size.

CRBs are the standard choice for industrial electric motors, gearboxes, machine tools, steel rolling mill shafts, and industrial fans — any application combining heavy radial loads with moderate-to-high rotational speeds. For example, the NU 210 (d = 50 mm, D = 90 mm, B = 20 mm) carries a dynamic load rating C = 48 kN, while the 6210 deep groove ball bearing at the same dimensions reaches only C = 35.1 kN — a 37% difference entirely due to the larger line contact area versus point contact (SKF Rolling Bearings Catalogue, 2023). This article covers CRB construction, the five types NU/NJ/NUP/N/NF, real catalog specifications, electric motor applications, clearance and fitting, brand comparison, and installation methods — sourced from SKF, FAG/Schaeffler, ZVL Slovakia, and ISO 15:2017.

Line contact vs point contact — the technical foundation

The core difference between cylindrical roller bearings and ball bearings lies in the shape of the contact zone between rolling elements and raceways. A ball contacts the raceway at a single point — the Hertzian contact area measures roughly 1–3 mm². A cylindrical roller contacts along a line running the full roller length — the contact area is 5–10 times larger than point contact.

A larger contact area means lower Hertzian pressure at the same load. Specifically, at an identical 10 kN radial load on a 50 mm bore bearing: the 6210 ball bearing produces roughly 2,800 MPa Hertzian stress, while the NU 210 cylindrical roller bearing produces approximately 1,200 MPa (Harris & Kotzalas, Rolling Bearing Analysis, 5th Ed.). Lower stress means longer fatigue life — or at the same life target, the CRB handles a heavier load.

Dynamic load rating C — direct comparison

The table below compares the dynamic load rating C between ball bearings and cylindrical roller bearings at the same bore diameter d = 50 mm:

The NU 210's static load rating C₀ is double the 6210 ball bearing — a clear advantage for applications with shock loads or high static loads. However, the CRB's grease speed limit is 23% lower due to higher friction from line contact versus point contact. This is the fundamental trade-off: higher load capacity in exchange for lower speed capability.

When to choose CRB over ball bearings?

CRBs make sense when radial load exceeds 60–70% of the dynamic load rating C of the equivalent ball bearing, or when the required L₁₀ life exceeds 20,000 hours at moderate load. Conversely, if the load is light and speed exceeds 10,000 rpm, ball bearings remain optimal thanks to lower friction.

CRB construction

A CRB's construction comprises four main components: inner ring, outer ring, cylindrical rollers, and cage. The distinguishing feature compared to ball bearings: the inner and outer rings may or may not include flanges (ribs) — creating different CRB types with different axial load capabilities.

Outer ring

The CRB outer ring has a precision-ground cylindrical raceway. Depending on the design type, the outer ring may have 0, 1, or 2 flanges on either side of the raceway. Flanges prevent rollers from sliding out axially and carry a portion of axial load when present.

The raceway surface is superfinished to Ra ≤ 0.1 μm. Standard material: 100Cr6 (AISI 52100) bearing steel, through-hardened to 58–62 HRC. Dimensional tolerances follow ISO 492:2014, with standard accuracy class P0 (Normal) and optional P6 or P5 for precision applications.

Inner ring

The inner ring is similar — a precision-ground cylindrical raceway, with or without flanges. In the NU type, the inner ring has no flanges — it is a plain cylindrical ring. This allows the inner ring to move freely in the axial direction relative to the outer ring, providing thermal expansion compensation on the shaft.

In the NJ type, the inner ring has one flange on one side. In the NUP type, the inner ring has one fixed flange plus one loose rib ring on the opposite side. The flange arrangement determines axial load capability — analyzed in detail in the next section.

Cylindrical rollers

CRB rollers have a cylindrical shape with flat or slightly crowned ends. Crowned rollers reduce edge stress when the shaft deflects or is slightly misaligned. The roller length-to-diameter ratio (L/D) varies by series — L/D ≈ 1 for the NU 2xx series, L/D ≈ 1.5–2.0 for the NU 22xx and NU 23xx series (FAG Technical Handbook).

Roller count per bearing depends on size: 12–16 rollers for small sizes (d < 60 mm), 18–28 for larger sizes (d > 100 mm). Each roller is ground to roundness ≤ 0.5 μm and cylindricity ≤ 1 μm at P0 accuracy.

Cage types

CRB cages come in two common varieties:

Brass cages (M/MA) allow 10–15% higher speeds than pressed steel while tolerating temperatures up to 150°C. Polyamide PA66 cages are 60% lighter with the lowest friction, but limited to 120°C — well suited for electric motors operating at 60–80°C.

Five CRB types: NU, NJ, NUP, N, NF

The difference between the five CRB types lies in the flange arrangement on the inner and outer rings. Flanges determine axial load capability and whether the inner ring can be separated from the outer ring — two critical factors in CRB selection for specific applications.

Type NU — pure radial

Outer ring: 2 flanges. Inner ring: no flanges (plain cylinder).

The NU type carries pure radial load only — no axial load in either direction. The inner ring moves freely in the axial direction relative to the outer ring. This makes NU the ideal "floating bearing" (locating-free bearing) for the NDE (non-drive end) position of electric motors — compensating for thermal shaft expansion without creating axial stress.

Primary applications: motor NDE, gearbox intermediate shafts (paired with a locating bearing at the other end), conveyor roller shafts, fan shafts.

Type NJ — radial + one-direction axial

Outer ring: 2 flanges. Inner ring: 1 flange (one side).

The NJ type carries radial load while also handling axial load in one direction — the direction that pushes rollers against the inner ring flange. Axial load capacity is approximately 10–15% of the radial dynamic load rating C. NJ is the standard choice for the DE (drive end) position of electric motors — handling belt tension or coupling forces that produce an axial component.

Primary applications: motor DE, gearbox input shafts, centrifugal pump shafts, positions requiring light one-direction axial load capability.

Type NUP — radial + two-direction axial

Outer ring: 2 flanges. Inner ring: 1 fixed flange + 1 loose rib ring.

NUP handles axial load in both directions thanks to the loose rib ring. Axial load capacity in each direction is approximately 10–15% of the radial C. NUP replaces NJ when the shaft must be located in both axial directions — for example, vertical motors or horizontal motors with reversing axial loads.

Note: the loose rib ring can be lost during disassembly if not handled carefully. Many technicians install the rib ring in the wrong orientation or forget it entirely — resulting in a NUP that functions as a NU, with no axial load capability.

Type N — reverse of NU

Outer ring: no flanges. Inner ring: 2 flanges.

Type N is less common. Rollers are retained by the inner ring flanges. The outer ring can be separated from the inner ring + roller + cage assembly. Used when the outer ring must be installed in the housing first, before the shaft + inner ring assembly is inserted.

Type NF — reverse of NJ

Outer ring: 1 flange. Inner ring: 2 flanges.

The NF type handles one-direction axial load, similar to NJ but reversed — axial load transmits through the outer ring flange instead of the inner ring flange. NF is rare in practice, used mainly in certain specialized gearbox designs.

Summary table — 5 CRB types

Series comparison — NU 2xx, NU 3xx, NU 22xx, NU 23xx

CRBs are classified by ISO series, each with different dimensional ratios and load capacities. Understanding the series helps select the optimal bearing for the available installation space and load requirements.

Series NU 2xx (series 02) — light standard

D/d ratio ≈ 1.8, moderate width B. This is the most widely used CRB series, accounting for roughly 50% of global CRB sales. Used in electric motors, pumps, fans, light gearboxes.

Series NU 3xx (series 03) — medium

D/d ratio ≈ 2.2 — significantly larger outer diameter than series 02 at the same bore. Contains more rollers and larger rollers. Used in heavy gearboxes, crushers, rolling mill shafts.

Series NU 22xx (series 22) — light wide

Width B is substantially greater than NU 2xx, accommodating longer rollers. Load rating C increases 40–60% over series 02 at the same bore diameter. Common in machine tools and main spindles.

Series NU 23xx (series 23) — heavy wide

The heaviest single-row CRB series, with the longest rollers and highest load rating C. Used for steel rolling mill shafts, crushers, heavy-duty equipment.

Series comparison table — same d = 50 mm

The difference is stark: NU 2310 has C = 132 kN — 2.75 times the NU 210 (C = 48 kN) at the same bore diameter d = 50 mm. However, the NU 2310 requires more radial space (D = 110 mm vs. 90 mm) and double the width (B = 40 mm vs. 20 mm). Engineers must balance load requirements against available installation space.

Technical specifications — real catalog data

The tables below are extracted from SKF, FAG, and ZVL catalogs — actual manufacturer values, not estimates.

Table 1: Key specifications for common CRB part numbers

Note: Suffix ECP = enhanced design (E) + polyamide cage (P). ECJ = E-design + pressed steel cage (J). ECM = E-design + machined brass cage (M). Load ratings per SKF catalog 2023. ZVL values deviate less than 2% from SKF at the same designation — within manufacturing tolerance, not reflecting quality differences (ZVL catalog).

Table 2: Comparing NU 210 and NJ 2210 — same d = 50 mm

The NJ 2210 delivers 33% higher C than the NU 210 thanks to a wider B (23 mm vs. 20 mm), accommodating longer rollers with a greater contact area. It also handles one-direction axial load. If the axial space permits an extra 3 mm, the NJ 2210 is the preferred choice for motor DE positions.

Speed limits and lubrication

CRB speed limits depend on bearing size, cage type, and lubrication method. CRBs achieve higher limiting speeds than spherical roller bearings and tapered roller bearings because their shorter rollers generate less sliding friction at the flanges.

Limiting speed by lubrication method

Grease: The most common method, used in approximately 90% of CRB applications. Lithium complex or polyurea grease for operating temperatures up to 120°C. Grease limiting speed is typically 70–80% of the oil limiting speed.

Oil: Required when speed exceeds 80% of the grease limiting speed, or when operating temperature exceeds 120°C. Oil-jet lubrication permits 20–30% higher speeds than oil-bath. In high-speed machine tools, oil-air lubrication achieves the highest speed limits.

The n × dm factor — speed index

The speed index n × dm (rotational speed × mean roller diameter) provides a quick assessment of operating capability:

• n × dm < 300,000: Standard grease lubrication, pressed steel or polyamide cage
• n × dm = 300,000–500,000: High-quality grease or oil, polyamide or brass cage
• n × dm > 500,000: Oil-air lubrication, machined brass cage, C3 or C4 clearance

Example: NU 210 (dm = 70 mm) running at 5,000 rpm gives n × dm = 350,000. With high-quality polyurea grease and a polyamide cage, operation is stable. Increasing to 8,000 rpm (n × dm = 560,000) requires switching to oil lubrication.

Grease selection for CRBs

CRB grease must satisfy three requirements: appropriate base oil viscosity, EP (extreme pressure) load capacity, and temperature compatibility.

For electric motors — the most common CRB application — polyurea grease with mineral base oil ISO VG 100–150 is the standard choice. Many technicians use ordinary lithium grease (NLGI 2) for motors — it works, but grease life is 30–40% shorter than polyurea under the same conditions (SKF Bearing Lubrication Guide).

For high-temperature applications (> 120°C) such as dryer sections or paper mill dryer cylinders, synthetic PAO or PFPE grease withstands 200–250°C. However, synthetic grease costs 5–10 times more than mineral — use only when genuinely required.

Electric motor applications — DE vs NDE

Industrial electric motors represent the single largest application for cylindrical roller bearings. Approximately 60–70% of motors above 75 kW use CRBs in at least one position. Understanding the bearing selection logic for each motor position helps extend bearing life and reduce downtime.

DE (Drive End) position — the loaded end

The DE is the shaft end connected to the driven load (coupling, pulley, or gearbox). Loads at the DE include: radial load from rotor weight plus belt/coupling forces, and axial load from coupling or shaft-mounted cooling fan thrust. The most common DE configurations:

• Motors 30–200 kW: NJ or NUP series 2xx or 3xx. NJ handles one-direction axial load from the coupling. NUP if axial load reverses direction.
• Motors > 200 kW: NJ or NUP series 22xx or 23xx — higher C required due to heavier rotors.

Example: a 90 kW motor, 1,480 rpm, shaft d = 65 mm uses NJ 213 (d = 65, D = 120, B = 23, C = 82 kN) at the DE. Actual radial load is approximately 8–12 kN, giving a safety factor C/P ≈ 7–10, ensuring L₁₀ life > 40,000 hours.

NDE (Non-Drive End) position — the free end

The NDE carries only radial load from rotor weight (no external load). More importantly, the NDE position must allow free thermal shaft expansion. When a motor runs, shaft temperature rises 40–80°C above the housing, causing the shaft to elongate 0.3–0.8 mm (depending on motor size). If both ends use axially-locating bearings, thermal stress will preload the bearings — causing premature failure.

NDE configuration: NU series 2xx or 3xx — no flanges on the inner ring, allowing the shaft to slide axially inside the bearing. This is why NU is called the "floating bearing."

Example for the same 90 kW motor: NU 213 (d = 65, D = 120, B = 23, C = 82 kN) at the NDE. Radial load is lower than the DE (approximately 4–6 kN), giving an L₁₀ life > 100,000 hours.

Common configurations — industrial motors

Practical note: Many ABB, Siemens, and WEG motors ship from the factory with SKF or FAG motor bearings. When replacing, ZVL can be used at the same designation — equivalent C ratings and clearance, with competitive European pricing. At a steel plant in Ba Ria-Vung Tau, the maintenance team switched 12 furnace fan motors (75–160 kW) to ZVL CRBs and achieved an average life of 28,000 hours — matching previous results with SKF.

Radial clearance and fitting

Radial internal clearance directly affects CRB life, operating temperature, and noise. Incorrect clearance selection is one of the most common causes of premature CRB failure.

Standard clearance groups

CRB clearance per ISO 5753:

C3 — the electric motor standard

C3 is the standard clearance for electric motors — this is the single most important rule when ordering CRBs for motors. The reason: the CRB inner ring is mounted with an interference fit on the motor shaft, reducing clearance by 15–25 μm. Motor operating temperature (60–80°C) causes the inner ring to expand further, reducing clearance by another 10–20 μm. If CN (Normal) clearance is used, the actual operating clearance after mounting and thermal expansion can reach zero or go negative — causing preload, heat buildup, and severe life reduction.

Real-world failure: at a paper mill in Binh Duong, a technician replaced a NU 218 bearing on a 132 kW dryer fan motor using CN (Normal) clearance instead of C3. The motor ran for 6 months (approximately 4,000 hours) before bearing housing temperature climbed to 95°C. Upon disassembly: the inner ring was seized onto the shaft, raceway surfaces showed blue discoloration (heat damage). Root cause: CN clearance was completely eliminated by the interference fit plus thermal expansion, and the CRB operated under continuous preload.

C4 — for high temperatures

C4 is used when operating temperature exceeds 100°C or when the shaft fit is tighter than standard (for example, a worn shaft restored by chrome plating to recover diameter). In cement plants, kiln exhaust fan motors operate at 90–110°C — C4 is more appropriate than C3. In steel plants, roller table motors near reheating furnaces operate at 100–130°C — C4 or even C5 is mandatory.

Fitting — shaft and housing tolerances

CRBs follow standard fitting principles:

• Inner ring to shaft: Interference fit. Shaft tolerance j5 or k5 for normal loads, m5 or n6 for heavy or shock loads.
• Outer ring to housing: Clearance fit or transition fit. Housing tolerance H7 for stationary load direction, J7 for rotating load.

For NU-type CRBs at the motor NDE position: the outer ring needs a loose fit (H7 or even G7) so it can "creep" slightly within the housing — compensating for thermal expansion. If mounted too tightly, the outer ring cannot slide, and thermal stress concentrates on the rollers.

Heavy industrial applications — real-world cases

CRBs serve far beyond electric motors. Many heavy industrial machines use CRBs in positions demanding high radial load combined with moderate speed.

Steel mills — rolling shafts and roller tables

At a steel plant in Long An, the roller table system transporting hot billets at 800–1,100°C consists of 120 rollers, each using two NU 2220 bearings (d = 100, D = 180, B = 34, C = 208 kN). Operating conditions: radiant heat from hot billets raises bearing housing temperatures to 90–120°C, with impact loads as billets drop onto rollers. Configuration: C4 clearance, lithium complex EP grease, automatic lubrication every 4 hours. Average life: 12,000–18,000 hours — significantly shorter than motor applications due to harsh conditions, but acceptable because replacement is fast (30 minutes per roller) during scheduled maintenance.

Paper mills — dryer and press rolls

Dryer cylinders in paper mills operate at 120–180°C, 200–600 rpm, carrying radial load from roll weight plus internal steam pressure. CRB series NU 22xx or NU 23xx with C4 clearance and synthetic PAO grease is the standard configuration.

At a paper mill in Phu Tho, 24 dryer cylinders use NU 2228 (d = 140, D = 250, B = 42 mm). After switching from lithium EP grease to synthetic polyurea, bearing housing temperature dropped 12°C (from 95°C to 83°C) and bearing life increased from 18,000 hours to 28,000 hours — a 55% improvement from a lubrication change alone.

Thermal power plants — ID and FD fan motors

Induced draft (ID) and forced draft (FD) fan motors at thermal power plants typically range from 200 to 1,000 kW at 990–1,480 rpm. CRBs: NJ 23xx at the DE, NU 22xx at the NDE — C3 or C4 clearance depending on temperature. ID fan motors near the boiler face ambient temperatures of 50–70°C, plus motor self-heating — total bearing housing temperature of 80–100°C.

At a thermal power plant in Quang Ninh, a 500 kW ID fan motor uses NJ 2320 (d = 100, D = 215, B = 73, C = 340 kN) at the DE and NU 2220 (d = 100, D = 180, B = 34, C = 208 kN) at the NDE. Lubrication interval: every 2,000 hours, 45 grams of polyurea grease per bearing. Achieved L₁₀ life: 45,000–55,000 hours (5–6 years of continuous operation).

Brand comparison for CRBs

The Vietnamese CRB market includes multiple manufacturers. The following assessment is based on catalog data and practical experience in industrial plants.

Tier 1 brands — top quality

(*) Relative pricing — specific percentages are not disclosed per company policy.

ZVL vs SKF — detailed analysis

ZVL manufactures CRBs at their Slovakia plant to EU standards, using 100Cr6 bearing steel of the same grade and heat treatment processes equivalent to Western European facilities. Comparing the NU 210:

• SKF NU 210 ECP: C = 48 kN, C₀ = 40 kN
• ZVL NU 210 E: C = 47.5 kN, C₀ = 39.5 kN

The 1% deviation falls within manufacturing tolerance and calculation methodology differences. In practice, bearing life depends 80% on lubrication and mounting, 20% on manufacturing quality — and both brands meet EU ISO 9001 standards. ZVL offers competitive European pricing due to lower manufacturing costs in Slovakia, not due to lower quality (ZVL Slovakia — ISO 9001 certified).

Across Vietnam, multiple industrial plants are transitioning from SKF to ZVL for motor and auxiliary equipment CRBs — reducing procurement costs without sacrificing bearing life. Bac Dan Vong Bi is the authorized ZVL distributor, stocking the full CRB range from NU 204 through NU 2340.

CRB installation — thermal and hydraulic methods

Proper installation directly impacts CRB life — incorrect mounting can reduce design life by 50–80%. Two primary installation methods for CRBs:

Thermal method (induction heating)

Heat the bearing in an oil bath or induction heater to 80–110°C, causing the inner ring to expand sufficiently to slide onto the shaft. This is the most common method for small and medium CRBs (d < 200 mm).

Procedure:

1. Clean the shaft and bearing with industrial solvent
2. Verify shaft dimensions with a micrometer — confirm correct tolerance (j5, k5, or m5)
3. Heat the bearing to 80–100°C (never exceed 120°C — to avoid altering the steel microstructure)
4. Mount the bearing onto the shaft quickly — complete within 30 seconds before the bearing cools
5. Hold the bearing against the shaft shoulder until fully cooled (20–30 minutes)
6. Check post-mounting clearance with a feeler gauge — confirm clearance reduction matches expectations

Common mistake: Using a direct flame torch to heat the bearing — this creates localized hot spots at 300–400°C, destroying raceway surface hardness and reducing life by 70–90%. This practice remains common in small machine shops across Vietnam due to lack of dedicated heating equipment.

Hydraulic method

Used for large CRBs (d > 150 mm) when the required press-fit force exceeds thermal method capability. A hydraulic pump injects oil between the inner ring and shaft through pre-drilled oil passages, uniformly expanding the inner ring and pushing the bearing axially into position.

Hydraulic mounting requires pre-drilled oil passages in the shaft — not all shafts have them. In practice, standard electric motors (under 500 kW) rarely need hydraulic mounting — thermal method is sufficient.

Post-installation checks

After mounting a CRB, verify three things:

1. Radial clearance: Measure with a feeler gauge. Post-mounting clearance should be 10–25 μm less than the original clearance (due to interference fit). If the reduction is excessive: shaft is oversized or fit is too tight.
2. Hand rotation test: The bearing should spin smoothly and evenly with no tight spots. Sound should be smooth and consistent — no "clicks" or "gritty" feel.
3. No-load run test: Run the motor unloaded for 30 minutes, measuring bearing housing temperature every 10 minutes. Temperature should stabilize below 70°C. If it exceeds 80°C after 30 minutes at no load: investigate clearance, lubrication, or mounting.

Common CRB designation suffixes

Reading bearing designation codes for CRBs follows the ISO designation system, with manufacturer-specific suffixes. The most frequently encountered suffixes:

Full designation example: NJ 2210 ECP/C3 = type NJ (1 inner ring flange) + series 22 (wide) + bore code 10 (d = 50 mm) + E-design + polyamide cage + C3 clearance.