Product Introduction

NU Type Cylindrical Roller Bearings is the most widely used "floating" variant among single-row Cylindrical Roller Bearings. Structurally, it features an outer ring with integral flanges on both sides and an inner ring that is a smooth, flange-less collar. The bearing consists of two main parts: an outer ring assembly (comprising the outer ring, cylindrical rollers, and cage) and the inner ring. The outer ring's flanges axially retain the rollers and cage, forming a separable assembly, while the smooth, flange-less inner ring can slide out independently from either side.

This configuration—flanges on the outer ring and no flanges on the inner ring—makes the NU-type bearing a classic floating bearing. Since the inner ring lacks retaining flanges, it can move freely in the axial direction relative to the outer ring assembly; consequently, NU-type bearings can only support radial loads and cannot accommodate any axial loads. In shaft system design, the NU-type bearing is typically positioned at one end of the shaft to serve as the "floating end," compensating for thermal expansion caused by temperature rises during operation and preventing the generation of additional axial forces due to thermal expansion or contraction.

Both NU-type and N-type bearings are pure floating bearings with identical functionality; the core difference lies in the placement of the flanges. The N-type features flanges on the inner ring and a smooth outer ring, whereas the NU-type features flanges on the outer ring and a smooth inner ring. This seemingly minor structural difference dictates distinct approaches to installation, maintenance, and application suitability.

Working Principle

During operation, the cylindrical rollers of the NU-type bearing undergo pure rolling motion between the outer and inner ring raceways, with radial loads transmitted through the line-contact zones between the rollers and the raceways. This line-contact structure grants the NU-type bearing a radial load-carrying capacity significantly higher than that of a deep-groove ball bearing of the same size—typically 1.5 to 3 times greater—and comparable to that of other single-row Cylindrical Roller Bearings such as the N-type and NJ-type. The flanges on both sides of the outer ring serve two purposes: first, they guide the roller end faces, preventing the rollers from tilting or shifting axially during operation; second, they retain the rollers and cage within the outer ring, allowing the outer ring assembly to be installed or removed as a single unit. The inner ring is a smooth, unflanged ring that provides only the raceway surface and offers no axial restraint.

Floating Characteristics

The floating capability of the NU type stems from the lack of axial restraint between the inner ring and the outer ring assembly. When the shaft undergoes thermal elongation due to rising operating temperatures, the inner ring slides axially relative to the outer ring assembly along with the shaft. The thermal expansion is absorbed internally by the bearing, preventing the generation of additional axial forces within the shaft system. This characteristic is the primary reason why NU type bearings are widely used at the floating end (non-locating end) of shaft systems. In long shaft systems (such as motor rotors, pump shafts, multi-support drive shafts, or internal combustion engine crankshafts), thermal elongation can reach several millimeters; if locating bearings were used at both ends, thermal expansion could cause the bearings to seize or the shaft system to deform. Therefore, a floating bearing must be installed at one end.

It should be noted that while the inner ring of the NU type can move axially relative to the outer ring, the extent of this movement is limited by the difference between the roller length and the raceway width (designated as "s" in product catalogs). Designers must ensure that the shaft's thermal elongation does not exceed this permissible displacement.

Model Designation Format

The model designation format for NU type bearings is: NU + Dimension Series + Bore Code, followed by suffixes indicating parameters such as cage material, precision class, and internal clearance group.

NU: Indicates a single-row cylindrical roller bearing with an outer ring having two integral flanges and an inner ring with no flanges.

Bore Code: 00=10mm, 01=12mm, 02=15mm, 03=17mm; for 04 and above, multiply the code by 5 to obtain the bore diameter in millimeters.

For example, NU2310 designates a single-row cylindrical roller bearing of the NU type, dimension series 23 (wide, medium-duty), with a bore diameter of 50mm. In the suffix codes: M denotes a solid copper alloy cage; EM denotes a reinforced copper cage; J denotes a pressed steel cage; TVP denotes a nylon cage; P6, P5, and P4 indicate precision classes; C1 through C5 indicate radial internal clearance groups; and W33 indicates an outer ring with a lubrication groove and oil holes.

Series Applications

Dimension series: Commonly used series include NU2 (light, narrow), NU3 (medium, narrow), NU4 (heavy, narrow), NU10 (extra-light, wide), NU22 (wide, light), and NU23 (wide, medium).

NU2 Series: Moderate loads and relatively high speeds; widely used in electric motors, pumps, compressors, and gearboxes; the most common NU series.

NU3 Series: Higher load-carrying capacity; suitable for heavy-duty equipment such as speed reducers, internal combustion engine crankshafts, and construction machinery.

NU4 Series: Extra-heavy loads and low-to-medium speeds; used in mining machinery, metallurgical equipment, and rolling mill auxiliary rolls.

NU10 Series: Small outer diameter and large width; suitable for applications with limited radial space, such as machine tool spindles and precision transmission units.

NU22 & NU23 Series: Increased width and further enhanced load-carrying capacity; suitable for applications with ample installation width and high rigidity requirements.

Key Advantages

High radial load-carrying capacity: The line-contact structure ensures a large load distribution area, making it suitable for heavy and shock loads; the dynamic load rating is far higher than that of deep-groove Ball Bearings of the same size.

High limiting speed: Pure rolling occurs between the rollers and raceways, and the sliding contact surfaces between the outer ring ribs and roller ends are precision-machined; the overall friction coefficient and heat generation are low, resulting in limiting speeds close to those of deep-groove ball bearings of the same size.

Outer ring assembly removable as a unit: Double ribs on the outer ring retain the rollers and cage, allowing the outer ring assembly to be installed or removed as a single unit—a key maintenance advantage of the NU type. The NU design is highly convenient when the outer ring needs to be frequently removed from the bearing housing while the inner ring remains on the shaft. Since the inner ring stays on the shaft and the outer ring assembly can be removed as a single unit, maintenance efficiency is high; this makes it particularly suitable for split-type bearing housings and crankcase assemblies.

Simple inner ring structure and low cost: The inner ring is a plain ring without flanges, requiring fewer machining steps and resulting in lower manufacturing costs compared to flanged inner rings; additionally, the inner ring can be replaced independently.

Thermal expansion compensation: Acting as a non-locating (floating) bearing, it accommodates the shaft's thermal expansion, preventing additional axial forces caused by temperature rises and avoiding premature fatigue failure of the bearing.

Limitations

Completely incapable of supporting axial loads; it cannot serve as the locating bearing for the shaft system and must be paired with a locating bearing.

Sensitive to installation coaxiality; the permissible misalignment between the inner and outer ring axes is minimal—typically no more than 2 to 4 arc-minutes. Exceeding this limit causes stress concentration at the roller edges, significantly shortening service life.

The lack of inner ring flanges means the rollers are not guided on the inner ring side; consequently, roller stability at high speeds is slightly lower than in designs with flanged inner rings, placing higher demands on cage design and lubrication conditions.

While the outer ring assembly offers good rigidity, the inner ring's rigidity is lower; as it is a plain ring, care must be taken regarding the effect of inner ring expansion on internal clearance when using a tight interference fit.

Application Scenarios

NU-type bearings are among the most widely used cylindrical roller bearings in industry, with applications including:

Internal combustion engine crankshafts: This is the most representative application for NU-type bearings. Crankcases often feature a split design, requiring the outer ring assembly to be removed from the crankcase during maintenance while the inner ring remains on the crankshaft; the NU type's ability to have its outer ring assembly removed as a single unit perfectly matches this maintenance requirement. Gasoline and diesel engines: NU-type bearings are widely used for crankshaft main bearings and connecting rod bearings.

Speed reducers and gearboxes: In cylindrical gear reducers and planetary gearboxes, NU-type bearings are used on high-speed, intermediate, and low-speed shafts. They leverage their radial load-carrying capacity and floating characteristics to compensate for thermal expansion of the shafts; the separable outer ring assembly also facilitates gearbox maintenance.

Electric motors: Used at the non-drive end (floating end) of small and medium-sized AC motors—paired with deep-groove ball bearings or angular contact ball bearings at the drive end—to compensate for rotor thermal expansion. Both NU and N types are widely used in the motor industry, with some manufacturers standardizing on one type based on traditional design practices.

Pumps and compressors: Used for rotor support in centrifugal pumps, vacuum pumps, and screw compressors, utilizing their high-speed performance and floating characteristics; the separable outer ring assembly facilitates maintenance of the pump body.

Mining and metallurgical machinery: Used for drive shafts in crushers, vibrating screens, conveyors, and rolling mill auxiliary rolls. These applications involve heavy radial loads and shock loads; heavy-duty series such as the NU4 are widely used in this field.

Construction machinery: Used in the transmissions and travel mechanisms of loaders, excavators, and bulldozers, where they must withstand variable loads and shocks in harsh operating environments.

Railways and vehicles: Used in some railway freight car axle boxes and subway wheelsets, utilizing the radial load-carrying capacity and floating characteristics of the NU type.

In these applications, NU-type bearings typically serve as the floating-end bearing. Axial positioning is handled at the other end of the shaft system by locating bearings—such as angular contact ball bearings, Tapered Roller Bearings, or NJ/NUP types—creating a complete shaft support solution.

Distinctions from other types

The five basic types of single-row cylindrical roller bearings share the same rolling element and raceway designs; the core differences lie in the rib configuration and axial positioning capability. As the quintessential floating bearing, the NU type differs from other types as follows:

NU type vs. N type: This is the comparison requiring the most careful distinction. Both are purely floating bearings with identical functionality; they support only radial loads, and their radial load-carrying capacity, limiting speeds, and dimensional series are essentially the same. The key differences lie in which ring features the integral ribs and the method of separability: Type N features integral ribs on the inner ring and a ribless outer ring; the inner ring assembly (inner ring, rollers, and cage) can be removed as a unit, while the outer ring is a separate, smooth-surfaced ring. Type NU features integral ribs on the outer ring and a ribless inner ring; the outer ring assembly (outer ring, rollers, and cage) can be removed as a unit, while the inner ring is a separate, smooth-surfaced ring. Selection depends entirely on installation and maintenance requirements: Type N is more convenient if the inner ring needs frequent removal from the shaft while the outer ring remains in the housing bore; Type NU is more convenient if the outer ring needs frequent removal from the housing bore while the inner ring remains on the shaft. Type NU is the preferred choice for applications requiring frequent removal of the outer ring assembly, such as internal combustion engine crankcases and split gearboxes; for applications requiring frequent removal of the inner ring assembly, such as electric motor rotors and pump shafts, both Type N and Type NU are used, with the choice depending on manufacturer preference.

Type NU vs. Type NJ: Both feature integral ribs on the outer ring, allow the outer ring assembly to be removed as a unit, and share the same separability method. The difference lies in the inner ring: Type NU has a ribless inner ring and acts as a pure floating bearing, incapable of supporting axial loads; Type NJ has an inner ring with a single integral rib, allowing it to support axial loads in one direction (acting as a unidirectional locating bearing). Type NU has a simpler structure and lower cost, making it suitable for floating-end applications requiring no axial location; Type NJ is suitable for applications requiring some axial constraint and can be paired with an HJ angle ring to achieve bidirectional location.

Type NU vs. Type NF: Type NF is a unidirectional locating bearing with integral ribs on both the inner ring and a single rib on the outer ring, differing significantly from Type NU. The inner ring assembly of Type NF can be removed as a unit, and it can support axial loads in one direction; conversely, the outer ring assembly of Type NU can be removed as a unit, but it cannot support axial loads at all. They differ in both separability methods and axial locating capabilities.

Type NU vs. Type NUP: Type NUP is a bidirectional locating bearing featuring integral ribs on the outer ring and an inner ring with a single integral rib plus a loose flat rib ring; while the outer ring structure is identical to that of Type NU, the inner ring structures differ significantly. NUP-type bearings can withstand axial forces in both directions but feature a complex structure and higher cost; NU-type bearings are fully floating, featuring a simple structure and lower cost. NUP-type bearings are suitable for applications requiring compact, bidirectional axial location, whereas NU-type bearings are suitable for use at the purely floating end.

Installation and Removal

The installation of NU-type bearings follows the assembly procedures for separable bearings. The inner ring typically has an interference fit with the shaft; for small bearings, a press and sleeve are used to press the ring onto the shaft, ensuring the pressing force is applied to the inner ring's end face. For medium and large bearings, induction heating is used to uniformly heat the inner ring to 80–100°C before quickly mounting it onto the shaft journal; the heating temperature must not exceed 120°C to prevent the bearing steel from tempering and softening. The outer ring assembly usually has a transition or clearance fit with the housing bore and can be installed using a press or a cold-fitting method; ensure the outer ring's end face sits flush against the housing shoulder during installation. When assembling, align the outer ring assembly with the inner ring and push it in steadily, paying attention to the alignment of the rollers with the inner ring raceway to avoid tilting or jamming.

Since the outer ring assembly can be removed as a unit, it can be dismantled during maintenance while the inner ring remains on the shaft, facilitating inspection and replacement. To remove the outer ring assembly, use a press and a spacer block to push it out via the disassembly notches in the housing shoulder, or use a specialized removal tool. To remove the inner ring, use a hydraulic puller with the jaws applied to the inner ring's end face; if the interference fit is tight, induction heating may be used to heat the inner ring before pulling it off while hot. During disassembly, it is strictly prohibited to strike bearing components directly with a hammer or to transmit impact forces through the rolling elements.

Lubrication and Maintenance

Proper lubrication and maintenance are crucial for the long-term, reliable operation of NU-type bearings. Grease lubrication is used for low-to-medium speed applications, with a fill volume of one-third to one-half of the bearing's internal free space; overfilling can cause heat generation due to churning. Oil lubrication is used for high-speed applications, utilizing methods such as circulating oil, oil jet, or oil mist lubrication. Since the inner ring lacks integral flanges, the guidance of the rollers on the inner ring side relies entirely on the cage. High-speed operation places significant demands on cage strength and lubrication; therefore, solid copper alloy cages (M-type) or reinforced copper cages (EM-type) are preferred.

Operational monitoring should cover three key indicators: temperature, vibration, and noise. Normal operating temperatures generally do not exceed 80°C, and the rate of temperature rise should not exceed 15°C per hour. Vibration monitoring can be performed by measuring the RMS value of bearing housing vibration using accelerometers or by employing the shock pulse method to detect early-stage defects. Abnormal noise is often an early sign of bearing failure and requires prompt investigation. During periodic inspections, particular attention should be paid to the inner and outer ring raceways for signs of pitting, spalling, scratches, or discoloration (blue or black hues indicate overheating/burning); the roller ends and outer ring flanges for wear or scuffing; and the cage for cracks, deformation, or loose rivets.

The selection of radial internal clearance directly affects the bearing's operating condition and service life. The standard clearance for NU-type bearings is the C0 group (normal clearance). However, if there is a significant interference fit between the inner ring and the shaft, high operating temperature rises, or high housing rigidity, C3 or C4 clearance groups should be selected to compensate for clearance reduction caused by the fit and changes resulting from thermal expansion. Insufficient clearance can lead to heat generation and roller jamming, while excessive clearance causes increased vibration and noise, as well as uneven load distribution.

Summary

NU Type Cylindrical Roller Bearings is a single-row, "pure floating" bearing featuring an outer ring with two integral flanges and an inner ring without flanges. It supports only radial loads and is primarily used at the floating end of a shaft system to accommodate thermal expansion. Its key structural feature is that the outer ring assembly can be removed as a unit, while the inner ring is a separate, smooth-surfaced component. This design makes it particularly suitable for maintenance scenarios requiring frequent removal of the outer ring assembly, such as in internal combustion engine crankcases and split-type gearboxes. NU-type and N-type bearings are functionally identical; the difference in selection lies solely in which ring features the integral flanges and the method of separability—choose the N-type if the inner ring assembly requires frequent removal, and the NU-type if the outer ring assembly requires frequent removal. NU-type bearings offer advantages such as high radial load capacity, high limiting speeds, simple and cost-effective construction, and ease of maintenance. Widely used in applications ranging from internal combustion engines and gear reducers to electric motors, pumps, and mining machinery, they are among the most widely utilized cylindrical roller bearings in the industrial sector.