Product Introduction

The NNU type double-row Cylindrical Roller Bearing is a high-rigidity, precision bearing featuring an outer ring with integral ribs, an inner ring without ribs, and two rows of internal rollers. Its radial load-carrying capacity and radial rigidity are approximately 1.8 to 2 times those of a single-row bearing of the same size, and it is capable of withstanding a certain degree of tilting moment.

The NNU type is a mainstream model among double-row cylindrical Roller Bearings. Structurally, it features an outer ring with ribs, a smooth inner ring without ribs, and two rows of cylindrical rollers. The bearing consists of two main parts: an outer ring assembly (comprising the outer ring, two rows of rollers, and cages) and the inner ring. The outer ring's ribs axially retain the two roller rows and cages, forming a separable outer ring assembly; the inner ring is a smooth ring with two raceways but no ribs, allowing it to be separated independently.

Both NNU and NN types belong to the double-row cylindrical roller bearing family, sharing comparable radial load capacity, rigidity, and precision grades. The core difference lies in the placement of the ribs: the NN type features ribs on the inner ring and a ribless outer ring, whereas the NNU type features ribs on the outer ring and a ribless inner ring. This structural difference dictates their respective separation methods and application suitability. The NNU type allows the outer ring assembly to be removed as a single unit, while the inner ring is a simple smooth ring; this offers distinct advantages in applications requiring frequent removal of the outer ring assembly or where cost-effective inner ring manufacturing is prioritized.

Structural Features

The outer ring of an NNU type bearing typically features three ribs (one on each side and a central rib) that separate the two rows of rollers into distinct raceways; the central rib also guides the inner end faces of both roller rows. Some series utilize a design with side ribs and a central spacer ring, which serves an equivalent function. The inner ring is a smooth ring without ribs, providing only the raceway surfaces without offering axial retention. Consequently, the outer ring assembly can be removed as a unit while the inner ring remains separable—a configuration similar to the separation method of the single-row NU type bearing. Regarding the cage, NNU-type precision bearings typically utilize solid brass cages (MB type) guided by the rollers; this design minimizes friction between the cage and the guide ribs, thereby enhancing limiting speeds and operational precision. Certain heavy-duty series employ steel or bronze cages.

Operating Principle

In terms of operation, the two rows of rollers undergo pure rolling motion between the outer and inner raceways. Radial loads are shared by both rows of rollers and transmitted through line-contact zones. The symmetrical double-row arrangement grants the NNU type a radial load-carrying capacity approximately 1.8 to 2 times that of a single-row bearing of the same size, along with significantly higher radial rigidity. Furthermore, the two rows of rollers can generate a couple, enabling the bearing to withstand certain overturning and bending moments—capabilities not found in single-row bearings.

The inner ring of the NNU type lacks integral ribs, allowing it to move axially relative to the outer ring assembly and providing a "floating" characteristic. In practical applications, however, NNU bearings are typically paired with angular contact Ball Bearings or thrust bearings; the latter handle axial positioning, while the NNU bearing focuses on providing high-rigidity radial support. The design featuring integral ribs on the outer ring enhances the overall rigidity of the outer ring assembly, ensuring stable operation even when the machining precision of the bearing housing bore is moderate. This is a key reason why NNU bearings are widely adopted in industrial equipment such as gearboxes.

Common Models and Series

The designation format for NNU bearings is: NNU + dimension series + bore diameter code. Suffixes are used to indicate parameters such as tapered bore, cage type, precision class, and internal clearance. - NNU: Indicates a double-row cylindrical roller bearing with an outer ring featuring integral ribs.

- Dimension series: Common ones include the 49 series (extra-light wide type) and 30 series (ultra-light wide type); others include the 48 series, 39 series, etc.

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

Unlike the NN type, where the NN30 series is the primary offering, the commonly stocked series for the NNU type are more varied.

The NNU49 series is the most widely used NNU variant, with bore diameters ranging from 20mm (NNU4904) to 240mm (NNU4948); it is extensively applied in machine tool spindles, gearboxes, and precision transmission units.

The NNU30 series competes with the NN30 series in certain machine tool spindle and precision equipment applications. For example, NNU4920K/P4 designates a double-row cylindrical roller bearing of the NNU type, 49 series, with a 100mm bore, a tapered bore, and P4 precision class.

Regarding supplementary codes, the NNU type largely follows the same conventions as the NN type: K indicates a tapered bore (1:12 taper), MB denotes a solid brass cage, P4 and P5 indicate precision classes, C1 and C2 denote reduced internal clearance groups, and W33 indicates an outer ring with an oil groove and lubrication holes. Because the NNU type features integral ribs on the outer ring, machining the outer ring's oil groove and lubrication holes is more convenient; consequently, the W33 feature is applied more frequently in NNU bearings than in NN bearings.

Key Advantages and Disadvantages

The advantages of NNU-type double-row Cylindrical Roller Bearings are primarily reflected in the following aspects:

High radial load-carrying capacity and rigidity: With two rows of rollers sharing the load, the dynamic load rating is approximately 1.8 to 2 times that of a single-row bearing of the same size; radial rigidity is significantly higher than that of single-row bearings, and they can withstand a certain degree of tilting moment. Outer ring assembly can be removed as a unit: The outer ring features integral ribs that retain the rollers and cage, allowing the outer ring assembly to be installed or removed as a single unit. The NNU type is highly convenient when the outer ring must be frequently removed from the housing while the inner ring remains on the shaft; it is particularly suitable for split gearboxes, crankcases, and similar structures requiring frequent maintenance of the outer ring assembly.

Simple inner ring structure and cost-effective manufacturing: The inner ring is a smooth, ribless component with two raceways; it requires fewer machining steps and costs less to manufacture than inner rings with ribs, and it can be replaced independently. Cost savings on the inner ring are even more significant for large-sized bearings.

High outer ring rigidity and adaptability to housing bores: With ribs on both sides, the outer ring offers superior overall rigidity compared to ribless designs; it maintains good operating accuracy and load distribution even when housing bore rigidity is insufficient or machining precision is only moderate.

High precision class and adjustable preload: NNU bearings are commonly produced to P5 or P4 precision classes. The tapered bore design (K-type) allows for precise adjustment of radial clearance and preload based on the inner ring's axial displacement along the tapered shaft journal, meeting the requirements of precision equipment.

Limitations

Weak axial load-carrying capacity: As the inner ring lacks ribs, the NNU type cannot withstand axial loads on its own and must be paired with angular contact ball bearings or thrust bearings.

Inner ring assembly cannot be removed as a unit: Since the inner ring is a smooth component and the rollers/cage are retained by the outer ring, removing the inner ring from the shaft while leaving the outer ring in the housing is less convenient than with the NN type.

Lower limiting speed than single-row bearings: Friction and heat generation from the double row of rollers and solid cage are higher than in single-row designs; consequently, the limiting speed is usually lower than that of a single-row cylindrical roller bearing of the same size.

Higher cost than single-row bearings: The double-row structure, high precision class, and brass cage result in a price significantly higher than that of standard single-row bearings. Application Scenarios

Applications 

NNU-type double-row cylindrical roller bearings center on the combination of "high-rigidity radial support" and "separable outer ring assembly for maintenance." Typical scenarios include:

- Machine Tool Spindles: A key application area for NNU-type bearings. Spindle support for lathes, grinders, milling machines, and machining centers often employs a combination of "NNU-type double-row cylindrical roller bearings + angular contact ball bearings."

- Gearboxes and Speed Reducers: NNU-type bearings are used more frequently than NN-type bearings in industrial gearboxes, planetary reducers, and wind turbine gearboxes. Gearboxes often feature a split-housing design; during maintenance, the outer ring assembly must be removed from the housing while the inner ring remains on the shaft. The NNU type's ability to have its outer ring assembly removed as a single unit perfectly meets this requirement. Additionally, the rigid double-flanged outer ring accommodates the machining conditions of gearbox housing bores.

- Precision Transmission Devices: Output shafts for harmonic reducers, precision gearboxes, and servo motors, which require high rigidity and low rotational error.

- Papermaking and Printing Machinery: Support for drying cylinders and calender rolls in papermaking machines, as well as printing press cylinders. These applications leverage the high rigidity and load-sharing characteristics of double-row bearings, while the separable outer ring assembly facilitates the maintenance of large rolls.

- Wind Turbine Gearboxes: Planetary carriers, intermediate shafts, and high-speed shafts in wind turbine gearboxes, which are subject to heavy radial loads and impacts. The separable outer ring assembly of the NNU type facilitates on-site maintenance.

- Mining and Metallurgical Equipment: Drive shafts for crushers, auxiliary rolls in rolling mills, and conveyors. Heavy-duty series, such as the NNU4 series, are used for these high-load operating conditions.

- Measuring Instruments and Testing Equipment: Precision shafts for certain high-precision rotary tables and coordinate measuring machines (CMMs), which demand high rigidity and rotational accuracy.

In machine tool spindle applications, the choice between NNU-type and NN-type bearings often depends on the maintenance strategy: choose the NN type if the design requires frequent removal of the inner ring assembly from the shaft for maintenance; choose the NNU type if the outer ring assembly needs to be frequently removed from the spindle housing. The performance difference between the two is minimal; ease of maintenance is the primary factor in the decision-making process.

A Brief Comparison: Double-Row vs. Single-Row, and NNU vs. NN

Double-row NNU-type vs. single-row cylindrical roller bearings: The dynamic load rating of the NNU type is approximately 1.8 to 2 times that of a single-row bearing of the same size. It offers significantly higher radial rigidity and can withstand tilting moments; however, its limiting speed is slightly lower, and its cost is 2 to 4 times that of the single-row version. Single-row bearings are designed for general industrial transmissions, prioritizing versatility and cost-effectiveness, whereas NNU-type bearings are intended for precision, high-rigidity applications, emphasizing accuracy and stiffness.

NNU-type vs. NN-type: Both are double-row cylindrical roller bearings with comparable radial load capacity, rigidity, and precision grades. The core differences lie in the rib configuration and separability: the NN type features ribs on the inner ring and no ribs on the outer ring, allowing the inner ring assembly to be removed as a unit; conversely, the NNU type features ribs on the outer ring and no ribs on the inner ring, allowing the outer ring assembly to be removed as a unit. The selection logic mirrors the relationship between N-type and NU-type single-row bearings: choose the NN type if the inner ring assembly requires frequent removal, and the NNU type if the outer ring assembly requires frequent removal. Additionally, the NNU type offers simpler inner ring manufacturing, lower costs, superior outer ring rigidity, and better adaptability to housing bores, leading to higher usage rates in industrial equipment such as gearboxes and wind turbines. The NN type, however, is more prevalent in machine tool spindles, with some manufacturers maintaining a long-standing tradition of using this type.

Installation and Maintenance Essentials

The installation process for precision NNU-type bearings is similar to that of the NN type. When installing bearings with a tapered bore (K-type), first heat the inner ring to 80–90°C (using induction or an oil bath; open-flame heating is prohibited) and mount it onto the tapered journal. Use a lock nut to drive the inner ring forward to the specified torque while monitoring the reduction in radial clearance until the design-specified preload or clearance value is achieved. The outer ring is installed into the housing bore using a transition or clearance fit; during assembly, align the outer ring assembly with the inner ring and push it in steadily, ensuring proper alignment between the two rows of rollers and the inner ring raceways.

Since the outer ring assembly can be removed as a single unit, it can be dismantled first during maintenance while the inner ring remains on the shaft, facilitating inspection and replacement. To remove the outer ring assembly, use a spacer and a press to push it out via the disassembly notches in the housing shoulder, or use a specialized removal tool; the inner ring is removed using a hydraulic puller, or via induction heating if the interference fit is tight. Never transmit impact forces through the rolling elements.

Regarding maintenance, NNU-type precision bearings have stringent lubrication requirements. Machine tool spindles typically utilize oil lubrication (circulating oil, oil-air, or oil-mist lubrication), while industrial equipment such as gearboxes employs oil bath or circulating oil lubrication. Because the NNU type features an outer ring with integral ribs, machining the outer ring's lubrication groove and holes (W33) is more convenient; specifying the W33 configuration can improve lubrication conditions. Lubricant cleanliness directly impacts bearing service life, so precision filtration should be employed. Monitor temperature, vibration, and noise during operation, and shut down for inspection immediately if abnormalities are detected. During periodic inspections, check for pitting, spalling, scratching, or bluing on raceway and roller surfaces, as well as wear on outer ring ribs and the condition of the cage. Precision bearings should be stored horizontally with oil seals intact, protected from moisture and impact.

Summary

The core structural feature of the NNU type is that the outer ring assembly can be removed as a unit, while the inner ring is a simple, smooth-surfaced ring. This characteristic makes it particularly suitable for maintenance scenarios requiring frequent removal of the outer ring assembly, such as in split-housing gearboxes and wind turbine gearboxes. Compared to the NN type, performance is essentially equivalent; the choice depends on the separation method—select the NN type if the inner ring assembly needs to be removed, and the NNU type if the outer ring assembly needs to be removed. The NNU type offers more economical inner ring machining and superior outer ring rigidity, leading to a higher rate of application in industrial gearboxes. NNU-type bearings are widely used in applications requiring high rigidity and precision—such as machine tool spindles, gearboxes, wind power equipment, and papermaking and printing machinery—serving as core supporting components for precision drive systems and heavy-duty industrial equipment.