Product Introduction

NN Type Cylindrical Roller Bearings is the most widely used precision product among double-row Cylindrical Roller Bearings. Its structural characteristics include an inner ring with integral ribs, a smooth outer ring without ribs, and two internal rows of cylindrical rollers. The bearing consists of two main parts: an inner ring assembly (comprising the inner ring, two rows of rollers, and the cage) and the outer ring. The inner ring features ribs that axially retain the two rows of rollers and the cage, forming a separable inner ring assembly; the outer ring is a smooth, ribless ring that can be separated independently.

Compared to single-row Cylindrical Roller Bearings, the fundamental difference of the NN type is the side-by-side arrangement of two roller rows, which share the radial load. Consequently, the radial load-carrying capacity and radial rigidity are significantly higher than those of single-row bearings of the same size. NN-type bearings are primarily designed for high-precision, high-rigidity applications and serve as core supporting components for equipment such as machine tool spindles and precision transmission units.

Structural Features

The inner ring of an NN 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 featuring side ribs combined with a central spacer ring to achieve the same function. The outer ring is a smooth, ribless component providing only the raceway surfaces, without axial retention capabilities. Therefore, the inner ring assembly can be removed as a unit, and the outer ring can be separated—a configuration similar to the separable nature of single-row N-type bearings.

Regarding the cage, NN-type precision bearings generally employ solid copper alloy cages (MB type) that are roller-guided (rather than rib-guided). This design minimizes friction between the cage and the ribs, thereby enhancing limiting speeds and operational precision. Certain high-speed series may utilize nylon or light alloy cages.

Working Principle

In terms of operation, the two rows of rollers undergo pure rolling motion between the inner and outer raceways, with radial loads transmitted simultaneously through the line-contact zones of both roller rows. Due to the symmetrical arrangement of the two rows of rollers, the load is distributed evenly, resulting in a radial rigidity approximately 1.8 to 2 times that of a single-row bearing of the same size. Furthermore, the double-row structure can withstand a certain degree of tilting moment (when the shaft is subjected to a bending moment, the load increases on one row of rollers and decreases on the other, creating a couple that resists tilting)—a capability single-row bearings lack.

NN-type bearings feature an outer ring without ribs; consequently, the inner ring can move axially relative to the outer ring, providing a "floating" capability. In practice, however, NN-type bearings are often paired with angular contact Ball Bearings or thrust bearings. The latter handle axial positioning, while the NN-type bearing focuses on providing high-rigidity radial support. In some applications, the NN-type bearing may also be used independently at the floating end.

Model Designations and Series

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

- NN: Indicates a double-row cylindrical roller bearing with ribs on the inner ring.

- Dimension Series: The most common is the 30 series (extra-light wide type); others include the 49 series (ultra-light wide type) and variants of the 30 series.

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

The NN30 series is the primary NN-type bearing series, with bore diameters ranging from 20mm (NN3004) to 240mm (NN3048); it is widely used in machine tool spindles. For example, NN3016K designates an NN-type, 30-series, double-row cylindrical roller bearing with an 80mm bore and a tapered bore. The suffix code is crucial for selecting NN-type bearings; common codes include:

- K: Tapered bore (taper 1:12); used for direct mounting on a tapered shaft journal or with an adapter sleeve; standard configuration for precision spindle bearings.

- MB: Solid brass cage, roller-guided; the most common cage type for precision bearings.

- P4, P5: Precision classes; P4 is widely used for machine tool spindles, while P5 is used for general precision equipment.

- C1, C2: Reduced internal clearance groups; precision bearings are often adjusted to zero clearance or a preload state after mounting by driving the bearing further onto the tapered shaft, so manufacturers often supply them with C1 or C2 clearance.

- W33: Outer ring features a lubrication groove and three lubrication holes; suitable for oil lubrication applications.

- SP, UP: Special precision classes (e.g., from SKF) corresponding to P4 and higher-than-P4 precision levels, respectively.

Key Advantages and Limitations

Advantages

High radial load-carrying capacity: Two rows of rollers share the load; the dynamic load rating is approximately 1.8 to 2 times that of a single-row bearing of the same size, making it suitable for heavy-load applications.

Extremely high radial rigidity: The symmetrical double-row arrangement results in significantly higher radial rigidity compared to single-row bearings, effectively minimizing spindle deflection under cutting forces and enhancing machining accuracy.

Capable of withstanding tilting moments: The two rows of rollers form a couple, enabling the bearing to withstand bending and tilting moments; particularly suitable for cantilevered spindles and long shaft systems.

High precision class: NN-type bearings are commonly manufactured to P5, P4, or even higher precision levels; their rotational and dimensional accuracy far exceeds that of standard Industrial Bearings, meeting the requirements of precision machine tools.

Separable mounting and adjustable preload: The inner ring assembly and outer ring are separable; the tapered bore design (K-type) allows for precise adjustment of radial clearance and preload by controlling how far the inner ring is driven onto the tapered shaft journal, thereby achieving the optimal balance of rigidity and accuracy. Limitations

Weak axial load-carrying capacity: The outer ring lacks integral ribs, meaning the NN type cannot support axial loads on its own; it must be paired with angular contact ball bearings or thrust bearings.

Lower limiting speed than single-row types: Friction and heat generation associated with the double-row rollers and solid cages are higher than in single-row versions; consequently, the limiting speed is usually lower than that of single-row cylindrical roller bearings of the same size.

High cost: Factors such as the double-row structure, high precision class, use of brass cages, and tapered bore machining result in a price significantly higher than that of standard single-row bearings.

Extremely high requirements for installation coaxiality and journal precision: The performance of these precision bearings relies on the machining accuracy of the journal and housing bore; improper installation can lead to a loss of precision and premature failure.

Application Scenarios

Applications for NN Type Cylindrical Roller Bearings center on "high-precision, high-rigidity radial support." Typical scenarios include:

Machine tool spindles: This is the primary application area for the NN type. Spindle support systems for lathes, grinders, milling machines, machining centers, and boring machines typically employ a combination of an NN-type double-row cylindrical roller bearing and an angular contact ball bearing—with the NN bearing providing high-rigidity radial support, while the angular contact ball bearing handles axial positioning and preload. NN30 series P4-grade bearings are widely used in applications such as grinding machine wheel spindles, lathe spindle heads, and machining center spindles.

Precision transmission devices: Input and output shafts for harmonic reducers, planetary gear reducers, and precision gearboxes require high rigidity and low rotational error.

Measuring instruments and inspection equipment: Precision rotary axes in coordinate measuring machines (CMMs), roundness testers, and gear inspection centers require extremely high rotational accuracy.

High-speed rotating equipment: Supports for certain high-speed centrifuges, turbomolecular pumps, and high-speed grinding heads require a balance between high rigidity and high rotational speed.

Printing and papermaking machinery: Supports for high-precision printing press rollers and papermaking machine drying cylinders utilize the high rigidity and load-sharing characteristics of double-row bearings.

Textile machinery: Roller shafts in high-speed spinning machines and texturing machines require high rotational speeds and low vibration.

In machine tool spindle applications, NN-type bearings typically employ a **tapered bore (K-type) and tapered journal** mounting configuration; the axial displacement of the inner ring is adjusted via a locknut to precisely control the bearing's radial clearance and preload. The selection of the preload level directly impacts spindle rigidity, temperature rise, and service life, making it a critical process in spindle assembly.

Comparison between double-row and single-row bearings

Load-carrying capacity: The dynamic load rating of double-row NN-type bearings is approximately 1.8 to 2 times that of single-row bearings of the same size, and the static load rating is also significantly higher, making them suitable for heavy-load applications.

Rigidity: The symmetrical double-row arrangement significantly enhances radial rigidity and allows the bearing to withstand certain overturning moments; single-row bearings offer lower rigidity and cannot withstand overturning moments.

Rotational speed: Single-row bearings have higher limiting speeds; double-row bearings have slightly lower limiting speeds due to increased friction surfaces and heat generation.

Precision: NN-type bearings are commonly manufactured to high-precision P4 and P5 grades, whereas single-row bearings are predominantly P0 and P6 (standard precision) grades, with high-precision versions being relatively rare.

Cost: Due to their complex structure and high precision requirements, double-row bearings typically cost 2 to 4 times as much as single-row bearings of the same size. Application Positioning: Single-row bearings are designed for the floating end or unidirectional locating end of general industrial equipment, emphasizing versatility and cost-effectiveness; double-row NN-type bearings are intended for applications requiring high precision and rigidity, such as machine tool spindles.

In short: choose single-row bearings for standard industrial drive applications, and double-row bearings for high-rigidity, precision spindle applications.

Installation and Maintenance

Proper installation is critical to the performance of NN Type Cylindrical Roller Bearings. For bearings with a tapered bore (K-type), the installation procedure is as follows: first, heat the inner ring assembly to 80–90°C (using induction or an oil bath; open-flame heating is strictly prohibited) and mount it onto the tapered journal. Then, 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. Refer to the bearing catalog for the relationship between the axial drive distance and the reduction in clearance; precision spindles typically require a zero-clearance or light-preload state after installation. The outer ring is fitted into the housing bore using a transition or clearance fit; ensure proper alignment between the two rows of rollers and the outer ring raceways during assembly.

Installation of cylindrical-bore NN-type bearings is relatively straightforward: the inner ring is mounted via an interference fit (press-fit or thermal mounting), and the outer ring is simply fitted into the housing bore. However, preload cannot be adjusted during installation, so the bearing must rely on the factory-set clearance class. For disassembly, first loosen the lock nut on tapered-bore bearings, then release the inner ring from the tapered journal using a hydraulic nut or the oil injection method (for models with oil holes) before pulling it off; the outer ring is pressed out of the housing bore. Never transmit impact forces through the rolling elements.

Regarding maintenance, NN-type precision bearings have stringent lubrication requirements. Machine tool spindles typically utilize oil lubrication (circulating oil, oil-air, or oil-mist lubrication) to control temperature rise and ensure precision; grease lubrication is used in some low-speed applications, but high-speed precision grease is required. Lubricant cleanliness directly impacts bearing service life; precision filtration (typically NAS Class 7 or better) should be employed. Monitor spindle temperature (precision spindles typically require a temperature rise of no more than 15–25°C), vibration, and noise during operation, and shut down the machine for inspection immediately if any abnormalities are detected. During periodic disassembly and inspection, check for signs of pitting, spalling, scratching, or bluing on the raceways and roller surfaces; inspect the cage for wear; and examine the tapered mating surfaces for scuffing. Precision bearings should be stored horizontally with oil seals intact and protected against moisture and impact.

Summary

NN Type Cylindrical Roller Bearings is a high-rigidity precision bearing featuring an inner ring with integral ribs, a ribless outer ring, and two rows of internal rollers. Its radial load-carrying capacity and radial rigidity are approximately 1.8 to 2 times those of single-row bearings of the same size, and it can withstand a certain degree of overturning moment. The NN30 series P4-class tapered-bore bearing is a standard component for machine tool spindles; the preload can be precisely adjusted by controlling the axial drive-up distance on the tapered bore, thereby achieving an optimal balance of rigidity and accuracy. Compared to single-row cylindrical roller bearings, the NN type offers distinct advantages in load capacity, rigidity, and accuracy, though it entails higher costs and a slightly lower limiting speed; furthermore, it must be paired with angular contact ball bearings to handle axial loads. NN-type bearings are widely used in applications demanding exceptional rotational accuracy and rigidity—such as machine tool spindles, precision transmission systems, and measuring instruments—serving as core foundational components in precision manufacturing equipment.