General Basics

Functions of Rolling Bearings

Rolling bearings are machine elements and serve to support shafts and axles. Depending on their design, they absorb radial and/or axial loads and simultaneously enable rotation of the shaft or the components mounted on an axle. The force is transmitted via spherical or roller-shaped rolling elements. The advantages include the following points:

• low friction even when starting from standstill

• low cooling and lubrication requirements, grease lubrication usually sufficient

• radial, axial and combined loading capacity achievable with little effort

• almost backlash-free or preloaded operation possible

• rolling bearings are available worldwide as ready-to-install standard part series

Bearing Types

Rolling bearings are divided into different bearing types and designs. Based on the ball bearing which represents the starting point, the starting point for the technical development of rolling bearings, today there are a large number of bearing types used for very specific operating conditions. Any design of a rolling bearing, however, will be a technical compromise determined by a wide variety of criteria. Rolling bearings are selected according to:

• the available installation conditions,

• the type and extent of the load,

• speeds or motion cycles in general,

• the required guidance accuracy of the machine and plant components,

• the rigidity of the bearings,

• the ambient conditions,

• the installation and removal options

Apart from the technical parameters of a rolling bearing, such as static and dynamic load carrying capacity and permissible speeds, the bearing clearance or preload, the appropriate cage modification and the required lubrication method must be considered when selecting the bearing type.

Deep Groove Ball Bearing

Angular Contact Ball Bearings

Four Point Contact Ball Bearings

Cylindrical Roller Bearings

Tapered Roller Bearings

Spherical Roller Bearings

Cylindrical Roller Thrust Bearings

Deep Groove Thrust Ball Bearings

Special Bearings

Technical Terms in Rolling Bearing Technology

Rolling Bearing Element Designations

Standardisation and Nomenclature

Rolling bearings are internationally standardized in their dimensions (bore, outside diameter, width). The rolling bearing designations consist of logically structured combinations of letters and numbers reflecting the bearing’s design, size and properties. In addition to the standard bearings, there are special bearings or standard bearings in special designs whose designation system varies depending on the manufacturer. The DIN standard DIN 623 defines the basic principles for the designation and marking of rolling bearings.

Standardization of bearing types

*)  Further designs can be derived from the designs of cylindrical roller bearings listed in DIN 5401-1. Technical properties such as load ratings and speeds remain unchanged.

Rolling Bearing Designation System

In accordance with DIN 623-1, each rolling bearing is uniquely identified by a standard designation. The next figure shows an example of a bearing designation.

The order within the prefixes and base characters shall be as set out in Section I and Section II. The sequence of the suffixes and supplementary symbols may vary depending on the bearing design and modality. The order shown in Section III should be used for orders, but manufacturer specific variations are permitted.

Base characters must always be specified in full. The prefixes, suffixes and supplementary characters may then be missing from the abbreviation, if

• according to Section I.2, only materials for the normal case are used,

• the characteristics designated by them are not present,

• no suffixes for special design variants (e.g. PN, CN and SN) are specified for the normal case in accordance with Section III,

• no specifications have been agreed for these characteristics; the design shall then be determined at the choice of the manufacturer in accordance with the standard numbers.

Prefixes and suffixes can be added beyond the standard character strings. The statement of the standardized character string must be retained, it may only be supplemented.

f.e.:    JP3    window cage made of sheet steel, manufacturer variant 3

The names of rolling bearing types introduced according to DIN ISO 5593 are used to form names in accordance with standards, indicating the type of rolling element and the raceway geometry, e.g. deep groove ball bearings, spherical roller bearings.

In order to keep the designation short, the "radial" prefix contained in customary designations is generally suppressed. Abbreviations are used for the same reason. The standardized designations and their abbreviations are listed in DIN 623-1. This table also contains the standard numbers. When the standard designation is formed, only the main standard number is specified.

Note: Related character blocks can be divided against each other by blanks or the graphic characters hyphen (-), slash (/), horizontal cross (x) or dot (●).

I Prefixes

I.1 Bearing Parts

The prefixes are used to identify parts of complete rolling bearings.

For some rolling bearing types (e.g. cylindrical roller bearings, tapered roller bearings) the free rings or the roller and cage assemblies with the non-detachable rings can also be ordered separately. These parts are identified by the prefixes before the base character.

The parts marked L and R of a particular bearing type form a complete bearing arrangement. However, its full functionality can only be guaranteed if the parts are supplied by the same manufacturer. If the free ring consists of several parts, e.g. inner ring and flanged pulley on the cylindrical roller bearing NUP, the prefix L for the inner ring with associated flanged pulley applies accordingly.

I.2 Material

Inner and outer rings and rolling elements are normally made of rolling bearing steel according to DIN EN ISO 683-17.

Rolling bearings made of stainless steel usually have the prefix S (e.g. S 6205) or W (e.g. W 6205). A comparison of conventional rolling bearing material is shown in chapter "Rolling bearing material in comparison".

II Base Character

The base character indicates the type and size of the bearing. It usually consists of one character or character group for each (exceptions see below):

• Bearing series (see section II.1)

• Bearing bore (see section II.2)

The above system does not apply to needle roller/axial cylindrical roller bearings, needle roller/axial ball bearings, drawn cup needle roller bearings, drawn cup needle bushes, drawn cup needle roller bearings, radial needle roller and cage assemblies, axial needle roller and cage assemblies and axial washers. Here the base character is composed of characters for :

• Bearing design

• Characteristic dimension(s)

The corresponding assembly system is shown in DIN 623.

II.1 Bearing Series

The bearing series consists of the bearing type and the dimensional series. Each bearing series is identified by a group of numbers or letters or by a combination of numbers and letters.

There are two different designation systems for tapered roller bearings according to DIN 720 and ISO 355. DIN 720 corresponds to the rules of this standard, ISO 355 contains its own systematics.

II.2 Bearing bore

The symbol for the bearing bore consists of digits and is generally appended directly to the symbol for the bearing series, but in defined cases also with a slash.

Symbols for bearing bore:

III Suffixes

Suffixes are added after the base character. They are used to identify

• NG 1: internal construction

• NG 2: external shape

• NG 3: cage design

• NG 4.1: tolerances

• NG 4.2: bearing pairs

• NG 4.3: bearing clearance

• NG 5.1: material

• NG 5.2: heat treatment

• NG 6: lubrication

NG 1  - Internal Construction

NG 2  - External Shape

NG 3 - Cage Design

NG 4.1 - Tolerance

NG 4.2 - Bearing Pairs

NG 4.3 - Bearing Clearance

NG 5.1 - Material

NG 5.2 - Heat Treatment

NG 6 - Lubrication

According to customer specification

Here you can find the KRW Suffix List (PDF).

IV Supplementary character

For specifications that go beyond the symbols from I to III, manufacturer-related supplementary symbols can be specified. The commitments of the relevant product standards must be kept. With addition signs, specifications beyond the product standard are made or tolerances are narrowed.

NG 7.1 - Technical Specification

NG 7.2 - Supplementary symbol according to manufacturer's specification

Main Dimensions

Rolling bearings can be used universally as ready-to-install machine elements. This is mainly because the main dimensions of the common bearings are standardized. ISO 15 applies to radial bearings (except tapered roller bearings), ISO 355 to metric tapered roller bearings and ISO 104 to axial bearings. The dimension plans of the ISO standards were adopted in DIN 616 and DIN ISO 355 (metric tapered roller bearings).

In the dimension plans of DIN 616, several outside diameters and widths are assigned to one bearing bore. Common diameter series are 7, 8, 9, 0, 1, 2, 3, 4 (outer diameter increasing in this order). Within each diameter row there are several width rows 8, 0, 1, 2, 3, 4, 5 (with larger number increasing width).

The two-digit number for the dimension series contains first the number of the width series (for axial bearings of the height series), and second the number of the diameter series. When using the width and diameter series in practice, figures are omitted for some bearing types, e.g. for cylindrical roller bearings of width series "0" (NU0220 à NU220)

Designation Example

The following applies to the designation of a spherical roller bearing:
 

Way of Speaking

The base characters are to be separated between the bearing series and the bearing bore. The separation of the character block for the dimension series and the connection of the separated character with the character for the bearing type are not permitted when speaking.

Correct Speech:

Cage Types and Designs

General Information

The bearing cage is an important component in the rolling bearing with the following functions:

• keeps the rolling elements at a distance to prevent contact

• ensures an equal distance between the rolling elements, thus ensuring even load distribution

• guides the rolling elements

• transmits circumferential forces

• prevents the rolling elements from falling out in bearings that can be dismantled and pivoted

A distinction is made between sheet metal cages, one-piece, and multi-piece solid cages made of different materials.

Pressed Steel Cage

Pressed steel cages are bearing cages which are almost always punched or pressed from sheet steel. Rarely, brass sheet is also used. A lower weight and a good lubricant supply to the bearing interior are advantages over solid cages.

Solid Cages

Solid cages are usually used for high cage strength requirements and temperatures up to 250°C. Brass, steel, bronze, aluminum, sintered iron, plastics or hard fabric are the materials used. Solid cages made of metal or hard fabric are produced by turning and milling, solid cages made of plastic by injection molding into molds.

For large, highly loaded bearings and for small series, solid cages, primarily made of brass and steel, are an advantage. Relatively low inertial forces have solid cages made of light metal, plastic or hard tissue and are therefore used in high-speed applications in an outboard version.

Cages made of glass fiber reinforced polyamide (PA66 GF25) are installed in many large series bearings, which have a low weight, high elasticity and good sliding and emergency running properties. This has a positive effect on the life of the bearing.

Special operating conditions require a specially selected cage. However, the operating temperature limit is 120 ° C.

Pin Type Cages

Rolling elements with a central axial bore are used for rolling bearings with a welded pin cage, mostly cylindrical roller bearings, tapered roller bearings and more rarely spherical roller bearings. A steel bolt leads through this hole, which connects two side cage washers and thus ensures high strength.

Cage Guides

A distinguishing feature is that cages can be guided differently depending on the application. The cages are most frequently guided by the rolling elements (without suffixes). Cage guide on the bearing outer ring, or also known as outboard guidance, is indicated with the suffix A. Cage guide on bearing inner ring, also known as inboard guidance, is indicated by the suffix B.

Bearing Tolerances

Bearing Tolerances ensure the rolling bearing interchangeability. The values ​​of the dimensional and running tolerances are given in DIN 620. Bearings are generally manufactured in tolerance classes PN, other tolerance classes are available on request or are selected depending on the application.

Symbols Bore Diameter

Symbols Outer Diameter

Symbols Widths and Height

Symbols Run-out Tolerance

The tolerances of our bearing types can be found in the overview "Bearing Tolerances" until further notice.

Bearing Clearance

The bearing clearance is the measure about how much a bearing ring can be moved radial and axial from one end position to another. It will be distinguished between the clearance of the disassembled bearing (bearing clearance) and the clearance of the mounted, at operating temperature bearing (operation clearance, operation play). The clearance must be as low as possible for an ideal shaft guidance. During assembly, the bearing clearance is reduced by a tight fit of the bearing rings. Therefore, the bearing clearance must be higher than the necessary operation clearance. During operation, the radial clearance is reduced, if the inner ring is - as it is usually the case - warmer than the outer ring. The DIN standard 620 specifies the radial clearance standard values of roller bearings. Thereby, the standard clearance (clearance group CN) is so assessed, that the bearing has an appropriate clearance at common assembly and operation conditions. The DIN standard 620 additionally defines clearance groups with higher bearing clearance. The ISO standard 5753 for spherical roller bearings contains additional values for the clearance group C5. Divergent assembly and operation conditions, e.g. tight fitting for both bearing rings or a temperature difference greater 10 K, require broader radial clearance groups that are available upon request. A suitable clearance group must be chosen through an assessment of the fittings. The clearance values are indicated for the prime bearing designs.

Tolerances for bearing clearance:

Please use our bearing clearance calculator, to calculate the bearing clearance tolerances of our bearing designs.

Radial clearance reduction by temperature differences

The radial clearance reduction ∆e by temperature differences ∆T between the inner ring and outer ring, amounts for not adjusted bearings approximately:

If warmth is applied or deprived from the bearing surface, a stronger change in the radial bearing clearance must be estimated. The radial clearance is reduced, if warmth is applied on the shaft or deprived over the housing. A greater radial clearance results from heat supply over the housing or heat removal over the shaft. While fast start-up on operation speed, greater temperature differences result between the bearing rings as during steady state. The rotational speed must increase slowly that the bearing does not deform, or a greater radial clearance must be chosen for the as it theoretically would be necessary for the warmed-up bearing.

Radial clearance reduction by tight fits

The inner ring race widening of 80% fit oversize and an outer ring race necking of 70% fit oversize can be assumed by approximation (requirements: solid steel shaft, steel housing with normal wall thickness).

Rolling Bearing Materials in Comparison

The roller bearing capability is largely influenced by materials and heat treatment used. The material for the roller bearing rings and rolling bodies normally is low-alloy chromium steel, in special cases case-hardened steel. It a matter of high-grade steels with a high standard of purity. Steels according DIN EN ISO 683-17 are used for the roller bearing rings and rolling bodies. Based upon customer wishes, rolling bodies (balls and cylinders) made of ceramic materials (e.g. silicon nitride) In so called hybrid bearings the low density, good tribological behaviour, low thermal expansion as well as high insulation capacities of ceramic materials are used. Silicon nitride as well is used for coating the roller bearing rings (current-insulated bearings).

Dimensioning of Rolling Bearings

Through the overall machine structure or device, the roller bearing bore diameter is already determined. The final determination of the bearing size should be proven by a dimensioning calculation, if the requirements on service life, static safety and the necessary for profitability are fulfilled. Based on this calculation, the bearing load with its load capacity are compared amongst the working condition chosen.

The roller bearing technology distinguishes between static and dynamic load. Static load defines as the loaded bearing stands still, spins very slow or performs a slow pivoting movement. In these cases, the safety to avoid plastic deformation of the bearing race and rolling body is examined. The following requirement for the static load applies:

Static Load

The static load rating C₀ is the determinant of a roller bearings static load capacity. The static load rating is defined by DIN ISO 76 for a Hertzian load of rolling bodies on the bearing races of

4.200 MPa for ball bearings (punctate load)
4.000 MPa for roller bearings (linear load)

The static load rating C₀ is stated in the measurements tables for each roller bearing.

During roller bearing load with C₀ on the most loaded contact point, a plastic deformation of the roller body and race of about 1/10.000 of the roller bodies diameter occur. The static figure f_s calculated for the verification that an enough sustainable bearing has been chosen.

Equivalent static load P₀

P₀ is a calculated value that defines the radial load for radial bearings and the axial centrically load for axial bearings. P₀ causes the same load in the centre of the highest loaded contact point between rolling body and race like the actual active combined load.

Nominal lifetime calculation

The standardised calculation procedure (DIN ISO 281) for dynamically loaded roller bearings relies on the material fatigue (pitting) as failure cause. The following life span formula is used for the calculations:

The life time exponent p differs for ball and roller bearings.

p = 3  for ball bearings

p = 10/3  for roller bearings

If the bearing life time is constant, the life time can be framed in hours.

If e.g. traffic engineering requires the declaration in miles, the mean wheel diameter D_R must be included in the life span calculation, so that following applies:

Extended nominal life time calculation

In most cases, the nominal life span L_10h is the adequate criteria for bearing capacity. In most cases of application, a more reliable calculation method is necessary. Therefore, the extended life span calculation according DIN ISO 281 is used It extends life span mentioned above about two factors. The formula shows the correlations.

The life time coefficient for reliability a₁ is defined according to DIN ISO 281 for following values:

The life span coefficient from the system assessment a_ISO is a function from the relation of fatigue load and actual occurring load.

Solid particles within the lubricant cause permanent wear in the bearings race (scoring) This leads to a life span reduction. The contamination factor e_c was introduced, to depict this impact in the extended life span calculation. The following table states indications for the e_c value according DIN ISO 281:

Contamination caused by water or other fluids that are not included in the e_c value.

The viscosity ratio κ is further explained here. The kinematic operation viscosity and the reference kinematic viscosity ν₁ are necessary for the calculation.

The lubricant must hold a certain minimum viscosity at operation temperature, to provide an appropriate lubrication film between the bearing elements. Life span limits can be extended by increasing the operation viscosity ν.
The kinematic reference viscosity ν₁ can be determined with the aid of the diagram in the following figure as a function of the speed n and the mean roller bearing diameter d_m. The operation viscosity ν can be determined in the following diagram.

The a_ISO coefficient can be determined from the diagram in the following figures containing values of the contamination factors e_C, the critical fatigue load c_U and the equivalent dynamic load P as well as the viscosity ratio _K.

The a_ISO value is defined according DIN ISO 281 as ≤ 50. This limit also applies for (e_C · Cu)/P > 5. The calculation must be made with the value k = 4, if the viscosity ratio _K exceeds 4.

Critical fatigue load

The critical fatigue load C_u is the load for the highest loaded contact point within the bearing, where the critical fatigue load is reached. The calculation for the critical fatigue load is defined in DIN ISO 281. The critical fatigue load C_u must not be the exclusive criteria for the determination of bearings. Roller bearings do not necessarily have an infinite life span, if the bearing load is below the fatigue limits. In practical use cases thin film or partial lubrication and lubrication contamination of roller bearings lead to increased load of the bearing race material, so that even for a bearing running below the critical fatigue load the fatigue limit is exceeded locally on the bearing race surface. These effects of lubrications and lubricant contaminations are considered by the life span calculation method.