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Summary
This article provides a basic introduction to plain and rolling element bearings by outlining issues of friction, load, bearing life, lubrication, and maintenance. The remainder of the article discusses plain bearing and rolling element bearing components, construction, and materials.
Bearing Basics
An Overview
RB SKF 24 pages April 2002 Revised: January 2004
SKF Reliability Systems @ptitudeXchange 5271 Viewridge Court San Diego, CA 92123 United States tel. +1 858 496 3554 fax +1 858 496 3555 email: [email protected] Internet: www.aptitudexchange.com
ANEXO 8.F
(Gentileza SKF)
1 Introduction
Bearings are designed to overcome friction to provide ease of rotation. Friction is a force that resists the relative motion of two bodies in contact. There are a couple of ways to reduce friction between two surfaces. One way is to change the contact environment by adding lubricant or changing to a material with a lower coefficient of friction. The other way to overcome friction is utilizing rolling elements. Rolling elements change sliding friction to rolling contact friction. Friction is reduced as things roll easier than they slide. The ancient world discovered the benefits of rolling elements thousands of years ago. During the construction of the pyramids, Egyptian builders used tree logs as rolling elements between the massive stones and the ground.
Figure 1. Bearings Reduce Friction.
This article provides a basic understanding and appreciation of plain and rolling element bearings. Part of this article comes from selections of SKF's general catalogue.
2 Bearing Load
Bearings are designed to support shafts and allow free rotation on applied loads. There are three basic types of load:
x Radial loads are applied perpendicular to the shaft (the bearing’s axis of rotation).
x Axial (thrust) loads are applied parallel to the axis of rotation.
x Combination load is encountered when the bearing simultaneously experiences a radial and axial load.
Axial load
Radial load Combined load
Fa
Fr
Figure 2. Bearing Loads.
3 Plain Bearings
Also referred to as journal or sleeve bearings, the plain bearing is cylindrical in shape and designed to fit tightly in the housing and on the shaft.
A plain bearing is constructed of material with a low coefficient of friction, which means the material structure and the smooth surface texture ensures low frictional losses. This results in low pressure drops and low resistance on sliding surfaces. The advantages of plain bearings include: smaller outside diameter (as opposed to rolling element bearings), quiet operation, absorption of shock loads, enduring oscillating motion (repetitive movement back and forth), and low cost.
3.1 Plain Bearing Designs Based upon the performance envelope, plain bearings are divided into two basic designs:
Hydrodynamic
Hydrodynamic (Fluid Film Bearings) – These bearings are used in large machine applications. Lubrication is drawn into the region between the bearing’s moving parts. When there is relative motion, the viscosity, combined with the bearing surface shape, generates pressure to keep surfaces separated.
Hydrostatic
Hydrostatic – Offering very low coefficients of friction at all speeds, the main characteristic of hydrostatic plain bearings is a high load capacity. The load is carried by fluid pressure, which creates decreased friction.
3.2 Bushings
Bushings are cylindrical plain bearings suitable for oscillation movements and, to an extent, linear and rotational movements. Cylindrical bushings are used in applications where misalignment does not appear.
3.3 Spherical Plain Bearings
Spherical plain bearings are standardized, ready to mount, mechanical components that are self aligning to enable multi-directional alignment movement. The forces acting on the bearing may be static or may occur when the bearing makes oscillating or recurrent tilting and slewing movements at relatively slow speeds.
Figure 3. Spherical Plain Bearing (Example).
Figure 4. Spherical Plain Bearing (Example).
Figure 5. Spherical Plain Bearing Movement.
Rod ends are spherical plain bearing units that consist of a bearing fitted into a housing equipped with a male or female thread or a welding shank. They are primarily used on the ends of piston rods or together with hydraulic and pneumatic cylinders to join the cylinder to associated components.
3.4 Plain Bearing Materials Plain bearing materials are grouped into two categories:
Lubricated
x Aluminum Based Alloys
x Copper Based Alloys
x Whitemetal
advantages and disadvantages. While ball elements are capable of running at higher speeds and adjust for minor misalignment, their load carrying capacity is limited.
“Line” contact rolling elements distribute the load across a larger surface between the ring’s raceway. The ability to disperse the load across a larger surface allows for heavier loading than that of a ball rolling element. In addition, the dispersed area permits impact loading. The main disadvantage of line contact rolling elements is diminished speed caused by increased friction and heat generation.
Figure 8. Rolling Element and Raceway.
4.2 Rings
The inner and outer rings provide a raceway for rolling elements to circulate. Each raceway is ground and polished to ensure the least amount of friction. The inner ring’s bore serves as the interface between the rolling element and the shaft, while the outer ring’s outer diameter interfaces with the housing.
4.3 Rolling Bearing Materials
Through-hardening steels
The most common through-hardening steel used for rolling bearings is a carbon chromium steel that contains approximately 1% carbon and 1.5% chromium. For bearing components with large cross sections, steel alloyed with manganese and molybdenum is used.
Case-Hardening Steels
Case-hardened rings are used when tough rings are necessary in combination with hard raceway surfaces. They are typically used in highly loaded applications. Chromium-nickel and manganese-chromium alloyed steels with a carbon content of approximately 0.15% are those case-hardening steels most commonly used for rolling bearings.
4.4 Cage The cage performs many functions in a rolling element bearing. It prevents immediate contact between rolling elements to minimize friction and heat generation, guides rolling elements, provides space for lubricant, and retains the rolling elements when bearings of separable design are mounted or dismounted.
Figure 9. Cage Ball Spacing.
Depending on bearing series and size, ball bearings are fitted with one of the following cage types:
x pressed steel
x pressed brass
x machined brass
x polyamide (fabric reinforced synthetic resin)
Cage Variants
Polyamide cage
Window-type sheet steel cage
Window-type solid brass cage
Figure 10. Cage Variants.
Polyamide cages, especially in high-volume standard series bearings, replace steel and brass cages. In the case of low-volume, specialized bearings, brass cages are often used. Use bearings incorporating pressed steel or brass cages for bearing arrangements operated at continuously high temperatures or under arduous conditions. Before ordering cages, it is advisable to check availability.
4.5 Seals
Bearing seals are found mainly on single and double row ball bearings. Seals are made of rubber with steel reinforced backing. Affixed to the outer ring, the rubber lip of the seal rides tightly on the surface of the inner ring. This seal keeps contaminants out of the bearing assembly while keeping the lubrication from leaving the raceway area. Yet, due to the friction of the lip seal contacting the inner ring, the sealed bearing is limited to slower operating speeds.
Two types of seals are normally used in conjunction with rolling bearings: non- rubbing (without contact), and rubbing (contacting) seals.
Non-Rubbing Seals
The effectiveness of non-rubbing seals depends in principle, on the sealing action of narrow gaps between rotating and stationary
components. The gaps may be arranged radially, axially, or a combination of axially and radially.
Non-rubbing seals generate almost no friction and do not wear. They are generally not easily damaged by solid contaminants and are particularly suitable for high speeds and high temperatures. Pressing grease into the gap(s) can enhances their sealing efficiency.
Rubbing Seals The action of rubbing seals depends on the seal exerting a certain pressure on its counter- face, usually by a relatively narrow sealing lip or surface. The penetration of solid lubricants or moisture and/or the loss of lubricant are prevented by this pressure. The pressure may be produced either by the resilience of the seal, which results from the elastic properties of the sealing material and the designed interference between the seal and its counter- face, or from a tangential force exerted by a garter spring incorporated in the seal.
Rubbing seals generally provide very reliable sealing, particularly when wear is kept to a minimum by producing an appropriate surface finish for the counter-face, and by lubricating the sealing lip / counter-face contact. The friction of the seal on its counter-face and the subsequent rise in temperature is a disadvantage; therefore, rubbing seals are only useful for operation up to certain peripheral speeds. They are also susceptible to mechanical damage as a result of improper mounting or solid contaminants. To prevent damage by solid contaminants it is customary to place a non-rubbing seal in front of the rubbing seal for protection.
Shields Bearing shields are made of steel. They too are affixed to the bearing’s outer ring but unlike the seal, the shield does not make contact with the inner ring. Shields allow for
Deep groove Angular contact Self-aligning
Radial load Axial load Speed Accommodates carrying capacity carrying capacity capability misalignment
Figure 13. Angular Contact Bearing.
5.3 Self-Aligning Ball
The inside surface of the outer ring of a self- aligning ball bearing is part of a sphere. This means the bearing is self-aligning and adapts to shaft misalignment. SKF was founded on Sven Wingquist's revolutionary invention of the double row self-aligning ball bearing.
Deep groove Angular contact Self-aligning
carrying capacityRadial load^ carrying capacityAxial load^ capabilitySpeed^ Accommodatesmisalignment
Figure 14. Self-Aligning Ball Bearing.
5.4 Cylindrical Roller
Cylindrical roller bearings are bearings with cylindrical rolling elements. These bearing variants have the highest load and speed capacity as compared with other line contact rolling elements (needle, spherical, and tapered). The load carrying capacity advantages of cylindrical roller bearings over ball bearings is sacrificed in its sensitivity to misalignment.
Another advantage to cylindrical roller bearings is their ability to deal with axial displacement (movement in the axial direction). NJ design (see Figure 15) cylindrical roller bearings and some full complement designs are used to compensate for axial displacement. These bearings permit roller axial displacement with respect to one of the raceways so both inner and outer rings can be mounted with interference fits. The cylindrical rollers are guided between two integral flanges on the inner ring, whilst the low flange on the inboard side of the outer ring holds the bearing together until it is mounted. Values for the permissible axial displacement within the bearing are given in the manufacturer’s product tables.
Cylindrical Roller Bearing Variants NU NJ NUP
N NU +^ HJ NJ +HJ
Figure 15. Cylindrical Roller Bearing.
5.5 Tapered Roller Tapered roller bearings are a four-part bearing assembly consisting of: tapered rolling elements, outer ring or cup , inner ring or cone , and cage. Designed for combination loads, tapered roller bearings are available with steep and shallow angles to accommodate larger thrust and radial load combinations respectively.
Taper Roller Bearing Capabilities
F igure 16. Tapered Roller Bearing.
5.6 Needle
Needle bearings are similar to cylindrical roller bearings, but have long, thin cylindrical rollers with a small length-to-diameter ratio. A hardened shaft or sleeve acts as the inner race / ring for the needle bearing, which allows for an overall lower bearing profile that requires little space. Caged needle bearings are available for higher speed applications.
Figure 17. Needle Bearing.
Figure 18. Needle Bearing.
5.7 Spherical Spherical bearings are roller bearing where the inside surface of the outer ring is part of a sphere. This means the bearing is self-aligning to adapt itself to shaft misalignment. Because the spherical bearing is internally self- aligning, it is well suited for heavy-duty applications. There are many different designs on the market, which results in large performance differences.
Spherical Roller Bearing Capabilities
Figure 19. Spherical Roller Bearing.
5.8 Toroidal The CARB is a toroidal roller bearing - a completely new type of radial roller bearing. This compact, self-aligning roller bearing was developed by SKF as a single row toroidal roller bearing with long, slightly crowned rollers. The raceways of both inner and outer rings are concave and symmetrical. This optimum combination of raceway profiles guarantees a favorable load distribution and low friction.
The CARB rollers are self-guiding (i.e. they always adopt a position to evenly distribute the load over the roller length) irrespective of whether the inner ring is axially displaced and/or misaligned with respect to the outer ring. The load carrying capacity of the CARB is very high even when it has to compensate for angular misalignments or axial displacements. This results in operationally
together with the external forces, generally exceed the requisite minimum load.
6.2 Load Carrying Capabilities and Life
Initially select the bearing size on the basis of its load carrying capacity in relation to load, life, and reliability requirements. Numerical values termed basic load ratings are used to express load carrying capacity. Values for the basic dynamic load rating C and the basic static load rating C 0 are quoted in the manufacturer’s product tables. The SKF Formula for Rolling Bearing Life is an excellent article on theory and calculations of bearing life.
6.3 Basic Load Ratings
The basic dynamic load rating C is used for calculations involving dynamically stressed bearings, or bearings that rotate under load. It expresses the bearing load as an ISO (International Organization for Standards) basic rating life defined under Life (Load Carrying Capability and Life) of 1,000, revolutions. It is assumed that the load is constant in magnitude and direction and is radial for radial bearings, and axial (acting centrically) for thrust bearings.
The basic static load rating C 0 is used in calculations when bearings rotate at very slow speeds, perform very slow oscillating movements, or are stationary under load for certain periods.
Load Capacity Basic static load rating C 0
s 0 = static safety factor P 0 = equivalent static bearing load, N C 0 = basic static load rating, N
With the load Pthe static safety factor s 0 = C (^0) 0 will be 1
The static safety factor
Figure 24. Load Capacity.
The basic static load rating is defined in ISO 76-1987 as static load that corresponds to a calculated contact stress at the center of the most heavily loaded rolling element / raceway contact of:
x 4,600 MPa for self-aligning ball bearings
x 4,200 MPa for all other ball bearings
x 4,000 MPa for all roller bearings
This stress produces a total permanent deformation of rolling element and raceway, which is approximately 0.0001 of the rolling element diameter. The loads are purely radial for radial bearings and centrically acting axial loads for thrust bearings.
6.4 Comparative Load Ratings For rolling mills and similar applications, particularly when multi-row roller bearings are used, it is common practice to calculate using dynamic load ratings (determined by a method that deviates from that specified in ISO 281:1990 and is based on a rating life of 90 million revolutions - 3,000 operating hours at 500 rpm) instead of the 1 million revolutions specified by ISO.
Load Capacity Basic dynamic load rating C
L 10 = basic rating life, millions of revolutions CP = basic dynamic load rating, N= equivalent dynamic bearing load, N p = exponent of the life equation With the load P = Cthe L 10 life will be 1 million revolutions
The ISO life equation
Figure 25. Load Capacity.
There are obvious differences in bearing load ratings quoted by different manufacturers for bearings, as load ratings depend on the use of materials, material quality, design details, number of rolling elements, etc.
6.5 Bearing Life
Practical experience and modern research shows that, under special conditions, bearings can attain a much longer life than predicted by standardized life calculation methods, particularly when loads are light. These special conditions apply when the surfaces in rolling contact (raceways and rolling elements) are effectively separated by a lubricant film and deterioration from contaminants is largely non-existent. Under ideal conditions it is possible to speak of infinite life.
It is well known that the calculated life is often longer than the service life actually achieved for large, multi-row bearings (i.e. heavily loaded rolling mill bearings). Therefore, a comparison of these bearings based solely on load ratings is questionable. The know-how of a bearing manufacturer in respect to typical operating conditions for a particular application is of considerable importance when satisfactory performance in that application is of concern.
The SKF Life Theory introduces the concept of a fatigue load limit Pu analogous to that used when calculating other machine components. This fatigue load limit represents the load below which fatigue will not occur in the bearing under ideal conditions.
The SKF theory represents an extension of the work of Lundberg and Palmgren, taking into account the fatigue load limit and several other factors related to lubrication and contamination. It is possible to predict a realistic bearing service life as referred to above with this theory. A precise prediction can only be made if the operating conditions are accurately known and if the full theory is applied, which requires the use of a computer.
Service life: The actual life achieved by the bearing before it fails.
Finite life Infinite life
Life Life
Load P Load P
P (^) U
Bearing Load
Figure 26. SKF New Life Theory.
Due to the complexity, a detailed description of the theory is beyond the scope of this article. However, to pass on the benefits inherent in the deeper understanding of bearing behavior on which the SKF Life Theory is based, a simplified catalog approach has been devised and presented in the IEC. This enables users to exploit the realistic life potential of bearings, to undertake controlled downsizing, and to recognize the influence of contamination on bearing life.
Most bearing manufacturers base their information on dynamic load ratings (except the comparative ratings), which are founded
8.1 Cylindrical Bore
Bearings with cylindrical bore are easier to mount and dismount if the design is separable rather than non-separable, particularly if interference fits are required for both rings. They are also preferable if frequent mounting and dismounting are required. One ring of these separable bearings (i.e. cylindrical, needle, and taper roller bearings) can be fitted independently of the other ring.
8.2 Tapered Bore
Bearings with a tapered bore can easily be mounted on a tapered journal or cylindrical shaft seating using an adapter or withdrawal sleeve.
8.3 Interference Fit
The tolerances for the bore and outside diameters of rolling bearings are specified in ISO 5753:1991 and shown in Figure 29. To achieve an interference or a clearance fit, suitable shaft and housing seat tolerance ranges have to be selected.
Correct Fits
Tolerance range for bearings manufactured to ISO
Figure 29. Correct Fits (ISO).
9 Bearing Selection Criteria
Each type of bearing displays characteristic properties that depend on design and designate the appropriate application. For example, deep groove ball bearings can accommodate moderate radial loads and axial loads. They
have low friction and can be produced with high precision and quiet running variants. Therefore, they are preferred for small and medium-sized electric motors.
Spherical roller bearings can carry very heavy loads and are self-aligning. These properties make them popular for applications in heavy engineering, where loads are heavy and deformations and misalignments are common.
In many cases however, several factors must be considered when selecting bearing type. The information provided serves to indicate the most important points to consider when selecting bearing type.
Operating conditions
Available space Misalignment
Speed Life
Load/Direction
Figure 30. Bearing Selection Criteria.
9.1 Available Space In many cases, one principal bearing dimension (generally the bore diameter) is predetermined by machine design. For small- diameter shafts all types of ball bearings can be used. For large-diameter shafts, cylindrical, spherical, taper roller bearings, and deep groove ball bearings are available.
When radial space is limited, bearings with a small cross section, particularly those with a low cross-sectional height, should be chosen. When space is limited in the axial direction, certain series of single row cylindrical roller bearings, deep groove ball bearings, and
various types of combined needle roller bearings can be used for combined loads.
For purely axial loads, needle roller and cage thrust assemblies (with or without washers), and certain series of thrust ball bearings and cylindrical roller thrust bearings can be used.
9.2 Load / Direction
Please refer to section 6, Load Capabilities on page 11.
9.3 Misalignment
Angular misalignment between shaft and housing occur when the shaft bends (flexes) under operating load, when bearing seats in the housing are not machined at a single setting, or when shafts are supported by bearings in separate housings too far apart.
So-called rigid bearings cannot accommodate any misalignment, or can only tolerate very minor misalignments, unless by force. Self- aligning bearings, on the other hand, are suitable to accommodate misalignment produced under operating loads and can compensate for errors of alignment resulting from machining or mounting.
9.4 Precision
Bearings with higher precision than normal are required for arrangements that call for high running accuracy (machine tool spindle arrangements) and very high speed operation.
1μ
Rolling Bearings are Precision Products
How much is a μ?
A human hair
Standard 0,06 mm
Normal Precisionbearing High precisionbearing P6 P6SP P4AUP PA9APA9B Toleranceclasses
0/-15 μm 0/-12 μm^ 0/-9 μm^
0/-7 μm 0/-2,5 μm
Figure 31. Precision.
Most bearing manufacturers produce a comprehensive range of high precision bearings, including single row angular contact ball bearings, single and double row cylindrical roller bearings, and single and double direction angular contact thrust ball bearings. See your bearing manufacturer’s catalogue for specific precision products.
9.5 Speed The permissible operating temperature limits the speed at which rolling bearings can be operated. Bearing types with low friction and correspondingly low heat generation are most suitable for high speed operation.
Relative Speed Limitations
Oil lubricationspeed rating
Grease lubricationspeed rating Bearing speed limit
r/min
Figure 32. Relative Speed Limitations by Type.
The highest speeds can be achieved with deep groove ball bearings when loads are purely radial. This is particularly true of high
dimensions of metric rolling bearings (ISO 15:1998 for radial bearings except taper roller bearings; ISO 355-1977 for radial metric taper roller bearings, and ISO 104:1994 for thrust bearings).
10.1 ISO Dimension Plan
The ISO dimension plan for radial bearings (except taper roller bearings) contains a progressive series of standardized outside diameters for every standard bore diameter arranged in diameter series 7, 8, 9, 0, 1, 2, 3, and 4 (in order of increasing outside diameter).
Within each diameter series, different width series have also been established (width series 8, 0, 1, 2, 3, 4, 5, 6, and 7 in order of increasing width). The width series for radial bearings correspond to the height series for thrust bearings (height series 7, 9, 1, and 2 in order of increasing height).
By combining a diameter series with a width or height series, dimension series, designated by two digits, are created. The first digit identifies the width or height series and the second, the diameter series.
In the ISO dimension plan for single row metric taper roller bearings, the boundary dimensions are grouped for certain ranges of the contact angle, known as the angle series (angle series 2, 3, 4, 5, 6, and 7 in order of increasing angle). Based on the relationship between outside and bore diameters, and between the total bearing width and the cross- sectional height, diameter and width series are laid down. Here, dimension series are obtained by combining the angle series with a diameter and a width series. These dimension series are designated by a combination of one figure (for angle series) and two letters (the first for the diameter series and the second for the width series).
Experience shows that the vast majority of bearing application requirements can be met using bearings with these standardized dimensions.
In addition to metric bearings, some inch-size ball bearings and cylindrical roller bearings are available.
10.2 Manufacturer Designations Most bearing manufacturers’ standard bearings have a characteristic basic designation, which generally consists of 3, 4, or 5 figures, or a combination of letters and figures. The figures and combinations of letters and figures have the following meaning:
The first figure or first letter, or combination of letters, identifies the type of bearing; the actual bearing type is identified from the presentation and listed under the heading. The following two figures identify the ISO dimension series; the first figure indicates the width (B or T) or height (H or T) series and the second the diameter series (D). The last two figures of the basic designation give the size of the bearing. When the size is multiplied by five, the bore diameter in millimeters is obtained.
Figure 35. Manufacturer Designations.
In some cases, the figure for the bearing type and/or the first figure of the dimension series identification is omitted. These figures are given in brackets.
For bearings with a bore diameter smaller than 10 mm and equal to, or greater than 500 mm, the bore diameter is generally given in millimeters direct, which means the size identification is separated from the rest of the bearing designation by an oblique stroke, e.g. 618/8 (d = 8 mm) or 511/530 (d = 530 mm). This is also true of standard bearings to ISO 15, which have bore diameters of 22, 28 and 32 mm, for example 322/28 (d = 28 mm). Bearings with bore diameters of 10, 12, 15, and 17 mm have the following size identifications:
00 = 10 mm
01 = 12 mm
02 = 15 mm
03 = 17 mm
For some deep groove, self-aligning, and angular contact ball bearings with a bore diameter smaller than 10 mm, the bore diameter is also given in millimeters (un- coded) but is not separated from the basic designation by an oblique stroke; for example, 629 or 129 (d = 9 mm).
Bore diameters that deviate from the standard bore diameter of a bearing have always been specified un-coded in millimeters with up to three decimal places. This bore diameter identification is part of the basic designation and is separated from the basic designation proper by an oblique stroke; for example, 6202/15.875 (d = 15,875 mm). However, such bearings will, in the future, be identified by a drawing number.
10.3 Series Designations
Each standard bearing belongs to a given bearing series, which is identified by the basic
designation without size identification. Series designations often include a suffix A, B, C, D, or E, or a combination of these letters. The series designations are used to identify differences in internal design, such as the contact angle.
The most common series designations are shown above bearing drawings. The figures in brackets are not included in either the basic designation or the series designation.
11 Lubrication and Maintenance
If rolling bearings are to operate reliably they have to be adequately lubricated to prevent direct metallic contact between the rolling elements, raceways, and cages. Keeping metallic components from contacting each other prevents wear, but the lubricant also protects the bearing surface from corrosion.
Thus, the choice of a suitable lubricant, method of lubrication, and lubricant maintenance for each bearing application is important. The various references in this paper provide insight into methods of lubrication, changing of oil, proper bearing handling, and servicing. For damaged bearing analysis, SKF can offer an online bearing diagnostic advisory system (see the @ptitudeXchange site) where the user inputs bearing and condition information and the program provides an analysis of probable causes.
The following information relates to bearings without integral seals or shields. Bearings and bearing units with integral seals or shields are supplied greased. The standard greases used for these products have operating temperature ranges and other properties to suit the intended application areas and filling grades appropriate to the bearing size. The service life of the grease often exceeds bearing life so that, with some exceptions, no provision is made for re-lubrication.
