An Introduction To Rolling Contact Bearings , Types , selection , And Applications

An Introduction To Rolling Contact Bearings , Types , selection , And Applications 

Introduction

The fundamental purpose of a bearing is to reduce friction and wear between rotating parts that are in contact with one another in any mechanism. The length of time a machine will retain its original operating efficiency and accuracy will depend upon the proper selection of bearings,
the care used while installing them, proper lubrication, and proper maintenance provided during actual operation.

Rolling-contact bearings are designed to support and locate rotating shafts or parts in machines. They transfer loads between rotating and stationary members and permit relatively free rotation with a minimum of friction. They consist of rolling elements (balls or rollers) between an outer and inner ring. Cages are used to space the rolling elements from each other. Figure 1 illustrates the common terminology used in describing rolling-contact bearings.

FIG 1

COMPONENTS AND SPECIFICATIONS

Rings The inner and outer rings of a rolling-contact bearing are normally made of SAE 52100 steel, hardened to Rockwell C 60 to 67.The rolling-element raceways are accurately ground in the rings to a very fine finish (16 min or less).Rings are available for special purposes in such materials as stainless steel, ceramics, and plastic. These materials are used in applications where corrosion is a problem.

Rolling Elements Normally the rolling elements, balls or rollers, are made of the same material and finished like the rings. Other rolling-element materials, such as stainless steel, ceramics, Monel, and plastics, are used in conjunction with various ring materials where corrosion is afactor.

Cages Cages, sometimes called separators or retainers, are used to space the rolling elements from each other. Cages are furnished in a wide variety of materials and construction. Pressed-steel cages, riveted or clinched and filled nylon, are most common. Solid machined cages are used where greater strength or higher speeds are required. They are fabricated from bronze or phenolic-type materials. At high speeds, the phenolic type operates more quietly with a minimum amount of friction.

Bearings without cages are referred to as full-complement.

Seals—Standard materials used in bearing seals are generally nitrile rubber. The material is bonded to a pressed steel core or shield. Nitrile rubber is unaffected by any type of lubricant commonly used in antifriction bearings. These closures have a useful temperature range of -70° to +225°F (-56° to 107°C). For higher operating temperatures, special seals of high temperature materials can be supplied. 

Lubricant—Prelubricated bearings are packed with an initial quantity of high quality grease which is capable of lubricating the bearing for years under certain operating conditions. As a general rule, standard greases will yield satisfactory performance at temperatures up to 175°F (79°C), as long as proper lubrication intervals and lube quantities are observed. Special greases are available for service at much higher temperatures. Estimation of grease life at elevated temperatures involves a complex relationship of grease type, bearing size, speed, and load. Volume 4 of this series can provide some guidance, although special problems are best referred to the product engineering department of major bearing manufacturers.

FIG 2

PRINCIPAL STANDARD BEARING TYPES

The general types are usually determined by the shape of the rolling element, but many variations have been developed that apply conventional elements in unique ways. Thus it is well to know that special bearings can be procured with races adapted to specific applications, although this is not practical for other than high volume configurations or where the requirements cannot be met in a more economical manner. “Special” races are appreciably more expensive. Quite often, in such situations, races are made to incorporate other functions of the mechanism, or are “submerged” in the surrounding structure, with the rolling elements supported by a shaft or housing that has been hardened and finished in a suitable manner.

In general, most ball, spherical roller, and cylindrical roller bearings made to metric boundary dimensions have standardized boundary plans, dimensions, and tolerances according to the International Standards Organization (ISO). Therefore, bearings from all subscribing manufacturers throughout the world are dimensionally interchangeable. Most taper roller bearings are made to inch dimensions and have standardized boundary dimensions and tolerances according to the Anti-Friction Bearing Manufacturers Association (AFBMA), a U.S. standards organization. Metric taper roller bearings utilizing ISO boundary plans are also made. Dimensionally interchangeable taper roller bearing components are thus available from several manufacturers. 

FIG 3

Ball Bearings

Single-Row Radial (Fig. 4) This bearing is often referred to as the deep groove or conrad bearing. Available in many variations—single or double shields or seals. Normally used for radial and thrust

loads (maximum two-thirds of radial).

Maximum Capacity (Fig. 4) The geometry is similar to that of a deep-groove bearing except for a filling slot. This slot allows more balls in the complement and thus will carry heavier radial loads. However, because of the filling slot, the thrust capacity in both directions is reduced drastically.

Double-Row (Fig. 4) This bearing provides for heavy radial and light thrust loads without increasing the OD of the bearing. It is approximately 60 to 80 percent wider than a comparable single-row bearing. Because of the filling slot, thrust loads must be light.

Internal Self-Aligning Double-Row (Fig.4) This bearing may be used for primarily radial loads where self-alignment (64°) is required. The self-aligning feature should not be abused, as excessive

misalignment or thrust load (10 percent of radial) causes early failure).

Angular-Contact Bearings (Fig. 4) These bearings are designed to support combined radial and thrust loads or heavy thrust loads depending on the contact-angle magnitude. Bearings having large contact angles can support heavier thrust loads. They may be mounted in pairs (Fig. 3.6) which are referred to as duplex bearings: back-toback, tandem, or face-to-face. These bearings  may be preloaded to minimize axial movement and deflection of the shaft.

Ball Bushings (Fig. 4) This type of bearing is used for linear motions on hardened shafts (Rockwell C 58 to 64). Some types can be used for linear and rotary motion.

Split-Type Ball Bearing (Fig.4) This type of ball or roller bearing has split inner ring, outer ring, and cage. They are assembled by screws. This feature is expensive but useful where it is difficult to install or remove a solid bearing.

FIG 4

Roller Bearings

Cylindrical Roller (Fig. 5) These bearings utilize cylinders with approximate length/diameter ratio ranging from 1 : 1 to 1 : 3 as rolling elements. Normally used for heavy radial loads. Especially useful for free axial movement of the shaft. Highest speed limits for roller bearings.

Needle Bearings (Fig.5) These bearings have rollers whose length is at least 4 times their diameter. They are most useful where space is a factor and are available with or without inner race. If shaft is

used as inner race, it must be hardened and ground. Full-complement type is used for high loads, oscillating, or slow speeds. Cage type should be used for rotational motion. They cannot support thrust loads.

Tapered-Roller (Fig. 5) These bearings are used for heavy radial and thrust loads. The bearing is designed so that all elements in the rolling surface and the raceways intersect at a common point on the axis: thus true rolling is obtained. Where maximum system rigidity is required, the bearings can be adjusted for a preload. They are available in double row.

Spherical-Roller (Fig.5) These bearings are excellent for heavy radial loads and moderate thrust. Their internal self-aligning feature is useful in many applications such as HVAC fans.

FIG 5

Thrust Bearings

Ball Thrust Bearing (Fig. 6) It may be used for low-speed applications where other bearings carry the radial load. These bearings are made with shields, as well as the open type.

Straight-Roller Thrust Bearing (Fig.6) These bearings are made of a series of short rollers to minimize the skidding, which causes twisting, of the rollers. They may be used for moderate speeds and loads.

Tapered-Roller Thrust (Fig 6) It eliminates the skidding that takes place with straight rollers but causes a thrust load between the ends of the rollers and the shoulder on the race. Thus speeds are limited because the roller end and race flange are in sliding contact

FIG 6

FIG 7

LOAD RATINGS

All manufacturers of rolling bearings establish a dynamic and static load rating for each bearing

produced. An ISO method for calculating this method exists, but not all manufacturers adhere to

the method. The unfortunate situation therefore exists that two almost identical bearings produced

by different manufacturers can have different published load ratings. Ratings are expressed as a

load which will provide a basic rating life of a defined number of revolutions. Basic rating life is

the number of revolutions (or the number of operating hours at a given constant speed) which the

bearing is capable of enduring before the first sign of fatigue occurs in one of its rings or rolling

elements. The basic rating life in millions of revolutions is the life 90 percent of a sufficiently large

group of apparently identical bearings can be expected to obtain or exceed under identical operating

conditions. In other words, this is a reliability or statistical rating, the only mechanical component

so rated. The ISO definition of the basic rating life is the most common and is at 1 million

revolutions. Some taper roller bearings are rated on the basis of 90 million revolutions, or 500

rev/min (rpm) for 3000 hours. Hence it can be easily seen that comparing manufacturers’ ratings

as published in catalogs can be misleading if appropriate adjustments are not made to published

values.

There are several other “bearing lives,” including service life and design or specification life.

Service life is the actual life achieved by a specific bearing before it becomes unserviceable. Failure

is not generally due to fatigue, but due to wear, corrosion, contamination, seal failure, etc.

The service life of a bearing depends to a large extent on operating conditions, but the procedures

used to mount and maintain it are equally important. Despite all recommended precautions, a bearing

can still experience premature failure. In this case it is vital that the bearing be examined carefully

to determine a reason for failure so that preventive action can be taken. The service life can

either be longer or shorter than the basic rating life.

Specification life is the required life specified by the equipment builder and is based on the hypothetical load and speed data supplied by the builder and to which the bearing was selected. Many times this required life is based on previous field or historical experiences.

ROLLING-CONTACT BEARINGS’ LIFE, LOAD, AND SPEED RELATIONSHIPS

An accurate knowledge of the load-carrying capacity and expected life is essential in the proper selection of ball and roller bearings. Bearings that are subject to millions of different stress applications fail owing to fatigue. In fact, fatigue is the only cause of failure if the bearing is properly lubricated, mounted, and sealed against the entrance of dust or dirt and is maintained in this condition. For this reason, the life of an individual bearing is defined as the total number of revolutions or hours at a given constant speed at which a bearing runs before the first evidence

of fatigue develops

Definitions

Rated Life     : The number of revolutions or hours at a given constant speed that 90 percent of an apparently identical group of bearings will complete or exceed before the first evidence of fatigue develops; i.e., 10 out of 100 bearings will fail before rated life. The names Minimum life and  life are also used to mean rated life.

Basic Load Rating C The radial load that a ball bearing can withstand for one million revolutions of the inner ring. Its value depends on bearing type, bearing geometry, accuracy of fabrication, and bearing material. The basic load rating is also called the specific dynamic capacity, the basic dynamic capacity, or the dynamic load rating.

Equivalent Radial Load P Constant stationary radial load which, if applied to a bearing with rotating inner ring and stationary outer ring, would give the same life as that which the bearing will attain under the actual conditions of load and rotation.

Static Load Rating   : Static radial load which produces a maximum contact stress of 580,000 lb/in² (4,000 MPa).
Static Equivalent Load      Static radial load, if applied, which produces a maximum contact stress equal in magnitude to the maximum contact stress in the actual condition of loading.
Bearing Rated Life
Standard formulas have been developed to predict the statistical rated life of a bearing under any given set of conditions. These formulas are based on an exponential relationship of load to life which has been established from extensive research and testing.

(1)

where = rated life, r; C = basic load rating, lb; P =equivalent radial load, lb; K = constant, 3 for ball bearings, 10/3 for roller bearings.
To convert to hours of life , this formula becomes

 (2)  

where N = rotational speed, r/min. Table 1 lists some common design lives vs. the type of application. These may be altered to suit unusual circumstances

Load Rating
The load rating is a function of many parameters, such as number of balls, ball diameter, and contact angle. Two load ratings are associated with a rolling-contact bearing: basic and static load rating.
Basic Load Rating C This rating is always used in determining bearing life for all speeds and load conditions
Static Load Rating  This rating is used only as a check to determine if the maximum allowable stress of the rolling elements will be exceeded. It is never used to calculate bearing life.
Values for C and  are readily attainable in any bearing manufacturer’s catalog as a function of size and bearing type. Table 2 lists the basic and static load ratings for some common sizes and types of bearings.

Equivalent Load
There are two equivalent-load formulas. Bearings operating with some finite speed use the equivalent radial load P in conjunction with C [Eq. (1)] to calculate bearing life. The static equivalent load is used in comparison with in applications when a bearing is highly loaded in a static mode.
Equivalent Radial Load P All bearing loads are converted to an equivalent radial load. Equation (3) is the general formula used for both ball and roller bearings.

(3)

where P = equivalent radial loads, lb; R = radial load, lb; T = thrust (axial) load, lb; X and Y 5 radial and thrust factors (Table 3). The empirical X and Y factors in Eq. (3) depend upon the geometry,
loads, and bearing type. Average X and Y factors can be obtained from Table 3. Two values of X and Y are listed. The set X1 Y1 or X2 Y2 giving the largest equivalent load should always be used.

Static Equivalent Load  The static equivalent load may be compared directly to the static load rating . If  is greater than the
rating, permanent deformation of the rolling element will occur. Calculate  as follows:

  (4)

where  = static equivalent load, lb; X0 = radial factor (see Table 4); Y0  thrust factor (see Table 4); R = radial load, lb; T = thrust (axial) load, lb.

Required Capacity

The basic load rating C is very useful in the selection of the type and size of bearing. By calculating the required capacity needed for a bearing in a certain application and comparing this with known capacities, a bearing can be selected. To calculate the required capacity, the following formula can be used:

 (5)

where C = required capacity, lb;  = rated life, h; P= equivalent radial load, lb; K =constant, 3 for ball bearings, 10/3 for roller bearings; Z =constant, 25.6 for ball bearings, 18.5 for roller bearings;

 N =rotation speed, r/min.

LIFE ADJUSTMENT FACTORS

Modifications to Eq. (2) can be made, based on a better understanding of causes of fatigue. Influencing factors include

1. Reliability factors for survival rates greater than 90 percent

2. Improved raw materials and manufacturing processes for ball bearing rings and balls.

3. The beneficial effects of elastrohydrodynamic lubricant films Equation (2) can be rewritten to reflect these influencing factors:

 (6)

where A1= statistical life reliability factor for a chosen survival rate, A2 = life-modifying factor reflecting bearing material type and condition, and A3 = elastohydrodynamic lubricant film factor.

Factor A3

This factor is based on elastohydrodynamic lubricant film calculations which relate film thickness and surface finish to fatigue life. A factor of 1 to 3 indicates adequate lubrication, with 1 being the minimum value for which the fatigue formula can still be applied. As A3 goes from 1 to 3, the life expectancy will increase proportionately, with 3 being the largest value for A3 that is meaningful. If A3 is less than 1, poor lubrication conditions are presumed. 

Speed Limits

Many factors combine to determine the limiting speeds of ball and roller bearings. It depends on several factors, like bearing size, inner- or outer-ring rotation, contacting seals, radial clearance and tolerances, operating loads, type of cage and cage material, temperature, and type of lubrication. A convenient check on speed limits can be made from a dn value. The dn value is a direct function of size and speed and is dependent on type of lubrication. It is calculated by multiplying the bore in millimeters (mm) by the speed in r/min.

dn = bore (mm) × speed (r/min)

Friction

One of the assets of rolling-contact bearings is their low friction. The coefficient of friction varies appreciably with the type of bearing, load, speed, lubrication, and sealing element. For rough calculations the following coefficients can be used for normal operating conditions and favorable lubrication:

        Single-row ball bearings               0.0015

        Roller bearings                             0.0018

Excess grease, contact seals, etc., will increase these values, and allowances should be made.

Selection of Ball or Roller Bearing

The selection of the type of rolling-contact bearing depends upon many considerations, as evidenced by the numerous types available. Furthermore, each basic type of bearing is furnished in several standard ‘‘series’’ as illustrated in Fig. 8. Although the bore is the same, the outside diameter, width, and ball size are progressively larger. The result is that a wide range of load-carrying capacity is available for a given size shaft, thus giving designers considerable flexibility in selecting

standard-size interchangeable bearings. 

FIG 8

Selection of the type of rolling-element bearing is a function of many factors, such as load, speed, misalignment sensitivity, space limitations, and desire for precise shaft positioning. However, to determine if a ball or roller bearing should be selected, the following general rules apply:

1. Ball bearings function on theoretical point contact. Thus they are suited for higher speeds and              lighter loads than roller bearings.

2. Roller bearings are generally more expensive except in larger sizes. Since they function theoretically on line contact, they will carry heavy loads, including shock, more satisfactorily, but are limited in speed.

Bearing Size Selection

Known type and series:

1. Select desired design life (Table 1).

2. Calculate equivalent radial load P [Eq. (3)].

3. Calculate required capacity Cr [Eq. (5)].

4. Compare Cr with capacities C in Table .2. Select first bore size having a capacity C greater than Cr .5. Check bearing speed limit [Eq. (7)].

Bearing-Type Selection Known bore size and life:

1. Select ball or roller bearing

2. Calculate equivalent load P [Eq. (3)] for various bearing types (conrad, spherical, etc.).

3. Calculate Cr [(Eq. (5)].

4. Compare Cr with capacities C in Table 3, and select the type that has a capacity equal to or greater than Cr .

5. Check bearing speed limit [Eq. (7)].

Bearing-Life Determination Known bearing size:

1. Select ball or roller bearing

2. Calculate equivalent radial load P [Eq. (3)].

3. Select basic load rating C from Table 2.

4. Calculate rated life  [Eq. (1) or (2)].

5. Check calculated life with design life.

Use Fig. 9  as a general guide to determine if a ball or roller bearing should be selected. This figure is based on a rated life of 30,000 h.

FIG 9

BEARING MOUNTING

Correct mounting of a rolling-contact bearing is essential to obtain its rated life. Many types of mounting methods are available. The selection of the proper method is a function of the accuracy, speed, load, and cost of the application. The most common and best method of bearing retention

is a press fit against a shaft shoulder secured with a locknut. End caps are used to secure the bearing against the housing shoulder (Fig.10). Retaining rings are also used to fix a bearing on a shaft or in a housing (Fig. 10). Each shaft assembly normally must provide for expansion by allowing one end to float. This can be accomplished by

FIG 10

allowing the bearing to expand linearly in the housing or by using a straight roller bearing on one end. Care must be exercised when designing a floating installation because it requires a slip fit. An excessively loose fit will cause the bearing to spin on the shaft or in the housing. Table 8 lists shaft and housing tolerances for press fits with ABEC 1 precision applications (pumps, gear reducers, electric motors, etc.) and ABEC 7 precision applications (grinding spindles, etc.).