Description

A wide variety of bearings are required in mechanical engineering. Depending on the application, plain bearings or rolling bearings are used. In the case of plain bearings, the relative movement takes place between two surfaces. Comparatively large forces can therefore be transmitted. Depending on the friction pairing, the resistance can be correspondingly high. Exceptions are hydrostatic or hydrodynamic plain bearings. In rolling bearings, the motion is realized by rolling elements rolling on corresponding raceways. The power loss is therefore usually lower than in plain bearings.

The forerunners of today's rolling bearings were found during excavations of Celtic chariots dating back to 700 BC. They were small cylinders made of hardwood, which were intended to improve the running properties of the wheels and extend their service life. However, it was not until industrialization that today's rolling bearings came into being.

Structure

A rolling bearing consists of an outer ring, the rolling elements, a cage for the rolling elements and an inner ring. Due to the shape of the rolling elements, the contact area to the raceways is very small. This results in large loads on the smallest surfaces, which in turn requires the best materials and the highest precision in the manufacture of the rolling bearings. The materials used for this purpose must have hard surfaces such as rolling bearing steel (100Cr6), stainless steel (AISI 440C) or even ceramic materials.

In the meantime, there is a suitable rolling bearing for almost every technical application. Whereby rolling bearings is an umbrella term. Depending on the design of the rolling element, they are divided into roller bearings, needle roller bearings, tapered roller bearings, spherical roller bearings and ball bearings.  The best known rolling bearing is the ball bearing. Here, too, there are further subdivisions such as deep groove ball bearings, axial deep groove ball bearings, self-aligning ball bearings, angular contact ball bearings, miniature ball bearings and others.

NORM

Roller bearings and ball bearings are subject to different standards. DIN 625-1 applies to deep groove ball bearings, DIN 628 to angular contact ball bearings, DIN 628 to spindle bearings, DIN 630 to self-aligning ball bearings, DIN 635-2 to spherical roller bearings, DIN ISO 355 / DIN 720 to tapered roller bearings, DIN 5412 to cylindrical roller bearings, DIN 628 to four point contact bearings and DIN 626-1 to radial insert ball bearings.

AREAS OF APPLICATIONS

Ball bearings are installed at high speeds, e.g. in motors, gearboxes and in drive axles. The compressive load is absorbed via the balls. Since the contact areas are very small, large loads cannot be applied. Cylindrical roller bearings or roller bearings in general can support higher compressive loads via their cylindrical rollers. In tapered roller bearings, the rolling bearings are tapered. This allows them to support lateral forces. Rolling bearings are as diverse as the areas in which they are used.

Technical information

 

Ball bearings are installed at high speeds, e.g. in motors, gearboxes and in drive axles. The compressive load is absorbed via the balls. Since the contact areas are very small, large loads cannot be applied. Cylindrical roller bearings or roller bearings in general can support higher compressive loads via their cylindrical rollers. In tapered roller bearings, the rolling bearings are tapered. This allows them to support lateral forces. Rolling bearings are as diverse as the areas in which they are used.

 

Designs

 

Rolling element designs

Rolling bearings are first and foremost categorized as ball bearings or roller bearings according to the shape of the roller body. Rolling bearings can generally withstand far greater forces than ball bearings. On the other hand, ball bearings have higher speeds than roller bearings.

 

     Ball           Cylinder roller      Tapered roller       Spherical roller      Needle roller

 

Types of rolling bearings

Ball bearing (radial bearing)

Ball bearings are categorized according to their typical design characteristics:

Deep groove ball bearings

One or two grooves in the inner and outer ring.

Angular ball bearings:

the load is transferred from one ring to the other via an angle.

Self-aligning ball bearings:

are able to compensate for misalignments of the shaft to the casing to a certain extent.

 

 

Roller bearing (radial bearing)

Roller bearings are categorized according to the shape of the roller:

 

Cylinder roller bearings:

Can withstand great forces radially exerted on the bearing.

Needle roller bearings:

Can withstand forces radially exerted on the bearing: considerably smaller outer diameter with the same nominal bore.

Tapered roller bearings:

Can withstand forces radially and axially exerted on the bearing.

Self-aligning roller bearings

Formerly called spherical roller bearings;  able to compensate for misalignments of the shaft to a certain extent.

 

Axial bearing

Only the axial bearings are marked with the prefix axial. l.e. , if a designation does not contain this ward, the bearing is radial!

Axial - deep groove ball bearing

Axial - cylinder roller bearing

Axial - tapered roller bearing

Axial – swivel-joint roller bearing

 

Material

 

Outer and inner rings are made of ball bearing steel

 

Standard:          GCr15; 100Cr6; (1.3505) or alternative material

NIRO:               AISI 440C; X105CrMo17; (1.4125) or alternative material

Bearing clearance

 

The bearing clearance is the gap between the parts in a roller bearing axially or radially. The bearing clearance is standardized. Roller bearings are offered with various bearing clearances depending on the type of bearing. Generally, one speaks of standard clearance, greater than standard clearance, and less than standard clearance.

 

 

 

Tolerances

 

The tolerances of roller bearings are standardized according to DIN 620 T1 to T6 and divided into various accuracy classes. The accuracy classes are marked in the DIN norm as normal, P6, PS, P4.

The accuracy increases as the number decreases!

Tolerances are the admissible values that deviate from the nominal sizes.

Selection

 

The design engineer can select the best type of bearing based on the characteristic properties of the various types of roller bearings and the operating conditions of the arrangement of the bearings. Here, the special or most important conditions of each case for the bearings that could influence the selection of the best bearing for the task are to be taken into consideration. Various types of bearings may be best for the given operating conditions in many cases .

The effective outer forces and the demands for service life and operating safety determine the size of the bearing needed. Above all, the decisive factors for the selection of the type and size of the bearing are the size, direction and type of loads that will be exerted on the bearing and the operating speed. Space limitations often require the selection of bearings with small cross-sections or even multiple rows of bearings to attain the necessary load-bearing capacity and service life.

lf the bearings have to be very accurate, bearings with great accuracy, especially ball bearings and cylinder roller bearings, should be used as they are produced in the highest accuracy classes.

The operating temperature of the bearings influences the design of the arrangement of the bearings both from the viewpoint of the selection of lubricant and the model of the bearing when the operating temperature continually rises above 100°C. The inner bearing clearance must be suited to the operating conditions, which are mostly determined by the temperature difference between the inner and outer ring, by the effects of the heat fed to the bearing, or by the effect of the high speed. Furthermore, the selection of the bearing is also influenced by the simplicity of the installation, demands on lubrication and seals, and demands for low friction and low running noise.

Service life

 

The service life of a bearing is the number of revolutions or the running number in operating hours that a bearing works at a set speed before the first signs of material fatigue (peeling) appear on the roller bodies or the running paths. There can, however be great differences in the service life of the same type and size of bearing under the same operating conditions. For this reason, the term service life was clearly defined for a calculation, and the nominal service life was used as the basis with an eye to operating safety and to fu lfill the ISO recommendation. That means that this service life is met or exceeded by 90% of a large number of the same bearings under the same operating conditions. We thus do not use the term service life to mean the time until a bearing fails due only to the dynamic material fatigue of the bearing rings or the roller bodies. Unforeseen failures due, for instance, to improper installation, errors in the design of the bearing, errors in maintenance, and the entrance of dirt and moisture.

 

Load rating

 

A distinction is made for roller bearings between static and dynamic loads and as parameters there are the static and dynamic load ratings.

 

Static load rating

The static load rating CO corresponds to the load under which the entire remaining deformation of roller bodies and running paths is maximally 0.0001 of the roller body's diameter. The calculation is based on surface pressure in the center of the pressure field. The following values result depending on the type of bearing:

4600 N/mm' for self-aligning ball bearings
4200 N/mm' for all other ball bearings
4000 N/mm' for roller bearings

 

Dynamic load rating

The dynamic load rating C corresponds to the load under which 90% of a !arge number of the same roller bearings attain a nominal service life of 1 million revolutions before they fail due to the fatigue of the roller surface. For radial bearings, the dynamic load rating refers to the purely radial, unchanging load and the rotating inner ring, while the purely axial, unchanging load is used for axial bearings. For each bearing, the dynamic load rating C is indicated in the bearing tables. This number depends on the dimensions of the bearing, the number of roller bodies, the material, and the model of the bearing.

Dimensioning

 

Bearings under static loads

For static loads, make sure that the bearing can withstand the loads. The static safety is calculated with the following formula:

So = static safety
Co = static load rating (KN)
Po = statically equivalent bearing load (KN)

The following values are to be aimed at for the static safety relative to the operating conditions and the demands on the smoothness of running:

So = 0,7 - 1,0 for low demands
So = 1,0 - 1,5 for normal demands
So = 1,5 - 2,5 for great demands

Axial self-aligning roller bearings are exceptions!

The statically equivalent bearing load Po is calculated as follows:

Yo = axial factor

P0 = statically equivalent bearing load (KN)

Fr = radial load (KN)

Fa = axial load (KN)

Xo = radial factor

The factors X0 and Y0 are given in the bearing tables where necessary.

 

Bearings under dynamic loads

The standardized calculation method (DIN ISO 281) for roller
bearings under dynamic loads is the following formula:

lf the speed of the bearing is constant, the service life of the bearing can be stated in hours:

L10h =  nominal service life in operating hours
n   =  speed
P   =  dynamically equivalent bearing load (kN)
p   = service life exponent for ball bearings p=3
p  =  service life exponent for roller bearings p=10/3

 

The dynamically equivalent bearing load P is calculated with the following formula:

P   = dynamically equivalent bearing load (kN)
Fr  =  radial load (kN)
Fa =  axial load (kN)
X   =  radial factor
Y   =  axial factor

The factors X and Y are given in the bearing tables where necessary.

 

Modified nominal service life

 

In the determination of the nominal service life L10, the effect of a load on the service life of a bearing is taken into consideration. Normal operating conditions such as good lubrication and proper installation are assumed here. This calculation context suffices in most cases. In some cases, however, influences on the service life of a bearing may have to be studied more carefully

ISO recommendations allow for improvements in the rollingbearing steels and the production process or more exact knowledge of the influence of lubrication on the fatigue process tobe taken into account in the calculation. Here, the fatigue service life Lna is calculated with the following formula:

The conditions of use must be known exactly to serve as the basis for the determination of the modified nominal service life, and the loads on the bearing must be determined exactly.

lf the normal survival probability of 90% is tobe applied, i.e. the bearings are made of known roller bearing materials and operating conditions are normal with sufficient cleanliness and maintenance, then:

Under these conditions, the same results are reached with both calculation methods.

a1 = correction value for survival probability. This correction value allows the survival probability of 90% to be changed.

a2  = correction value for bearing material

a3  = correction value for operating conditions


As both factors are interdependent, both values should be given together:

a23 = (2.2 for ball bearings, provided the lubrication film is perfectly formed)

a23 = (1 .5 for roller bearings, provided the lubrication film is perfectly formed)

The modified nominal service life Lna expanded with the factors  a1 , a2  and a3 also only considers the material fatigue as the cause of failure. Thus, the calculated service life of the bearing can only correspond to the actual usable life if the assumed lubrication condition remains constant throughout the operating time, the assumed load data, temperatures, etc. really correspond to the actual operating conditions, and no dirt can enter the bearing during the entire operating time. Under such conditions, the influence is 30% for ball bearings (a23  = 2,2) and 12.5% for roller bearings 12,5% (a23  = 1,5). The influence of geometry is not taken into consideration here.

This gives us the modified dynamic load rating Cmod• The survival probability thus remains at 90%.

This load rating is then used to calculate the increased nominal service life:

Lubrication

 

Rolling bearings must be lubricated for three reasons:

 

  1. To prevent metallic contact between the roller bodies, bearing rings, and cages.
  2. To prevent corrosion.
  3. To prevent wear.

Normally, roller bearings are lubricated with grease. During installing, only 30-50% of the empty space where the bearing is being installed should be filled with grease.

In the course of time, lubricants lose their lubrication. Used or dirty lubricants therefore have to be renewed or replaced. For this lubrication, only greases and oils suitable for bearings may be used. 

Sealed bearings are generally lubricated with highperformance lithium-saponified greases. These greases normally have a temperature range from -25°C to + 120°C,
withstanding operating temperatures of + 120°C for short times. Under constant operation above 70°C, these standard lithium-saponified greases tend to be effective for shorter periods.

For sufficient values under constant use at higher temperatures, use special greases.

Keep in mind, though, that the use limits of the contact seals used is + 110°c. For use above this limit, make sure that seals made of heat-resistant materials can be used.

Non-marked bearings are generally dimensionally stable up to a limit temperature of 150°C. Operating temperatures above 150°C require special heat treatment (stabilization) to prevent inadmissible changes in the bearing dimensions due to crystallization. Such treated bearings are marked with the following suffixes:

As already mentioned, always make sure that the use limit of the contact seals used in the standard is at + 100°C. For use above this limit, make sure that seals made of heat-resistant materials can be used.

 

Regreasing intervals                    

The usable life of the grease is influenced by many factors. The regreasing intervals in our table thus can only be seen as very rough estimates.

Experience with comparable bearings or ones already used is therefore very important as not all operating conditions and influential factors that affect the service life of a lubricant - and hence also the bearing - are known or determinable in many cases.

Regreasing amounts

In large housing spaces, when a grease amount regulator or at low speeds, the danger of overlubrication and thus of an inadmissible temperature increase is low. In these cases, plentiful regreasing is possible to improve the lubrication exchange.

lf these conditions are not met, especially at high speeds, the lubricant volume can only be increased to prevent overlubrication. The following table shows the standard values for regreasing amounts.

D =outer bearing (mm)
B = bearing width (mm)

Speed limit

 

The bearing load, the bearing clearance, the lubricant, and the heat elimination and heat supply influence the speed limit. The speed limit given in the tables apply for purely radial loads on radial bearings and purely axial loads on axial bearings, normal tolerance of the bearing clearance, no external heat, no excessive operating temperatures, and low, absolutely shock-free loads.

Bearing data

 

Tolerance symbols, DIN ISO 1132, DIN 620

 

Bore diameters

d: nominal bore diameter (theoretically small diameter with conical bore)

ds: bore measured at one spot

dmp:  1. mean bore; arithmetic mean of the largest and smallest bore measured in a radial plane
          2. mean theoretical small diameter for conical bores; arithmetic mean of the largest and smallest bore

d1mp:  mean theoretical large diameter for conical bores; arithmetic mean of the largest and smallest bore

∆dmp = dmp – d: deviation of the mean bore from the nominal size

ds = ds – d:  deviation of the bore measured at one position from the nominal size

d1mp = d1mp – d1: deviation of the mean large diameter of conical bores from the nominal size

Vdp:   fluctuation of the bore; difference between the largest and smallest bore measured in a radial plane

dmp = dmp max – dmp min: fluctuation of the mean bore; difference between the largest and smallest bore

 

Outer diameter

D:  nominal size of the outer

Ds :  outer measured at one position

Dmp:   mean outer ᴓ; arithmetic mean of the largest and smallest outer measured in a radial plane

Dmp = Dmp – D:  deviation of the outer ᴓ from the nominal size

∆Ds = Ds – D:  deviation of the outer measured at one position from the nominal size

VDp:  fluctuation of the outer; difference between the largest and smallest outer measured in a radial plane

VDmp = Dmp max – Dmp min:  fluctuation of the mean outer; difference between the largest and smallest outer

 

Width and height

Bs Cs:  width of the inner ring and outer ring measured at one position

Bs – B, ∆Cs – C:  deviation of the width of the inner or outer ring measured at one position from the nominal size

VBs = Bs max – Bs min, VCs = Cs max – Cs min:  fluctuation of the width of the inner or outer ring; difference between the largest and smallest measured ring width

Ts :  overall width of a tapered roller bearing measured at one position

T2s : overall width of a tapered roller bearing measured at one position via inner ring and outer ring normal

∆Ts = Ts –T, ∆Ts1 = T1s – T1, ∆ T2s = T2s – T2: deviation of the overall width of a tapered roller bearing measured at one position from the nominal size

Hs, H1s, H2s, H3s, H4s:  overall height of an axial bearing measured at one position

 

Running accuracy

Kia:       True running of the inner ring on the assembled radial bearing (radial deviation)

Kea:      True running of the outer ring on the assembled radial bearing (radial deviation)

Sd:        Axial run-out of the inner ring's lateral surface to the bore (wobble)

SD:       Fluctuation of tilt of the surface line to the reference lateral surface (wobble)

Sia:       Axial run-out of the inner ring's running path on the assembled radial bearing (wobble)

Sea:      Axial run-out of the outer ring's lateral surface to the outer ring 's running path on the assembled radial bearing (wobble)

 

Tolerances of the radial bearings (without taper roller bearing)

 

 

 

 

Tolerances of the taper roller bearings

 

 

Tolerances for axial bearings

 

 

Radial bearing clearance of single-row deep groove ball bearings in accordance with DIN 620

 

 

Radial bearing clearance of self-aligning ball bearings in accordance with DIN 620

 

 

Radial bearing clearance of self-aligning roller bearings in accordance with DIN 620

 

 

Fits

 

Fits provide sufficient radial stability for the roller bearings to prevent sliding in the bearing's position. As a rule, set fits have to be used for this.
Set fits have the benefit of supporting the whole of the relatively thin-walled bearing rings, which positively affects the exploitation of the service life.
This is not, however, always possible as requirements such as for movable bearings or simple installation and removal have to be taken into consideration.

The following factors must be taken into consideration when selecting fits:

 

The type and extent of the load

A distinction is made between circumferential loads, spot loads, and undefined load directions.

 

Circumferential loads

occur when the ring is running and the load is standing still or when the load is running and the ring is standing still, i.e. when force is exerted on every point in the running path once during each revolution.

Bearing rings subjected to circumferential loads tend to move in the direction of the circumference, which means that a fit should always be used. lf one is not provided, frictional rust will occur due to the motion, i.e. dry friction occurs between the contact surfaces, leading to the abrasion of the two surfaces. The greater the loads and shocks, the more firmly set the fits have tobe.

 

Spot loads

occur when the load and the ring are standing still or the load rotates with the ring. Bearing rings subjected to spot loads to not tend to move. Thus, a loose fit is admissible in this case.

 

Uncertain load directions

occur when both spot loads and circumferential loads occur. Both bearing rings should be equipped with set fits.

 

Temperature

The temperature fluctuations in the bearing position influence the fits, primarily in the direction of the heat flow.

 

The following shaft tolerances (full shaft) have proven their worth for use with axial bearings with cylindrical bares.

 

The following housing tolerances have proven their worth for use with axial bearings with cylindrical bares.

 

The following shatt tolerances (full shaft) have proven their worth for use with radial bearings with cylindrical bores.

 

The following housing tolerances have proven their worth for use with radial bearings: