-
Rolling bearings. This document can be used for the selection,
calculation and check of rolling bearings of the company SKF. The
programme provides solutions to the following tasks:
1. Selection and check of a suitable bearing. The document
includes a database of approx. 10,000 different rolling bearings
SKF in all basic types and design.
2. Calculation of basic bearing parameters (life, static safety,
etc.).
3. Calculation of adjusted bearing life acc. to the new
methodology of ISO 281.
4. Calculation of load with a pair of tapered roller bearings or
a pair angular contact ball bearings resp.
5. Support of 2D and 3D CAD systems.
In addition to the above given basic calculations, the document
also includes several other auxiliary calculations (e.g. a
calculation of lubricant operational viscosity, calculation of mean
loads for bearings loaded by variable loads, calculation of
permitted bearing speed, etc.).
The programme uses data, procedures, algorithms and other
information from specialised literature, catalogues of rolling
bearings SKF, ISO, ANSI, SAE standards and other sources.
Related standards: ISO 15, ISO 76, ISO 104, ISO 281, ISO 355,
ISO 1132, ISO 5593, ISO 5753, ISO 3448, ISO 15312, DIN 615, DIN
620, DIN 625, DIN 628, DIN 630, DIN 635, DIN 711, DIN 715, DIN 720,
DIN 722, DIN 728, BS 290, BS 292, BS 3134
Hint: When selecting a suitable type of bearing, you can use the
comparative document "Selection of a rolling bearing".
Control, structure and syntax of calculations.
Information on the syntax and control of the calculation can be
found in the document "Control, structure and syntax of
calculations".
Information on the project.
Information on the purpose, use and control of the paragraph
"Information on the project" can be found in the document
"Information on the project".
Theory - Fundamentals.
Rolling bearings are produced in a wide scope of different
designs and sizes. They usually consist of two rings, rolling
elements and a cage. The bearings are divided into several basic
types according to their inner design, the shape of rolling bodies
and directions of the forces that can be retained. A comparison of
individual types of rolling bearings can be found in the document
"Selection of a rolling bearing".
Basic types of rolling bearings are internationally
standardized. Within the scope of each type the bearings are
produced in various designs whose properties may differ from the
basic design. Detailed technical parameters of rolling bearings are
given in catalogues of individual producers.
Calculation of rolling bearings. Selection of suitable
dimensions of the bearing is determined by the amount, direction
and type of load on the bearing and its speed. Depending on the
type of load on the bearing in operation, the bearings may be
divided into two groups for calculation purposes:
Bearings loaded dynamically In case of a dynamic loading the
loaded bearing rotates and selection of a suitable bearing is
determined by its life due to contact fatigue of the material.
Bearings loaded statically In case of a static loading the
bearing is loaded at standstill, at very slow rotation or slow
swinging movements. Selection of a suitable bearing is determined
by its static load rating.
Basic bearing life.
-
The life of a rolling bearing is understood as the number of its
revolutions (or the period of its operation at the given speed) to
the moment when the first traces of fatigue of material on rolling
elements or orbital paths appear. Practical tests show that the
life of identical bearings differs under the same operational
conditions. In order to assess the service life of bearings, the
so-called basic life measurement has been introduced.
The basic life of rolling bearings is the life that is achieved
or exceeded by 90% of identical bearings under the same operational
conditions provided that commonly used materials were used, usual
production quality achieved and bearings are operated under normal
operational conditions. The basic life is defined by the
equation:
where: C ... basic dynamic bearing load rating [N, lb] P ...
equivalent bearing dynamic load [N, lb] n ... bearing speed [1/min]
p ... exponent (p=3 for ball bearings, p=10/3 for other
bearings)
Basic dynamic load rating of the bearing is defined as a
constant non-variable load at which the bearing reaches the basic
life of 1 million revolutions. Values of dynamic loading capacities
are given for each bearing in the respective catalogue.
Equivalent dynamic load rating of the bearing is defined
exclusively as a radial load (with radial bearings) or axial load
(with axial bearings), at which all bearings of the same type show
the same life as reached under conditions of a real load. The
amount of the equivalent load is described in the relation:
where: Fr ... radial component of the real load [N, lb] Fa ...
axial component of the real load [N, lb] X ... coefficient of
radial dynamic load Y ... coefficient of axial dynamic load
Values of the coefficients X, Y depend on the type, design and
size of the bearing; with some types of bearings, also on the
direction and amount of the real load. These values are given for
each bearing in the respective catalogue.
Hint: Guiding values of the life can be found in par.
[1.13].
Adjusted bearing life. The basic life assesses the life of the
rolling bearing only in view of loads acting on it and does not
take into account any other effects such as operational conditions,
production quality or properties of the materials used. Efforts to
enhance the quality and reliability of the designs lead to the
requirement to calculate the life of the bearing more precisely and
therefore the standard ISO has introduced a modified equation of
the life:
where: a1 ... coefficient of the life for the required
reliability (see the table below) a2 ... coefficient of the life
for the given material properties and level of production
technology a3 ... coefficient of the life for the given operational
conditions
Values of coefficient a1
Reliability [%] 90 95 96 97 98 99
a1 1.00 0.62 0.53 0.44 0.33 0.21
-
Due to the mutual dependence of coefficients a2 and a3 producers
of bearings usually introduce the common value a23. The value of
this coefficient will depend, above all, on the quality of
lubrication and according to recommendations in ISO 281 it is
determined in dependence on the type of bearing using the
respective diagram (see the picture).
Values of the coefficient a23 for radial roller bearings
where: ... viscosity ratio (gives the rate between operational
and rated lubricant viscosity =/1 - see the chapter on lubrication
of bearings) ... coefficient of the level of contamination of the
lubricant (see par. [3.10]) P .... equivalent dynamic load PU ...
fatigue load limit (given for each bearing in the respective
catalogue)
In case the producer does not give these values of limit fatigue
loads with the bearings, you can use approximate values in
calculations as given in the following theoretical relations:
... for ball bearings
... for self-aligning ball bearings
... for other bearings
Load of the bearing. The external system of forces acting on the
seating must be distributed with a calculation of the bearing into
the forces acting in the radial and axial directions. The
intersection of normal lines at
-
contact points of rolling bodies and orbital paths with the axis
of the bearing (see the illustration) is considered the centre of
the acting forces.
Additional dynamic forces (vibrations and surges) that increase
loading on bearings usually occur with machines in operation. These
additional forces cannot usually be calculated or measured
precisely. Their effects are therefore expressed by various
empirical factors that multiply the calculated radial and axial
forces. In case of toothed gears, the amount of these additional
forces depends on the accuracy of toothing and in case of machines
connected to belt drives, on the type of belt and its prestressing.
Values of the respective coefficients are usually given in
documents of producers of belts and gears, orientation values can
be found in par. [1.15].
Fluctuating load. The above-mentioned calculations of the life
of rolling bearings are based on the presumption that the bearing
is operated under constant non-variable operational conditions.
However, in practice this presumption is often not fulfilled. In
applications where the amount of direction of the load or speed,
temperature, conditions of lubrication or level of contaminations
varies over the course of time, it is not possible to determine the
bearing life directly. In such cases it is necessary to divide the
bearing working cycle into several time periods in which the
operational conditions are approximately constant (see the
picture).
It is necessary to calculate the bearing life separately for
each such period. The total bearing life can be determined using
the relation
where: Lmhi ... partial bearing life for individual time periods
with constant operational conditions [h] ti ....... time portions
of individual periods in the bearings total working cycle [%]
In an effort to design a bearing quickly, practical procedures
use a simplified way of calculation of the bearing life for some
types of loads. In this calculation the external load of the
bearing is replaced by a virtual mean permanent load that shows the
same effects on the bearing as an
-
actually acting variable load. The procedures for determination
of the mean load for some common types of loads are given in the
table.
Calculation of the bearing mean load Fm
Fluctuating load with linear change of the amount, at constant
speed
Fluctuating load with sinusoidal course, at constant speed
Rotating load, at constant speed
Fluctuating load, at constant speed
Fluctuating load, at variable speed
where mean speed:
-
Oscillating motion
Oscillating motion is replaced by virtual rotation at the speed
equal to the frequency of oscillation:
where: Fi ... partial non-variable load [N, lb] ni ... constant
speed during acting of partial loads [1/min] ti ... time portions
of acting of partial loads in the bearings total working cycle [%]
p ... exponent (p=3 for ball bearings, p=10/3 for other
bearings)
Note: The simplified calculation method gives sufficiently
accurate results with calculations of basic life provided that a
variable load of constant direction is applied. Use of the
simplified calculation is not suitable in case of a load with
variable amounts and directions and with calculations of modified
life.
Effects of temperature on the bearing load rating. Commonly
produced and delivered rolling bearings are designed for
operational temperatures up to 120 C (100 C for sealed bearings).
In case of use of a bearing at permanently higher temperatures it
is necessary to modify it during production to ensure its
dimensional stability under operation. Bearings for use at high
temperatures are produced with thermal treatment, usually with
greater clearances and a differently designed cage, possibly with
the use of special materials.
Requirements for the use, production and delivery of stabilized
bearings must usually be consulted with the producer, where you can
find detailed technical parameters of the bearing. For the purposes
of preliminary designs it is possible to use the following
orientation table.
Approximate load rating of stabilized bearings compared with
common bearings of the same sizes
Limiting temperature 150 200 250 300 350
Supplementary designation
S0 S1 S2 S3 S4
Load rating [%] 90 - 100 75 - 90 60 -75 50 - 60 45 - 50
Safety of bearings at static load. A bearing at static load is
loaded by forces at standstill, at very slow speed or slow swinging
movements. The load rating of the bearing is determined by
permissible permanent deformations of orbital paths and rolling
bodies. The coefficient of safety s0 gives the standard of safety
of static-loaded rolling bearings and is defined by the following
relation:
where: C0 ... basic bearing static load rating [N, lb] P0 ...
equivalent bearing static load rating [N, lb]
-
Basic static load rating of the bearing is defined as the
external load that causes a permanent deformation of 0.0001 of the
diameter of the rolling body at the contact point of the most
loaded rolling body. This permanent deformation usually has no
adverse effects on the bearing function. Values of static load
ratings are given for each bearing in the respective
catalogues.
Equivalent static load rating of the bearing is defined
exclusively as radial load (with radial bearings) and axial load
(with axial bearings) respectively, which causes a permanent
deformation in the bearing and this deformation is of the same size
as under actual conditions of loading. The amount of the equivalent
load is described by the relation
where: Fr ... radial component of the real load [N, lb] Fa ...
axial component of the real load [N, lb] X0 ... coefficient of
radial static load Y0 ... coefficient of axial static load
Values of the coefficient X0,Y0 depend on the type, design and
size of the bearing. These values are given for each bearing in the
respective catalogue.
Hint: Guide values of the coefficient of safety can be found in
par. [1.14].
Friction and warming of bearings. The friction moment of rolling
bearings depends on many factors (design of the bearing, method of
lubrication, speed, etc.) and it is very difficult to determine
exactly. Practical calculations therefore use a simplified model
with the use of an estimated coefficient of friction. Under the
assumption of normal operational conditions and good lubrication an
approximate friction moment can be calculated with rolling bearings
operated at mean speed using the equation
where: P ... equivalent dynamic load of the bearing [N] d ...
diameter of the bearing hole [mm] f ... coefficient of friction
(depending on the type of bearing, f=)
In case of sealed bearings the moment from the friction sealing
must be added to the calculated friction moment. The resulting
friction moment further determines the power loss NR that is equal
to the heat produced in the seating:
where: n ... speed of the bearing [1/min]
Calculation of bearings with angular contact In case the shaft
is seated in two single row angular contact ball bearings or in two
tapered roller bearings, a mutual inner axial force is produced
with radial load in the bearings. This force will naturally affect
the bearing load rating and therefore it must be included in the
calculation. The amount of the axial load of one bearing depends on
the contact angle and arrangement of both bearings, on the amount
of radial forces FrA, FrB and on the direction and amount of the
external axial force Ka.
-
The calculation must also consider the seating as a unit and
both bearings must be designed at the same time.
Operational conditions. Required minimum load rating of the
bearing. Higher speeds create a danger of rolling elements slipping
between the orbital paths of the rings with unloaded bearings due
to centrifugal forces. This may adversely affect wear of the
bearing and thus reduce its life. The bearing should be loaded by a
certain minimum force under operation to ensure correct rolling.
The amount and size of this force depends on the type, design and
size of bearing and operational conditions. The relations for
determining the minimum load are usually given in catalogues of
individual producers.
Operating temperature. The heat that is produced by friction
must be dissipated to achieve thermal balance. The operational
temperature depends on many factors; its calculation is very
complicated and leads to a system of non-linear equations. The
following relation can be used for fast orientation:
where: t0 ..... ambient temperature [C] NR .... power loss [W]
WS ... coefficient of cooling [W/C]
The coefficient of cooling gives the amount of heat being
dissipated into the ambient air at a temperature drop of 1 C. For
bearings seated in frame machines it can be determined
approximately using the relation
where: D ... outer diameter of the bearing [mm] v ... velocity
of air [m/s] (v~1-2 for bearings inside the buildings, v~2-4 for
bearings in the open air)
Limiting speed. The speed of rolling bearings cannot be
increased without any limitation. Centrifugal forces of the bearing
increase its loading, inaccuracy of its run causes vibrations and
friction in the bearing causes warming. Limit speed depends on the
type, design and size of bearing, its accuracy, and the design of
the cage, inner clearances and operational conditions in its
seating and, above all, the highest permissible temperature of the
lubricant.
No specific and generally applicable limit of permissible speed
can be determined exactly for rolling bearings. Producers give in
their dimensional tables guide values of limit speeds for
individual bearings for the purposes of fast orientation. These
values are based on practical experience and are
-
applicable for bearings with normal clearances and produced at
normal levels of accuracy provided that they are operated under
normal conditions and with usual cooling. The given limit speeds
can be exceeded in certain individual cases, however, it is
advisable to consult this with the producer.
In addition to limit speeds, some producers also state in their
catalogues of rolling bearings values of so-called thermal
reference speeds. The reference speed gives the limit permissible
speed of the bearing under exactly defined conditions and serves as
an initial value for determining the permitted speed of the bearing
for the given operational conditions.
where: nr ... reference speed [1/min] fp ... adjustment factor
for the given type, size and load of bearing fv ... adjustment
factor for the chosen conditions of lubrication
The method of determining adjustment factors is described in
catalogues of individual producers or in ISO 15312. The reference
speeds given in the dimensional tables are defined for the
following operational conditions:
Bearing temperature 70 C
Ambient temperature 20 C
load P=0.05*C0
Lubricant kinematic viscosity
= 12 [mm2/s] ... oil lubricated radial bearings = 24 [mm2/s] ...
oil lubricated thrust bearings 40 = 100-200 [mm
2/s] ... grease lubrication
Lubrication of rolling bearings. The reason for lubricating
rolling bearings is to create a carrying lubrication film on
contacts between rolling bodies with orbital paths of the rings. In
addition, the lubricant protects the bearing from corrosion,
improves its sealing, exhibits cooling effects and lubricates the
surfaces of the bearing with sliding friction.
Rolling bearings can be lubricated by plastic or liquid
lubricants. Selection of a suitable lubricant is determined, above
all, by the speed, operational temperature, position of the shafts,
general concept of seating and economy of operation. If permitted
by operational conditions, greases are preferred with rolling
bearings.
Grease lubrication.
Grease lubrication is preferential particularly as regards easy
operation, economy and sealing of bearings against dirt and
moisture. It enables a simple arrangement of seating and is better
suited for high and surge loading. Greases must show good
lubrication capability and high chemical, thermal and mechanical
stability. The market offers a wide range of suitable greases. In
addition, most producers of rolling bearings offer their own ranges
of lubricants.
Greases offered by SKF
Design.
Description Viscosity [mm2/
s] Temperature [
C] 40 C 100 C
LGMT2 All purpose industrial and
automotive 110 11 -30 ... 120
LGMT3 All purpose industrial and
automotive 120 12 -30 ... 120
LGEP2 Extreme pressure, high load 200 16 -20 ... 110
LGLT2 Low load and temperature,
high speed 15 3.7 -55 ... 100
-
LGHP2 High performance and high
temperature 96 10.5 -40 ... 150
LGFP2 Food compatible 130 7.3 -20 ... 110
LGGB2 Biodegradable and low
toxicity 110 13 -40 ... 120
LGLC2 Low temperature and high
speed 24 4.7 -40 ... 120
LGWA2 Wide temperature range 185 15 -30 ... 140
LGHB2 High viscosity and high
temperature 450 26.5 -20 ... 150
LGET2 Extreme temperature 400 38 -40 ... 260
LGEM2 High viscosity 500 32 -20 ... 120
LGEV2 Extreme high viscosity 1000 58 -10 ... 120
LGWM1
Extreme pressure, low temperature
200 16 -30 ... 110
Grease has a limited life in the bearing. The reason is its
leakage from the bearing and impairment of its properties over the
course of time. Therefore, it is necessary to refill or replace the
lubricant at certain time intervals. The refill intervals will
depend on the type and size of bearing and operational conditions.
The recommended refill periods are given for individual bearings in
catalogues of the producers
Oil lubrication.
Lubrication of rolling bearings by oil is not so good and is
usually used only in the following cases:
Speed of the bearing is so high that refilling intervals for
lubrication by grease would be too short
Operational temperature is higher than the permissible
temperature for greases
The adjacent parts are lubricated by oil
Lubrication by oil flow is required for intensive cooling of the
bearing
Lubrication of spherical roller bearings
Depending on the operational conditions and desired design of
seating several different types of oil lubrication of rolling
bearings are used (oil bath, circulation of oil, spraying of oil,
oil mist). Bearings are usually lubricated by mineral oils.
Kinematic viscosity is the decisive property of oil; it decreases
with increasing temperature. Practical experience shows that in the
case of common seating the viscosity of oil should not drop below
12 mm2/s at operational temperatures. The rated viscosity that is
determined in dependence on the mean diameter and speed of the
bearing is the guiding factor for the selection of an oil with
suitable operational viscosity.
Rated viscosity 1
-
The qualitative standard of lubrication of rolling bearings is
given in the viscosity ratio:
where:
.... viscosity of the lubricant at operational temperatures
[mm2/s] 1 ... rated viscosity [mm
2/s]
For the viscosity ratio
-
Clearance of the bearing is the amount of free shift of one ring
against the other from one margin position to the other. Correct
run of the bearing is influenced, above all, by its radial
clearance. Bearings with normal radial clearance, C0, which is not
marked in the name of the bearing, are designed for normal
operational conditions. Smaller clearances, C2, or greater
clearances, C3, C4, C5, are chosen for significantly different
operational conditions.
Detailed information can be found in the respective catalogue of
bearings.
Matching of rolling bearings. Selection of correctly matching
bearing rings on the shaft and in the body has great importance as
for the life of the rolling bearing. When selecting suitable
tolerances, the following conditions are critical:
Amount and way of loading
Dilatation of the seated parts
Thermal conditions in the bearing
Requirements for accuracy, installation and dismounting of
bearings
Material and rigidity of the parts
Orientation values for the selection of tolerances can be found
in the following tables; exact data for individual types and sizes
of bearings can be found in the respective catalogue.
Tolerances of diameters of shafts for radial bearings
Operating conditions
Tolerance for bearings
ball cylindrical and taper
roller
spherical and toroidal
roller
Stationary inner ring load Light and normal loads g6
Heavy and shock loads h6
Rotating inner ring load or direction of load indeterminate
Light and variable loads (P0.07*C) j5, k5, k6,
m5, m6, n6 k5, k6, m5, m6, n6, p6
k5, k6, m5, m6, n6, p6,
r6, r7
Very heavy loads, shock loads (P>0.15*C)
n6, r6, p6 n6, r6, p6
High mounting precision, light loads h5, j5, k5 j5, k5
Axial loads only j6, js6 j6, js6
Tolerances of diameters of holes for radial bearings
Operating conditions Tolerance
Rotating outer ring load Very heavy loads, shock loads
(P>0.15*C) P7
Normal and heavy loads (P>0.07*C) N7
Light and variable loads (P0.07*C) K7
Light and normal loads (P
- All loads (P
-
1.3 Bearing design. Within the range of each type, rolling
bearings may be produced in a different design with some properties
different from the basic design. In case the producer delivers
various designs of the selected type [1.2], the programme offers
the respective selection lists in rows [1.4 .. 1.6]. Set up the
desired design of the bearing in these lists.
1.7 Bearing load. In this paragraph enter the radial and axial
components of external loads of the bearing and its speed at
constant non-variable operational conditions.
Hint: In case the actual load of the bearing is fluctuating, use
the auxiliary calculation in par. [5] to determine the mean
non-variable load. Detailed information on calculations of bearings
operated under variable operational conditions can be found in the
theoretical section of the Help.
1.12 Required parameters of bearing.
In this paragraph enter the required physical properties of the
bearing. In case of bearings loaded dynamically their life will be
critical; in case of bearings loaded statically their safety
coefficient will be critical.
1.13 Bearing life. Enter the desired life of the bearing.
Guide values of the life of rolling bearings
Bearing life [hours]
Machine type
300 - 3000 Household machines, agricultural machines,
instruments,
technical equipment for medical use
3000 - 8000 Machines used for short periods or intermittently:
electric
hand tools, lifting tackle in workshops, construction equipment
and machines
8000 - 12000 Machines used for short periods or intermittently
where
high operational reliability is required: lifts (elevators),
cranes for packaged goods or slings of drums etc.
10000 - 25000
Machines for use 8 hours a day, but not always fully utilized:
gear drives for general purposes, electric motors for industrial
use, rotary crushers
20000 - 30000
Machines for use 8 hours a day and fully utilized: machine
tools, woodworking machines, machines for the engineering industry,
cranes for bulk materials, ventilator fans, conveyor belts,
printing equipment, separators and centrifuges
40000 - 50000
Machines for continuous 24 hour use: rolling mill gear units,
medium-sized electrical machinery, compressors, mine hoists, pumps,
textile machinery
30000 - 100000
Wind energy machinery, this includes main shaft, yaw, pitching
gearbox, generator bearings
60000 - 100000
Water works machinery, rotary furnaces, cable stranding
machines, propulsion machinery for ocean-going vessels
> 100000 Large electric machines, power generation plant,
mine
pumps, mine ventilator fans, tunnel shaft bearings for
ocean-going vessels
In case of wheeled vehicles, their life is usually given in
millions of driven kilometres.
Bearing life [106 km]
Type of vehicle
0.1 - 0.3 Road vehicles
0.8 Railway vehicles - freight wagons
1.5 Railway vehicles - underground carriages, tramway
vehicles
3 Railway vehicles - passenger coaches
3 - 5 Railway vehicles - diesel and electric locomotives
For recalculation use the following relation:
-
where: n ... speed of the bearing [1/min] D ... diameter of the
vehicle wheel [m]
1.14 Static safety factor. Enter the desired safety at static
loading of the bearing.
Minimum permissible values of the static safety coefficient
Operating conditions Ball
bearings Other
bearings
Rotation movement, only requirements regarding quiet running
Smooth operation, vibration-free 0.5 1
Normal operating conditions 0.5 1
Pronounced shock loads 1.5 2.5
Rotation movement, normal requirements regarding quiet running
Smooth operation, vibration-free 1 1.5
Normal operating conditions 1 1.5
Pronounced shock loads 1.5 3
Rotation movement, high requirements regarding quiet running
Smooth operation, vibration-free 2 3
Normal operating conditions 2 3.5
Pronounced shock loads 2 4
Non-rotating bearings Smooth operation, vibration-free 0.4
0.8
Normal operating conditions 0.5 1
Pronounced shock loads 1 2
Oscillating motion great oscillation amplitude with small
frequency and with approximately steady periodic loading
1.5 2
small oscillation amplitude with high frequency and with shock
uneven loading
2 3
Note: In case of axial spherical roller bearings it is
recommended to use the minimum value of the coefficient s0=4.
1.15 Additional dynamic forces. Additional dynamic forces
(vibrations and surges) that increase loading on bearings usually
occur with machines in operation. These additional forces cannot
usually be calculated or measured precisely. Their effects are
therefore expressed by various empirical factors that multiply the
calculated radial and axial forces.
In this paragraph define the individual factor depending on the
type of machine used. The resulting factor of additional forces is
calculated additionally in [1.11].
1.17 Additional forces from geared transmission. In case of
transmissions with toothed gears the amount of additional forces
will depend on the accuracy of the toothing and machines connected
to the transmission.
The factor of additional forces fk, resulting from inaccuracy of
toothing, should be entered in row [1.19]. The recommended values
for the selected type of toothing [1.18] are given in the green
field.
The factor of additional forces from the connected machines fd
should be entered in row [1.21]. The recommended values for the
selected type of machine [1.20] are given in the green field.
-
Note: When ticking the checkboxes [1.19, 1.21] the calculation
automatically introduces the mean values of factors.
1.22 Additional forces from belt drives. In case of belt drives,
the amount of additional forces will depend on the type of belt and
its pre-stressing. The factor of additional forces fp should be
entered in row [1.24]. Data on its amount are usually given in
materials from the producers of the belts. If the data are not
available, use the recommended values that are given for the
selected type of belt [1.23] in the green field. Higher values in
the given range should be used for short lengths of shafts, surge
loads or large pre-stressing of belts.
Note: When ticking the checkbox [1.24] the calculation
automatically introduces the mean value of the factor.
Selection of bearing size. [2] This paragraph can be used for
selection of a bearing of a suitable size. Dimensions of the
bearing should be selected in par. [2.1]. Physical properties,
dimensional and operational parameters of the selected bearing are
calculated in par. [2.2] in real time.
Hint: The programme provides a function of automatic searching
for a bearing of a suitable size to facilitate the design.
Automatic selection of the bearing can be activated using the
buttons in row [2.1].
2.1 Bearing size. In the selection list select a bearing with
the desired dimensions. Individual bearings are listed in ascending
order according to inner diameter. The table parameters of the
bearing are arranged in columns in the following order: - Main
dimensions of the bearing (inner and outer diameter, width of the
bearing) - Basic dynamic and static load rating of the bearing (C,
C0) - Reference and limit speeds (nr, nmax) - Marking of the
bearing
Automatic selection of the bearing The programme provides a
function of automatic searching for a bearing of a suitable size to
facilitate the design. After pressing the button "Find first" the
programme finds the first bearing that meets the requirements for
life and static safety as defined in par. [1.12]. In case some
recommended values are exceeded with the proposed bearing or this
bearing does not meet the desired requirements, use the button
"Find next" to find another bearing.
When searching for a suitable bearing, the programme also checks
any possible exceeding of the permitted load [2.9, 2.10]. In case
the calculation cannot find a suitable bearing, select another type
[1.2] or design of bearing [1.3] and repeat the calculation.
Note: Bearings that are marked with "*" belong to a new range of
high quality bearings, "SKF Explorer".
2.2 Parameters of the selected bearing. Basic parameters of the
selected bearing are calculated additionally in this paragraph in
real time. Physical properties and operational parameters of the
bearing are given in the left part, its dimensions in the right
part.
Hint: The meaning and a detailed description of individual
parameters can be found in the theoretical section of the Help.
2.9, 2.10 Permissible radial or axial load. Not all types of
rolling bearings can carry combined loads. Some types are designed
only for retaining radial forces, other types for axial forces;
some types may carry only limited loads in the given direction. The
recommended amounts of permitted loads are prescribed for the given
types by producers and calculated additionally for the selected
bearing in row [2.9] or [2.10] resp.
Note: In case the producer does not give any limitations to
carrying combined loads for the given type and design of bearing,
no values will be given in rows [2.9, 2.10].
2.13 Power loss. Reference value which is valid for given type
and size of the bearing with the assumption of standard operating
conditions, load P/C0.1 and good type of lubrication.
-
Operating parameters, adjusted bearing life. [3] The adjusted
life [3.12] and recommended amount of minimum load [3.6] are
calculated additionally for the given operational parameters
(lubrication) of the selected bearing in this paragraph.
3.1 Kinematic viscosity of the lubricant. In row [3.3] enter the
kinematic viscosity of the lubricant used at the operating
temperature. In case of plastic lubricants the kinematic viscosity
of its basic oil component is given.
Practical experience shows that in the case of common seating
the viscosity of oil should not drop below 12 mm2/s at operating
temperatures. The rated viscosity [3.2] that is determined in
dependence on the mean diameter and speed of the bearing is the
guiding factor for the selection of an oil with suitable operating
viscosity. The qualitative standard of lubrication of rolling
bearings is
given in the viscosity ratio [3.4]. For the viscosity ratio
-
Typical contamination - Typical conditions for bearings without
integrated sealing; coarse oil filter, lubricant contaminated by
particles rubbed from neighboring machine parts
Severe contamination - Strongly contaminated ambient
environment; arrangement of bearings with insufficient sealing
Very severe contamination - =0
Note: When ticking the checkbox [3.11] the calculation
automatically introduces the mean value of the coefficient
depending on the selected level of contamination of the lubricant
[3.10].
Auxiliary calculations. [4] This paragraph gives some auxiliary
calculations for approximate determination of some operational
parameters of rolling bearings (operating viscosity of the
lubricant, length of relubrication intervals, desired oil flow,
etc.).
4.1 Calculation of operating viscosity. This paragraph is
designed to determine the approximate kinematic viscosity of the
selected lubricant at the operating temperature [4.2]. The
calculation is divided into two parts:
Determination of operating viscosity of mineral oils [4.3]
Calculation of the operating viscosity is based on the known oil
viscosity [4.5] at the reference temperature 40 C (~100 F).
Determination of operating viscosity of other lubricants [4.7]
Calculation of the operating viscosity is based on two known values
of kinematic viscosity of the lubricant [4.9] at various
temperatures [4.8].
Note: Exact values of operating viscosity can be found in the
material sheets of the respective lubricants.
4.11 Bearing lubrication. The desired oil flow [4.13] or the
length of the relubrication interval [4.14] resp. are additionally
calculated for the selected bearing [2.1] and the selected method
of lubrication [4.12].
Note: The selected method of lubrication is also decisive in
calculating the permissible speed of the bearing [4.15].
4.13 Desired oil volume flow. The necessary flow of oil for
cooling the bearing with circulatory lubrication is determined for
the given warming of the bearing (power loss [2.13]) in this row.
The calculated oil flow is a theoretical table value that is
determined for the difference in temperatures at the oil inlet and
outlet, T=10 C.
Note: The calculation does not take into account any external
cooling of the bearing due to heat conduction, radiation or
convection. Practical experience shows that under normal cooling
conditions there will be sufficient oil flow approx. 20-40% lower,
under very good cooling conditions up to 70% lower.
4.14 Relubrication interval. The recommended length of the
relubrication interval is determined for the given load and speed
of the selected bearing. The given value is valid for loads
C/P>4, normal lubrication conditions and operational temperature
of the lubricant up to 70 C (~160 F). In case of higher
temperatures the additional lubrication interval is shorter.
4.15 Calculation of permissible speed. The permissible speed of
the bearing is determined for the given load, method of lubrication
[4.12] and viscosity of the lubricant [4.17] in this paragraph. The
calculated value is for orientation
purposes only and can be applied for a lubricant with reference
viscosity 40 up to 460 [mm2/s], at
normal cooling conditions, temperature of the lubricant 70 C and
ambient temperature 20 C.
Fluctuating bearing load. [5]
-
The used calculations of the life of rolling bearings are based
on the presumption that the bearing is operated under constant
non-variable operational conditions. However, in practice this
presumption is often not fulfilled.
The auxiliary calculation in this paragraph is designed to
determine the mean non-variable loading in applications where the
bearing is exposed to a loading of a variable amount in a constant
direction at a constant or variable speed.
When calculating the mean loading, proceed in the following
steps:
1. Divide the working cycle into several time periods in which
the operational conditions are approximately constant (see the
picture).
2. In the selection list [5.1] set up the number of these time
periods.
3. In the input table [5.2] define the operational conditions
for individual time periods.
4. The mean non-variable loading is additionally calculated in
par. [5.3]. Using the button "Transfer" then transfer data on the
loading to the main calculation.
Warning: This calculation is approximate only and gives
sufficiently accurate results with calculations of basic life
provided that the variable loading has a constant direction. For
calculations of a adjusted life (or if the bearing is exposed to a
load of variable amounts and directions) it is more suitable to
select a more complex method of calculating the life of rolling
bearings. Detailed information on calculations of bearings working
under variable operational conditions can be found in the
theoretical section of the Help.
Calculation of bearings with angular contact. [6] In case the
shaft is seated in two single row angular contact ball bearings or
in two tapered roller bearings, a mutual inner axial force is
produced with radial load in the bearings. This force will
naturally affect the bearing load rating and therefore it must be
included in the calculation. The amount of the axial load of one
bearing depends on the contact angle and arrangement of both
bearings, on the amount of radial forces FrA, FrB and on the
direction and amount of the external axial force Ka.
-
The calculation must also consider the seating as a unit and
both bearings must be designed at the same time. In case of the
design of bearings, proceed in the following steps:
1. Activation of a switch in Fig. [6.1] selects the respective
arrangement of bearings and direction of action of the external
axial force. The calculation assumes action of an external force in
the shaft axis. In case the external axial force is acting on the
bearing body, forces in the opposite direction in the shaft must be
considered.
2. In the selection list [6.2] select the desired bearing
type.
3. Enter the amount of the external axial force [6.3].
4. In the pop-up lists [6.5, 6.13] select the designs of both
bearings.
5. Enter the respective radial loads [6.6, 6.14] for both
bearings.
6. In the following step it is necessary to select both bearings
step-by-step. In case the entered data are definite, the programme
shows recommendations in rows [6.4] or [6.12] respectively, for
which bearing must be designed the first.
7. Activate the automatic search for a suitable bearing using
the buttons "Find first" in rows [6.7, 6.15]. The basic life of
both bearings will be additionally calculated in rows [6.10,
6.18].
8. Using the buttons "Transfer" in rows [6.11, 6.19] you can
transfer the selected bearings into the main calculation. Here
check the parameters of the designed bearing in par. [2] and
additionally calculate the adjusted life of the bearing in par. [3]
for known operational parameters, if necessary.
Warning: Here the performed calculation of the bearings works
with the following data from the introductory paragraph: - speed of
the bearing [1.8] - desired life [1.13] - additional dynamic forces
defined in par. [1.15] Therefore it is necessary to enter these
data in par. [1].
Graphic output, CAD systems.
Information on options of 2D and 3D graphic outputs and
information on cooperation with 2D and 3D CAD systems can be found
in the document "Graphic output, CAD systems".
Setting calculations, change the language.
Information on setting of calculation parameters and setting of
the language can be found in the document "Setting calculations,
change the language".
Workbook modifications (calculation).
General information on how to modify and extend calculation
workbooks is mentioned in the document "Workbook (calculation)
modifications".