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6.9.2 Rolling Element Bearings
Discussion: Table 6.9.-2, 6.9-5 and 6.9-6.
Table 6.9-2. lists the bearings used in API applications. Table 6.9-5 was developed to
illustrate each of these types and provide the industry Series designation for each type.
Table 6.9-6 compares the ABMA and ISO standards which cover these bearings.
Table 6.9-5 Types of rolling element bearings
Type
Series
Example
Radial
Single row deep groove
ball bearings
6 000
Double row deep groove
ball bearings (Duplex)
4 000
Cylindrical roller bearings
N, NU, NJ Second letter
describes configuration of
the races. I.e illustration is
NU which has “inner Flg”
on OD and non on ID.
Spherical roller bearings
20 000
Tapered roller bearings
Different for ABMA and
ISO
Single or double (shown)
ABMA
ISO
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EH,EL,H,H
H,HM,J,L,
LL,LM,M
30 000
or XXX as
described in
(These may
ISO
be
inch 355:1977Pa
series)
ra 3
(Metric
size)
Radial / Thrust
Single row angular contact
ball bearing
7 000
Double row angular contact
ball bearing
5 000 (US Nomenclature)
or 3 000 (European
Nomenclature)
For reference, a comparison of the ABA Standards and ISO standard scopes appear in
Table 6.9-6
Highlighted standards are presently referred to in the SP.
As typical between the US customary standards and the ISO equivalents, the ISO
standards tend to be more focused and cover individual subjects or types of equipment.
The typical US references tend to have a wider scope and cover more subjects or
equipment in one publication. Thus the ABMA Standard 20 covers the same content as
the following ISO standards:
ISO 15:1998 (Rolling bearings-Radial bearings-Boundary dimensions –
General)
ISO 464:1995 (Rolling bearings with locating snap rings-Dimensions)
ISO 492:2002 (Radial bearings – tolerances)
ISO 582:1995 (Rolling bearings – Metric series – Chamfer dimensions)
ISO 5753:1991 (Rolling bearings – Radial internal clearances)
ISO 8443:1999 (Radial ball bearings with flanged outer ring – Flange
dimensions)
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ISO 12043:1995 (Rolling bearings – Single-row cylindrical roller bearings –
Chamfer dimensions for loose rib and non-rib sides)
ISO 12044:1995 (Rolling bearings – Single-row angular contact ball bearings –
Chamfer dimensions for outer ring non-thrust side)
Bearings made to either the ISO standards or ABMA 20 are equally acceptable.
Discussion: Types of Tapered Roller Bearings.
There are three types of tapered roller bearings. In the US there is the metric (J class))
covered by ABMA 19.1 and the inch series, covered by ABMA 19.2,
Tapered roller brgs in the US provide different part numbers for the cone and cup .In
ordering a bearing, both the cone and cup part numbers need to be specified. In the
ISO series, the cups and cones together are identified by one single part number. Thus
if you order an ISO bearing you will have one part number and get both the cup and
cone. If you purchase an bearing per ABMA there are three potential part numbers,
One for the bearing assembly, one for just the inner cone and rollers , and one for just
the outer cup. The assembly number needs to be specified, otherwise you may only get
the inner cone and rollers or outer cup. The machinery manufacturer should provide
a part number which will result in obtaining a complete bearing assembly.
Discussion: Table 6.9-6
TABLE 6.9-6 ABMA STANDARDS AND ISO STANDARDS
ABMA
Content
ISO
Content
DIMENSIONS-RADIAL BEARINGS
Standard
7
Shaft/bearing & bearing/housing fits
for radial ball & roller bearings
(except tapered roller bearings)
None
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20
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Radial Brg of Ball, Cylindrical
Roller and Spherical Roller Types.
For tapered roller Brgs. See ABMA
19.1 & 19.2 below.
For tapered roller bearings see
ISO 355:1997 below.
For thrust bearings see ABMA 21.4
below.
For thrust bearings see ISO 104
below.
Boundary Dimensions:
Boundary Dimensions:
Section 2. Radial ball, cylindrical
roller, spherical roller Brgs.
ISO 15:1998 Brg Boundary
Dimensions: Radial ball, roller
brgs.(Cylindrical & Spherical)
Section 3 Radial Brgs with locating
snap rings
ISO 464:1995 Radial Brg with
locating snap ring.
Section 4 Radial Brgs with flanged
outer ring
ISO 8443:1999 Radial Ball Brg.
with flanged outer ring – Flange
dimensions
Section 5 Special case chamfer
dimension limits.
ISO 582:1995 Rolling Brg
chamfer dimen. Maxium Values
– bearings and housings
Boundary Tolerances:
Boundary Tolerances:
Section 6 Brg Boundary Tolerances:
Radial ball, roller Brgs.
ISO 492:2002 Brg Boundary
Tolerances: Radial ball, roller
Brgs. - Section 5.1
ABEC,RBEC,1 – ISO Normal
ISO Normal
ABEC,RBEC,3 – ISO 6
ISO 6
ABEC,RBEC,5 – ISO 5
ISO 5
ABEC,RBEC,7 – ISO 4
ISO 4
ABEC,RBEC,9 – ISO 2
ISO 2
Section 6.4 Tolerance for tapered
bore Brgs 1:12 and 1:20
Section 5.4 Tolerance for
tapered bore Brgs. 1:12 and 1:30
Internal Clearances
Internal Clearances
Section
6.6
Radial
Internal
ISO 5753:1991 Rolling Bearings
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Clearances and designated as
Groups 2,N,3,4,5 depending on the
amount of clearance.
- Radial Internal Clearances.
Groups 2,N,3,4,5 depending on
the amount of clearance
Table 19 Radial contact ball bearings
Table1 Radial contact ball
bearings with cylindrical bore.
with cylindrical bore.
Table 24 Double row self aligning
ball bearings with cylindrical bore.
Table 25 Double row self aligning
ball bearings with tapered bore
Table 20 Cylindrical roller bearing
with cylindrical bore.
Table 22 Double row self aligning
roller bearings with cylindrical bore.
Table 23 Double row self aligning
roller bearings with tapered bore
Table 2 Double row self aligning
ball bearings with cylindrical
bore.
Table 3 Double row self aligning
ball bearings with tapered bore.
Table 4 Cylindrical roller
bearing with cylindrical bore.
Table 5 Double row self aligning
roller bearings with cylindrical
bore.
Table 6 Double row self aligning
roller bearings with tapered
bore.
Chamfer dimensions of bearing,
shaft and housing
Chamfer
dimensions
bearing, shaft and housing
Metric bearings:
ISO 582:1995 Rolling bearings
–
chamfer
dimensions,
maximum values.
ABMA 20 Section 5 and section
6.5.
ABMA 19.1 Tapered roller bearings
Inch Bearings:
ABMA 19.2 Tapered roller bearings
of
Table 1 Brgs in accordance with
ISO 15:1998 (Radial ball, roller
brgs)
Table 2 Brgs in accordance with
ISO 246 (Cyl roller brgs –
Separate thrust colars), ISO
464:1995 (Radial Brg with
locating snap rings, ISO
12043:1995
(single
row
cylindrical roller brgs chamfer
dim for loose rib and non-rib
side.
Table 3 ISO 12043:1995 and
12044:1995
Table 4 ISO 355:1997 Tapered
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roller brgs.
Section 5 Shaft and housing filet
dimensions.
19.1(Metr
ic)
TAPERED ROLLER BRG’S
TAPERED ROLLER BRG’S
19.2
(Inch)
Radial Tapered roller Brgs:
Metric Radial Tapered Roller
Brgs:
Types:
(TS) Single Row Straight Bore
Types:
ISO 355:1997 Single
Tapered roller Brgs
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(TSF)Single Row Straight Bore
Flanged Cup
ISO 355:1997 Addendum 2:
Single Row Tapered roller Brgs
Flanges Cup
ISO 355:1997 Addendum 1:
(TDO) Double row straight bore,
Two Single cones, One double cup,
With Lubrication Hole and groove.
Double row straight bore, Two
Single cones, One double cup,
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(2TS) Double Row, Straight Bore,
Two Single cones, Two single cups
ISO 355:1997 Addendum 1:
Double Row, Straight Bore,
Two Single cones, Two single
cups
Shaft/Brg. & Brg/Housing fits
ABMA 19.1 (Metric Design)
Shaft/Brg. & Brg/Housing fits
Section 6 and Table 11
There are no ISO standards
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ABMA 19.2 (Inch Design)
covering these fits.
Section 6 and Table 3
Boundary Dimensions:
ABMA 19.1 (Metric Design)
Boundary Dimensions:
Boundary dimensions in Section 4
and Tables (2-5) are a combination
of those in common use, selected
sizes from ISO 355, and bearings to
meet anticipated future needs.
Type TS (Single row) Table 2
Type TSF (Single row flanged cup)
Table 3
Type TS (Single row)- ISO
355:1997, Tables 2-6
Type TSF (Single row flanged
cup)- ISO 355:1997 Addendum
2, Tables 1-4 and Base ISO
Standard 355 Tables 2-6
Type TDO (Double row)Table 4
Type 2TS (Double row)Table 5
ABMA 19.2 (Inch Design)
Boundary dimensions are not
specified in this standard. Consult
bearing manufacturer for boundary
dimensions.
Type TDO & 2TS (Double row)
- ISO 355:1997 Addendum 1
Tables 2-6 and Base ISO
Standard 355:1997 Tables 2-6
Inch Design – Not available
Boundary Tolerances:
ABMA 19.1 (Metric Design)
Section 5.3 and Tables 6-10
Std Class, K&N
Precision Class C,B & A
Boundary Tolerances:
ISO 492:2002 Brg Boundary
Tolerances: Radial ball, roller
Brgs. - Section 5.2
ISO Normal
ISO 6X
ISO 5
ISO 4
ISO 2
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ABMA 19.2 (Inch Design)
Inch Design – Not available
Section 5.0 & Table 2
Standard class: 2 & 4
Precision class: 3,0,00.
Section 6.4 Tolerance for tapered
bore Brgs 1:12 and 1:20
Section 5.4 Tolerance for
tapered bore Brgs. 1:12 and 1:30
Internal Clearances:
Since
there
is
a
definite
relationshiop between radial and
axial clearance in a tapered roller
bearing only on clearance, usually
the axial is specified. It is generally
set on assembly and the actual value
depends on the anticipated operating
conditions.
Internal Clearances:
Since there is a definite
relationship between radial and
axial clearance in a tapered
roller bearing only on clearance,
usually the axial is specified. It
is generally set on assembly and
the actual value depends on the
anticipated operating conditions.
DIMENSIONS-THRUST BEARINGS
ABMA 24.1.
Thrust Bearings of the Ball,
Cylindrical Roller and Spherical
roller types – Metric Design (Part
1 of each table) - Soft conversion
to US Customary in Part 2 of
each table
Boundary Dimensions:
ISO 104
Rolling
Bearings-Thrust
bearings
Boundary
Dimensions-General Plan
Boundary Dimensions:
Boundary Tolerances:
Internal Clearances:
Boundary Tolerances:
Internal Clearances:
ABMA 24.2
Thrust Bearings of the Ball,
Cylindrical Roller types – Inch
Design
Inch Design – Not available
Boundary Dimensions:
Boundary Tolerances:
Internal Clearances:
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Inch Design – Not available
Thrust bearings of tapered Roller
Type, Inch design
Boundary Dimensions:
Boundary Tolerances:
Internal Clearances:
Metric Design – Not available
ISO 104
Thrust bearings of tapered
Roller Type- Metric Design
Boundary Dimensions:
Boundary Tolerances:
Internal Clearances:
LOAD RATING
Standard 9
1990
Load rating and fatigue life for
Ball bearings
281 First edition
1990
Applies to ABEC 1
Load rating and rating life for:
Radial ball Brgs
Thrust ball Brgs
Radial Roller bearings
Thrust roller bearings
Standard 11
Load rating and fatigue life for:
Roller bearings
Calculation of L10h and Lna.
281 First edition
1990
Standard 9
1990
The ABMA standard 9 does not
address these issues and uses the
a1 ,a2 , a3 factors. The ABMA
Standard was published in 1990
and
these
ISO
revisions
published in 2 000.
Admendment 1 2
2 000
281 First edition
1990
Admendment 2 2
2 000
Since International Standard IS0
281 was published in 1990 more
knowledge has been gained
regarding the
influence on bearing life of
contamination, lubrication,
internal stresses from mounting,
stresses from heat
treatment, fatigue load limit of the
material etc. It is therefore now
possible to take into consideration
factors
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influencing the life calculations in
a more complete way. In this
amendment is specified how new,
additional knowledge can be put
into practice in a consistent way
in the life formula. The life
calculated with this extended life
formula is called modified rating
life and replaces the adjusted
rating life, Lna, in IS0 281:1990.
The modified rating
life has received a new
designation, Lnm, to avoid
confusion with Lna.
Rolling element bearing life Life
modification factors Replaces
Clause 9 of 281:1990
Replaces Lna Adjusted rating
life with the Modified rating life
Lnm..Still does not give any
number for modifying fators
other than reliability.
15:1998
(Rolling
bearings-Radial
bearings-Boundary dimensions –
General Plan)
12043:1995
Single row cylindrical roller
bearings – chamfer dimensions
for loose rib and non rib sides
12044:1995
Single row angular contact ball
bearings- chamfer dimensions
for outer ring non thrust side
6.9.2.1 Rolling element bearings shall be located, retained and mounted in accordance
with the following:
a. Bearings shall be located on the shaft using shoulders, collars or other positive
locating devices; snap rings and spring-type washers are not acceptable.
Discussion: Bearing location on shaft. When fixing a bearing in position on a shaft or
housing, there are many instances where the interference fit alone is not sufficient to
hold the bearing in place. Bearings must be fixed in place so that they do not move
axially when placed under load. Even bearings which are not subject to axial loads
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(such as cylindrical roller bearings etc.) must be fixed in place due to momentary loads
and resulting shaft flexure which may cause damage.(NTN)
Although the use of snap rings makes construction simple, they are not allowed since
they are sensitive to chamfers, bearing installation dimensions, and susceptibility to
axial loads.
b. The device used to lock thrust bearings to shafts shall be restricted to a nut with a
torque-type lock washer. [610]
c. Tapered bore bearings should have a positive axial stop on the large and small end of
the taper bore.
NOTE The large end stop will prevent movement of the bearing up the shaft. Movement up the shaft will
result in decreased internal clearance.
6.9.2.2 Shaft / Bearing and Bearing / Housing Fits
6.9.2.2.1 For inner race rotation, radial ball and roller bearings shall be retained on the
shaft with an interference fit and fitted into the housing with a diametrical clearance, both
in accordance with the recommendations of ABMA Standard 7. Tapered roller bearing
fits shall be in accordance with ABMA 19.1 (Metric sizes) and 19.2 (Inch sizes).
Interference fits and clearances shall consider the complete range of tolerances and
operating conditions for each component.
NOTE ABMA Standard 7 Figure 1 and Table 1 list the various class fits between the shaft and inner race
and Figure 2 and Table 3 lists the various fits and clearances between the outer race and bearing housing.
The actual fits and clearances used in a design depend on many factors such as load, bearing size and type,
material properties and other design and performance requirements.
Greater interference fits than normal can be required if the bearing is mounted on a hollow shaft such as
those used on gearing with quill shafts. Lighter interference fits can be required for some stainless steel
shafts that have a high coefficient of thermal expansion.
Bearing and shaft material with different coefficients of thermal expansion or parts with the same
coefficient of thermal expansion operating at different temperatures could either increase or decrease the
interference fit.
Discussion: No ISO equivalent for ABMA 7, 19.1 or 19.2.
There is no equivalent ISO standard for ABMA 7, 19.1 or 19.2.
ISO 13709 [API 610] did not reference an equivalent ISO standard, nor is any
referenced in ISO 5753:1991(Roller bearings Internal Clearances). The ISO Web site
does not list any standard with the same title. However some product literature and web
sites refer to the above class fits as ISO class m5, etc. This results from the content of
ISO 286:1988 Part 1 & 2 which provides standard shaft tolerances and fits. This
standard however does not contain the requirements of the ABMA standards with
regard to fits vs load, and types of bearings.
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Discussion: ABMA Standard 7 Figure 1.
ABMA Standard 7 Fig 1 (Reproduced below) lists 8 classes of fits identified as k5 to r7
which provide interference between the shaft and inner race. The actual interference is
determined by the class fit and the diameter of the shaft
ABMA 7 Figure 1
The last sustenance of 6.9.2.2.1 which was previously R22 6.9.2.1 b) has been added to
include the requirement that the interference fit needs to consider the complete range
of tolerances for the shaft OD and the bearing ID.
The recommended class fit to use in a design i.e k5, k6, m5 etc is based on shaft
diameter, type of bearing, load and other design requirements. These recommendations
are covered in Table 1 of ABMA 7 which is reproduced below for convenience:
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Table 1 ABMA 7
Thus for heavy loads and ball bearings, a class k5 for shafts up to just under 4 in. and
class m5 for shafts above 3.94 in. are recommended. To get an appreciation for the
interferences being discussed, Table 2 of ABMA 7 is partially reproduced for
convenience below. It can be seen that the interference for a 1 in. shaft would be from
0.0001in tight(T) to 0.0008 T and for a 3 inch shaft it would be from 0.0001 T to
0.0012 in. T
Table 2 ABMA 7
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One of the rolling element bearing manufacturers recommends a k5 class fit for ball
bearings 18-100 mm.
Discussion: ABMA 7, ISO 5753:1991 and ABMA 20.
ABMA 7 (Shaft – Housing Fits) allows roller bearings of the Cylindrical AND
Spherical design. It appears that in ISO 5753:1991 and ABMA 20 the cylindrical and
spherical roller bearings are grouped together under roller bearings.
6.9.2.2.2 The vendor shall advise on the data sheets the interference fit between the shaft
and the inner ring of the bearing and the clearance between the outer ring of the bearing
and the housing used in the design.
NOTE This paragraph applies to all rolling element bearings, including both ball and roller types. For
certain roller bearings, such as cylindrical roller types with separable races, bearing housing diametric
clearance may not be appropriate. [This Note was previously a discussion paragraph but it was changed to a
Note in API 610. Therefore it was made a Note in these SP]
Discussion: Describes reason for requirement.
This information is required for inspection and subsequent rebuilding of the
equipment.
6.9.2.2.3 Bearings shall be mounted directly on the shaft; bearing carriers (sleeves) are
not acceptable.
Discussion: Describes reason for requirement.
Bearing carriers and sleeves under the bearing result in additional interference fit
considerations which can develop into improper internal bearing clearances. When
mounting a bearing, the interference fit between the inner race and shaft, increases the
inner race ID and subsequently reduces the bearing internal clearance. This affect is
doubled when the bearing is mounted on a sleeve, and then the sleeve is mounted on
the shaft. Note consider rewording considering the case of using tapered adapters.(
tapered bore bearings)
-----------------------------------------------------------------------------------------------------------6.9.2.3 Internal Bearing Clearances [ISO 5753:1991, ABMA Standard 20]
Discussion: Bearing internal clearance
Bearing internal clearance (initial clearance) is the amount of internal clearance a
bearing has before being installed on a shaft or in a housing.
As shown in Fig. 6.9-1 when either the inner ring or the outer ring is fixed, and the
other ring is free to move, displacement can take place in either an axial or radial
direction. This amount of displacement (radial or axially) is termed the internal
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clerance and depending on the direction is called radial internal clearance or the axial
internal clearance. [NTN]
Discussion: Nomenclature.
The race is the inside surface of the ring. [CPC]
Discussion: Figure 6.9-1Radial and Axial Clearance.
Figure 6.9-1 Bearing Internal Clearance
Radial Clearance = δ
Axial Clearance = δ1 , δ2
Internal Clearance Selection: The internal clearance of a bearing under operating
conditions (effective clearance) is usually smaller than the same bearing’s initial
clearance before being installed and operated. This is due to several factors including
bearing fit, the difference in temperature between the inner and outer rings, etc. As a
bearing’s operating clearance has an effect on bearing life, heat generation, vibration,
noise etc. care must be taken in selecting the most suitable operating clearance. [NTN]
The axial clearance is not specified in the ABMA or ISO standards since it is a
function of the ball diameter, radial clearance and raceway curvature of the inner and
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outer ring which are dependent on the bearing manufacturer. Manufacturers catalogs
illustrate on how to calculate the axial clearance.
Discussion: MEASURING MOUNTED CLEARANCES
When measuring the mounted clearance of a bearing in a machine, allowances need to
be made for the increased clearance due to the deflection of the rolling elements, the
clearance between the outer race and the housing and the reduced internal bearing
clearance due to expansion of the inner race when the bearing is shrunk onto the
shaft.
Since the bearings have either point or line contact the contact stresses can be very
high, and the elements can deflect depending on the lifting force exerted during the lift
clearance check. For example, a 50mm (2 in.) bearing with a lift force of 5 kg (11 Lbs)
can deflect the elements approximately 0.0002”. A C3 clearance for this bearing is in
the range of 15-33 microns (0.0006 -0.0012”.) and this 0.0002”deflection needs to be
subtracted from the measured reading.
When the bearing is shrunk on the shaft, the inner race expands which decreases the
bearing internal clearance. The decrease in clearance due to the shrinking of the
bearing on the shaft can range from approximately 70-90% of the effective
interference.
The class fit between the outer race and the bearing housing will determine this
clearance. This clearance needs to be subtracted from the lift check to determine the
bearings internal clearance.
The mounted clearance is therefore different than the unmounted clearance. The
machinery manufacturer should provide the procedure and value for this installed
measurement. Note that ABMA 4 “Tolerance Deflections and Gageing Practices for
Ball and Roller Bearings” provided procedures for measuring only UNMOUNTED
bearing dimensions and not installed bearings.
Discussion: Additional Definitions.
The following definitions were added as 3.56 & 3.57
3.56 Rolling element bearing effective clearance: The mounted bearings internal
clearance under operating conditions.
3.57 Rolling Element bearing initial clearance: The bearings internal clearance before
it is installed on a shaft or in a housing.
6.9.2.3.1 Single row deep-groove ball bearings shall have greater than normal initial
internal clearance according to ISO 5753:1991 Group 3.
NOTE 1 For the purpose of this provision, ABMA 20 Group 3 is equivalent to ISO 5753:1991 Group 3. [API 610]
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NOTE 2 The internal clearance of a bearing under operating conditions (effective clearance) is usually
smaller than the same bearing’s initial clearance before being installed and operated. This is due to several
factors including bearing shrink fit to the shaft or housing, the difference in temperature between between
the inner and outer rings, etc
NOTE 3 Group 3 clearances are also known as C3 clearances.
Discussion: Example of internal clearances.
To give a feeling for the clearances being discussed, and the differences between the
various Groups of clearances, the Table 6.9-7 was extracted from ABMA 20 Tables 19
& A-19 . It lists the internal clearances for a single row deep-groove 30-40 mm ID
bearing and for a 80-100 mm ID bearing for the 5 groups of internal clearances
tabulated. As required by SP 6.9.2.3.1 the Group 3 is the one required for API
applications.
RADIAL INTERNAL CLEARANCE VALUES FOR RADIAL CONTACT GROOVE BALL
BEARINGS WITH CYLINDRICAL BORE
Clearance values in microns (0.0001 in.)
Dia
mm (in)
over Incl.
30
40
80
100
Group 2
Group N
Group 3
Group 4
Group 5
min
1
(0.5)
max
11
(4.5)
min
6
(2.5)
max
20
(8)
min
15
(6)
max
33
(13)
min
28
(11)
max
46
(18)
min
40
(16)
max
64
(25)
1
(0.5)
18
(7.0)
12
(4.5)
36
(14)
30
(12)
58
(23)
53
(21)
84
(33)
75
(30)
120
(47)
Table 6.9-7
Additionally, the clearance range of a bearing depends on its type, as illustrated by
Figure 6.9-2
MISSING was 16 MG size
[NTN]
Figure 6.9-2 Internal Clearance for Various Rolling Element bearings
Since clearance is a function of the heat generated, double element bearings (Double
row self aligning bearings and spherical roller bearings ) require greater clearance
than single element bearings ( deep groove ball bearings, and cylindrical roller
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bearing) Cylindrical and spherical bearings with line contact require greater
clearance than the ball element bearings with point contact.
Group 3 = C3
6.9.2.3.2 The selection of the internal clearance Group from ISO 5753:1991 for double
row angular contact ball bearings, cylindrical roller bearings, double row self aligning
roller bearings, double row self aligning ball bearings, and spherical roller bearings shall
be determined for the individual application and stated on the data sheet.
NOTE 1 For the purpose of this provision, ABMA 20 Groups are equivalent to ISO 5753:1991 Groups.
NOTE 2 These clearances apply to non-preloaded bearings and of a design such that they can take purely
radial load. [ISO 5753:1991]
NOTE 3 Single row tapered roller bearings and single row angular contact ball bearing mounted back-toback or face-to-face clearance are not covered in ISO 5753:1991 and ABMA 20 since clearances for these
bearings are set during assembly in the machine,
6.9.2.3.3 Preload for angular contact ball bearings, and taper roller bearings shall be
determined for the individual application, and stated on the data sheet. Preload shall be
set by grinding of the race faces rather than by spacers or springs.
NOTE Preload of angular contact ball bearings, and tapered roller bearings, is set during the installation of
the bearing in the machine. The faces of these bearings are ground by the bearing manufacturer during
manufacturing so as to result in the specified preload when finally assembled in the back-to-back or face to
face configuration in the machine.
Discussion: Preload
Preload : The compressive force acting on a rolling element bearing when the bearing
has zero internal clearance. Normally bearings are used with a slight internal
clearance under operating conditions as required by paragraph 6.9.2.3.2 and 6.9.2.3.3.
However, in some applications, the bearings internal clearance is removed, in fact it is
negative i.e the races, and rolling elements are in compression before operation. This is
analogous to a spring that is compressed between your right and left hand. Once the
spring is compressed, your hands are preloaded against each other. (NTN) Axial
preload is applied most commonly to angular contact ball bearings and tapered roller
bearings
Purpose of preload: The primary reason for providing a preloaded bearing is to prevent
unloading and skidding of an inactive lightly loaded bearing. Giving preload to
bearings results in the rolling element and raceway surfaces being under constant
elastic compressive forces at their contact points. This has the effect of making the
bearing extremely rigid so that even when load is applied to the bearing, radial or axial
shaft displacement does not occur. In addition to preventing skidding, preload may be
used to suppress runout, shaft vibration, noise, improve running accuracy, and
increase the critical speed of the shaft. (NTN)
Application of preload: The most common method of preloading is to apply an axial
force to two bearings having the same contact angle, which are mounted back-to-back
or face-to-face (duplex set of bearings) so that their two respective inner rings are
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pushed towards each other or their two respective outer rings are pushed towards each
other.
A schematic representation of component displacement and loading is presented in
Figure 6.9-3, through 6.9-8.
Figure 6.9-3
Duplex mounted angular contact bearings necessitate special face grinding. Face
grinding makes preload possible by allowing the bearing rings to be pushed together.
The duplex back-to-back mounting is illustrated below:
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Figure 6.9-4
The inner and outer races of two angular contact bearings are pushed axially to
eliminate all the internal clearance. With the components just touching, the preload =
0.
Figure 6.9-5
The above illustration is same two bearings with 0 preload except the excess ring
widths have been ground exactly flush. To add preload and allow compression of the
bearing components, additional stock is ground off the inside faces of inner race. This
is illustrated by the following sketch:
Figure 6.9-6
The inner ring separation indicates no preload. Once the inner rings are pushed
together, the rings and balls are compressed and the bearing becomes preloaded. This
is illustrated in the following sketch:
Thin Face
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Thick Face
Figure 6.9-7
Due to the high modulus of elasticity of steel, and the relatively low axial loading
(Preloading) imparted by the compression of the components, the original face-to-face
clearance is in the range of 0.0001 - 0.0002”.
For the back-to-back arrangement illustrated above, the inner races are pushed
together. For the face-to-face arrangement illustrated below, the outer races are
pushed together.
Thick Face
Thick Face
Figure 6.9-8
The bearing preload is determined by the equipment manufacturer into which the
bearings are going to be installed. It is important that the correct operating
temperature be used in determining the preload. If the bearing operating temperature
is higher than anticipated, the preload will be increased, additionally increasing the
operating temperature. This temperature spiral can continue until the bearing fails.
In addition to the above illustrated face grinding, preload can be set by either shims
and spacers (fixed position) or by springs (constant pressure).
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For fixed position preload the bearing position relative to each other remains constant
during operation and is usually obtained by one of the following three methods.
a. By installing duplex bearings with stand-out dimensions premachined by
the bearing manufacturer
b. By using a spacer or shim between the inner races or between the outer
races.
c. By adjusting a threaded screw which axially positions the inner race with
relation to the outer race.
For constant pressure preload the preload remains relatively constant, when the
distance between the two bearings fluxuates under the influence of operating heat and
load. Constant pressure preload is accomplished by coil, leaf or belleville springs acting
on the inner or outer race.
Generally about 95% of the API pumps do not require preload and the preload is
accomplished by the fixed preload method by requiring the bearing manufacturer to
premachine the races faces. The use of spacers at the inner race or inner and outer
race is not allowed in 6.9.2.3.3 since they can be easily improperly installed or lost. A
very small percentage of pumps have constant pressure preload and this is
accomplished by belleville springs.
Preload is calculated and specified as a force. The force required to compress the
bearing to remove the axial race face clearance is the bearing preload. The amount of
preload is determined by this initial gap. The larger the gap, the more compression is
required to close it and thus the higher preload. In all cases, the gap is reduced to zero
when mounting for all amounts of preload. For increased preload additional material
is removed to increase the axial race face clearance.
6.9.2.4 Rolling Element Bearing Cages
6.9.2.4.1 Unless otherwise specified, double row or single row angular contact ball
bearings and two single row ball bearings mounted back-to-back , face-to-face or tandem
shall be provided with machined cages.
Discussion: Reason for requirement.
The experience of the 610 ISO 13709 Joint working group is that machined brass or
bronze cages are required for angular contact ball bearings.(Series 7000). Max speed machined better, probably more difficult from lubrication stand point, machined brass
may not be available, heat transfer better with brass, cages are used in lighter loaded,
vibration less for machined cages.
6.9.2.4.2 Non-metallic cages shall not be used. [610]
Discussion: Reason for requirement.
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Non metallic cages are easily damaged if excessive heat is used to mount and
dismount the bearing also Non-metallic cages also tend to give no warning of trouble
and therefore fail “suddenly”. With oil lubrication, additives contained in the oil may
lead to a reduction of the cage service life. [FAG]
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6.9.2.4.3 Except as indicated in 6.9.2.4.1 and 6.9.2.4.2 and depending on availability, the
following types of cages are preferred:
Bearing Type
Single row deep groove ball
bearing (Conrad Type)
Preferred cage
Pressed Steel (Grease)
Machined (Oil)
Single row cylindrical roller
bearings
Machined (Grease and Oil
Lubricated)
Comments
It is often thought that machined cages will
eliminate most wear difficulties. In the case of
single row deep groove ball bearings, pressed
cages usually outlast machined cages when
grease lubrication is used because of the
difficulty in lubricating the riding surfaces of
the machined cage. Pressed cages have larger
clearances between the cage and balls which
allows better distribution of the grease. The
pressed cages also do not take up as much room
as the machined cage and therefore provides a
larger reservoir for grease.
More robust than pressed cages and not
restricted by the above lubrication concerns.
However, machined cages are generally not
available in size 100mm (4 In.) and below.
More robust than pressed cage.
The machined cage for this type of bearing has
more reservoir capacity for grease than the
Single row deep groove bearing and is adequate
to supply the required grease lubricant.
Double row spherical roller
bearing
Machined (Grease and Oil
Lubricated)
More robust than pressed cage.
The machined cage for this type of bearing has
more reservoir capacity for grease than the
Single row deep groove bearing and is adequate
to supply the required grease lubricant.
Tapered roller bearings
Pressed Steel (Grease and
Oil Lubricated)
Manufacturers only make pressed gages for this
type of bearing.
Discussion : Cage Function & Materials
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Cage Function:
The function of the cage is to keep the bearing elements apart so they do not rub
against each other so as to keep friction and heat at a minimum, to keep them evenly
spaced for uniform load distribution, guide the elements in the unloaded zone of the
bearing, and prevent the elements from falling out of separable bearings.
Transmission of forces is not one of the functions of the cage. [FAG]
Cage Materials:
Bearing cage materials must have the strength to withstand rotational vibrations and
shock loads. These materials must also have a low friction coefficient, be light weight,
and be able to withstand bearing operation temperatures. [NTN]
Bearing cages are either machined, pressed or injection molded. Machined cages are
made of brass or steel. Pressed cages are made of brass, hot, cold rolled or stainless
steels. Injection molded cages are made of various types of plastics or PEEK material.
Nonmetllic cages have a lower operating temperature limit than metallic cages. One
manufacturer limits the outer race operating temperature to 100 C (212 F) when
cages are non metallic. Pressed metallic cages are sensitive to operation above ndm of
250 000. [SKF]
For special applications such as for aircraft engine bearings, cages may be tensile
brass, midcarbon nickel, chrome, or molybdenum steel is used after undergoing
various heat treatments and high temperature tempering. The sliding properties of
these materials may also be enhanced when silver plated. [NTN]
Discussion: Illustrations of various types of cages
Figures 6.9-9 to 6.9-13 illustrate various types of cage construction for the different
types of bearings.
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Figure 6.9-9 Deep Groove Ball Bearings – Machined and Pressed Cages [FAG]
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Figure 6.9-10 Cylindrical Roller Bearings – Machined and Pressed Cages [FAG]
Figure 6.9-11 Tapered roller bearings – Machined and Pressed cages [FAG]
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Figure 6.9-12 Double Row Spherical Roller Bearing- Pressed Steel Cage [FAG]
Figure 6.9-13 Single Row Angular Contact Ball Bearing – Machined and Pressed
Cage [FAG}
Discussion: Bearing shields and seals
Seals are used primarily to retain grease in the bearing.
Shields are used to control the flow of grease.
Discussion: Temperature rise with seals and shields
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Temperature rise shielded vs sealed an example: Shielded Brg 7 º C vs contact seal
bearing 20 º C temperature rise.
Discussion: Figure 6.9.-14 illustrates various types of seals& shields
Shield does not
touch race
Seal touches
race
Figure 6.9-14 Schematics of Shields and Seals
Discussion: Sealed for life bearings
Grease lubricated bearings supplied with seals may be referred to as “sealed for life”
bearings. This life is dependent on the grease life rather than the 50 000 Hour bearing
life requirement. This grease life is a strong function of the operating temperature. The
catalog values for SKF is based on lithium base grease at 70º C. An increase in 15 ºC
will reduce the grease life by half.
Discussion: Relubrication of greased bearings
The length of time during which a grease lubricated bearing will function satisfactorily
without relubrication depends on the type of bearing, bearing size, load, speed, and
operating temperature. Regreasing intervals can be obtained from bearing vender
literature. The following is an excerpt from SKF catalogue “Bearing Installation and
Maintenance Guide Publication 140-710 Pg 76. Each bearing manufacturer has
developed criteria for regreasing of their bearings and the specific bearing
manufacturers’ recommendations should be followed. i.e. this chart is only applicable
to SKF bearings. Individual bearing manufacturers regreasing intervals can differ by a
factor of 2. Other manufacturers such as NTN factor in bearing load and operation
above 80 C.
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For a 100 mm diameter bearing running at 3600 r/min the regreasing interval
recommendation depends on the type of bearing.
A – radial ball bearing = 300 Hrs.
B – Cylindrical and needle roller bearing= 150 Hrs.
C – Other bearings as describes = 30 Hrs.
Regreasing can be performed by installing the bearing with no seals or shields and
inserting grease into the bearing cavity through the grease supply fitting. The grease
supply and drain may be on the same side of the bearing or on opposite sides of the
bearing.
Shielded bearings are sometimes used in motors. For that application, there’s not
unanimous agreement on the arrangement of the shields. H. Block in “Practical
Lubrication for Industrial Facilities” pg 346 indicates one user prefers the shield to
face the grease supply. The shield serves as a baffle and the shield - to-inner-ring
annulus serves as a metering device to control grease flow.
Some motor manufacturers favor a double-shielded design which has been
prelubricated. Packing the housing with grease next to the bearing prevents dirt and
moisture from entering the bearing. Oil from this grease reservoir seeps into the
bearing and revitalizes the grease within the bearing. A grease retainer labyrinth
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located on the inner side of the bearing is designed to prevent grease from reaching the
motor winding.
A bearing with seals will leak more grease than a shielded bearing. There is more heat
buildup in the sealed bearing, due to the rubbing seals, with subsequent reduction in
grease viscosity and generation of internal pressure. This will force grease out between
the seal and inner ring.
Sealed bearings are typically not used on API 610 Pumps.
Oil mist and ring lubricated bearings should not be supplied with seals or shields.
Polyeura, in motors Don’t want to mix lubricants. Should have compatable greases for a train of
equipment which needs to be regreased.
Don’t want EP additives in the grease.
Greases are specified by an ISO number which indaicates the viscosity of the oil in the
grease, and NLGI which indicates the “stiffness of the grease’.
Revisit the highlighted topics when lubrication section is reviewed.
6.9.2.5 Bearing Boundary Dimensions [ABMA Standard 20, ISO 492:2002]
6.9.2.5.1 Radial ball and roller (cylindrical and spherical) bearing boundary dimensions
shall be in accordance with ISO 15:1998 and ISO 582:1995. Tapered roller bearing
boundary dimensions shall be in accordance with ISO 355:19 NOTE -97
NOTE For the purpose of this provision, ABMA 20 Section 2.0 and Section 5 are equivalent to ISO
15:1998 and ISO 582:1995 and ABMA 19.1 and ABMA 19.2 are equivalent to ISO 355:1997.
Discussion: Explanation of Note
Referring to Table 6.9-6, ABMA 20 has several sections on bearing boundary
dimensions covering various types of bearing configurations. Sections 2 & 5 as
referenced in the note are the only ones applicable. Section 3 covers radial bearings
with locating snap rings which are not allowed per SP 6.9.2.1.a, and Section 4 covers
bearings with flanged outer ring.
Discussion: Reason for requirement.
Boundary dimensions have been standardized in order to allow bearing
interchangeability. The listing of “standard” size bearing is extensive. The dimensions
describing the bearing envelope i.e bearing bores, bearing OD’s and bearing widths
have all been standardized and can be found in ABMA 20. ABMA 20 lists 86 different
bearing bore diameters from 0.6 to 2000 mm, 8 different bearing ODs and 9 different
bearing widths. All standardized combinations of these dimensions are presented
Table 1 of ABMA 20.
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6.9.2.5.2 Radial ball and roller (cylindrical and spherical) bearing boundary tolerance
class shall be in accordance with ISO 492:2002 Section 5.1 Normal tolerance class.
Tapered roller bearing boundary tolerance shall be in accordance with ISO 492:2002
Section 5.2 Normal tolerance class.
NOTE 1 For the purpose of this provision, ABMA 20 Section 6-ABEC 1 (Ball bearings) and RBEC 1
(roller bearings) are equivalent to ISO 492:2002 Section 5.1Normal tolerance class.
NOTE 2 ABMA 19.1 Class K and 19.2 are equivalent to ISO 492:2002 Section 5.2 Normal tolerance class.
Discussion: Reason for requirement.
Paragraph 6.9.2.5.2 was added since there was no tolerance on bearing boundary
dimensions.
ABMA Standard 7 as required by 6.9.2.5.1 covers the general selection of shaft and
housing fits ( i.e. interference or clearance fits) for metric radial ball and roller
bearings (cylindrical & spherical). Since the amount of interference or clearance
depends on the tolerances of the mating dimensions, a tolerance has to be applied to
these boundary dimensions. ABMA 20 specified that bearings be manufactured in
accordance with tolerance classes as describe as ABEC 1 or RBEC 1. These tolerances
apply only to the bearings mounting dimensions and not any internal components such
as the balls, rollers or races.
ABEC stands for Annular Bearing Engineering Committee (not American or
Antifriction Bearing Engineering Committee) and refers to ball bearing tolerances.
RBEC refers to roller bearing tolerance and stands for Roller bearing Engineering
Committee )
A listing of these BOUNDARY tolerance (ABEC 1) can be found in ABMA Standard
20 and ISO 492:2002.
Discussion: ABMA & ISO Classes of Boundary tolerances
Rolling element bearing manufacturing BOUNDARY tolerances are divided into 5
classes. A listing of these classes and the corresponding ABMA and ISO designations
are listed in table 6.9-8
Tolerance Classes (From ABMA 20
para 6.3)
ABMA 20 Class
ISO
492:2002
Class
ABEC 1, RBEC 1
Normal
ABEC 3, RBEC 3
6
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ABEC 5, RBEC 5
ABEC 7, RBEC 7
ABEC 9, RBEC 9
5
4
2
Table 6.9-8 Tolerance Class
Airframe bearings, instrument ball bearings, needle roller bearings, tapered roller
bearings and thrust bearings and other bearing types and series not conforming to the
requirements in ABMA 20 are covered in other ABMA Standards.
The tolerances in ISO 492:2002 & ABMA 20 apply to the inner ring ID and width and
outer ring OD and width and not to the OD of the inner ring nor the ID of the outer
ring or the balls or rollers. The tolerances in ISO 492 & ABMA 20 thus only apply to
the mounting dimensions and runouts.
Discussion: Examples of bearing tolerances
What are the actual tolerances required by the ABEC, RBEC or ISO designation?
A comparison of the tolerance for a 30-50 mm ID bore bearing for the inner and outer
ring and the various classes is presented in Table 6.9-9. The highlighted ABEC 1,
RBEC 1 ISO Normal class is the specified requirement in 6.9.2.5.2
Bearing size (30-50 mm Bearing Bore) From ABMA 20
Tolerance Class
Inner Ring
Outer Ring
ABMA 20 Class
ISO
ID (Bore of
OD
5753:1991
Bearing)
Variation of mean
Class
Variation of mean outside dia. (VDmp
bore dia.(Vdmp )
)
Microns 0(.0001
Microns (0.0001
in.)
in)
ABEC 1, RBEC 1
ABEC 3, RBEC 3
ABEC 5, RBEC 5
ABEC 7, RBEC 7
ABEC 9, RBEC 9
Normal
6
5
4
2
9 (3.5)
8 (3.0)
4 (1.5)
3 (1.0)
1.5 (0.5)
8 (3.0)
7 (3.0)
4 (1.5)
3 (1.0)
2 (1.0)
Table 6.9-9 ABEC & RBEC Tolerance for 30-50 mm Bearing Bore
Discussion: Bearing manufacturers tolerances
Some bearing manufacturers supply, as standard, bearings to a higher tolerance such
as ISO 6 (ABEC 3). Since the API standards are a minimum standard, these tighter
tolerance bearings meet the requirements of 6.9.2.5.2.
6.9.2.5.3 Bearing chamfers shall conform to ISO 582:1995 and ABMA 19.2
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NOTE For the purpose of this provision ABMA 20 and ABMA 19.1 are equivalent to ISO 582.
Discussion: Reason for referencing ABMA 19.2
ABMA 19.2 is the inch series for tapered roller bearings and there is no ISO
equivalent.
Discussion: Reason for requirement.
In addition to the boundary dimensions and tolerances, rolling element bearing
chamfers on the outside corners of the OD and ID races and the mating components
have been standardized. Standardization eliminates the possibility of stress risers
developing when the radius or chamfer on the bearing is smaller than on the mating
component.
Discussion: Shaft filets.
Paragraph 6.6.7 in the shafting section of standard paragraphs has been added to
cover the filets on shaft shoulders against which rolling element bearings seat.
6.9.2.5.4 Finished steel balls for rolling element bearings shall be in accordance with ISO
3290:2001 with a maximum of Grade 16.
Discussion: Reason for requirement.
ISO 3290 is the only standard we have found which addresses the internal
configuration of a rolling element bearing.
Discussion: Content of ISO 3290:2001.
ISO 3290:2001 lists standard size balls, form, surface roughness tolerance, and
sorting tolerance and gages. The ball grade, as identified by the letter G, is a specific
combination of dimensional, form, surface roughness and sorting tolerances for the
balls. Sorting tolerance and gage grades go from G3, the most stringent, to G 200 the
least stringent.
Discussion: AFBMA STD 10.
AFBMA STD 10 previously covered the same subject but the AFBMA standard has
been superseded by ISO 3290:2001.
6.9.2.6 Rolling element thrust bearings shall be selected in accordance with the
following:
a. A rolling element bearing may be a single-row deep-groove ball bearing provided
the combined axial thrust and radial load is within the capability of such a bearing
and requirements of 6.9.1 are satisfied.
Note to TF Chairs: Only include this paragraph if the equipment covered by your
specification generates very light axial loads since the deep groove 6 000 series ball
bearings have very limited axial load carrying capacity.
Discussion: API 610 Experience.
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This single row deep groove option has been eliminated from 610 as a thrust bearing
and used only as a radial bearing (see 610 9th edition para 5.10.1.4. and 5.10.1.5) since
pumps generally have axial loads which exceed the capability of the 6 000 series deep
groove ball bearings.
b. Where the loads exceed the capability of a single-row deep-groove bearing, a
matched pair of single-row, 40° angular contact type (7 000 series) bearings shall be
used. Unless otherwise specified, bearings shall be mounted in a paired arrangement
back-to-back. The need for preload shall be determined by the vendor to suit the
application and meet the bearing life requirements of 6.9.1.3.[677]
Discussion: Reason for 40°contact angle.
These 7 000 series bearings come in various contact angles such as 25 º, 30º, and 40º.
The higher the angle, the larger the thrust capacity of the bearing. Mandating the
highest contact angle provides bearings with the highest thrust load carrying
capability, thus reducing spare parts inventory. With less spare parts, the possibility of
inadvertently installing a lower thrust capability bearings on rework is also reduced.
There are no mounting dimensional differences between the same “size” bearings with
different contact angles.
Discussion: Reason for preload determination.
With the back-to-back arrangement one set of balls will be thrust loaded the other will
be unloaded. Unloaded balls can skid, thus the need to determine if preload is
necessary. For a discussion on preload refer to the discussion paragraphs after
6.9.2.3.3
Discussion: Pump Pac SKF bearings.
One manufacturer sells a back-to back bearing set which does not comply with this
requirement and it is marketed as a “high thrust load” bearing . One of the bearings
has a 40º contact angle and the other bearing in the back-to-back pair has a 15º
contact angle. It is easy to install these bearings in incorrectly with the low thrust 15º
contact angle side actually handling the high thrust load. This can lead to premature
failure. The advantage of this bearing is that the 15º contact angle bearing on the
lightly loaded side has less of a tendency to skid than if it were 40º with no preload.
The thrust capability of a 40 º bearing is 25% greater than a 15º bearing.
Discussion:Reason for Back-to-back requirement.
Angular contact bearings can be installed back-to-back ,face-to face or in tandem.
Figure 6.9-7 illustrates the back-to-back arrangement, and Figure 6.9-8 illustrate the
face-to-face arrangement. Bearings mounted back-to-back provide a stiffer bearing
arrangement than the face-to face arrangement and can accommodate tilting moments
better than the face-to-face arrangement.
Discussion: Proper mounting for back-to-back arrangement.
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Since it is possible to mount the angular contact bearings in either the back-to-back or
face-to face arrangement, it is important that the bearings be installed properly. For 7
7 000 series angular contact bearings mounted back-to-back, the two thick sides of
the inner race are on the outside of the mounted pair. In face-to face arrangements,
the two thick sides of both the inner and outer race are on the outside of the mounted
pair. Refer to Fig 6.9-7 and 6.9-8.
c. If loads exceed the capability of pared angular contact bearings alternative rolling
element bearing types or arrangements may be proposed. Alternative bearing types shall
meet the criteria of Table 6.9-2 and 6.9.1.3.
Alternate bearing arrangements are arrangements where identical bearings are mounted
adjacent to each other with the thrust capability acting all in the same direction. A tandem
arrangement is illustrated in Fig 6.9-9.
Figure 6.9-9
The load carrying capacity of these arrangements is defined per the following formula:
C= I 0.7
Where C = Dynamic load capacity of the tandem arrangement
I = number of bearings adjacent to each other.
The Ndm in Table 6.9-2 shall be reduced by 20% for each additional bearing in the
arrangement. [SKF]
6.9.2.7 Bearings shall be located on the shaft using shoulders, collars, locking nuts with
tongue type lock washer or other positive locating devices. Snap or spiral type rings and
spring-type washers are not acceptable.[610 5.10.1.3.c]
Discussion: Reason for requirement.
Snap or spiral type rings are not allowed since they can roll over, and have a sharp
corner stress riser in the shaft.
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6.9.2.8 Four-point contact (split race) ball bearings and bearings with filling slots shall
not be used.
Discussion: Reason for requirement.
Four-point contact (split race) ball bearings are unique and easily misapplied. They
are usually applied in special thrust load situations and should not be considered for
general use. The inner ring is split and the outer ring with ball and cage assembly can
be mounted separately from the two inner ring halves. These bearings need to float
axially for the bearing to contact in either direction. The axial float can be in the range
of 0.030”. This amount of float can adversely affect a seal or the stiffness of the
assembly. They need less axial space than a double row angular contact bearing or two
angular contact bearings mounted back-to-back. The contact angle is 35 º. Refer to
Figure 6.9.10.
Fig 6.9-10
Discussion: Reason for requirement.
Filling slot bearings are configured to increase the number of balls that can be
installed within the races and are used for special high radial load only applications.
They tend to operate at higher temperatures than conventional single row deep groove
ball bearings, and they are sensitive to axial loads that might result in the balls passing
over the filling slot with subsequent bearing failure. Because of these conditions,
filling slot bearings should not be considered for general use.
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