12/31/2009 1 of 12
R 26
6.9 Bearings and Bearing Housings
6.9.1 Bearings—General
6.9.1.1 Each shaft shall be supported by two radial bearings and one double acting axial (thrust) bearing which may or may not be combined with one of the radial bearings. Unless otherwise specified, the bearing type and arrangement shall be selected in accordance with the limitations in Table 6.9-1, & 6.9-2. and 6.9.1.2
Note to Task Force Chairman: Include any restrictions or alternative bearing arrangements appropriate to the class of equipment being considered.
Table 6.9-1—Bearing SelectionCondition / Bearing Type and Arrangement
Radial and thrust bearing speed and life within limits for rolling element bearings
and
Machine energy density below limit / Rolling element radial and thrust.
Radial bearing speed or life outside limits for rolling element bearings
and
Thrust bearing speed and life within limits for rolling element bearings
and
Machine energy density below limit / Hydrodynamic radial and rolling element thrust
or
Hydrodynamic radial and hydrodynamic thrust
Radial and thrust bearing speed or life outside limits for rolling element bearings
or
Machine energy density above limit / Hydrodynamic radial and thrust
Table 6.9-2 Rolling Element Bearing Limiting Ndm/1 000
Bearing Type / Oil Bath or Splash Lubricated / Circulating Oil or Oil Mist / Grease Lubricated
Note 4
Bearings with or without shields or bearings with no seals / Bearings with seals
Radial
Single row deep groove ball bearing / 500 / 500 / 380 / 230
Single Row Cylindrical roller bearings /
340 /
450 /
290 / Note 2
Single Row Tapered roller bearings /
290 /
350 /
210 / Note 2
Double Row Spherical roller bearings /
230 / 315 /
175 / Note 2
Double Row angular contact ball bearings (Two sets of balls in the same races) /
290 / 360 /
225 / 145
Radial/Thrust
Single Row Angular Contact ball bearing. (40º contact angle)
Note 1 /
450 / 500 / 330 / Note 2
2 - single Row angular contact ball bearings mounted Back-to-Back, Face -to- Face, or tandem i.e (Duplex) 40 º contact angle. No preload
Note 1 /
360 /
450 /
270 / Note 2
dm = (d+D)/2 (mm)
Where :
D = Outside diameter of outer race (mm)
d = inside diameter of inner race (mm)
N = rotational speed (r/min)
NOTE 1 The Ndm limits for single row angular contact ball bearings and single row angular contact ball bearings mounted back-to-back, face-to-face or tandem are based on machined cages as required by 6.9.2.4.1. The Ndm limits for all other bearings in the table are based on pressed cage designs. Refer to 6.9.2.4 for bearing cage requirements.
NOTE 2 These bearings are generally not available with shields or seals.
NOTE 3 Rolling element bearing selected with Ndm greater than that indicated in table 6.9-2 requires purchasers and bearing manufacturer’s approval with special consideration being given to cooling and confirmation that the temperature requirements of 6.9.4.3 are met.
NOTE 4 Limits for Grease lubricated bearings and references to grease lubrication in Section 6.9 are presented for reference only since 6.10.1 requires bearings to be oil lubricated using mineral oil.
Discussion: Background information used to generate 6.9-2
The limits outlined in 6.9-2 are based on calculations and tabulations presented in Informative Annexes 5 & 6.
Discussion: Table 6.9-2 and cage design
Except for angular contact ball bearings (single, Back-to-Back, Face-to-Face or Tandem), tapered roller bearings and the exclusion of non-metallic cages, no cage design is specified for the other types of bearings listed in Table 6.9-2. The manufacturer’s catalogue ratings were modified, per their catalogue directions, so as to use speed ratings for pressed cages in Table 6.9-2. In many instances the catalogue ratings are tabulate for machined cages which generally are rated at the highest speed. A footnote then instructs the purchaser to reduce these speed limits if the bearings have pressed cage or resin designs even though the pressed or resin cage identification is next to the machined cage speed limit. One could easily misinterpret that the speeds listed next to the bearing designation applies to that bearing. Whereas they may have to be reduced by 20-25% if other than machined cages are used. To preclude this potential misapplication, the lower pressed cage limits were developed.
Discussion: Modification of machined cage design.
It is difficult to insert a note or paragraph increasing the speed limits if machined or other design cage is used, since these factors are “all over the country side” and appear unique to each manufacturer, and type of bearing. In some instances, one manufacturer may apply no corrections, and another manufacturer may apply a correction of over 1.5.
Discussion: Basis for circulating or oil mist limits
The column for circulating oil or oil mist is based on the industry wide speed limit number referenced in the previous paragraph. The NSK catalogue E1101g Pg A 37 states: “ When the speeds are more than 70% of the listed limiting speed, it is necessary to select oil or grease which has good high speed characteristics” . SKF in their publication 4560 E “ Rolling bearings in industrial gearboxes pg 83 states: “ Bearing speeds which are higher than 70-80% of the catalogue speed ratings are considered high. In such cases the following influences must be taken into consideration. Heat produced as the result of the friction…temperature differential between the inner and outer races…higher minimum load..”
As the result of these caveats, the speeds listed in the circulating oil and oil mist column were raised to the industry wide speed limit number referenced in the previous paragraph. It should be noted that these industry wide speed limits closely agree with the corresponding numbers for circulating oil and oil mist already appearing in API 673.
Discussion: Self Aligning ball bearings
Self aligning ball bearings are not included in the table since they are a very special case i.e. 90-10% rule.
Discussion: Graphical description of each type of bearing.
For graphical description of each type of bearing refer to Table 6.9-5
6.9.1.2 When the product of machine rated power, kW (HP), and rated speed, r/min, is 4,0 million (5.4 million) or greater, hydrodynamic radial and thrust bearings are required.
Note to TF Chairs: Consider modifying this paragraph to reflect the equipment being specified.
6.9.1.3 Rolling Element Bearing Basic Rating Life
Rolling element bearing’s basic rating life, L10h ,shall be at least 50 000 hours with continuous operation at rated conditions, and at least 32 000 hours at maximum radial and axial loads and rated speed. The basic rating, L10h life, shall be calculated in accordance with ISO 281 first edition 1990.
Discussion: Superseded ISO Standards.
Since ISO 281;1990 has been superseded is this and other superseded ISO standards archived? How do you get back to a 1990 issue?
The following is an e-mail from Alain Same of ISO to Graham Thomas addressing this question.
----- Original Message -----
From: Alain Samne
To: Graham A N Thomas
Cc:
Sent: Monday, July 26, 2004 9:25 AM
Subject: Back issues of ISO documents
Graham,
Withdrawn documents can be purchased directly from the Central Secretariat, butthey aren't and won't bemade available through the Online Store. We have started to keep old editions on ISOSTD server a few time ago now and have some ideas to prepare a product, under the form of a CD-ROM, which would contain these old documents
NOTE 1 - The basic rating life, L10h , is the number of hours, at the operating conditions, that 90 percent of a group of identical bearings, will complete or exceed before the evidence of failure. [API 611]
NOTE 2 - ISO 281:1990 defines basic rating life L10 in units of millions of revolutions. Industry practice is to convert this to hours and refer to it as L10h. Where:
L10h = (1,000.000/60N ) L10
N=Revolutions per minute
NOTE 3 - For the purpose of this provision , ABMA 9 - 1990 is equivalent to ISO 281- 1990 for ball bearings and ABMA 11 - 1990 is equivalent to ISO 281- 1990 for roller bearings.
NOTE 4 - The operating life of a bearing can deviate significantly from the calculated basic rating life. This may be due to wear and/or fatigue as a result of :
· Deviating operating conditions from that used in the life calculation
· Misalignment between the shaft and housing
· Insufficient or excessive operating clearances
· Inadequate lubrication
· Excessive operating temperature
· Oscillating bearing movement with very small pivoting angles-leading to false brinelling
· Vibration and false brinelling
· High shock loads
· Damage prior to and during installation [INA Web Pg]
· As a result of these factors, an estimated 95% of all bearing failures are classified as premature bearing failures [Timkin NTN Introduction to Ball bearings]
NOTE 5 - Bearing life calculations in accordance with ISO 281:1990 and ABMA Standard 9 do not yield satisfactory results for bearings subjected to such application conditions which cause deviations from a normal load distribution in the bearing, for example misalignment, housing or shaft deflection, rolling element centrifugal forces or other high speeds effects, and preload or extra large clearances in radial bearings. Where there is reason to assume that such conditions prevail, the user should consult the bearing manufactrurer for recommendations and evaluation of equivalent load and life. [ISO 281:1990 & ABMA Standard 9]. This life may also not be achieved where the bearings are operated in fluids other than clean lubricating oil. [API 676]
Discussion: Reason for revising hours
The hours were revised to agree with the run time specified in 6.1.1 .
A summary of the L10h lives specified in the various API standards follows:
Continuous operation at rated conditions : 25 000 – 610, 674/675
50 000 - 619, 611, 618, 677, 681
80 000 - 673
Maximum radial and axial loads and rated speed: 16 000-610,674/675
32 000-619, 611, 677, 681
25 000-618
Note that 50,000 Hrs = 6 years.
Discussion: Information in Annex 3
Refer to Informative Annex 3 for:
1)A description of the calculation of L10h
2)The relative load carrying capabilities of several typical radial bearings
3 The affect on bearing selection if the L10h life is increased from 25 000 Hrs to 50 000 Hrs.
4)The affect on bearing selection if the L10h probability of reaching the required life was raised from 0.9 to 0.95. This is necessary when accounting for multiple bearing failures in a machine if the overall machine probability of reaching its 5 year running life is kept at 0.9.
· 6.9.1.4 If specified, the L10h life for a system of bearings shall be calculated as follows:
L 10h (System) = [ ( 1/ L 10hA) 3/2 + ( 1/ L 10hB) 3/2 + ···· + ( 1/ L 10hN) 3/2 ] – 2/3
Where: L 10hA = Basic rating life, L10h per ISO 281:1990 for bearing A
L 10hB = Basic rating life, L10h per ISO 281:1990 for bearing B
L 10hN = Basic rating life, L10h per ISO 281:1990 for bearing N
The purchaser shall specify the system L 10h life of the system.
NOTE 1 A machine with two bearings each with an L10h of 50,000 hours has a .9 X .9 = .81 probability of reaching 50,000 Hours
NOTE 2 Based on actual load, and selected bearing, each bearing may not be at the 50,000 limit. The actual bearing L10h may be used.
NOTE 3 The System L10h life will not be greater than the lowest component L10h Life.
Discussion: System L10h life for a two and four bearing machine configuration.
Refer to SP Informative Annex 4 for an illustration of System L10h life for a two and four bearing machine configuration.
Discussion: Justification for 6.9.1.4 & 6.9.1.5
6.9.1.4 and 6.9.1.5 have been introduced to allow the user to calculate the probability of a machine with two or more bearings reaching the specified L 10h life. If desired, the user can then use 6.9.1.5 to modify the L10h life of the individual bearings for the machine to meet its desired L10h life. There are no tables given in ISO 281:1990 for the a1 a2 factors in 6.9.1.5.1.
6.9.1.5 Rolling Element Bearing Adjusted Rating Life
·6.9.1.5.1 If specified, the adjusted rating life Lna per ISO 281:1990 Section 9 may be used in lieu of the basic rating life L10h.
Where:
Lna = a1 a2 a3 L10h
a1 = Life adjustment factor for reliability
a2 = Life adjustment factor for special bearing properties
a3 = Life adjustment factor for operating conditions
· 6.9.1.5.2 For the purposes of this standard, a1 shall be in accordance with Table 6.9-3. The reliability and corresponding a1 shall be specified; a2 shall be 1.0 and a3 shall not exceed 1.0. The owner and vendor shall agree on the a3 factor used in the calculation. In no case shall the bearing selected be rated less than that determined by the basic rating life L10h. The Lna life may be used to calculate a system life when specified.
Table 6.9-3 – Life adjustment factor for reliability, a1Reliability
% / Lna / a1
90 / L10a / 1
95 / L5a / 0.62
96 / L4a / 0.53
97 / L3a / 0.44
98 / L2a / 0.33
99 / L1a / 0.21
NOTE If a bearing has a 90 % change of operating for 50,000 Hrs it has a 99% chance of operating for 0.21 x 50,000 = 10,500 hrs. Alternatively, if the bearing is required to operate for 50,000 Hrs with a 99% probability its L10h life would need to be 50,000 / .21 = 238,000 Hrs.