1 3 Symbols and Units 4 ISO Series and SNFA production 5 Bearing identification code 6 Precision 7 Assembly Tolerances 8 Form Errors 9 Diameter of Abutment Shoulders and Corner Radii of Seatings 11 Diameter of Bearing Shoulders and Corner Radii 14 Marking 15 Lubrication 15 Grease 18 Oil 21 Nozzle Position 23 Bearing handling 24 Clamping of bearing rings 25 Calculation of the axial clamping force 26 Tightening procedure 28 Excessive loads 28 Overheating 29 Brinnelling 29 False Brinnelling 30 Fatigue 30 Reverse Loading 31 Contamination 31 Lubrication 32 Corrosion 32 Misalignment 33 Excessive Radial Play 33 Excessive Ring Fit 34 Electrical Arc Damage 35 Natural Frequencies ASSEMBLY MANUAL SUMMARY BEARING DAMAGE ANALYSIS
36
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Transcript
1
3 Symbols and Units
4 ISO Series and SNFA production
5 Bearing identification code
6 Precision
7 Assembly Tolerances
8 Form Errors
9 Diameter of Abutment Shoulders and Corner Radii
of Seatings
11 Diameter of Bearing Shoulders and Corner Radii
14 Marking
15 Lubrication
15 Grease
18 Oil
21 Nozzle Position
23 Bearing handling
24 Clamping of bearing rings
25 Calculation of the axial clamping force
26 Tightening procedure
28 Excessive loads
28 Overheating
29 Brinnelling
29 False Brinnelling
30 Fatigue
30 Reverse Loading
31 Contamination
31 Lubrication
32 Corrosion
32 Misalignment
33 Excessive Radial Play
33 Excessive Ring Fit
34 Electrical Arc Damage
35 Natural Frequencies
ASSEMBLY MANUAL
S U M M A R Y
BEARING DAMAGE ANALYSIS
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SNFA angular contact, super precision ball bearings
are recognised for their high performance capabilities,
especially where the demands of precision and speed
are at their greatest.
SNFA bearings satisfy ISO dimensional requirements
(18, 19, 10, 02) as well as AFBMA international
regulations (Std. 20), These tolerances are listed in the
general SNFA catalogue.
The content of this publication shall be viewed as a
supplement of, and complimentary to, the data that is
contained in the “SNFA General Catalogue” and it is
intended for SNFA bearing users.
It provides a useful set of assembly instructions, but
does not presume to provide specific instructions as
each application has its own particular requirements.
3
Symbols and units of measurementd : Bearing bore diameter mmD : External bearing diameter mmB : Bearing width mmα : Contact angle degreesdm : Average bearing diameter mmC33 : Dynamic load capacity daNCo : Static load capacity daNRa : Axial rigidity daN/µmVh : Maximum speed of a single, spring preloaded,
oil lubricated bearing, α = 15° (Series BS200 and BS α = 62°) revs /minCr : Low speed bearing assembly rolling torque daN • mmM : Mass Kgn : Rotational speed rpmndm : Speed factor rpm • mm
Other symbols appearing within the text are described in the section in which they are found.
This table is provided for purely indicative purposes and is not binding as regards the technical characteristics and performance.
Comparison Table of SNFA Bearings with other makes
SNFAseries ISO Characteristics
SEA 18Average load capacitySpeed up to 1,500,000 ndm (oil)
SEB 19Good load capacitySpeed up to 1,500,000 ndm (oil)
VEB 19Good load capacitySpeed in excess of 2,000,000 ndm (oil)
Speed > 2,000,000 ndm, complete with oil lubricationHB 19 via the outer race and integral O-rings (... / GH)
Grease lubrication complete with seals (... / S)
EX 10High load capacitySpeed up to 1,500,000 ndm (oil)
VEX 10
Good load capacitySpeed in excess of 2,000,000 ndm complete with oil lubricationvia the outer race and integral O-rings (... / GH)Speed up to 1,600,000 ndm completewith grease lubrication and seals (... / S)
Speed > 2,000,000 ndm, complete with oil via the outer raceHX 10 and integral O-rings (... / GH)
Grease lubrication complete with seals (... / S)
E 200 02High load capacitySpeed up to 1,500,000 ndm (oil)
BS 200 02Mainly axial loadHigh rigidity and axial load capacity
VBs, VCs = Ring width variation: the difference between thelargest and Smallest measurements of the inner racewidth and outer race width measurements respectively.
∆Bs, ∆Cs = Deviation from the nominal value of a single innerrace or a single outer race width (∆Bs = Bs - B ecc.).
∆B1s, ∆C1s =Deviation the nominal value of a single inner racewidth or a single outer race width of a setmanufactured for paired mounting or a universalbearings (∆B1s = Bs - B ecc.).
∆C1s ABEC 7 Values identical to those of the corresponding inner ring of the same bearing
ABEC 9
7
Bearing assembly and fitting tolerances areextremely important for both assembly andbearing operation.
The values shown in the following table are aguideline for the design of the shaft, housing andbearing location.
Given that precision angular contact ball bearingsare widely used in machine tools, the tolerancesshown are mainly for this field of applicationwhere the shaft rotates whilst the outer race isstationary. If the application is reversed, i.e. theinner race stationary and the housing/outer racerotating, then the fit between outer race and
housing will need to have increased interferenceto prevent creep during operation.
The same applies to any shafts that are subjectedto high rotational loads (e.g. winding shafts).
The values given in the following table are validfor steel shafts and housings.
Critical situations may occur where there is a hightemperature gradient between shaft/housing andthe bearing raceways and these will requirespecial consideration. Thermal effects need to becarefully analysed and assembly tolerancesadjusted to prevent either excess bearing preloador loss of preload and subsequent failure.
d VEB - HB SERIES VEX - HX SERIES BS 200 SERIES BS (special) SERIESD damin DLmax rmax Rmax D damin DLmax rmax Rmax D damin DLmax rmax Rmax D damin DLmax rmax Rmax
d BS 200 SERIES BS (SPECIAL) SERIESD d1 D1 rmin Rmin D d1 D1 rmin Rmin
Values in mm
Shoulder diameterand corner radii of bearings
BS (SPECIAL) - BS 200 SERIES
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Marking
The “V” marking applied to the outerdiameter of the bearing indicates thedirection of the thrust applicable to theinner rings of the bearing set.
The arrow is located at the point ofmaximum eccentricity (maximum radialthickness) of the outer rings.
In assemblies of large and medium diameterbearings, a complete description includingvariant codes (contact angle, level ofprecision, coupling type, etc.) is applied toeach bearing in the set. For smaller bearingsfull marking may be applied to one bearingonly and the others partially marked withbase type, trademark and country of origin(e.g. SNFA, Italy).
The deviation in microns from thenominal value, for both the bore and
outside diameter of each bearing, ismarked at the highest point ofeccentricity for that ring.By positioning that point diametricallyopposite the point of maximum eccentricityof the shaft, optimum assembly concentricitywill be achieved.
Other symbols that might also appear onthe ring faces include manufacturingreferences: e.g. date of manufacture, setnumber, etc.
BEARINGPART NUMBER
SNFA TRADE NAMESIZE CODING AT POINTOF MAXIMUM ECCENTRICITY
COUNTRY OF ORIGIN
Fa
15
This is the most common form of lubri-cation as it is simple and easy to use.
When operating conditions (speed,temperature and cleanliness) are within thelimits stipulated by the grease manu-facturers, bearings require no specialmaintenance or subsequent topping up.This is often called life-long lubrication.
Selection of grease type is critical forbearing operation and depends on:
• operating temperature,
• life,
• protection,
• noise level.
Greasing of bearingsTo reduce the risk of contamination during spindle assemblyand to ensure correct lubrication it is recommended forcustomers to have bearings greased by SNFA.This operation is preformed in a clean room using specialistequipment immediately after the bearings have beenwashed. In this way the cleanliness of the bearing, thecorrect amount of grease and its uniform distribution areensured.
APPLICATION GREASE
Speed (ndm)
Up to 600.000
Up to 600.000
Up to 900.000
Over 900.000
Load
Light / medium
High
Medium
Light
NLGIConsistency
2
2
2
2
Soap
Lithium
Calcium / lithium
Calcium / barium / lithium
Calcium / barium / lithium
Lubrication Lubrication reduces friction and hence heat generation inside the bearing by separatingthe rolling and sliding surfaces and works even under high contact stress. Lubricants willalso protect the metal surfaces against corrosion.
A wide range of commercially availablehigh quality synthetic greases is nowavailable. Products satisfying the limits inthe table below are most frequently used.
Bearings operating at high temperatures,such as electrospindles, must be lubricatedwith long-life grease that has an adequatebase oil viscosity and high wear resistance.
Grease
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This activity is extremely important ifspindle operation is to be guaranteed and
especially so if the lubricant is grease.Running-in ensures that all spindle components “settledown” after assembly and, if grease lubrication is beingused, that is uniformly distributed.
Bearing and lubricant life and performance are directlylinked to the correct running in of a spindle and theprocedures used. In the case of grease based lubricationit is important to adhere to the following guidelines:
1 - Start off with a reduced rotation speedn1 ≤ nmax • 0,1
2 - Gradually increase the speed, in steps that areapproximately 15% of the maximum speed:
∆n ≅ nmax • 0,15
The effectiveness of the greasereduces in time due to operating
conditions such as temperature, stress and contaminationlevels and its chemical and physical characteristics.
Grease life However, these parameters are hard to estimate, so thegrease life hours “Lg” are calculated using statistical data.
Figure 1 the elements needed for assessing the life ofgood quality synthetic greases in optimum operatingconditions. The upper part of the life curve relates tooperating conditions at moderate temperatures (e.g.spindles fitted with a belt transmission). The lower part ofthe life curve, on the other hand, relates to applicationswhere there is another heat source (e.g. electro-spindles)that significantly increases the temperature of the bearingwith negative effects on the lubricant.From the diagram it is clear that, in applications thatfeature high operating temperatures, bearing life is moresignificantly dependent upon the grease life than it is onmaterial fatigue properties.
Speed factor ndm
Grease life (Lg in hours)
(Lg
in h
ours
)
Fig.1
Fig.2
0
4000
8000
12000
16000
20000
24000
0 40 80 120 160 200 240 280 320 t [min]
Tn
~ 15' min.
∆n
n1
nMax
[rp
m]
Wait at least 15 minutes after the bearing operatingtemperature has stabilised before increasing the speed.
During the run-in period it is essential to monitor thebearing temperature, using a probe that is in contact withthe bearings (figure 2).If at any time the temperature should rise to 55oC thenthe running in should be stopped, the spindle allowed tocool and the process restarted from the pervious stage,with the rotation speed being increased in half steps.
The temperature of 55°C is precautionary in nature. The bearing is in fact capable of handling uniformtemperatures up to approximately 100°C, but it is a goodidea not to exceed this limit as the temperature mightreach a far higher level for a short time within the bodyof the bearing itself.
Running-in
Running-in a SNFA VEX40 9CE1 DDL complete with grease lubrication
17
Reference quantitycm3
Hole (mm)
In the case of smaller bearings, where theamount of grease used is small, it isrecommended first to immerse the bearingin a mixture of solvent and grease (3 - 5%),then to dry it by evaporation in the open air,before finally adding the lubricant that isneeded.This will guarantee that the lubricantspreads uniformly across all bearingsurfaces.
Very often, SNFA bearings are supplied withthe type and quantity of grease requestedby the customer.
This solution offers operating and economicadvantages for the customer as the bearingis greased during the manufacturing routewith greater control over the cleanness,quantity and distribution.
Grease Quantity The quantity of grease used varies accordingto the type of bearing and operating speed.As such, the quantity is calculated bymultiplying the factor K (a function of the
maximum anticipated rotation speed,expressed in “ndm” - figure 3) by the valueof the “reference quantity” highlighted intable 4.
Factor KFig. 3
FactoryGreasedBearings
Table 4 - Basic quantity of grease
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2.423.525.067.48
11.0016.5024.2035.2050.6074.80
110.00165.00
Oil lubrication is mandatory when greasing limits are exceeded. A number of different oil basedlubrication systems are available and for the machine tool sector, the best known are:
• Oil injection • Oil mist • Air-Oil.
Oil injection is preferred for bearings havingto operate at very high speed, high load andwhere conditions do not allow “oil mist”lubrication owing to the need to cool thebearings.
Oil is injected into the bearings throughnozzles placed so as to lubricate the ball/racecontacts with minimum churning. Drainagechannels must be provided to prevent oil fromstagnating and/or churning and hencegenerating heat.
Besides ensuring proper lubrication the oilcrossing the bearing also removes the heatgenerated by the bearing operation and by
Oil injection entering from external sources and willmaintain temperatures at an acceptable level.
The assembly should also include oil filtering,a heat exchanger to dissipate heat removedfrom the bearings and an adequate oilreserve. A suitably sized reservoir facilitatesheat dispersion and the settling out of anydebris, it also avoids early lubricant ageing.
This type of lubrication system requiresaccurate and proper analysis. Precise rules forcalculating oil flow take account of thebearing type and the assembly. The oilviscosity range for an oil injection system isusually ISO VG10 or VG15.
19
Oil mist lubrication is widely used, especially in high-speed applications as it provides thefollowing advantages:
• A reasonable level of efficiency, even with a complex bearing arrangement.
• Low temperatures, reduced power absorption.
• Low cost assembly.
• Simple construction (channels, spacers, etc.)
• Good protection for the bearing against outside contamination (pressurised environment).
Oil mist lubrication equipment also needs to be designed in accordance with precise standardsthat take into account the design features and speed of the bearings being lubricated.
Oil Mist control unit manufacturers can provide the specific data that is needed.
The recommended oil viscosity for oil mist lubrication is ISO VG32.
A significant characteristic of this system isthe use of high viscosity synthetic oil(generally ISO VG68) that, even in smallquantities, ensures the presence of a resistantoil film between the rolling elements and thebearing raceways.
This provides both reduced ball rollingresistance and, simultaneously, good bearingbehaviour even under high stress.
Fig. 5
Oil mist
Air / oil The system is only moderately polluting as it has:
• A low level of oil consumption,• A controlled atomisation effect.
Indeed, in this system, the air (the carrier) andthe oil are supplied to the bearing via sidenozzles (figure 5) or via holes in theexternal ring of the bearing itself (pleaserefer to the following page for “H1” and “G1”bearings), without any mixing en route.
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CONVENTIONAL
H1
G1
Any system capable of sending the quantityof oil to a bearing that is strictly needed inorder for it to operate correctly is consideredto be “minimum” in nature.This type of lubrication can also be used in
Minimum oil high-speed bearings, by injecting smallquantities of oil directly into the bearing itself.A control unit and circuit that guaranteescontinuity of pressure and flow controls thetype and dose of oil used.
Notable results have been achieved in the high frequency and high power electro-spindle sectorusing air / oil lubrication.
High speed VEB and VEX bearings with the NS/H1 or NS/G1 designation (ceramic material ballsand outer ring with radial lubrication holes), and air / oil lubrication are capable of achieving highrotation speeds in excess of 2,500,000 ndm.
An approximate calculation of the quantity of oil (Q) that is needed is obtained using thefollowing formula:
Q = 1.2 · dm mm3/h for each bearingwhere dm is the bearing mean diameter (in mm).
The air / oil flow to the bearings must be homogeneous and without any lossesalong the way. It is therefore strongly recommended that each bearing besupplied individually even if, at times, a more complex delivery system isrequired.
“P” and “S” dimensions in mm
6789
10
1215172025
3035404550
5560657075
80859095
100
105110120130140
150160170180190
200220240260
280
13.40
15.4018.4020.4024.5029.50
34.5039.5044.5050.0055.60
61.3066.4072.4077.4082.40
87.4094.1099.10
104.10109.10
114.60120.90130.90144.00153.20
165.60
0.30
0.300.300.300.350.35
0.350.350.350.350.45
0.550.650.650.650.65
0.650.900.900.900.90
1.401.101.102.201.85
2.20
12.10
14.80
16.8019.8022.0026.7031.80
36.8043.0048.7054.2058.40
64.6069.6074.5081.5086.50
91.5098.60
103.50108.50115.40
125.40137.40149.80159.80
173.30183.30193.30207.40217.30
231.10251.10271.00298.90
318.3
0.85
0.75
0.751.150.901.051.05
1.001.251.451.451.65
1.851.851.752.202.15
2.152.552.502.503.00
2.953.403.403.35
4.654.654.655.655.60
6.306.306.207.95
7.30
10.1011.3013.3014.8016.50
18.2021.9024.1028.7033.50
39.7045.7051.1056.6061.60
68.1073.1078.1085.2090.20
97.00102.00109.50113.60118.80
126.00132.80142.80157.10167.10
178.90190.80204.50219.50229.00
240.30264.10283.60
0.901.001.201.200.90
1.201.201.351.501.75
1.901.901.902.302.30
2.302.302.302.802.80
3.003.003.103.103.25
4.004.254.255.405.40
5.655.755.506.556.05
6.206.906.40
10.1011.3013.3014.8016.50
18.2021.9024.1028.7033.80
40.3046.1051.6057.6062.30
69.6074.6079.3086.5091.50
98.50103.50111.00115.40120.40
135.40144.90
0.901.001.201.200.90
1.201.201.351.501.65
1.651.901.851.852.10
1.901.852.052.152.25
2.702.702.653.053.05
3.053.50
13.1013.8016.1017.90
19.6022.3025.7030.8035.50
42.4049.9055.8060.9065.20
72.2080.2086.0091.0095.80
102.70110.00116.00123.80130.30
137.20144.40157.20168.60182.50
1.000.701.301.55
1.601.451.551.751.65
2.052.052.502.102.75
3.152.803.003.002.75
3.304.002.053.703.75
4.854.654.855.705.50
SEAP S P S P S P S P S
Diameterd SEB - VEB EX VEX E 200
Nozzle position.
Maximum performance is achieved for all oil lubrication systems whenthe lubricant flow reaches the bearing contact areas with minimumturbulence.
Nozzle positioning, as indicated in the table below, is therefore stronglyrecommended.
“P” and “S” values for nozzle position
SERIES
21
The level of bearing cleanliness affects both
bearing life and efficiency. It is therefore
extremely important to achieve an application
where the bearings operate free of external
contamination.
18/13
13/10
10/7
ISO 4406 CONTAMINATION LEVELNumber of particles
in1 cm3
in100 cm3
Of oil
Numberof fields
Particle dimensions, microns
Particle dimension, micronsFig. 6
Influence of the amount of lubricantcontamination on bearing
behaviour and life.
With grease lubrication it is essential that
all precautions are taken to prevent the
ingress of contaminants both during the
greasing process, assembly and operation.
Spindle sealing has a significant role to play in
keeping the bearing system free from debris
during normal operation. The new range of
SNFA sealed bearings can also offer designers
new options in ensuring longer life.
In the case of oil lubrication the basic
demands for cleanness also apply but there is
the added requirement of ensuring that the
oil remains adequately free from conta-
minating particles. The contamination
level will need monitoring. The frequency
of monitoring will be governed by; the rate of
contamination, the effectiveness of the
sealing and the standard of the filtration and
filter size.
Apart from particulates, oils are also
contaminated by the ingress of cutting oils
and coolants etc. The oil properties are
reduced so adversely affecting bearing life.
This problem should be minimised by good
sealing of the spindle.
Contaminating particle classifications are
available that specify size limits and amounts
per 100cm3 of oil.
With reference to classification ISO 4406 and
ISO 4572 (figure 6) and the high precision
sector, especially the high performance
electro-spindle sector, it is advisable not to
exceed a maximum contamination level of
11/8 and a filtering efficiency of B3 ≥ 200.
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Angular contact super precision SNFA ball bearings are manufactured and packaged under strictlycontrolled environmental conditions.
The end user can only take full advantage of bearing performance by using them properly andobserving the following advice very carefully:
Bearing handling
• Store the bearings in the originalpacking and in a dry environment.
• Plan the assembly sequences carefully.
• Operate in a suitable environment.
• Inspect components close to thebearings and check their cleanness.
• Check on the drawing that the bearingdesignation on the box is correct.
• Open the package only when thebearings are required for installation
• In the case of grease lubrication,introduce the correct amount of grease
and distribute it evenly. In the case ofsynthetic grease, issues might arise relatingto incompatibility with the protective oil.Whenever possible wash the bearing in wellfiltered products compatible with theenvironment and bearing materials and dryit immediately using dry and filteredcompressed air. On no account should the bearing be spunusing the air jet.
• Assemble the bearing in accordancewith the instructions enclosed in thepackaging (excessive force must beavoided).
• If necessary preheat the bearing boreor outer housing to ease assembly.
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Bearings are tightened axially on journals orinto housings with either ring nuts or endcaps. These must be designed andmanufactured to have:
The clamping force, Pa, which is obtained eitherby tightening the ring nuts or end caps, is ofsignificant importance and shall be able to:
The value for Pa can be obtained from:
Pa = Fs + ( Ncp ˙ Fc ) + Pr Where: Pa Axial clamping force (daN)Fs Minimum axial clamping force (daN)Fc Axial fitting force (daN)Pr Bearing preload (daN)Ncp Number of preload bearings
Values for Fs and Fc are listed on the following page by bearing series and bore diameter.
The preload value Pr, is specified in the bearing data table or, when dealing with a special preload, in the bearing
designation.
For a more accurate calculation please contact the SNFA Technical Office.
With values for Pa the value of the tightening torque C (daN mm) can be calculated:
C = K ˙ Pa for the locking nut
C = K ˙ Pa / Nb for bolts in the end cap.
K is a based on the screw thread (see the table on page 26) and Nb is the number of screws on the end cap.
Details and recommendations on the tightening procedures are included in the “SNFA bearings assembly” manual.
Tighteningtorquecalculation
Calculation of theaxial tighteningforce, Pa
N.B.: The tightening torque value C calculated using the above method is only valid for:
- Locking bearing sets that comply with the tolerances that are recommended in this catalogue.- Locking bearings and spacers only and not other components (e.g. gearwheels).- A maximum axial workload of less than 2 • Pa.- Good quality ring nuts or end caps where the thread is lightly oiled.
The SNFA technical department can provide the requisite advice if the above conditions cannot met.
Fitting andclamping ofbearing rings.
• Prevent any relative movements of thecomponents and so avoid any frettingcorrosion during operation.
• Guarantee correct bearing location withoutresulting in any kind of deformation.
• Minimise material fatigue.
Correct assessment of the force Pa is difficultgiven the uncertainty of the parameters thatare in play. However, as a general guide, thetightening force Pa and the resultant value ofthe tightening torque C for the ring nuts andend caps can be calculated using thefollowing rules:
24
25
6789
10
1215172025
3035404550
5560657075
80859095
100
105110120130140
150160170180190
200220240260280
37
43556095
120
140160180240290
330330470500550
550750800800850
9001100120017001600
2100
24
2118162521
1821181918
2324262423
3055504846
4560609080
100
33
50
606575
130160
190260310380310
410450480650650
700900950
10001200
1300160023002400
27002800300037003900
48005200570077008300
28
28
2828284034
3044504838
4340375048
65908585
100
90120160150
180170160220260
320290270400400
Fs Fc Fs Fc Fs Fc Fs Fc Fs Fc
Calculation of the axial tightening force
SEASERIES
E 200SERIES
BS 200 - BS (SPECIAL)SERIES
SEB - VEB - HBSERIES
VEX - EX - HXSERIESd
2631456065
70100100160180
250330410450500
600650700850900
11001100160014001500
2000220027002900
34003800510064006800
660079008600
4341494955
4749496550
5575757565
8075708075
120140170150140
180190270250
270290350450500
550600550
49496585
10095
130230240
340550600700600
7501100130014001500
17001900190027002700
31003700450048005900
55606070
7060708575
80120120120100
110130130130130
190250250300310
330360430450500
120140190260320
480650800900
1000
1500
2100
7575809595
95130140130130
150
210
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THREADFACTOR “K” FACTOR “K”
NUTS BOLTS NUTS
M 4M 5M 6M 8
M 10
M 12M 14M 15M 16M 17
M 20M 25M 30M 35M 40
M 45M 50M 55M 60M 65
M 70M 75M 80M 85M 90
M 95M 100M 105M 110M 120
M 130M 140M 150M 160M 170
M 180M 190M 200M 220M 240
M 260M 280
1.4
1.61.92.02.12.2
2.63.23.94.55.1
5.86.47.07.68.1
0.81.01.21.62.0
2.42.72.93.1
9.09.6
10.011.011.0
12.012.013.014.015.0
16.017.018.019.021.0
22.023.024.026.027.0
29.032.0
THREADCoefficient “K”used to calculatethe tighteningtorque
N.B.: The “K” values in the table are for fine pitch threads only.
• Use a torque spanner initially to tightenthe ring nut to a level that is approximatelythree times greater than C (this operationis important).
Tightening procedure
Closure usingring nuts
Closure usingend caps andbolts
Spacers
• Loosen off the ring nut.• Retighten the ring nut to a torque of C.• Close the anti-locking device according to
the manufacturer’s instructions.
It is important to remember that the spigot depth obtained using the above technique is validfor that set of bearings only. It is important to always repeat the spigot depth measurementprocedure when assembling new/replacement bearings.
The spacer configuration that is given in the above figure is recommended in any case where thespindles operate out of the horizontal when it is important to guarantee that the grease remainsclose to the bearings.
A residual gap must remain between the endcap and the housing face (figure 7) once theforce Pa has been applied and the tighteningprocedure is complete.• Use a torque spanner to tighten the bolts
to a torque that is 2- 3 timesgreater than the recommendedvalue of C. The operationshould be carried out graduallymoving across the diameter forthe next bolt.• Loosen off the bolts.
• Re-tighten the bolts to the specified torqueC (in the same manner as before)
• Measure the residual gap “L” between theend cap and the front face of the housing(see figure 7).
• Reduce the spigot depth by an amountthat is equal to the residual gap “L” orcompensate for the gap using spacers.
• Tighten the screws gradually to theMAXIMUM torque as recommended bymanufacturers of the components.
Fig. 7
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BEARING DAMAGEANALYSIS
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28
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Excessive loading of the bearings demonstrates itself inmany ways. The first is for a wide contact trackingband that may be discoloured by the heat generated,the second is fatigue spalling developing aroundmicroscopic pits and scratches in the raceway and thethird is spalling starting from inclusions within thematerial body. The first will generate in to the second and theresultant spalling will develop as shown on the ring.Spalling originating from inclusions will also developaround the ring and they could also appear as shown.Whatever the beginning, the life of the bearing will beshort.The problem can be resolved by reducing the externalloads or by using bearings with a higher load capacity.
OVERHEATING
Overheated rings and balls display colouring thatvaries from golden yellow through to blue.Overheating occurs because there is an applicationproblem, because the bearing is overloaded, becausethe lubrication is not good enough or because there isno way the heat developed within the bearing canescape.If the bearing runs at temperatures in excess of thetempering temperature for any period of time not onlywill they begin to discolour they will begin to softenand eventually become misshapen. Bearing fatigue lifewill be reduced.The most common cause for this problem is related tolubrication. As shown, the ball tracks are discolouredbrown indicating that the track surface has been inexcess of 200°C. At this temperature the lubricationwill be poor if not destroyed. This leads to more heatgeneration and eventual premature failure.To control this problem, confirm that the lubrication isadequate for the operating conditions (loads,rotational speeds and temperature) and try to ensure agood heat path away from the bearing.
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BRINNELLING
When a bearing is subjected to very high loads (it is notimportant if they are applied gradually or are impact loads) andthe contact stresses are in excess of the elastic limit,indentations are formed. This is Brinnelling.Brinnelling can appear as discrete indents if the bearing has notrotated or as high wear if the bearing has been running duringthe time of the high loading.Brinnelling of a bearing is often first noted by high noise levels.
The most common causes of Brinnelling are:• Assembly and / or disassembly using inappropriate tools
(e.g. hammer).• Accidentally dropping previously assembled components.• Incorrect assembly and / or disassembly procedures.
Never assemble bearings onto the shaft by applying pressure tothe external ring, but rather ensure that pressure is applieddirectly to the internal ring. This prevents the balls and the ringsfrom being subjected to excessive static loads.
FALSE BRINNELLING
False Brinnelling resembles brinnelling but it is generateddifferently. When the static bearing is vibrated the ball/trackcontacts begin to suffer fretting corrosion. The products of thismechanism are abrasive so they tend to accelerate the process.As the bearing is static any lubricant present is ineffective.To stop this happening there is a need to locktogether the shaft and housing to prevent relativemovement or fully isolate the part from thevibration source.
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Fatigue-related failure takes the form of spalling of theraceway surface. It generates either from the surfacewhere there are high contact stresses aroundmicroscopic pits or scratches or from below the surfacewhere stress concentrations occur around inclusions,leading to crack propagation. Fatigue spalling usuallypropagates gradually during operation and is evident onboth the inner and outer rings as well as the balls. Theproblem is usually detected through increased vibrationand noise levels.
REVERSE LOADING
Angular ball bearings are designed to support axialloads that act in one direction only. If a reverse load isapplied the contact area between the ball and the outerring moves towards the non-thrust side which has alower shoulder height. The result is that theball/raceway contact ellipse becomes truncatedresulting in high contact stresses and rapid failure.Not all reverse load situations result in the bearingactually attempting to take thrust in the wrongdirection. Most often the reverse load is sufficient toovercome the preload. This is termed off-loading. Whenthis occurs the balls are allowed to spin and take upanother preferred axis of rotation and hence developanother tracking pattern.Where complete reverse loading occurs the signs ofdamage will be excessive bearing noise and poorspindle operation. This may be confused with otherfailure causes, however, on disassembly and inspectionof the balls, a deep line will be witnessed in the trackingband (caused by running over the shoulder) and thetrack/smaller shoulder corner radius of the bearing ringwill be damaged.
Contact stress
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CONTAMINATION
Contamination can be one of the main reasons for bearing rejection. Thepresence of particles in the bearing leads to indentations in the raceway asthe balls roll over them. These indentations then increase the general noiselevel of the bearing. The indentations also act as stress raisers from whichfatigue spalls can generate. Wear rates, and all that that brings with it, areenhanced.Contaminants may include:• Dust that is blown in by the air supply,• Machining debris left behind after spindle or housing manufacture,• Abrasive particles from grinding wheels etc normally found in a workshop.Typically, bearings may be contaminated if the person handling them has dirtyhands or uses dirty tools, or if they are located in dirty surroundings, or indeedif contaminated lubricants and washing liquids are used.It is good practice to provide assembly areas away from any machines andpreferably in an area that is enclosed with a controlled atmosphere. Bearingsshould be stored in their original packaging until they are needed. Shouldbearings need to be washed prior to fitting or greasing, then wellfiltered liquids must be used.Seals play a significant role in preventing bearing contamination, andshould always be damage free and hence effective.
LUBRICATION
Tracking bands on rings and balls that are discoloured blue or brown area good sign of lubrication problems. This happens because the lubricantfilm has been unable to maintain sufficient thickness to prevent surface tosurface contact. Lubrication failure could mean that it is wrong for theapplication or that the supply is marginal and hence a full film can notdevelop. It is necessary to always ensure that the specified lubricant,delivery system and quantity is correct for the application.A matt tracking band indicates that wear is taking place but there is nosignificant heating. This will progress very slowly to rejection. If thetracking bands are discoloured then the heat build up is more significantand the rejection will happen earlier. Diagnosis may be difficult as only asmall part of the machine’s duty cycle may cause the problem. It istherefore necessary to look at the worst case and decide if it is significant.Bearing failure caused by lubrication problems can be dramatic. The cagecan burn or melt and the track becomes red hot and material deformedand pushed out of the way by the passing balls. When rotation stops, theballs which are likely to be completely misshapen, become welded to theraceway.Lubrication issues can be resolved by selecting the optimum lubricant thatis suited to the specific application and also by eliminating any causes thatcould lead to an abnormal increase in the operating temperature.
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Corrosion displays itself in the form of red-brownmarks on the ball and the rings. This happens whenthe bearing is exposed to environmental or chemicalcorrosive agents The result is a significant increase inwear and vibration levels which together act to reducethe pre-load. In some cases, corrosion can actually giverise to fatigue-related failure. Keeping the bearing dryand avoiding contact with corrosive agents is the bestprevention.
MISALIGNMENT
A tracking band that does not run parallel to thestationary ring shoulder is the result of misalignment.The tracking band on the rotating ring will be widerthan normal.Misalignment is a problem associated with poormanufacturing or assembly. Abutment shoulders mustalways be square to the bearing seat and seats inhousings or on shafts must always be concentric. Ifburrs or machining debris are not removed from theassembly they can become trapped between the partsand also lead to misalignment.
The maximum acceptable misalignment dependsgreatly on the bearing, the type of application and willcertainly need to minimised as speeds increase.
As is shown here with the tracking band being wideron one part of the ring than on another, misalignmentcan develop over time as parts move or duringoperation as parts deflect under load.
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EXCESSIVE RADIALCLEARANCE
Incorrect selection of the fit between the bearing outer ringand the housing or the inner ring and shaft can result inrelative vibratory movement between the surfaces leading tofretting corrosion. Fretting corrosion generates small metallicoxide particles that are brown in colour. These particles are abrasive and wear the surfaces.This increases the play even further and an everrapidly increasing problem occurs.
Wear of the bearing side faces and wear of theraceway by intruding debris causes a loss ofpreload. Couple this with a loss of bearing fit andsubsequent ring rotation and the result is poorspindle performance and spindle rejection.
EXCESSIVE RING FIT
When fits on bearing rings are excessive, the radial play of thebearing may be reduced to the point where there has been asignificant change in contact angle. Reducing the contactangle in a predominantly axially loaded bearing means thatthe contact load is increased and that, in turn, means a wideand often discoloured tracking band.
High interference also means high hoop stresses that,when added to the contact stress, effectively reducesbearing fatigue life.
Always ensure that the fits are adequate at operatingconditions and take account of any thermal gradientsas well as any speed effects.
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ELECTRIC ARC DAMAGE
When an electrical current passes through abearing, it tends to arc between non-contactingballs and raceways leaving visual patterns thatrange from random pitting to fluted patterns.Bearings that have suffered this sort of damageproduce vibrations and noise and may have a shortfatigue life.
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Faults on rolling elements generate vibration peaks at the spin rotationfrequency and subsequent harmonics (BSF, 2xBSF, 3xBSF, etc). In addition, thecage rotation frequency (FTF) often modulates the frequencies in question,creating smaller peaks corresponding to BSF±FTF, 2xBSF±FTF etc.
The frequency of vibration varies continuously. In addition, cage guidedrolling elements generate vibrations that deviate from BSF.
When a defect is present on the rolling track, the balls generate a vibrationthat corresponds to their pass frequency, BPFI and BPFO respectively, if thedamage is to the inner or outer ring. In general, the phenomenon developswith time and also damages the rolling element that in turn begins togenerate signals at BSF and its harmonics (see above)
In any situation in which insufficient lubrication is provided, peaks can becreated in the field of a few kHz of frequency. This is due to contact betweenthe micro-unevenness of the surfaces.
The most common cause of rotor vibration is the presence of an unbalancedrotating mass. This is when the rotor axis of rotation does not coincide withthe geometric axis, thereby creating major vibrations at the rotationfrequency.
Another common cause of vibration is the imperfect axial alignment of therotor supports. Rotor supports that are not perfectly coaxial in naturegenerate vibrations that increase in magnitude according to the degree towhich the supports are misaligned and as the speed increases. The generatedvibrations reflect the rotor rotation frequency and its subsequent harmonics.
When there is excessive movement between two components (e.g. a bearingand its journal) major vibrations will be generated at the shaft rotationfrequency and sub-harmonics (0.5 x n/60).
Demaged rollingelement
Damage cage
Damaged rings
Lubrication
Unbalanced rotor
Misalignment
Excessive play
BSF
BSF - FTF
BPFO -BPFI
Variable
n/60
n/60
0.5 x n/60
Radial
Radial - Axial
Radial
Radial - Axial
Radial
Radial - Axial
Radial - Axial
Fault Dominantfrequency
Vibration measurementdirection
Comments
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The vibrations produced by a bearingare significant indications of its’condition and, more generally of thecondition of the machine in which it isfitted.Indeed, damaged bearings or failingmachinery often first present asincreased vibration levels.Using vibration analysis equipment andcomparing the spectrum theequipment produces with the bearingnatural frequencies, it is possible towork out if the vibrations are the resultof damage to the bearing or to othermachine components.
The bearing natural frequencies arefunctions of their geometry and aredetermined using the followingformulae:
Outer ring ball pass frequency:
Inner ring ball pass frequency:
Ball Spin Frequency:
Cage Rotation Frequency (Fundamental Train Frequency):
n: Internal ring rotation velocity [revs / minute]
α: Contact angle [degrees] - Z: Number of balls - Φ: Ball diameter [mm]
NaturalFrequencies n Z Φ
BPFO = — • — (1 - — • cos α) [Hz]60 2 dm
n ΦFTF = 0.5 • — (1 - — • cos α) [Hz]
60 dm
n Z ΦBPFI = — • — (1 + — • cos α) [Hz]
60 2 dm
n dm ΦBSF = 0.5 • — (— - — • cos2 α) [Hz]
60 Φ dm
The table below provides an initial analysis of the reasons that give rise to anomalous vibrations:
SNFA - MANUALE DI MONTAGGIO E DANNEGGIAMENTI - 1° edizione - 10/2003 - italiano/2m