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A PRESENTATION BY
WG CDR ARVIND KUMAR
BEARINGS, FAILURES
CAUSES & REMEDIES
CONCEPT
CONSTRUCTION
BEARING MATERIALS
TYPES OF LOADS & BEARINGS
BEARING CLEARANCES
WHY BEARING FAILS!
FAILURES, CAUSES & REMEDIES :FEW EXAMPLES
PATH PATTERN INTERPRETATION
OTHER IMPORTANT POINTS
TV-2 STATISTICS
PRACTICAL DEMONSTRATION
CONCEPT OF ROLLING BEARING
• Things roll better than they slide. Hence the invention of WHEEL.
• Rolling friction is far lesser (>100 times less) than sliding friction.
• Microscopic contacts get peeled off and not sheared off in rolling.
Cage/Retainer
Inner ring / race
Rolling element
Outer ring / race
Side faces
Outer ring raceway
Shoulder
Inner ring raceway
Shoulder
7
ROLLING BEARING CONSTRUCTION
7
BEARING DIA
Outside Diameter
Bore
BEARING CONSTRUCTION-2
Seal Rolling Elements Inner RingOuter Ring Cage Seal
BEARING CONSTRUCTION-3Outer Ring
Cage
Inner Ring
Side faces W33 Lubrication Groove and
Hole
Rolling Element
Guide Ring
Inner Ring Raceway
VARIOUS ROLLING ELEMENTS
Spherical roller(asymmetrical)
Taper roller
Spherical roller(symmetrical)
Needle roller
Cylindrical roller
Ball
POINT / LINE CONTACT
PROPERTIES : BEARING MATERIALS
• High wear resistance• High rolling fatigue strength
• Non-metallic inclusions like O, S etc. increase fatigue cracking
• High dimensional stability• Heat treatable to high hardness in depth• High corrosion resistance• High wettability with oil• Low coefficient of semi-dry friction• Good heat conductivity• Good antiseize properties
MATERIALS-SKF BEARINGS
BEARING RINGS AND ROLLING ELEMENTS
THROUGH-HARDENING STEELS
CARBON CHROMIUM STEEL CONTAINING APPROXIMATELY1 % CARBON AND 1,5 % CHROMIUM
CASE-HARDENING STEELS
CHROMIUM-NICKEL ALLOYED STEEL AND MANGANESE-CHROMIUM ALLOYED STEEL CONTAINING APPROXIMATELY 0,15 % CARBON
SKF BEARING MATERIALS -2
• ~SAE 52100
• C-Cr Bearing steel ( ISO 683 -17:1999)
• Ceramics like Si3N4 for ultra high speed applications
• Stainless steels like X65Cr14 (ISO 683-17:1999)
BEARING MATERIALS -3
• Highly alloyed steels like 80MoCrV42- 16 for temperature > 250°C (SKF)
• Case Hardening Steel for shock loads
• DMRL analysis for Russian bearing (6-7000108B, Central Drive B.B on Ist support assy of TV-2 engine of Mi-8 heptr)
C~1.0% Cr~1.8%Si~0.23%Mn~0.33%steel nearly equivalent to AISI-52100
• Original Russian material is Sh Kh 15
SPECIAL FEATURES
• Hard Surfaces HRC 58-65 for C-Cr bearing steel (SKF) DMRL analysis of Russian Bearing
- HV/5Kg : 930 (HRC : 68)
• Perfectly round and incredibly smooth • Very high surface finish : CLA ~ 0.5 microns• Addition of Si to improve heat resistance
dimensional stability) at 150-200°C
Radial loads
Axial Loads
Combined loads
AXIAL / THRUST LOADS
BAR STOOL BEARINGS(Man’s wt. causes thrust load)
Thrust load
Bearings
RADIAL LOADS
Motors & Pulley Shaft Support Bearings
Radial loadMotor Tension
COMBINED LOADS
Car Wheel Bearing
CYLINDRICAL ROLLER BRG : Radial load only
(Heavy) BALL BRG : Both axial & radial
TAPERED ROLLER BRG : Radial load & Axial load
in one direction also
Axial load is also called as Thrust load
RADIAL LOAD
Cylindrical Roller Bearing
Deep grooveB.B
Angular contactB.B
Self-aligning B.B
Ball Bearings
Radial load Axial load Speed Accommodates
carrying capacity carrying capacity capability misalignment
Radial Load
Axial Load(one direction)
Tapered Roller Bearing
Radial Load
Thrust Thrust
Double Tapered Roller Bearing
Radial Load
Thrust Thrust
Double Tapered Roller Bearing
360°
~150°
LOAD ZONE
Radialclearance / play
Axialclearance / play
BEARING CLEARANCES
•C1 - less than C2 clearance
•C2 - less than normal clearance
•CN - normal clearance
•C3 - greater than normal clearance
•C4 - greater than C3 clearance
•C5 - greater than C4 clearance
•Russian System ?
Example:
A 6210 /C3 ball bearing has 18 - 36 µm (microns) or 0.0007” - 0.0014” radial internal clearance
CLEARANCE DESIGNATIONS
AXIAL INTERNAL CLEARANCE
Example:
A 5210 / C3 ball bearing has 33 - 54 µm or
0.0013” – 0.0021” axial internal clearance
25-126114P ball bearing used in TV-3 aeroengine 2nd Support should have 140-220 µm axial internal clearance
EFFECT OF TEMPERATURE ON CLEARANCE
COLD (by 5-10°C)
WARM
Reducedradial
clearance
Expansion
Compression
EFFECT OF FIT ON CLEARANCE
• INTERFERENCE FIT between shaft and bearing (inner race) reduces radial clearance by approx 80% of the fit.
• Because the inner ring expands and the outer ring contracts.
EFFECT OF LOOSE FIT
N = 3 000 10 hours/day 30 days = 18 000 min
Sliding motion
=0,013 x π x 3 000 x 18 000 = 2,2 x
106 mm = 2,2 km (1.4 miles) creep
LOAD DISTRIBUTION & CLEARANCE
Less Clearance More Clearance
Pressure between rolling element and races can reach 4 lakhs PSI
EXCESSIVE CLEARANCE
• Loading area reduces & stress increases
• Bearing rigidity reduces
• Alignment of rolling elements decreases
• Vibration increases at high speeds
• Noise increases
• Running and locating accuracy reduces
PRELOAD
• Small amount of loading before running of brg Causes negative clearance Elastic deformation & compressive stress at
contact area Bearing rigidity improves Vibration at high speeds reduces Noise reduces Alignment of rolling elements improves
(Thrust brgs) Running and locating accuracy improves
EFFECT OF CLEARANCE ON BEARING LIFE
Excessive preload causes high stress and heat generation
Life
Preload Clearance
BEARING LIFE
• Repeated compressive stresses (rolling fatigue) cause flaking of material of raceways and rolling elements and hence failure.
• Basic Rating Life L10 (90% reliability) of the bearing is the total number of revolutions in millions which 90% of the bearings out of the lot reach before or at which the first signs of flaking occur under identical operating conditions.
BEARING LIFE-2
• L10 = (C / P)n
C = Design Load {Basic dynamic rating load (constant radial or axial load when brg is rotating) in N that will give bearing life of 106 revs in accordance with ISO 281:1990}
P = Actual equivalent dynamic load in N
n = 3 for ball bearings
3.33 for roller bearings
BEARING LIFE-3
• If actual load is half of the rated load, life of a ball bearing will increase by 8 times. (Min. Load*)
• If actual load increases by 25% above the rated load, the life of a ball bearing gets nearly halved.
• Actual service life may be reduced even further below the calculated value because of factors like contamination, misalignment, improper installation or lubrication etc.
• Effect of overload is more severe on roller brg.
LIFE ADJUSTMENT FACTOR
Higher the expected reliability, the lesser is the bearing life.
L5 (95% reliability) = 0.62 L10
L3 (97% reliability) = 0.44 L10
L1 (99% reliability) = 0.21 L10
WHY BEARINGS FAIL!
•Study in Scandinavian countries indicates :
- ~1/3 brgs fail because of Poor Lubrication
- ~1/6 brgs fail because of Contamination
- ~1/3 brgs fail because of Fatigue - ~1/6 brgs fail because of Bad
Installation
WHY BEARINGS FAIL!-2
In India, there is likely to be greater percentage of failures because of :-
Contamination Improper installation and lubrication
Lesser percentage of failures because of :-
Fatigue
FAILURE MODE CLASSIFICATION-1
• As per ISO 15243:2004, there are 15 main failure modes classified as: Fatigue (Repeated compressive
stresses) -Subsurface
-Surface initiated (metal to metal contact) Wear
-Abrasive -Adhesive
FAILURE MODE CLASSIFICATION-2
Corrosion
- Moisture,
- Fretting
- False Brinelling
Electrical Erosion
-Excess voltage
- Current leakage
FAILURE MODE CLASSIFICATION-3
• Plastic Deformation
- Overload
- Indentation from debris
- Indentation by handling• Fracture
- Forced
- Fatigue
- Thermal Cracking
SYMPTOMS OF BEARING FAILURE
- Overheating- Noise- Vibration- Obstruction of movement (RDT reduces)- Shaft jammed- Others ?
FAILURES,
CAUSES & REMEDIES
– FEW EXAMPLES
CORROSION
CORROSION
Appearance Cause Action
Grey black streaks across the raceways, mostly coinciding with the rolling element spacing. At a later stage, pitting of raceways and other surfaces of the bearing.
Presence of water, corrosive substances in the bearing over a long period of time.
Improve sealing. Use lubricant with better rust-inhibiting properties.
WEAR-ABRASIVE PARTICLES
Fig 19 Outer ring of a spherical roller bearing with raceways that have been worn by abrasive particles. It is easy to feel where the dividing lines goes between worn and unworn sections.
WEAR-ABRASIVE PARTICLES
Appearance Cause ActionSmall indentations around the raceways and rolling elements. Dull, worn surfaces.
Lack of cleaning before and during mounting operation.
Ineffective seals.
Lubricant contaminated by worn particles from brass cage.
Do not unpack bearing until just before it is to be mounted. Keep workshop clean and use clean tools.
Check and possibly improve sealing.
Always use fresh clean lubricant. Wipe the grease nipples. Filter the oil.
WEAR-INADEQUATE LUBRICATION
Fig 20 Cylindrical roller with mirror-like surface on account of lubrication starvation
Fig 21 Outer ring of a spherical roller bearing that has not been adequately lubricated. The raceways have a mirror finish
WEAR-INADEQUATELUBRICATION
Appearance Cause Action
Worn, frequently mirror-like surfaces. At a later stage blue to brown discoloration
Lubricant has gradually been used up or has lost its lubricating properties.
Check that the lubricant reaches the bearing. More frequent lubrication.
WEAR DUE TO VIBRATION
WEAR DUE TO VIBRATION
Appearance Cause Action
Depressions in the raceways. These depressions are rectangular in roller bearing and circular in ball bearing. The bottom of these depressions may be bright or dull and oxidized.
The bearing has been exposed to vibration while it was stationary.
Secure the bearing during transport. Provide a vibration damping base. Where possible use ball bearing instead of roller bearing. Employ oil bath lubrication, where possible.
INDENTATIONS -FOREIGN PARTICLES
Fig 31 Indentations caused by dust in one of the raceways of a roller bearing-50 x Magnification
INDENTATIONS -FOREIGN PARTICLES
Appearance Cause Action
Small indentations distributed around the raceways of both rings and the rolling elements.
Ingress of foreign particles into the bearing.
Cleanliness to be observed during the mounting operation.
Uncontaminated lubricant.
Improved seals.
FRETTING CORROSION
FRETTING CORROSION
Appearance Cause Action
Areas of rust on the outside surface of the outer ring or in the bore of the inner ring. Raceway path pattern heavily marked at corresponding positions.
Fit too loose.
Shaft or housing seating with error of form.
Proper fit and housing.
SMEARING
SMEARING
Appearance Cause Action
Scored and discolored roller ends and flange faces
Sliding under heavy axial loading and with inadequate lubrication.
More suitable lubricants.
ELECTRIC CURRENT EROSION
ELECTRIC CURRENT EROSION
Appearance Cause Action
Dark brown or greyish black fluting (corrugation) or crater in raceways and rollers. Balls have dark discoloration only. Sometime zigzag burns in ball bearing raceways.
Localised burns in raceways and on rolling elements.
Passage of electric current.
Re-route the current to bypass the bearing.
Use insulated bearings.
When welding, arrange earthing to prevent current passing through the bearing.
• Working surfaces become dull after operation.
• ‘Path Pattern’ (dull surface) varies in appearance according to the rotational and loading conditions.
• By experience, we can learn to distinguish abnormal paths from normal paths and hence get clues to brg failure reasons.
PATH PATTERN INTPT-2
Uni-directional Radial Load
PATH PATTERN INTPT-3
Uni-directional Radial Load
PATH PATTERN INTPT-4
Uni-directional Axial Load
PATH PATTERN INTPT-5
Uni-directional Radial Load + Creeping* Outer Ring
PATH PATTERN INTPT-6
Uni-directional Radial Load + Tight Fit / Preloading
PATH PATTERN INTPT-7
Oval Compression of Outer Ring
PATH PATTERN INTPT-8
Outer Ring Misaligned
PATH PATTERN INTPT-9
Inner Ring Misaligned
PATH PATTERN INTPT-10
Combined Uni-directional Radial & Axial Loads
23-04-10 ©SKF Slide 80 [Code]
LUBRICATION MODES
Greased for lifesealed bearingsinitial grease selection important
Occasional relubrication / replenishingopen bearingsconsult SKF / General Catalogue
Continuous relubrication for highly demanding applications,
or for sealing purposesconsult SKF / General Catalogue
Lubrication modes:
SKF SKF
GREASE RUN-IN
AXIAL LOCATION
"Non-located" bearings that can move axially
CONDITIONS :
BEARINGS SHOULD BE STOCKED IN A SEPARATESTOCKING PLACE
* CLEAN (no dust)
* DRY (Max 60 % humidity)
* DRAUGHTS TO BE AVOIDED (no windows opened)
• AWAY FROM VIBRATION
• TEMPERATURE AROUND 20 DEGREES C.
* NO TEMPERATURE FLUCTUATIONS (avoid condensation)
“AIR CONDITIONED” IN HOT & HUMID CLIMATES “Never touch aviation bearing with hand.”
BEARING STORAGE ROOM
STORAGE AND REPACKING
1. FIRST IN, FIRST OUT
2. DO NOT PILE UP TOO HIGH
3A. STOCK LARGE BEARINGS FLAT (O.D. > 420 mm)
3B. MEDIUM SIZE BEARINGS
3C. SMALL BEARINGS
* OLDEST BEARINGS ALWAYS IN FRONT AND ON THE TOP OF THE PILE
* IT DOES DAMAGE THE PACKING
* AT THE BOTTOM OF THE RACKS* STOCK FLAT (not vertical)
* STOCK UNOPENED (individually packed in box)* NUMBER READABLE (up front)
* NUMBER (designation) UP FRONT)
* IN DRAWERS OR BOXES (keep away from dust)* DESIGNATION ON DRAWER (box)
STORAGE IN RACKS
STORAGE AND REPACKING
REMARK :
* BEARINGS ARE CLEAN AND ARE PROTECTED WITH A RUST INHIBITIVE OIL.
* BEARINGS ARE WRAPPED IN A SPECIAL PAPER OR PLASTIC
SHEETING FOR PROTECTION.
* BEARINGS ARE PACKED IN CARDBOARD BOXES FOR PROTECTION
* LARGE SIZE BEARINGS ARE INDIVIDUALLY PACKED IN WOODEN
BOXES FOR PROTECTION.
3 GOOD REASONS TO KEEP BEARINGS IN THE ORIGINAL PACKING :
STOCKING RECOMMENDATIONS
STORAGE AND REPACKING
* WHITE SPIRIT
* CLEAN AREA
* LET DRY
1. WASH
2. INHIBITING OIL PROTECTION
* 40 % QUAKER 5815 BASE AND 60 % WHITE SPIRIT
* DIP AND ROTATE
* DRY 7 HOURS
* CLEAN AREA
3. PACK
* WRAP IN POLYETHYLENE COATED "VPI" PAPER
* PACK IN (CARTON) BOXES
4. IDENTITY
* INDICATE FULL BEARING NUMBER
* INDICATE BRAND !!
RENEWING ANTI-RUST (AND PACKING)
STORAGE AND REPACKING