A PRESENTATION BY WG CDR ARVIND KUMAR.

A PRESENTATION BY WG CDR ARVIND KUMAR

BEARINGS, FAILURES CAUSES & REMEDIES
29

TYPES OF LOADS & BEARINGS BEARING CLEARANCES WHY BEARING FAILS!
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 29

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.

CONSTRUCTION

ROLLING BEARING CONSTRUCTION
Outer ring raceway Cage/ Retainer Shoulder Inner ring / race Inner ring raceway Rolling element Shoulder Bearing components SKF rolling bearings normally consist of five components: 1. Outer ring – which fits in the housing 2. Inner ring – which fits onto the shaft 3. Rolling elements – balls or rollers, which permit the rotation and transmit the loads from one ring to the other 4. Cage – which keeps the rolling elements separated from each other 5. Shields or seals (available e.g. on deep groove ball bearings) Rings The material of the outer and inner rings is the SKF specially developed clean martensitic hardened bearing steel. High surface finish combined with the hardness of 58–62 HRC reduces wear and gives low risk for fatigue, which means long service life. Rolling elements The same high quality steel as for the rings, is used for the rolling elements. They have the same high surface finish but have a hardness of 60–64 HRC. Outer ring / race Side faces 7 To think about Do not distribute any bearings or other pieces during your lesson. It will distrurb the concentration among the listeners. Instead place the exhibits on a table and when you have finished your presentation invite the participants to go to the table for a hands-on session of bearings, cages etc. 10

BEARING DIA Outside Diameter Bore Bearing components 7 To think about
SKF rolling bearings normally consist of five components: 1. Outer ring – which fits in the housing 2. Inner ring – which fits onto the shaft 3. Rolling elements – balls or rollers, which permit the rotation and transmit the loads from one ring to the other 4. Cage – which keeps the rolling elements separated from each other 5. Shields or seals (available e.g. on deep groove ball bearings) Rings The material of the outer and inner rings is the SKF specially developed clean martensitic hardened bearing steel. High surface finish combined with the hardness of 58–62 HRC reduces wear and gives low risk for fatigue, which means long service life. Rolling elements The same high quality steel as for the rings, is used for the rolling elements. They have the same high surface finish but have a hardness of 60–64 HRC. 7 To think about Do not distribute any bearings or other pieces during your lesson. It will distrurb the concentration among the listeners. Instead place the exhibits on a table and when you have finished your presentation invite the participants to go to the table for a hands-on session of bearings, cages etc. 10

BEARING CONSTRUCTION-2
Bearing Components Rings, rollers, cages and closures Purpose of rings and rollers: Carry the loads and transmit power Different shapes to accommodate various design requirements. Basic difference between rolling elements: balls vs. Rollers: balls have point contact, rollers have line contact Purpose of cages: Provide rolling element guidance Rolling elements generally slightly harder than rings Rc 60-66 Material generally steel 100 means 1% carbon, considered high carbon steel 52 is % of chromium Hardened to Rc 58-62 Stainless is available in some types and sizes Ceramic (Silicon Nitride) balls are an option Seal Rolling Elements Inner Ring Outer Ring Cage Seal

BEARING CONSTRUCTION-3
Outer Ring Cage Inner Ring Inner Ring Raceway Guide Ring Rolling Element Side faces W33 Lubrication Groove and Hole

VARIOUS ROLLING ELEMENTS
Spherical roller (symmetrical) Spherical roller (asymmetrical) Ball Cylindrical roller Taper roller Needle roller

POINT / LINE CONTACT

BEARING MATERIALS

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 APPROXIMATELY 1 % 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 :1999) Ceramics like Si3N4 for ultra high speed applications Stainless steels like X65Cr14 (ISO :1999)

BEARING MATERIALS -3 Highly alloyed steels like 80MoCrV for temperature > 250°C (SKF) Case Hardening Steel for shock loads DMRL analysis for Russian bearing ( B, 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 Perfectly round and incredibly smooth
HRC 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 °C

TYPES OF LOADS Radial loads Axial Loads Combined loads

(Man’s wt. causes thrust load)
AXIAL / THRUST LOADS Thrust load Bearings BAR STOOL BEARINGS (Man’s wt. causes thrust load)

Motors & Pulley Shaft Support Bearings
RADIAL LOADS Motor Tension Radial load Motors & Pulley Shaft Support Bearings

COMBINED LOADS Car Wheel Bearing

BEARINGS & TYPES OF LOADS
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

BEARINGS &TYPES OF LOADS-2
RADIAL LOAD Cylindrical Roller Bearing

Deep groove B.B Angular contact B.B Self-aligning B.B Radial load Axial load Speed Accommodates carrying capacity carrying capacity capability misalignment Ball Bearings

Radial Load Axial Load (one direction) Tapered Roller Bearing

BEARINGS & TYPES OF LOADS-5
Radial Load Thrust Thrust Double Tapered Roller Bearing Radial Load Thrust Thrust Double Tapered Roller Bearing

BEARING CLEARANCES

LOAD ZONE 360° ~150°

Radial clearance / play
BEARING CLEARANCES Radial clearance / play Axial clearance / play

CLEARANCE DESIGNATIONS
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 µm (microns) or ” ” radial internal clearance Radial or axial values are listed in tables **Reminder: Clearances are not the same as precision classes**

AXIAL INTERNAL CLEARANCE
Example: A 5210 / C3 ball bearing has µm or ” – ” axial internal clearance P ball bearing used in TV-3 aeroengine 2nd Support should have µm axial internal clearance Bearing Axial Internal Clearance For certain bearing types, radial clearance is sometimes presented in catalogs as Axial Clearance, also called End Play. Special Clearances are sometimes available (e.g., C2H) Sometimes Nomenclature is combined: C783 = C78 (ABEC 5) + C3 P62 = ISO P6 + C3

EFFECT OF TEMPERATURE ON CLEARANCE
COLD (by 5-10°C) Reduced radial clearance Compression Expansion WARM

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 x = 2,2 x 106 mm = 2,2 km (1.4 miles) creep Effects of Clearance: Rotating Shaft and Inner Ring The effect of a loose fit between the rotating inner ring bore and the shaft seating is shown in this example. The smaller circumference of the shaft relative to the bearing bore results in different rotational speeds. The bearing inner ring will lag behind the shaft by pi.d each revolution (d = difference in diameters). With a clearance of only 0,013 mm (0.0005") diameter between the bore of the ring and shaft seating, an application running at r/min for 10 hours each day over 30 days, the relative distance is: Relative distance = 0,0133,1423 000602430 = 2,2 kilometers (1.4 miles) 1 000  1 000 This relative difference between un-lubricated surfaces can give rise to wear and damage to both shaft and bearing bore. As the clearance increases due to wear, the problem will accelerate. To correct the problem, the bearing seating on the shaft should be machined to the relevant SKF tolerance. Other attempts to fix this problem can cause additional problems: If commercial fluids, which set hard, are used to fill the gap: This can result in an eccentrically running bearing, causing vibration damage to the bearing and other machine components. Center punching or knurling to raise the surface of the journal: The resultant contact areas supporting the bearing are too small, and even light loads will rapidly flatten the raised areas. Trying to clamp the ring to the shaft shoulder using the retaining nut will not prevent movement of the ring to the shaft, and can give rise to smearing damage on the inner ring faces.

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
Preload vs. Clearance Preload is sometimes desirable, as in heavily loaded pumps. The designer will generally err on the side of safety, choosing an initial bearing clearance that leaves a slight operational clearance. Relative bearing life decreases very rapidly if the bearing is preloaded. Excess clearance reduces overall bearing life at a much lower rate. Preload Clearance Excessive preload causes high stress and heat generation

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!

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

Corrosion - Moisture, - Fretting - False Brinelling Electrical Erosion -Excess voltage - Current leakage

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 Action Small 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-INADEQUATE LUBRICATION
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.

PATH PATTERN INTERPRETATION

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

OTHER IMPORTANT POINTS

LUBRICATION MODES Text slide with title, image and text [object & text] The image is cropped top and bottom using the SKF Supergraphic, aligns left with the typographic grid (Ctrl G) and bleeds off the edge of the slide area. The image will be ‘masked’ by the SKF Supergraphic and should be of sufficient size to fit within this space at 96ppi resolution. To ensure that your image is masked by the SKF Supergraphic, select the image and set as ‘send to back’, accessed via the ‘Draw>order’ menu. Colour images should be saved as RGB (Red Green Blue) .JPG (JPEG) format from your image editing application. Black and white images should be saved as grayscale .JPG files. The page folio, SKF Supergraphic and SKF corporate brand mark are not part of the ‘slide master’ and are separate elements, otherwise the image would hide these ‘slide master’ elements. We therefore recommend that you cut and paste this slide for use in your own presentation, then delete the current image, insert your own and set as ‘send to back’, accessed via the ‘Draw>order’ menu. ©SKF Slide 80 [Code]

GREASE RUN-IN Text slide with title, image and text [object & text]
The image is cropped top and bottom using the SKF Supergraphic, aligns left with the typographic grid (Ctrl G) and bleeds off the edge of the slide area. The image will be ‘masked’ by the SKF Supergraphic and should be of sufficient size to fit within this space at 96ppi resolution. To ensure that your image is masked by the SKF Supergraphic, select the image and set as ‘send to back’, accessed via the ‘Draw>order’ menu. Colour images should be saved as RGB (Red Green Blue) .JPG (JPEG) format from your image editing application. Black and white images should be saved as grayscale .JPG files. The page folio, SKF Supergraphic and SKF corporate brand mark are not part of the ‘slide master’ and are separate elements, otherwise the image would hide these ‘slide master’ elements. We therefore recommend that you cut and paste this slide for use in your own presentation, then delete the current image, insert your own and set as ‘send to back’, accessed via the ‘Draw>order’ menu.

AXIAL LOCATION "Non-located" bearings that can move axially
A major consideration for any bearing application is to provide the means to compensate for any linear expansion of the shaft due to temperature rise. This is required to prevent additional stresses being imposed on the bearings. This slide illustrates a number of methods used: The taper roller bearing arrangement must be adjusted for axial clearance or preload on assembly. Information of these values must be obtained from the machine manual or SKF. The cylindrical roller bearings in this arrangement have complete axial freedom. The axial location of the shaft is provided by a QJ bearing. This bearing must have radial clearance in the housing. The spherical roller bearing arrangement shows one bearing axially located (fixed) and one non-located (axially free) bearing. Sufficient radial clearance must be provided in the non-locating (free) bearing housing for the outer ring to be able to move freely. The deep groove ball bearing is used to axially locate the shaft, while the NU type cylindrical roller bearing is able to accommodate axial movement. This arrangement is typical of large electric motors.

STORAGE AND REPACKING CONDITIONS :
BEARING STORAGE ROOM BEARINGS SHOULD BE STOCKED IN A SEPARATE STOCKING PLACE CONDITIONS : * 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) To stock bearings in a « good » way, a separate place needs to be foreseen. Although the bearings have a good protection (thixotropic oil, wrapping, box) it is important to stock them in such a way that favours a good protection. Dust and moisture will penetrate in « unprotected » bearings and after breaking through the oil film protection can give rise to damage by rust. Draughts will just add up to this. Temperature fluctuations should be avoided. A good warehouse temperature should not fluctuate more than 3 degrees over a period of 24h. “AIR CONDITIONED” IN HOT & HUMID CLIMATES “Never touch aviation bearing with hand.”

STORAGE AND REPACKING STORAGE IN RACKS 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) Stocks should not be too old (limited storage period). The principle of first in, first out to be applied. Not too many boxes on top of each other. The lower ones could get crushed and damaged. Larger bearings are (too) difficult to handle, hence a need to put them at the bottom. They should be stocked flat – not vertical. If stocked vertical, large bearings could ovalise by their own weight. For easy identification and to avoid « mixing », the number should always be up front. Small bearings are best packed in a drawer for good protection.

STOCKING RECOMMENDATIONS
STORAGE AND REPACKING STOCKING RECOMMENDATIONS 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 : THE 3 KEY REASONS why a bearing should be kept in its original packing at all times !

RENEWING ANTI-RUST (AND PACKING)
STORAGE AND REPACKING RENEWING ANTI-RUST (AND PACKING) * WHITE SPIRIT * CLEAN AREA * LET DRY 1. WASH 2. INHIBITING OIL PROTECTION * 40 % QUAKER BASE AND 60 % WHITE SPIRIT * DIP AND ROTATE * DRY 7 HOURS 3. PACK * WRAP IN POLYETHYLENE COATED "VPI" PAPER * PACK IN (CARTON) BOXES 4. IDENTITY * INDICATE FULL BEARING NUMBER * INDICATE BRAND !! When bearings need to be repacked, strict rules of cleanliness do apply. 4 points are important when renewing the packing. They are listed and commented.

STATISTICS TV-2 AEROENGINE BEARINGS

PRACTICAL DEMONSTRATION

THANK YOU

? ANY QUESTIONS?

A PRESENTATION BY WG CDR ARVIND KUMAR.

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