Slidmekanismer og slidforebyggelse - F M V · Slidmekanismer og slidforebyggelse Niels Bay...

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Institut for Mekanisk TeknologiDanmarks Tekniske Universitet

Slidmekanismer og slidforebyggelse

Niels Bay

DTU-Mekanik

FMV Temadag om Slid på Metaller

Scandic Hotel, København

15. november 2012.


Mechanisms of wear

Primary mechanisms

1. Adhesive wear

2. Abrasive wear

3. Corrosive wear

4. Fatigue wear

Secondary mechanisms

5. Fretting

6. Erosive wear

7. Cavitation wear

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Adhesive wear

Adhesive wear is due to strong attractive forces appearing when the atoms approach each other.

.

Arises when two rather smooth surfaces slide against each other and particles from one surface are torn out adhering to the other one.


Adhesive wearSize of wear particles

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Abrasive wear

Arises from the cutting action of a hard surfacesliding on a softer material (2-body abrasivewear)

or

when loose debris particles trapped betweenthe sliding surfaces are penetrating the softersurface and scratching a wear groove in the harder one (3-body abrasive wear)


Abrasive wear

Severe journal wear

Severe wear of bronze shaft due to failing soft packing

Severe tearing of bearing surface

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2-body abrasive wear 3-body abrasive wear


Corrosive wear

If sliding did not take place, the corrosiveproducts would form a protective film on the surface impeding further corrosion, but the slidingwears out the corrosive film thus allowing the corrosion to continue.

(Corrosion is the degradation of a surface by chemical reaction with the environment)

Corrosive wear arises when two surfaces slide against each other in a corrosive environment.

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Corrosive wearRemoval of lead phase from lead-bronze slide bearing


Fatigue wear

Arises when a surface is loaded cyclically due to repeated sliding, rolling or impacts.

The repeated loading and unloading causes crack formation in the surface or subsurface and subsequent breaking off fragments from the surface resulting in pit formation.

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Fatigue wear Shear stresses in the subsurface layer


Fatigue wear

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Fretting

Fretting occurs when two surfaces in contact under load and nominally at rest with respect to each other are subjected to slight oscillating tangentialmovement with small amplitude.

Typical examples are vibrations in:

Poorly aligned spline coupling

Loosely bolted machine parts

Riveted joints

Press fits

Surgical implants

Initial adhesive wear forms wear debris which may oxidize to form abrasivewear particles, which cannot readily escape due to close fit of the surface.

Often surprisingly large wear rates.


FrettingScavenge pump failed

by fretting fatigue at centre

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Fretting

Riveted joint

Screw joint


Erosive wear

Damage experienced by a solid body, when a fluid or gas containing solid particles impinges on to the surface of the body.

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Erosive wear

Compressor blade in Ti-alloyErosive wear of leading edgesdue to sand particles


Cavitation wearCavitation wear arises when a solid and a fluid are in relative motion, and bubbles formed in the fluid become unstable and implode against the surface of the solid.

Cavitation wear is closely related to fatigue wear and as such materialswhich are hard and ductile are resistive to cavitation wear.

The implosion creates a chock wave which can tear out particles from the surface.

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Cavitation wearShips propellers, centrifugal pumps


Modelling of adhesive wear

During sliding small asperities may come into contact and during passage there is a small possibility that separation does appear in the original interface.

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Adhesive wearArchards model for adhesive wear

r0 ApP

All contacts assumed to have same size and diameter d. The total number of contacts N :

0

2

r pP

4d

NA

20dpP4

N



20 dpP4

N

Every contact assumed to exist during a sliding length of d. If N contacts remain under load the number of new formed contacts per unit sliding length is:

dN

n 30 dpP4

n

Total number of contacts

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)lengthslidingunitpercontacts

formednewofnumbern(dp

P4n

30

vnkdxdV

k: probability of formation a wear fragment

1k0

Fragment shape is assumed to be semi-spherical

12d

nkdxdV 3

12d

dpP4

kdxdV 3

30

0pP

3k

dxdV

0p3xP

kV

v: volume of wear fragment


Archards model for adhesive wearThe dimensionless wear coefficient k

0p3xP

kV

Combinationin dry contact

Wear constant k

Zinc on zinc 16010-3

Low carbon steel on low carbon steel 4510-3

Copper on copper 3210-3

Stainless steel on stainless steel 2110-3

Copper on low carbon steel 1.510-3

Low carbon steel on copper 0.510-3

Bakelite on bakelite 0.0210-3

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Archards model for adhesive wearThe dimensionless wear coefficient k

0p3xP

kV

Surface conditionMaterial combination

Like Unlike

Clean (dry contact) 510-3 210-4

Poor lubrication 210-4 210-4

Average lubrication 210-5 210-5

Excellent lubrication 210-6-10-7 210-6-10-7


Archards model for adhesive wearPrevention of adhesive wear

0p3xP

kV

Small normal pressures

Small sliding lengths

Hard materials

Combine materials with small interaction (small k)

Chose one material to be non-metallic

Chose materials with low mutual adhesion energy

Apply lubrication

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Adhesive wear in spaceProblems with cargo hatch hinges on space shuttle

Adhesive wear in spaceprevented by ion sputtering of MoS2 on clean surfaces

High duty tribo-elements likeballs and gears are Au-plated


Abrasive wearExperimental studies of plowingand cutting mechanisms

Plowing

Cutting

Cutting

T. Abildgaard Petersen, T. Wanheim

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Model for abrasive wear

20r0 d

4pApP

0

2

p

P4d

Projected area of indented cone on plane perpendicular to sliding direction:

4tand

A2

rs

d

p0

P

tan2d

z

Volume removed when sliding the distance dx:

dx4tand

dV2

1

dx

ptanP

0



d

p0

P

tan2d

zVolume removed when sliding the distance dx:

dxptanP

dV0

1

Summing up for all contact points we get:

dxp

tanPdV

0

or

0ridge p

xPtanV

Assuming that only a fraction β of the load is carried by asperities with cutting action we get:

0wear p

xPtanV

0abrwear p3

xPkV

tan3

kabr

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0abrwear p3

xPkV

tan3

kabr

2-body abrasive wear: 0.02 < kabr < 0.2

3-body abrasive wear: 0.001 < kabr < 0.01

Experimental investigations confirms the model as regards the influence of the three main parameters P, x and p0


Prevention of abrasive wear

0abrwear p3

xPkV

tan3

kabr

Ensure that one of the materials is harder than the abrasive particles and the other so soft, that it can bury wear particles. Since these have about the same hardness as the harder surface a hardness ratio of minimum 3 is feasible.

Ensure filtering of lubricant

Be careful to avoid formation of large wear particles which are more dangerous than small ones. Avoid fatigue wear particles. Chose materials with good resistance against fatigue.

Apply very flexible materials like rubber as the softer material.

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Prevention of abrasive wear in hot forgingDiminish sliding

0abrwear p3

xPkV

tan3

kabr

Lubricated forging Unlubricated forging

Tool roughness Ra not too small


Abrasive wearMohs hardness scale

1. Talc

2. Gypsum

3. Calcite

4. Fluorite

5. Apatite

6. Felspar

7. Quartz

8. Topaz

9. Corundum

10. Diamond

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Off-line testing of new, environmentally friendly tribo-systems for sheet metal forming

Objective:

• To replace environmentally harmful lubricants such as chlorinated paraffin oils

• Sheet forming of tribologically difficult materials, e.g. AHSS, stainless steel, Al, Ti

Partners:

• Grundfos

• Uddeholm

• SSAB

• Outokumpu Stainless


Deep drawing in progressive tool - Grundfosa. Deep drawingb. 1st redrawingc. 2nd redrawing Tribologically the most severe operationd. Sharp pressing of flange

Step a Step b Step c Step d

Workpiece material: AISI 304

Step c E. Ceron, E. Madsen, N. Bay

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Simulation of deep drawing and 2 redrawings

LS-DYNA 2D implicit model

The blank is transferred from one process to the following updating flow stress and equivalent strain

E. Ceron, N. Bay


Distribution of radial stress in step c

workpiece

die

[MPa]

Maximum contact pressure pmax = 900 MPa

E. Ceron, N. Bay

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Simulation of BUT test

Round pin with radius R = 3,5Maximum contact pressure 360 MPawith maximum back tension 300 MPa

tool strip

MPa

E. Ceron, N. Bay


Distribution of radial stresses in BUT test

Workpiece strip

tool

[MPa]

By modifying the BUT test tool to a 45°contact instead sufficient contact pressure can bereachedMaximum contact pressure pmax = 1000 MPa with back tension 300 MPa

E. Ceron, N. Bay

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BUT

DBT

SRT

E. Ceron, N. Paldan, J. Gregersen, N. Bay

New, universal sheet tribo-tester


Universal sheet tribo-testerAutomatic PLC controlled running of repeated tests

Material feed from coil of more than 1000m

Adjustable sliding lengths, speed, cycle time and total number of strokes

Ensuring appropriate emulation of production conditions withheating and cooling cycle

Easy programming by Labview

E. Ceron, N. Paldan, J. Gregersen, N. Bay

BUT_test_running_detail.avi

BUT_test_running.avi

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Second screening test campaign

DP 800; Shell Ensis PQ 144• Back tension = 250 MPa; production speed = 85 stroke/min; Vanadis 4E; (test 1)

• EDS analysis of pick-upSpectrum In stats. C Si S V Cr Mn Fe Total

1 Yes 10.8 0.6 1.1 87.5 100.0

2 Yes 5.1 1.4 4.8 5.0 83.7 100.0

No Vanadium and Chromium in spectrum 1 but Manganese and Silicium.Spectrum 2 is tool surface


Second screening test campaign

DP 800; Fuchs PLS 100• Back tension = 300 MPa; production speed = 40 stroke/min; Vancron 40 (V40_C3); (test 26)

• Almost no pick-up on the tool surface

Exit edge Entrance threshold

Sliding direction

Slidmekanismer og slidforebyggelse - F M V · Slidmekanismer og slidforebyggelse Niels Bay...

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