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Tribology of Sealing elastomers

Braham PrakashDivision of Machine Elements

Email: braham.prakash@ltu.se

Tribology Days 2012 ÖrnsköldsvikNovember 6-8, 2012

1. Introduction

2. Friction and wear under dry sliding conditions

3. Lubricated sliding friction

4. Two body abrasive wear in lubricated condition

5. Concluding remerks

Outline

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Introduction

• Reciprocating and rotary seals are widely used in various technological systems

• Elastomers are the preferred choice of materials

Sealing elastomers

Elastomers are the most popular seal materials because of their:

low modulus of elasticity & high elongation-to-break; can deflect significantly, follow the irregularities and vibration of the sealed surface without giving high contact stresses.

high Poisson's ratio (close to 0.5) & low shear modulus G, enabling an elastomeric seal to create its own sealing force automatically in proportion to the pressure.

Unloaded coiled chains

Loaded intension

Elastomers are polymeric materials with high elasticity and fairly low cross-link density, with links at random intervals, usually of between 500 and 1000 monomers

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Friction

Leakage (due to wear)

Service life (limited by wear)

Seal-oil interaction (performance deterioration)

Why tribology of elastomeric seals?

Abrasive wear

Chemical degradation

*Seals account for major part of bearing friction (~ 75% of total bearing power loss)

*Patent, WO 2011/110360 A1

Challenges in tribological testing of elastomers!

• Large elastic deformation

• Low contact pressure, difficult to investigate the tribological behaviour of elastomer in boundary or mixed lubrication

• Edge effects

• Absorption of oil and leaching

• Effect of cleaning agents

• Thermal effect

High frequency tests Low frequency tests

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Dry friction of elastomers under unidirectional sliding conditions

Elastomeric materials Hardness(Shore A)

Tensile strength (MPa)

Elongation at break (%)

Density(g/cm3)

Nitrile rubber (NBR) 76.1 25.4 466 1.31

Hydrogenated nitrile rubber (HNBR) 71.3 17.5 303 1.24

Acrylate rubber(ACM) 73.4 7.8 171 1.49

Fluoro rubber (FKM) 72.8 15.6 >900 2.03

Surface roughnesss of steel rings

Normal load (gr)

Average contact pressure (Kpa)

150 ≈240

1000 ≈750

Experimental work

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Dry frictional behaviour at low load *(Normal load: 1.5 N, Speed: 10 r.p.m.)

• Friction coefficient decreased after a running-in period and

the wear was insignificant.

• Longest running-in periods observed during sliding against

fine surfaces

Dry frictional behaviour: Influence of sliding speed(Normal load: 1.5N, Ra: 0.35 ~ 0.55 μm)

Speed dependence of friction is quite anomalous; different for different elastomers!

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Dry friction and wear behaviour*(Normal load: 10 N, Speed: 10 rpm, Ra: 0.35 ~ 0.55 μm)

• Powdery worn particles observed on the ACM resulted lower friction coefficient but for FKM and HNBR, worn particles with roll shapes have been produced.

• Worn particles of FKM were significantly larger than those of the other tested materials

FKM Worn particles

Frictional behaviour of sealing elastomers in different lubrication regimes

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Experimental materials and lubricant Four different types of sealing elastomers NBR, HNBR, ACM and

FKM

(Rectangular sheets of 16 mm x 4 mm x 2 mm thickness)

• Paraffinic oil without any additive

(Viscosity: 34.1 cSt @40⁰C)

• Steel counterface

- AISI 52100 bearing steel

‐ Ra ≈ 380 nm, after removing

the cylindrical curvature

Experimental elastomersElastomeric

materials Hardness (Shore A)

Tensile strength (MPa)

Elongation at break (%)

Density (g/cm3)

NBR 76.1 25.4 466 1.31

HNBR 71.3 17.5 303 1.24

ACM 73.4 7.8 171 1.49

FKM 72.8 - - 2.03

Surface topography of elastomeric samples (Ra ≈ 80 nm)

Elastomers’ properties

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Test apparatus and configuration

Main lip

Lip contact

Dust lip

Garter spring

Case

Shaft

Lubricant

Bore tight coating

A block on ring test configuration waschosen to simulate radial lip sealoperating under unidirectional(rotational) sliding conditions

Test parameters and procedure

Running in for 50 minutes at a sliding velocity of 18.33 mm/s to reach more steady results and the sliding

Tests at both increasing and decreasing speeds of 0.24, 0.33, 0.58, 1.03, 1.83, 3.26, 5.79, 10.30, 18.33, 32.58 mm/s

Tests run for 10 min at each sliding speed

Tests performed at room temperature (22 ± 2 °C).

Experiments carried out at a normal load of 3.5 N

(contact pressure ~ 370 kPa)

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Friction results

Purple lines show friction coefficients in tests in which oil and elastomers have been in contact for longer duration.

Friction in full film (soft ehl) regime

Friction coefficients for different elastomers are different and this is due to the different viscoelastic behaviour of the elastomers.

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Two body abrasive wear and friction of sealing elastomers in unidirectional lubricated sliding

Materials and test parameters

Materials:• Four different types of sealing

elastomers (NBR, ACM and FKM)• The lubricant was monoester

Abrasive wear testsTear tests

The samples aged in the lubricant for 2 weeks at 120ºC and then the changes in their weights and tear strength were measured

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Abrasive wear

0

1

2

3

4

5

6

NBR-1 NBR-2 FKM ACM

Ab

rasiv

e w

ear

(m

g)

Dry Lubricated

Influence of lubricant on the abrasive wear, weight change, tear strength and friction coefficient is most for ACM!

Load: 10 N, Speed: 3.1 mm/s, Abrasive grit size: #500

Wear particles examination

Wear particles (particularly from ACM) in dry sliding get aggregated but in lubricated sliding dispersed!

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Friction coefficient

0

0.3

0.6

0.9

1.2

1.5

NBR-1 NBR-2 FKM ACM

Fri

ctio

n c

oef

fici

ent

Dry Lubricated

• Apart from FKM, presence of the oil resulted in a decrease in friction coefficient as well as the tear strength of the elastomers, especially for ACM

Load: 10 N, Speed: 3.1 mm/s, Abrasive grit size: #500

• In boundary lubrication, the ACM and FKM have shown the lowest and highest friction coefficients respectively.

• Friction coefficient of an elastomer in boundary lubrication may decrease or increase with time and it may be due to the oil absorption or extraction of some elastomer’s constituent respectively.

• In EHL regime, the friction coefficients of FKM and HNBR have the lowest and highest values respectively.

• Depending on the wear mechanism and the oil-elastomer compatibility, the presence of lubricant may decrease or increase the wear of elastomers.

Concluding remarks

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• Tribology of elastomer is highly complex; influenced by several factors such as counterface surface topography, interaction with lubricant etc.

Concluding remarks

In another study, we have found both leaching (extraction of elastomer constituents by the lubricant) & absorption of lubricant in elastomer material.

Salient publications

Mofidi, M., Prakash, B., “The influence of lubrication on two body abrasive wear of sealing elastomers”, Journal of Elastomers and Plastics, 2011, 4, 19-31.

Mofidi, M., Prakash, B., Two body abrasive wear and frictional characteristics of sealing elastomers under unidirectional lubricated sliding conditions, Tribology: Materials, Surfaces & Interfaces, 2010, 4, 1, 26-37.

Simmons, G. F., Mofidi, M., and Prakash B.,"Friction evaluation of elastomers in lubricated contact: a comparison of different test methodologies", Lubrication Science, 2009, 21, 10, 427-440.

• Mofidi, M., Prakash, B., Persson, B.N.J., Albohr, O., Rubber friction on (apparently) smooth lubricated surfaces, J. Phys.: Condens. Matter 20 (2008) 085223 (8pp).

• Mofidi, M. R., and Prakash, B., “Influence of counterface topography on sliding friction of some elastomers under dry sliding condition”, Proceedings of the Institution of Mechanical Engineers, Part J, Journal of Engineering Tribology (2008) 222, 5, 667-673.

• Mofidi, M. R., Kassfeldt, E. and Prakash, B., “Tribological behaviour of an elastomer aged in different oils”, Tribiology International (2008), 41, 9-10, 860-866.

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Thank you for your kind attention!