Effect of Gear Surface and Lubricant Interaction on Mild Wear

Abstract

In this study, a twin-disc test machine was used to simulate a rolling/sliding gear contact for three surface finishes, each run with two types of lubricants, thus seeking to develop insight into the tooth flank/lubricant tribological system. The test disc surfaces were case-carburised before the surfaces were produced by: transverse grinding followed by a mechanical abrasive polishing process; transverse grinding only; and transverse grinding followed by preheating as a final finishing step (intended to enhance the build-up of an easily sheared surface boundary layer using a sulphur additive). The twin-disc contact was lubricated with an ester-based environmentally adapted lubricant or a polyalphaolefin-based commercial heavy truck gearbox lubricant. To obtain information about the composition of chemically reacted surface layers, the specimens used were analysed using glow discharge-optical emission spectroscopy. The results indicate that the interactions between different surface finishes and lubricants have different impacts on friction behaviour, wear and the reacted surface boundary layer formed by the lubricant. Running a smooth (polished) surface with the appropriate lubricant drastically reduces the friction. Surface analysis of the ground surfaces gives clear differences in lubricant characteristics. The commercial lubricant does not seem to react chemically with the surface to the same extent as the EAL does. Micropitting was found on all ground discs with both lubricants, though at different rates. The highest amount of wear but less surface damage (i.e. micropits) was found on the preheated surface run with the commercial lubricant.

Appendices

Appendix

Film thickness equations for elliptical and rectangular conjunctions are applied to specific machine elements, other than the rolling-element bearings, by Hamrock [20] to illustrate how fluid film lubrication conditions can be analysed. Two disc pairs, i.e. ground/ground (elliptical contact, crowning set to 25 μm) reference discs and polished/polished (rectangular contact), are used as input with the set load. Note that the pressure viscosities were not measured at lubricant bulk temperature and pure rolling is assumed in the calculations. The EAL (ester) is assumed to have a similar pressure viscosity coefficient as the commercial lubricant (PAO), for which the pressure viscosity coefficient is taken from Höglund [30].

The minimum film thickness for the elliptical conjunction is given by:

$$ H = h/R = 3.63U^{0.68} G^{0.49} W^{ - 0.073} (1 - e^{{ - 0.68{\text{k}}}} ) $$

(A1)

The minimum film thickness for the rectangular conjunction is given by:

$$ H^{\prime } = h^{\prime } /R^{\prime } = 1.714(W^{\prime } )^{ - 0.128} U^{0.694} G^{0.56} $$

(A2)

The specific film parameter for the elliptical conjunction, the EHD film thickness for an ideal smoothness of the surfaces divided by the composite surface roughness, is given by:

$$ \Uplambda = h/({\text{Rq}}_{\text{a}}^{ 2} + {\text{Rq}}_{\text{b}}^{ 2} )^{ 1/ 2} $$

(A3)

The specific film parameter for the rectangular conjunction, the EHD film thickness for an ideal smoothness of the surfaces divided by the composite surface roughness, is given by:

$$ \Uplambda^{\prime } = h^{\prime } /({\text{Rq}}_{\text{a}}^{ 2} + {\text{Rq}}_{\text{b}}^{ 2} )^{ 1/ 2} $$

(A4)

List of Symbols

b :

Disc width, m

E :

Effective Young’s modulus, E = 2[(1−ν ²_a )/E _a + (1−ν ²_b )/E _b]⁻¹, Pa

E _{a, b} :

Young’s modulus, Pa

G :

Dimensionless material parameter, G = Eα

h :

Minimum film thickness, m

h′:

Minimum film thickness for rectangular conjunction, m

H :

Dimensionless film thickness, h/R

H′:

Dimensionless film thickness for rectangular conjunction, h′/R′

k :

Ellipticity parameter, k = (R _y /R _x)^2/π

R :

Effective radius ground discs, R ⁻¹ = [r ⁻¹_ax  + r ⁻¹_bx ]⁻¹ + [r ⁻¹_ay  + r ⁻¹_by ]⁻¹, m

R′ :

Effective radius polished discs, R ⁻¹ = [r ⁻¹_ax  + r ⁻¹_bx ]⁻¹, m

Rq:

RMS roughness, μm

U :

Dimensionless speed parameter, U = uη ₀ /E′R

u :

Mean surface velocity on the pitch line, u = (ω _a ·r _a  + ω _b ·r _b )/2, m/s

W :

Dimensionless load parameter, W = w _z /E′R ²

W′:

Dimensionless load parameter, W′ = w _z /E′Rb

w _z :

Normal load component, N

α :

Pressure–viscosity coefficient at 40 °C, Pa⁻¹

Λ :

Specific film parameter for elliptical conjunction

Λ′:

Specific film parameter for rectangular conjunction

η ₀ :

Dynamic viscosity at 40 °C, Pa s

ω:

Angular velocity, rad s⁻¹

2.1 Index

a, b :

Body a, b

0:

Reference value at atmospheric pressure

x :

Direction of motion

y:

Direction perpendicular to motion

z :

Normal direction

2.2 Input data

• b = 0.010 m

• E _{a, b} = 207 GPa

• r _{a, b} = 0.0253 m

• R = 0.01120 m

• R′ = 0.0118 m

• Rq = 1.1 μm

• Rq′ = 0.06 μm

• u = 1.04 m/s

• α = 15.5 GPa⁻¹

• ν _{a, b} = 0.293

• w _z = 7140 N

• η₀ = 0.0692 Pa s (EAL)

2.3 Results

• E = 226 Pa

• G = 3510

• h = 0.27 μm

• h′ = 0.19 μm

• H = 2.41 × 10⁻⁵

• H′ = 1.60 × 10⁻⁵

• k = 7.0

• U = 2.82 × 10⁻¹¹

• U′ = 2.70 × 10⁻¹¹

• W = 2.18 × 10⁻⁴

• W′ = 2.5 × 10⁻⁴

• Λ = 0.17

• Λ′ = 2.2