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Tribology International 43 (2010) 1615–1619

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/triboint

Experimental investigation of friction in entrappedelastohydrodynamic contacts

Adam Young, Scott Bair �

Georgia Institute of Technology, Center for High-Pressure Rheology, George W. Woodruff School of Mechanical Engineering, Atlanta, GA 30332-0405, USA

a r t i c l e i n f o

Article history:

Received 3 February 2010

Received in revised form

4 March 2010

Accepted 12 March 2010Available online 27 March 2010

Keywords:

Sliding friction

Rheology

Friction measurement

Elastohydrodynamic lubrication

9X/$ - see front matter & 2010 Elsevier Ltd. A

016/j.triboint.2010.03.007

esponding author.

ail address: scott.bair@me.gatech.edu (S. Bair

a b s t r a c t

Start-up friction is a performance-limiting aspect of hydraulic motor operation. This study was

conducted to gain a better understanding of the roles played by contact pressure, speed, and oil type to

start-up friction behavior for contacts containing trapped pockets of highly pressurized oil, also known

as elastohydrodynamic lubrication (EHL) entrapments. An apparatus was built to measure the start up

friction response for ball-on-plane sliding contact with simultaneous observation of the contact region

by optical interferometry. Baseline trials for all cases were conducted in the absence of any entrapment

and then repeated after forming an entrapment. An impact, activated by solenoids, was used to create a

small separation whereby oil would fill the gap and then become trapped as the load rapidly brought

the surfaces back into contact.

In all cases, entrapment substantially decreased the start-up friction. Additionally, the short-lived

entrapments provide the greatest reduction in start-up friction. Therefore, the method of entrapment

that may be implemented with least delay before the initiation of sliding has the greatest potential.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Certain hydraulic motors are currently sized at greater scalethan is necessary for steady operation because of the need toovercome start-up friction [1]. The increase of scale results ingreater cost and decreased efficiency [1]. It has been recentlyproposed [2] that elastohydrodynamic lubrication (EHL) entrap-ment may be a simple and effective means to reduce the startingfriction in these motors by mitigating the friction at non-conformal sliding contacts within the motor.

Entrapments may form following a sudden halt to rolling orsliding motion [3,4] or by impact [5,6]. In each situation thereexists, initially, a separating film of pressurized oil with pressuredecreasing from the maximum value at the center to nearambient pressure at the edges. To escape from the center of thecontact, oil must flow outwardly toward the edges. However,the viscosity of organic liquids is extremely sensitive to pressure.The relatively low viscosity near the edges allows rapid collapse ofthe film there while the much greater viscosity in the central regionprevents rapid flow and a dimple which contains pressurized oil isformed. The entrapment may persist for seconds or hours.

It has been established [2] through rheological measurement,numerical simulation and transient film thickness measurementthat pressure-fragility is the rheological property of overwhelming

ll rights reserved.

).

importance to entrapment and the persistence of the entrapment.Here, the friction response to controlled sliding is investigatedexperimentally for circular concentrated contacts, both with andwithout entrapment.

2. Experimental equipment

A new device, shown in Fig. 1, was purpose-built for theseexperiments. The configuration is ball-on-flat with normalapproach and sliding. The directions of the sliding displacementand speed are designated D and S, respectively. The normalapproach and force direction is represented by N. Themechanisms used to produce these conditions are omitted tosimplify the figure but it should be noted by the schematic thatthese mechanisms are interfaced with the motion platform suchthat they would not impart a significant force in an unintendeddirection by misalignment or friction. Also omitted in Fig. 1 arethe microscope and camera used to observe the interferencepattern at the ball-flat interface. Flexures are used to ensure allmotions would be stiction free. The flat is a 3 mm thick sapphiredisc with a semi transparent chromium coating. While a discshaped flat is employed for convenience, no rotation is involved.The ball is a grade 10, 12.7 mm diameter chrome steel bearing ballwhich is lightly press fit into a mounting cup. The ball surface wasmeasured to have arithmetic average roughness of 25 nm. The ballmount is fixed atop a pedestal with low stiffness in the directionof the sliding displacement so that the effective stiffness of the

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Fig. 1. Schematic of the experimental apparatus.

Fig. 2. Photograph showing the ball and disc, the microscope objective, miniature

load cell (at the end of wire) and structure.

A. Young, S. Bair / Tribology International 43 (2010) 1615–16191616

load cell is not substantially changed. The load cell with 9�105

N/m stiffness measures the friction force from the ball-flatinterface. The load cell signal is sampled at a rate of 5 kHz and alow-pass filter with 40 Hz cut-off is used to suppress noise.

A pair of solenoids acts against adjustable stop rods tomomentarily remove the normal force from the interface andcreate a small, 672mm, separation between the ball and disc. Therapid closing of this gap and re-application of the normal forceproduces the desired EHL entrapment. Normal force is generatedby elastic deflection of a cantilever not shown in Fig. 1.

The ball and disc, the optics and the load cell (at the right ofthe ball) are shown in the photograph in Fig. 2. A stepper-motor

based linear actuator resides to the left, out of view, in thephotograph and connects with the disc through the roller contactseen in the left of Fig. 2 to drive the disc from left to right in Fig. 2.A potentiometer measures displacement of the disc.

A microscope shown in Fig. 2 is equipped with verticalillumination through the objective lens. The light from themicroscope lamp is filtered with a 600 nm narrow band passfilter. A video camera allows recording of the sliding contact alongwith the fringe pattern resulting from entrapment.

3. Experimental liquids

Four liquids were selected for study. They are described inTable 1. Oils 1, 2 and 3 are hydraulic fluids,. A mineral oilemployed in Ref. [2] was included as an example of a liquid whicheasily forms an entrapment which will persist for hours [2]. Theviscosities were measured at 25 1C at pressures to 350 MPa or toviscosity of 104 Pa s in a routine falling-body viscometer [7]. Thepressure-viscosity coefficient that is tabulated is defined as

a¼Z 1

0

mðp¼ 0Þdp

mðpÞ

� ��1

ð1Þ

where m is the low-shear viscosity and p is pressure. Othercoefficients have been defined [8] as well.

4. Experimental procedure and conditions

The test oil was then applied to the ball–disc interface and theinterface centered with the microscope using an x�y stage. If anentrapment was required for a test, a momentary switch was usedto actuate the load-lifting solenoids just before the initiation ofsliding. Maximum (central) entrapment depth was measured byinterference fringe counting.

All tests were at room temperature, approximately 20 1C. Twoloads were considered, 9.7 N for Load 1 and 18.2 N for Load 2,resulting in Hertz pressures in dry contact of 0.98 and 1.21 GPa,respectively and resulting in dry contact radii of 0.069 and0.085 mm, respectively. Three sliding speeds were considered,0.20, 0.41, and 0.61 mm/s, Speeds 1, 2 and 3, respectively. Thecombinations of loads and speeds were investigated with andwithout an entrapment, the tests without entrapment serving asbaseline measurements. The ball and sapphire disc were cleanedcarefully between changing oils. The load cell signal for zerofriction force was established by averaging the signal forentrapment-free sliding in opposite directions, assuming thatthe magnitude of friction is independent of direction.

5. Results

Central entrapment depth was measured by fringe countingwith a resolution of 50 nm. Four trials were averaged and resultsare listed in Table 2 where it can be seen that the Heavy MineralOil produced entrapments far thicker than the hydraulic oils, 1, 2and 3. Further, entrapment depth for these liquids is correlatedwith pressure-viscosity coefficient, a, as greater a generatesgreater depth. However, it has been demonstrated that this willonly be true when a is an accurate predictor of the viscosity atHertz pressure [2] and this is not universal. There is evidence thata greater load traps a deeper pocket of oil, at least with thepresent entrapment process, possibly due to higher impactvelocity with higher load.

The evolution of the friction coefficient during sliding is shownin Fig. 3 for the Heavy Mineral Oil at Load 2 and Speed 1. Two

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Table 2Entrapment central depths in nm.

Load 1 (9.7 N) Load 2 (18.2 N)

Oil 1 380 380

Oil 2 205 255

Oil 3 180 180

Hvy Min Oil 830 930

Table 1Experimental oils and their properties.

Designation Hydraulic oil 1 Hydraulic oil 2 Hydraulic oil 3 Hvy. Min. Oil

Description Solvent refined mineral oil Hydrocracked mineral oil

with VI improver

Biodegradable

trimethylolpropane

trioleate

Severely hydrotreated

naphthenic mineral oil

Kinematic viscosity

(mm2/s) at 40 1C

46 50 48 144

Pressure (MPa) Dynamic viscosity

(Pa s) at 25 1C

Dynamic viscosity

(Pa s) at 25 1C

Dynamic viscosity

(Pa s) at 25 1C

Dynamic viscosity

(Pa s) at 25 1C

0.1 0.0851 0.0865 0.0868 0.383

25 0.144 0.136 0.125 0.886

50 0.238 0.210 0.176 1.87

100 0.622 0.475 0.335 9.26

150 1.57 1.03 0.610 40.7

200 3.86 2.16 1.08 183

250 9.45 4.47 1.87 1012

300 23.1 9.15 3.19 5580

350 56.9 18.6 5.36 –

a (1/GPa) 20.0 16.9 13.1 32.3

A. Young, S. Bair / Tribology International 43 (2010) 1615–1619 1617

curves are plotted for friction coefficient; one for sliding in theabsence of entrapment and one for sliding with entrapment.Entrapped sliding begins with significantly lower friction beforeincreasing to become identical to the non-entrapped case afterabout 0.35 mm of displacement. Ten micrographs of the contactare shown in Fig. 3 of the shape and position of the entrapmentwithin the contact area. Lines in the figure show the position onthe friction plot corresponding to each micrograph.

The initial entrapment diameter is approximately half of thecontact diameter which is 0.17 mm. If the pocket of oil were fixedto the sapphire flat, the last part of the original pocket wouldreach the contact edge at a displacement of 0.13 mm. Both thefriction plots and the micrographs show that slightly more thantwice this displacement is required to remove the entrapment.Therefore, it can be established that the oil pocket travels at avelocity that is intermediate to that of the two surfaces. The liquidfrom the pocket which is close to the ball surface requires thegreatest displacement to be drained from the entrapment.

The friction coefficient was sampled from each trial at adisplacement of 0.03 mm. This displacement was the maximumrequired to transition from the at rest condition to steady statevelocity for speed 3. The friction coefficients are listed in Table 3.The reduction in friction that results from entrapment is tabulatedas well. In every case, entrapment substantially decreased thefriction. The reductions (improvements) varied from 10% to 70%.

Friction improvement, as a percentage of the non-entrappedfriction, decreased with increasing speed in most cases andincreased with increasing load in most cases. Some exceptionswere observed for Oil 3.

Fig. 4 plots the friction versus time for all 4 oils for a commonset of conditions, Load 1 and Speed 2. Most interesting about Fig. 4is the shape of the friction curve for Oil 3 during start up. The

other oils produced an initial jump to a friction coefficient ofapproximately 0.07 and then diverged from this point to reach theindividual dynamic friction levels. Oil 3, in contrast, does notexhibit this initial jump. While most obvious for this combinationof load and speed, the anomalous behavior of Oil 3 is observed forall loads and speeds. There are at least two possible explanationsfor the different response of Oil 3, a fatty-acid ester. First, theunusually low viscosity at Hertz pressure [9] for this particularlow-fragility liquid results in rapid flow of liquid from theentrapment. This flow through the ostensibly solid contactregion of the contact patch may contribute to hydrodynamic liftwithin this region. Second, this liquid is known to be a goodboundary lubricant.

6. Conclusions

A specialized test apparatus was built to measure the start-upfriction response for ball-on-plane sliding contact with simulta-neous observation of the contact region by optical interferometry.Baseline trials for all cases were conducted in the absence of anyentrapment and then repeated after forming an entrapment. Animpact, activated by solenoids, was used to momentarily relievethe contact load and create a small separation whereby oil wouldfill the gap and then become trapped as the load rapidly broughtthe surfaces back into contact.

There is no clear trend with respect to the effect ofpressure-viscosity coefficient on friction, with or without entrap-ment. Film thickness depends upon the viscosity at lowpressures and, for Newtonian inlet behavior, the film thicknessshould correlate with pressure-viscosity coefficient. However,friction is determined by both the film thickness and the viscosityat Hertz pressure and the pressure-viscosity coefficient is notalways a good predictor of viscosity near the glass pressure.

There is also no clear trend with respect to the effect of load onfriction, with or without entrapment. Increasing load generallyreduced the friction for the hydraulic fluids but, on average,increased the friction for the heavy mineral oil.

In all cases, entrapment substantially decreased the start-upfiction. The viscosity at the highest pressures of the contactdetermines the persistence of any entrapment [2]. However, theseresults indicate that a greater reduction in friction from

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Table 3Friction coefficients at a displacement of 0.03 mm.

Without entrapment With entrapment % reduction

Oil 1 Load 1 (9.7 N) Speed 1 (0.20 mm/s) 0.1500 0.0852 43.22

Speed 2 (0.41 mm/s) 0.1691 0.1007 40.45

Speed 3 (0.61 mm/s) 0.1401 0.1168 16.60

Average 0.1531 0.1009 33.42Load 2 (18.2 N) Speed 1 (0.20 mm/s) 0.1342 0.0713 46.86

Speed 2 (0.41 mm/s) 0.1519 0.0889 41.48

Speed 3 (0.61 mm/s) 0.1355 0.1018 24.82

Average 0.1405 0.0873 37.72

Oil 2 Load 1 (9.7 N) Speed 1 (0.20 mm/s) 0.1586 0.0710 55.23

Speed 2 (0.41 mm/s) 0.1749 0.1025 41.40

Speed 3 (0.61 mm/s) 0.1557 0.1284 17.58

Average 0.1631 0.1006 38.07Load 2 (18.2 N) Speed 1 (0.20 mm/s) 0.1314 0.0482 63.31

Speed 2 (0.41 mm/s) 0.1546 0.0776 49.79

Speed 3 (0.61 mm/s) 0.1430 0.0970 32.15

Average 0.1430 0.0743 48.41

Oil 3 Load 1 (9.7 N) Speed 1 (0.20 mm/s) 0.1259 0.0385 69.39

Speed 2 (0.41 mm/s) 0.1338 0.0644 51.82

Speed 3 (0.61 mm/s) 0.1146 0.0872 23.93

Average 0.1248 0.0634 48.38Load 2 (18.2 N) Speed 1 (0.20 mm/s) 0.1065 0.0771 27.58

Speed 2 (0.41 mm/s) 0.1299 0.0508 60.85

Speed 3 (0.61 mm/s) 0.1065 0.0771 27.58

Average 0.1143 0.0683 38.67

Heavy mineral oil Load 1 (9.7 N) Speed 1 (0.20 mm/s) 0.0981 0.0741 24.46

Speed 2 (0.41 mm/s) 0.1132 0.0820 27.57

Speed 3 (0.61 mm/s) 0.0969 0.0869 10.38

Average 0.1027 0.0810 20.80Load 2 (18.2 N) Speed 1 (0.20 mm/s) 0.1249 0.0727 41.82

Speed 2 (0.41 mm/s) 0.1166 0.0830 28.80

Speed 3 (0.61 mm/s) 0.1078 0.0904 16.09

Average 0.1164 0.0820 28.90

Fig. 3. The displacement evolution of the friction coefficient for the heavy mineral oil for load 2 and speed 1, with and without entrapment. Micrographs of the entrapment

were obtained at the displacements indicated on the friction plot.

A. Young, S. Bair / Tribology International 43 (2010) 1615–16191618

entrapment often occurs for liquids with lower values ofpressure-viscosity coefficient. For many liquids a low value ofthe pressure-viscosity coefficient is an indication of low viscosityat Hertz pressure, but this rule is not universal. The findings here

suggest that the short-lived entrapments are more ideally suitedfor reducing start-up friction for the common pressure-viscositybehavior. Therefore, the method of entrapment that may beimplemented with least delay before the initiation of sliding is

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Fig. 4. Friction versus time of sliding contact with an entrapment at Load 1 and

Speed 2.

A. Young, S. Bair / Tribology International 43 (2010) 1615–1619 1619

best suited for friction reduction or, alternatively, liquidsshould be identified which have large pressure-viscosity coeffi-cient together with low viscosity at Hertz pressure. Furtherinvestigation of surface finish effects as well as entrapment

parameters are needed before the technology can be applied tohydraulic motors.

Acknowledgment

This work was supported by the National Science Foundationunder Grant number EEC#0540834.

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[5] Chang L. An efficient calculation of the load and coefficient of restitution ofimpact between two elastic bodies with a liquid lubricant. ASME Journal ofApplied Mechanics 1996;63:347–52.

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