Behavior of some vegetable oils in EHL contacts

Elastohydrodynamics '96 / D. Dowson e l al. (Editors)

0 1997 Elsevier Science B.V. All rights reserved.

243

Behavior of some vegetable oils in EHL contacts N. Ohnoa. A. Shiratakea, N. Kuwanoa and F. Hiranob aDepartment of Mechanical Engineering, Saga University, 1. Honjyo. Saga, 840. JAPAN bProfessor Emeritus of Kyushu University. 4-10- 12, Takamiya, Minamiku. Fukuoka, 815, JAPAN EHL oil film thickness measurements for rape seed oil, camellia oil. olive oil. castor oil and glycerol have been carried out by optical interferometry. The experimental results showed that the central film thickness of castor oil and glycerol under rolling conditions was 0.5-0.9 times thinner compared with the Hamrock - Dowson central film thickness formula. This fact was considered to be attributed to their poor wettability a t the Iiquid/solid interface, which was estimated by observing the contact angle with the aid of a goniometer. Furthermore. the largest contact angle of glycerol was lowered by adding surfactant and brought about an increase in film thickness. It is suggested that the interfacial phenomena near the inlet side of the EHL contact region affects the EHL central film thickness, in particular. in case of vegetable oils and glycerol. Further. comparing oils with equal viscosity grades. vegetable oils with lower pressure-viscosity coefficients show lower traction coefficients than the paraffinic mineral oils. The effect of wettability on the traction coefficient was unrecognized. 1. INTRODUCTION The main component of vegetable oils comprises glyceride of fatty acids, biodegradble oils have a superior to the performance for boundary lubrication. However. the limitation of vegetable oils has been their high cost, and their thermal and oxidation instability. Therefore, it was mainly used for metal working lubricants. Recently the demand for biodegradable lubricating oil is increasing, vegetable oils as environmentally acceptable fluids are noticed [ 11. So f a r in experimental research of elastohydrodynamic oil films mineral oils and synthetic oils were widely used. the use of vegetable oils was failed to notice. Thus. the traction and EHL film thickness measurements for vegetable oils were carried

out by optical interferometry. A s compared with mineral oils, behavior of vegetable oils show considerable deviation concerning the EHL film thickness, in particular. in maps of liquid/solid transition lubrication 121[3] and traction coefficient 141. It is confirmed t h a t t h e difference in wettability plays a significant role of this fact. Further, comparing oils with equal viscosity grades, vegetable oils with lower pressure-viscosity coefficients show lower traction coefficients t h a n mineral oil series. The effect of wettability on the traction coefficient was unrecognized. The present investigations were carried out to clarify merits or demerits of vegetable oils concerning EHL oil film thickness and traction. Furthermore, using the entrapment of oil between normally a p proaching elastic bodies, it is ascertained

244 the interfacial phenomena's effects on the EHL central film thickness.

2. EXPERIMENTAL METHOD The apparatuses used in the present investigation for measuring basic properties of lubricants are the viscometer of a falling hall type, and the high pressure cylinder for the density measurement at pressure up to 1.2 GPa and a t temperatures from 0 to 100°C as described in the a u thors' previous report in detail 151. Hence. the general views of these apparatuses are omitted. For measurement of traction forces a n d observation of EHL p a t t e r n s by means of the usual interferometric method. a rolling/sliding contact a p p a r a t u s was used. It was composed of a pyrex glass or a sapphire optical flat of 40 mm diameter and 5 mm thick and a 23.8 mm diameter bearing steel ball for ellipticity parameter k = l . For ellipticity parameter k = 3.6 a barrel shaped roller of 23.6 mm in maximum outer diameter and 8 4 mm in radius of curvature in the meridian plane w a s used. Fig. 1 shows a schematic diagram of the experimental apparatus used in this study. The experimental conditions in the traction measurements are as follows: Maximum rolling speed u : 1 100 m m / s Con tact load W : 86N Mean Hertzian contact pressure p in circular contact. k= 1 with pyrex glass : 0.45 CPa. with sapphire : 0.90 GPa. in elliptic contact, k=3.6 with pyrex glass : 0.26 GPa. with sapphire : 0.52 GPa Temperature : room temperature The film thickness measurements were carried out at mean Hertzian pressure 0.26 GPa for k = 3.6 and 0.45 GPa for k =

1.

The names and properties of tested lubricants are listed in Table 1. The main s e ries are classified into the following types: 1. Vegetable oils : rape seed oil. camellia oil. olive oil and castor oil 2. Glycerol C3H5(OH13 3 . Surfactant containing oil: Polyoxyethylene 5% by weight used in solution in glycerol. 4. Paraffinic mineral oils P60N. P1 5 0 N . P500N. PBSN Values of pressure-viscosity coefficient (K listed in Table 1 were all detcrmined as follows. Provided the validity of relation, In qi = In 110 t c r ~ (.I was derived from the slope of plot using method of least s q u a r e s . For t h i s , experimental d a t a points in range q i < 103 Pa.s arid pi < 0.33 GPa were used 151. 3. RESULTS AND DISCUSSIONS 3.1 Film Thickness Measurements Fig. 2 shows the interference pattern for paraffinic mineral oil PBSN. We c a n see the well-known film shape having the flat

p-

.I

~

Light source

belt

M

-

I

l

l

I

Lubricant reservoir Air bearing

'

Load

Fig. 1 Schematic diagram of rolling sliding apparatus

245

Table 1 Properties of Oils 14 g/mL rape seed oil camellia oil olive oil castor oil glycerol glycerol + surfactant 5%

15°C 0.9 189 0.9 168 0.9 137 0.9666 1.263 1

v, m / s 2 40°C 100°C: 35.7 8.06 39.3 8.28 39.6 8.24 241.0 17.50 209.3 12.00

(1,

CPa-I

40°C 9.0 7.0 8.1 11.3 3.7

4)"

q,.mN/m

23°C 12.0 12.9 11.8 18.7

23°C 34.1 33.2 33.1 33.1 40.1

48.1

181.0 11.45 3.4 23.4 8.0 2.38 11.7 12.2 29.9 5.28 12.7 11.0 P500N 89.8 10.99 14.1 14.2 PBSN 379.3 29.10 16.4 12.9 p : density, v : kinematic viscosity, u : pressure-viscosity coefficient 4 : contact angle, uL : surface tension, surfactant : polyoxyethylene

WON P 150N

1.2549 0.8552 0.8620 0.87 16 0.8772

plateau region bounded by the horseshoe shaped constriction and the wake at the outlet region. When we compare the theoretical film thickness and the experimental film thickness, it is based on the lubrication regime. It is common practice for identification of the lubrication regime of point contacts to use Hamrock-Dowson diagram [6] in which the regimes are divided as follows: isoviscous-rigid IR, piezo-viscous-rigid PR, isoviscous-elastic IE, a n d piezoviscous-elastic PE. The authors [3] previously pointed out that the most suited representation for estimated of the solidified film thickness at high pressure EHL contacts is concluded to use the liquid/solid transition lubrication diagram, where the product of pressure-viscosity coefficient a and mean Hertzian pressure p as the abscissa and dimensionless parameter Gull4 as the ordinate are used. It is composed by combining Greenwood diagram [21, Hamrock-Dowson diagram a n d liquid / solid transition condition given by up. i.e., for liquid txp < 13, for viscoelastic solid 13 < a p < 25, and for elastic-plastic

31.4 27.0 28.3 32.2 31.2

p = 0.45 GPa

U= 3 . 0 ~ 1 0 - l O

a p = 8.5

G u l l 4 = 8.3 h, = 1.3 pm

0.5 mm

Fig. 2 EHL pattern in rolling cotact (paraffinic mineral oil PBSN)

__ - 0cmnellla

rn olive oil

011

1-1 castor oil +glycerol

0 glycerol

v )I

+surlaclanl 5 P500N BS

I

I

I I

lVEl EP I

1 oo

I I

,

I

10' a P

Fig. 3 Experimental points in liquid/solid transition lubrication diagram

246 solid 25 < t r p [71. The author’s experimental ranges for circular contact k = 1 are plotted in Fig. 3. In our experimental range. the state of the lubricants is liquid and the lubrication regime is PE, therefore, the central film thickness calculated by the formula of Hamrock-Dowson for rlastohydrodynamic lubrication. Fig. 4 shows central film thicknesses obtained under conditions of pure rolling with a load of 86 N for the case of k=3.6, where pure rolling was obtained by removing the drive shaft from the barrel shaped roller and allowing the barrel shaped roller to be driven by the disk. For paraffinic mineral oil PBSN the measured film thickness is 1 . 1 times of the predicted value. The measured film thickness for castor is 0.9 times of the predicted value. In particular. for glycrrol that is main component of vegetable oil it is one half of the predicted value. So far with regard to the measured film thickness lower t h a n predicted value Wedeven. Evans a n d Cameron I81 a n d Chiu 191 observed the effects of oil starvation on film thickness. Hlrata a n d Cameron [lo]reported the viscosity loss of the polymer thickened oils. In this study experiments are done under fully flooded condition, we couldn’t observe the oil starvation. and also the tested lubricants contain no polymer. Therefore, we investigated the effects of interfacial phenomena on the film thickness. The contact angle was measured on the glass disk with a n aid of goniometer. Table 1 show the values of the contact a n gles. The cmntact angle of glycerol is so much higher. Therefore, it h a s poor wettability. Fig. 5 shows the relation between the contact angle (I, and the nondimensional measured film thickness h,/h, i.e.. the ratio

of the predicted central film thickness h to the measured central film thickness h., The nondimensional measured film thickness h,/h decreases with a n increase the contact angle @. This fact w a s considered to t x attributed to their poor wettability a t the liquid/ solid interface. Furthermore, the largest contact angle of glycerol was lowered by adding surfactant a n d brought about a n increase in film thickness. It. is suggested that 1.he interfacial phenomena near the inlet side of the EHL contact region effects the EHL central film thickness, in particular, in the case of castor oil and glycerol. Moreover, it was supported that the recovering time t of the wake after the sudden stop decreases with a decrease in the contact angle @. Fig. 6 shows the relation between nondimensional measured cen-

100; Y

Jz i(--lLLaL-LL-,-,.-&I lo-” 1o-’O 1J Fig. 4 Central film thickness in rolling con-

tact ( p = 0.26 GPa)

0

20

40 contact angel (P , deg

Fig. 5 Effect of contact angle on central film thickness (rolling contact)

247

tral film thickness h,/h and non-dimensional number for interfacial phenomena qa/uLt, where CTL is surface tension listed in Table 1. a is the semi-width of the contact ellipse In perpendicular to the direction of motion and t is recovering time of wake. I t is suggested that the behavior of the inlet region under the EHL condition is similar to the behavior of the outlet region. Next, we attempted to measure the entrapment of oil between normally approaching elastic bodies to verify the effects of the interfacial phenomena on the EHL film thickness. A schematic diagram of the experimental apparatus I1 11 is shown in Fig.7. An optical flat of pyrex glass is clamped to the upper part. On the other hand, a bearing steel ball is attached to the upper end of a spindle supported by an air bearing. The impact load is applied by releasing a n electromagnet at the end of a lever. The steel ball with a static load 86 N was separated from the glass disk with a clearance of 0.06 mm. Fig. 8 shows the changes of in'terference fringe patterns with the elapse of time for castor oil. The authors previously pointed out that under the condition O than 25, no leakage showing O L ~ greater occurs corresponding to the elastic-plastic solidification, where po is the maximum Hertzian pressure. When ap0 is lower than 13. leakage occurs within a short period. Under the intermediate condition 13 c up0 c 25. sealing effect is incomplete, but according to the value of clpo it maintains for a considerably long period [ 121. In this study, apo of tested oils is in the range of 3.7 to 12.5, leakage occurred within a short period. So we recorded the sequence of fringe patterns in a VTR and decided the maximum entrapped film thick-

'

" ; 3

'

'

""'

I

lo-*

r]

a/oLt

Fig. 6 Relation between central Alm thickness and non-dimensional number for interfacial phenomena

Tesl bdl.

Hcater

Fig. 7 Schematic diagram of impact test apparatus

248

ness hem,, of each oil. Fig. 9 plots hem, determined as such for all the 9 tested oils as a function of the lubricant parameter [131 aqo. The cross points in this figure rearranges for the lubricant parameter avo at the previous results [ 111. It shows that the maximum entrapped film thickness hem, of the mineral oil series, rape seed oil, camellia oil and olive oil tends to increase with increasing the lubricant parameter aqo, a single relation is obtained irrespective of oils. However, the experimental points of castor oil and glycerol with poor wettability show some deviation from the well-established relation in the case of the mineral oil series. Furthermore. the surfactant containing glycerol brought about an increase in the film thickness. Therefore, using the entrapment of oil between normally approaching elastic bodies, it was ascertained the interfacial phenomena's effects on the EHL central f€lmthickness.

increasing GU114, while in the range of lower G u l l 4 value the traction coefficient reach constant high level papma and to increase with increasing mean Hertzian pressure [41. A number of papma for each test oil at mean Hertzian pressure 0.26 GPa, 0.45 GPa, 0.52 GPa and 0.90 GPa are shown in Fig. 12. The results show the traction coefficients of vegerape seed oil carnellja oil olive oil castor oil

5I r

Kiirvniici. Olitio a i d Iiiieiio 11 I I

I 4

t

++'

t

''"'"1

0

V

0

tb

4

o

1oo '

' ' gl.l**l

Q

3.2 Traction Measurements The authors previously pointed out that the most suited representation to estimate the traction characteristics is concluded to use the liquid / solid transition lubrication diagram 141. The author's experimental ranges of elliptical contact k = 3.6 at mean Hertzian pressure p = 0.26 GPa are plotted in Fig. 10. From this flgure, it is recognized that the lubrication regime is piezoviscous elastic (PE). The absolute values of the traction coefflcient were measured varying slip ratios in positive and negative directions. Fig. 11 plots traction coefficients of castor oil as a function of G u l l 4 under mean Hertzian pressure 0.26 GPa (k = 3.6). 0.45 GPa (k= 1). 0.52 GPa (k = 3.6) and 0.90 GPa (k = 1). I t shows that traction coefficient under the constant pressure tends to decrease with

i++l

'"'"'1

v0,

'

'

1o2

10-'~

Fig. 9 Relation between entrapped Alm thickness and lubricant parameter

1oo

CrP

10'

Fig. 10 Experimental points in liquid/solid transition lubrication diagram

249 table oils with lower pressure-viscosity coefficients a shown in Table 1 is lower than the mineral oil series of equal viscosity grades at each mean Hertzian pressure. Furthermore, there is a linearly increasing at each relation between the papma mean Hertzian pressure and the pressureviscosity coefficient cx of oils. Then, the traction coefficients of glycerol and surfactant containing glycerol is shown about the same values at each mean Hertzian pressure. The effect of wettability on the traction coefficient was unrecognized. Fig. 13 shows the maximum traction coefficients papma of each oils and the product of the pressure-viscosity coef€icient a and mean Hertzian pressure p. It shows the /+mm tends to rise with increasing ap, where a p is a measure of free volume of lubricant [4,5,7I. niI

1

I

I

.

I

rape seed oil 0 camellia oil I 0.1 - olive oil - 0 castor oil -

'

I

'

I

-

z

glycerol

I ! 0.05

0

v

glycerol

x

+surfacrunt 5%

'P150N P500N PBSN

'

v

*d

-In

700

'

a p

x

0

Ova

v

no

10'

Fig. 13 Effect of a p on traction coefficient Vapmax up'3.6

1o2

10'

c;

u

1'4

Fig. 11 Effects of a p and G u l l 4 on traction coefficient p

4. coNcLu8ION8

Summarizing the experimental results, the following conclusion are drawn concerning the behavior of some vegetable oils in EHL contacts. (1) The central fllm thickness of castor oil and glycerol with poor wettability under rolling condition is 0.5-0.9 times thin-

250 ner compared with the predicted values. (2)The largest contact angle of glycerol is lowered by adding surfactant and brings about an increase in central film thickness. (3)Furthermore, using the entrapment of oil between normally approaching elastic bodies. it is ascertained. (4) These facts show the interfacial phenomena near the inlet side of the EHL contact region affects the EHL central film thickness. (5)In the full EHL regime, the traction coefficients of vegetable oils are lower than the paraffinic mineral oils with equal viscosity grades. I t is caused by lower pressure-viscosity coefficients of vegetable oils. Then, the effect of wettability on the traction coefficient is unrecognized. (6)The maximum traction coefficient under the constant txp tends to rise with increasing t r p both the vegetable oils a n d the paraffinic mineral oils. 5. ACKNOWLEDGMENTS

The a u t h o r s wish to express their thanks Mr. Y. Nakahara, Saga University, for his efforts in preparing the experimental apparatuses: and also to Messrs. T. lsobe and M. Funaguti for their cooperation. The paraffinic minerals and the s u r factant were supplied by Idemitsu Kosan Co., Ltd. REFERENCES 1. Product review, Biodegradable fluids a n d lubricants, Industrial Lubrication a n d Trlbology, 4 8 , 2 ( 1996) 17-26. 2. J.A. Greenwood, Film thickness in circular elastohydrodynamic contacts, Proc. Instn. Mech. Engrs., 202, C1 (1988) 1 1 17. 3. N. Ohno, N. Kuwano and F. Hirano. Di-

agrams for Estimation of the Solidified Film Thickness a t High Pressure EHD Contacts, Proceedings of the 20th LeedsLyon Symposium on Tribology, Lyon. September 1993:Disslpatiue Processes in Trfbology, Elsevier Science B. V., ( 19941 507-518. 4.N. Ohno, N. Kuwano a n d F. Hirano, Traction Control by Considering Free Volume of Lubricating oil, Proceedings of the International Tribology Conference, Yokohamo, ( 1995). 5. N. Ohno, N . Kuwano and F. Hirano, Effect of Bulk Modulus of Solidified Oils Under High Pressure on Tractional Behavior. J a p u n e s e Journal of Tribology 38,10 (1993)1361-1372. 6. B.J. Hamrock and D. Dowson. Ball Bear ing Lubrication, J o h n Wiley & S o n s 1986). 7. N. Ohno, N. Hattori, N. Kuwano a n d F Hirano, Some observations on the Relations hip between Rheological Properties of Lubricants a t High Pressure and Regimes of Traction (Part 11 The Rheological Properties of Lubricants a t High Pressure, J.JSLE, 33. 12 (19881922-928. 8.L.D. Wedeven, D. E v a n s a n d A. Cameron, Optical analysis of ball bearing starvation, Trans. ASME, JOLT, 93 ( 197 1)349-363. 9.Y.P. Chiu, An analysis a n d prediction of lubricant film starvation in rolling contact systems, ASLE Trans., 17 ( 1974)22-35. 10. M. Hirata a n d A. Cameron. The LJse of Optical Elastohydrodynamics to Investigate Viscosity Loss in Polymer-thickened Oils, ASLE Trans. 2 7 . 2 (19841 114-121. 11. N. Kuwano, N. Ohno and F. Hirano, lnvestigation on Entrapment of Mineral Oils under Impact. J. JSLE. 31.7 [ 1986) 477-484.

25 1 12.F. Hirano, N. Kuwano and N. Ohno, Fundamental Study of Static Sealing Characteristics of Solidified Oils at High Pressure. in Fluid Sealing, Nau, B. S.. ed.. Kluwer Academic Publishers, Dordrecht, The Netherlands (1992) 109-120. 13. Mobil EHL Guidebook, Mobil Oil Corporation, New York ( 1979)

Notation a semi-width of contact ellipse in y direction b semi-width of contact ellipse in x direction E effective elastic modulus / [2/ E'=(1- v I ~E) 1/+( 1- ~ 2 2 )E21 G material parameter, a E' h film thickness he entrapped film thickness k ellipticity parameter, a / h P mean Hertzian pressure

po maximum Hertzian pressure R, radius of curvature in x direction t recovering time of wake U speed parameter, qou/E'R, u mean entrainment velocity ( x pressure-viscosity coefficient 4 contact angle 710 absolute viscosity at atmospheric pressure p traction coefficient Y kinematic viscosity p lubricant density q, surface tension IE IR PE PR

isoviscous-elastic isoviscous-rigid piezoviscous-elastic, EHL piezoviscous-rigid

L liquid VE viscoelastic solid EP elastic-plastic solid