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T H E ROLE OF SCOSITY IN LUBRICATION PROCEEDINGS OF AN ASME SYMPOSIUM H E L D MARCH 10-11, 1958 SPONSORED BY THE LUBRICATION DIVISION OF T H E AMERICAN 50CIETY O F MECHANICAL ENGINEERS

E D I T E D BY OSCAR C. BRIDGEMAN PHILLIPS PETROLEUM COMPANY BARTLESVILL E, OKLAHOMA

THE 2 9

A M E R I C A N W E S T

3 9 t h

SOCIETY S T R E E T ,

O F

MECHANICAL

N E W

Y O R K

18,

ENGINEERS N E W

Y O R K

CONTENTS l ntroductory

3

Remarks

I. VISCOSITY CHARRCTERISTICS' OF LUBRICANTS Effect o f Temperature on Viscosity H. H. Zuidema Effect o f Pressure on Viscosity E. M. Barber Effect of Rate of Shear on Viscosity Alan Beerbower

XI. SENSITIVITY TO "ISCOSIITY OPERATING CONDITIONS

UNDER

Introductory Remarks Paul C. Warner

24

The Sensitivity of Equipment to Variation i n Lubricant Viscosity R. C. Garretson and J. Boyd

25

Sensitivity of Machines to Lubricant Viscosity Charles A. Bailey

33

Lubrication o f Roll Neck Bearings and Gear Drives i n Continuous Rolling M i l l s J. H. Hitchcock Effect o f Viscosity on Hydraulic Systems K. G. Henrikson

III. DESIGN CRITERZA FOR SELECTING VISCOSITY Viscosity and Related Problems i n Engine Design W. M. Kauffmann

53

Selecting Lubricant Viscosity for Design o f Helical and Worm Gears F. A. Thoma

61

Viscosity i n the Lubrication Mechanisms of RollingElement 8earings L. B. Sibley and J. C. B e l l

64

I V . RESEARCH C O ~ R I B U T I O N STO APPLIED LUBRICRTION Gear Lubrication and Viscosity E. E. Shipley Recent Research and Development Work in Rolling Bearings H. Hanau

85

The Effect of Temperature and Pressure on Viscosity as Related to Hydrodynamic Lubrication E. Saibel

105

Author Index

108 Copyright 1960 by The Amerlcan Society of Mechanical Engineers Printed In the United States of America

INTRODUCTORY REMARKS By Oscar

C. Bridgeman, Phillips Petroleum Co.

. Ihe tour Technical Committees of the A S M E Lubrication Division are the Lubricants Technical Committee, Builders & Operators Technic]a1 Committee, Design Technical Comnit;tee and Research Technical Committee. These cornmitItees organized and presented *I.C ..-- - - - -. L I I C ~ y r n p u s l mon the Role of Viscosity in Lubrication at the Socony hlobil Oil Company Training Center, New York, N.Y., on March 10 and 11, -1958. The papers presented covered both theory and practical applications of this subject and were followed by an active discussion. The presentations and di scussions were later revised and submitted for publication early in 1960. It is appropriate to include in this -n+-"duction a brief statement about viscosity per s e -- what it is and what it means Viscosity of a fluid is defined r--...lurlllsrlly as the shearing stress divided by the rate of shear. Further, Newton's law states that this ratio is constant, or in *her - _ - words that the viscosity is indepbnt of the rate of shear. Liquids obeying this law are frequently called Newtonian fluids Measurement of viscosity therefore requires means for evaluating shearing stress and rate of shear, or their equivalents. Many methods are available, such as use of a capillary tube, a rotational viscometer, a falling or rolling ball viscometer, and Q h nn. It will be found generally that it is necessary to apply corrections to the observed results in order to account for extraneous hydrodynamic disturbances. In *,.".,. such cases, there is lack of general agreement on the precise validity of these corrections, and sometimes it almost appears 1that Newton's law is being accepted by definjition, as a basis for the corrections. Further, in many cases, the magnitude of L1cne correctj.ons changes with rate of shear, making it possible to extrapolate back to an infinitely low rate of shear. Most of t h e available data on viscosity of fluids es to very low shear rates. is rare in practice to make viscosity cements directly in terms of the prim\ 1 variables, due to the uncertainties in &..ULV1

I.

C '\

--

-.a

IIJCUJ~

,,

.

the corrections for the hydrodynamic disturbances. Instead, most viscosity values are relative to the viscosity of some working standard, and the generally accepted standard is the value for water at 68OF and normal atmospheric pressure. Thus water is used to calibrate a suitable viscometer, i which is then used for measurement of a higher viscosity fluid. In turn, this lattf er fluid is usedto calibrate another viscometer, and so on up the scale. While this procedure does not eliminate corrections for hydrodynamic disturbances, it tends to make the magnitude of these corrections smaller. On the other hand, the errors are cumulative through this step-wise procedure and hence there may be considerable lack of precision in high viscosity values. One other point should be mentioned in connection with the viscosity of water. For years, the accepted value was 1.007 centistokes at 68OF. About two years ago, this value was changed to 1.0038 cs at 68OF, to bring it into accord with the measurements of the National Bureau of Standards. Thus, in terms of the new working standard, all viscosity values are now about 0.3% lower for the same identica1,fluids.This change may not be of much consequence in many applications, but it is important to keep it in mind when comparing literature values for pure liquids. Insofar as viscosity is confined to infinitely low shear rates, it can be considered as a property of the given fluid, as long as there is no change in composition of the fluid. In other words, if the symbol p is used to designate viscosity, this means that d p is a perfect differential, with all that this connotes. One important consequence is that the rate of change of the temperature coefficient with pressure must equal the rate of change of the pressure coefficient with temperature. Whether viscosity can continue to be considered a property of the fluid as shear rates increqse is possibly a metaphysical question. From one standpoint, the answer to this question involves such matters as whether Newton's law holds at these higher shear

r a t e s , and whether a d e q u a t e a c c o u n t h a s b e e n t a k e n o f hydrodynamic d i s t u r b a n c e s . C e r t a i n l y , t h e r e a r e some f l u i d s which show a temporary d r o p i n v i s c o s i t y t o an e x t e n t d e p a d e n t upon t h e s h e a r r a t e . A l s o , a t v e r y h i g h r a t e s o f s h e a r , i t seems p r o b a b l e t h a t many non-polymer l i q u i d s d e p a r t from Newton's law d u e t o c h a n g e s i n m o l e c u l a r o r i e n t a t i o n . A r e l a t e d phenomenon i s t h e m e a s u r e a b l e r e l a x a t i o n time, n a m e l y t h e t i m e for a liquid t o return t o its n o ~ m a l m o l e c u l a r o r i e n t a t i o n a £t e r b e i n g s u b j e c t e d t o a high r a t e o f p r e s s u r e a p p l i c a t i o n . C l a s s i c a l hydrodynamics d e a l s w i t h r a m i n a r flow, namely up t o Reynolds numbers o f a b o u t 2000. I n t h i s r e g i m e , p r e s s u r e d r o p i s p r o p o r t i o r l a l t o t h e f i r s t power o f t h e v i s c o s i t y . At Reynolds numbers above about 4000, t u r b u l e n t flow e x i s t s , and u n d e r t h e s e conditions t h e pressure drop is e s s e n t i a l l y proportional t o the one-quarter power o f t h e v i s c o s i t y . I n t h e i n t e r m e d i a t e r a n g e from 2000 t o 4000 R e y n o l d s number, t h e e f f e c t o f v i s c o s i t y v a r i e s i n unknown manner from t h e 1st t o t h e % power. I n c o n c l u s i o n , v i s c o s i t y i s a v e r y comp l e x c h a r a c t e r i s t i c o f a f l u i d . As u s e d i n

hydrodynamic s t u d i e s , i t i s a v a l u e a t - 9 a p p r o p r i a t e t e m p e r a t u r e and p r e s s u r e , r.,-. a t i v e t o a f i x e d v a l u e f o r w a t e r , b u t appl i c a b l e t o a low r a t e o f s h e a r . The e x t e n t t o which t h i s v a l u e i s a p p l i c a b l e t o h i g h r a t e s o f s h e a r may b e u n c e r t a i n . D e f i n i t e l y i t is apparent t h a t design t r e n d s towards h i g h e r t e m p e r a t u r e s , p r e s s u r e s , and Reyno l d s numbers, are f o r c i n g a t t e n t i o n t o t h e i n c r e a s i n g importance o f v i s c o s i t y e f f e c t s . T r e n d s i n new t y. p e s o f l u b r i c a n t s a r e a l s o c o m p l i c a t i n g t h e p i c t u r e . Hence, i t seems e v i d e n t t h a t w e may b e o u t - r u n n i n g o u r knowledge on v i s c o s i t y b o t h i n t h e o r y and i n service application. In years p a s t each o f t h e four Technical Committees mentioned above normally h e l d i n d i v i d u a l meetings covering t h a t s u b j e c t m a t t e r i n which e a c h was i n t e r e s t e d . On t h i s o c c a s i o n a l l f o u r g r o u p s met c o n s e c u t i v e l y t o discuss t h e broad s u b j e c t of v i s c o s i t y i n l u b r i c a t i o n from t h e i r r e s p e c t i v e viewpoints. This resulted in t h e Symposium which f o l l o w s , a n d i t i s hoped t h a t t h i s w i l l be a n o t h e r s t e p towards adva n c i n g o u r t e c h n i c a l p r o g r e s s , which i s t h e goal of the A S M E Lubrication Division. -

SESSION CHAIRMAN - A . R . B l a c k , Shell O i l Co. Chairman, Lubricants Technical Committee

V I S C O S I T Y CHARACTERISTICS OF ZUBRICANTS E F F E C T O F TEMPERATURE ON V I S C O S I T Y

H . H . Z u i d e m a , S h e l l O i l Co. E F F E C T OF P R E S S U R E ON V I S C O S I T Y

E . M . B a r b e r , T h e T e x a s Co. E F F E C T OF RATE O F SHEAR ON V I S C O S I T Y

Alan Beerbower, Esso Research & Engineering C o

EFFECT OF TEMPERATURE ON vIscasIn By 11.H. Zuidema, S h e l l O i l Co. V i s c o s i t y i s a measure o f t h e r e s i s t a n c e o f a f l u i d t o flow. \#en t h e temperature o f a l i q u i d i s changed, t h e d i s t a n c e between molecules changes, and t h i s i n t u r n a f f e c t s t h e v i s c o s i t y . L i q u i d s w i t h low c o e f f i c i e n t s o f expansion w i l l i n general have lower viscosity-temperature c o e f f i c i e n t s than t h o s e which have high c o e f f i c i e n t s o f expansion. Faust proposed a theory i n 1914 t h a t t h e v i s c o s i t y o f a given l i q u i d i s a function o f d e n s i t y a l o n e , i r r e g a r d l e s s o f tempera t u r e and p r e s s u r e . B r i d p a n l a t e r proved t h a t t h i s theory is an o v e r - s i m p l i f i c a t i o n . He made v i s c o s i t y measurements on a number o f l i q u i d s o v e r a range o f temperatures and p r e s s u r e s , and found t h a t change i n d e n s i t y does not account f o r a l l o f t h e e f f e c t o f t e m p e r a t u r e on v i s c o s i t y , a l t h o u g h i t i s responsible for a substantial p a r t of the total effect. The magnitude o f t h e e f f e c t o f tempera t u r e upon v i s c o s i t y i s v e r y s t r i k i n g . For example, an c i l w i t h a v i s c o s i t y o f 10 c s a t 210°F and a v i s c o s i t y iildex o f 100 w i l l have a v i s c o s i t y o f approximately 10,000 c s a t O°F. T h u s , a change i n t e m p e r a t u r e o f o n l y 210°F caused a tllousand-fold change i n v i s c o s i t y . Many l u b r i c a t i n g o i l s a r e c a l l e d upon t o f u n c t i o n o v e r a much wider tempera t u r e range than this. Furthermore, i f t h e o i l had been one o f z e r o v i s c o s i t y i n d e x r a t h e r t h a n o f one hundred, t h e change i n v i s c o s i t y would h a v e been by a f a c t o r o f tea thousand i n s t e a d o f one thousand. Even m u l t i g r a d e 10 W-30 motor o i l s , which a r e unique i n t h a t t h e y show a minimum v a r i a t i o n i n v i s c o s i t y with temperature, s u f f e r a change i n v i s c o s i t y o f a p p r o x i m a t e l y two

"--*

3

h u n d r e d - f o l d o v e r t h i s same t e m p e r a t u r e range. S i n c e tlie e f f e c t o f temperature on visc o s i t y i s so pronounced, i t i s necessary t o c o n t r o l t e m p e r a t u r e v e r y c l o s e l y , and t o measure i t a c c u r a t e l y , i n o r d e r t o a t t a i n good a c c u r a c y a n d p r e c i s i o n i n v i s c o s i t y d e t e r m i n a t i o n s . The ASTM r e q u i r e s a temp e r a t u r e c o n t r o l o f fO.OS°F i n Method D-88 f o r S a y b o l t v i s c o s i t y . The same t o l e r a n c e i s r e q u i r e d i n Method D-445 f o r k i n e m a t i c v i s c o s i t y a t t e m p e r a t u r e s below 60°F. At temperatures higher than t h i s , t h e to1erance i s f0.02°F. The m a t h e m a t i c a l r e l a t i o n s l i i p between v i s c o s i t y and t e m p e r a t u r e i s complex, and many e q u a t i o n s h a v e been d e v e l o p e d , b o t h t h e o r e t i c a l and e m p i r i c a l . Perhaps t h e b e s t known and most widely used o f t h e s e r e l a t i o n s , a t l e a s t i n t h i s country, i s t h e e p i r i c a l W a l t h e r e q u a t i o n which was f i r s t p u b l i s h e d i n 1929. I t may be w r i t t e n a s follows:

l o g l o g [V

+ kl

=A

+D

log T

where V i s t h e k i n e m a t i c v i s c o s i t y i n centistokes, T is the absolute t m p e r a t i ~ r e , k i s a ' u n i v e r s a l c o n s t a n t ' , and A and B a r e c o n s t a n t s f o r a given o i l . A valae of 0 . 6 i s g e n e r a l l y assigned t o k , although 0 . 8 h a s a l s o been used. For h i g h v a l u e s o f V, t h e e x a c t v a l u e o f k i s o f l i t t l e importance, but a s V decreases, the significance o f k i n c r e a s e s . The Walther e q u a t i o n h a s been found t o f i t t h e d a t a f o r mineral o i l s q u i t e well. I t i s t h e b a s i s f o r t h e ASTM v i s c o s i t y temperature c h a r t s , Method D-341. The f o l l owing f i v e c h a r t s a r e a v a i l a b l e .

CHART

VISCOSITY UNITS

V ~ S C O S IRANGE ~

TEMP. IMNGE

SIZE OF CHART

A

Saybolt Universal

33 t o 100,000,000

-30 t o +450°F

20 x 16 i n .

B

Saybolt Universal

33 t o 100,000

-30 t o +350°F

8%

C

Centistokes

2 t o 20,000,000

-30 t o +450°F

D E

Centistokes

0 . 4 t o 100

-30 t.o +450GF

20 x 16 i n . 20 x 20 i n .

Centistolies

2 t o 20,000,000

-100 t o +450°F

x 11 i n .

24 x 16 i n .

Ci

'These c h a r t s a r e c o n s t r u c t e d i n s u c h a A e r t h a t t h e d a t a f o r a given o i l can b e p l o t t e d l i n e a r l y . Tne o n l y s e r i o u s d e v i a t i o n normally encountered i n t h e c a s e o f mineral o i l s o r o t h e r petroleum f r a c t i o n s i s i n t h e c a s e o f non-homogeneous systems, a s f o r example a waxy o i l t h a t h a s b e e n c h i l l e d t o a t e m p e r a t u r e below i t s c l o u d p o i n t . High-temperature v i s c o s i t y d a t a f o r s u c h an o i l w i l l p l o t l i n e a r l y , b u t t h e curve w i l l bend upward a t low t e m p e r a t u r e s . Data f o r c e r t a i n s y n t h e t i c o i l s , on t h e o t h e r hand, show a d i s t i n c t c u r v a t u r e o v e r a wide t e m p e r a t u r e r a n g e . T h i s p o i n t i s i l l u s t r a t e d i n F i g . 1 which shows v i s c o s i t y temperature c u r v e s f o r two m i n e r a l o i l s and t h r e e s y n t l l e t i c l u b r i c a n t s , a l l p l o t t e d on a s i m p l i f i e d ASTM c h a r t . B o t h m i n e r a l o i l s a n d two o f t h e s y n t h e t i c s p r o d u c e d l i n e a r p l o t s although the slopes varied widely. However, t h e f i f t h l i n e r e p r e s e n t ing t h e p o l y a l k y l e n e g l y c o l shows a d e c i d e d curvatt~re. The method that. h a s b e e n u s e d t h e most

' .-

widely i n t h i s country f o r expressing t h e v i s c o s i t y - t e m p e r a t u r e e f f e c t i s t h e viscosi t y i n d e x , w h i c h was f i r s t p u b l i s h e d by Dean and Davis i n 1929, and which h a s been an ASTM method (D-567) s i n c e 1940. It i s based on two s e r i e s o f r e f e r e n c e o i l s . ?he f i r s t , a r b i t r a r i l y assigned v i s c o s i t y index v a l u e s o f 100, c o n s i s t s o f a s e r i e s o f f r a c t i o n s from a P e n n s y l v a n i a c r u d e . T h e second is s i m i l a r , except t h a t t h e s o u r c e was a G u l f C o a s t a l c r u d e , and t h e v a l u e o f v i s c o s i t y i n d e x a s s i g n e d was z e r o . The v i s c o s i t y i n d e x o f a n o i l i s c a l c u l a t e d from t h e equation:

v.

I. = 100 [L

- ul

/ [L

where U i s t h e v i s c o s i t y a t 100°F o f t h e o i l i n q u e s t i o n , and where L and H a r e t h e r e s p e c t i v e v i s c o s i t i e s a t 100°F o f t h e 0 V I and 100 V I s t a n d a r d s which match t h e unknown o i l i n v i s c o s i t y a t 210°F. V a l u e s o f L a n d o f (L - H) a r e c o n v e n i e n t l y r e a d from t a b l e s p u b l i s h e d a s a p a r t o f ASTM Method D-567.

FIG. 1

EFFECT O F TEMPERATURE ON V I S C O S I T Y

1 . MINERAL O I L 2 . MINERAL O I L

-

1 0 0 VI 0 VI

3 . SILICONE 4 . FLUOROCARBON 5.

POLYALKYLENE GLYCOL

- HI

4

i s i l l u s t r a t e d i n Fig. 2. V i s c o s i t y i n d e x can be r a i s e d by L/ i n c o r p o r a t i o n o f c e r t a i n polymeric additi v e s , commonly known a s V I i m p r o v e r s . These polymers r a i s e t h e v i s c o s i t y throughout t h e temperature range, but t h e i r r e l a t i v e e f f e c t i s g r e a t e r a t t h e higher tempera t u r e s . Thus t h e y f l a t t e n t h e v i s c o s i t y temperature curve and r a i s e t h e v i s c o s i t y index. 'The e f f e c t of t h r e e d i f f e r e n t polymers upon t h e v i s c o s i t y i n d e x o f t h r e e d i f f e r e n t base o i l s i s shown i n Fig. 3. I t w i l l be observed t h a t t h e degree of improvement i n v i s c o s i t y index a t t a i n a h l e with a given p e r c e n t a g e o f polymer depends upon t h e c h o i c e o f b a s e o i l a s well a s t h e c h o i c e of polymer. Large q u a n t i t i e s o f V I improvers a r e used i n t h e manufacture o f multigrade motor o i l s which have become s o p o p u l a r d u r i n g t h e l a s t few years.

'The v i s c o s i t y index of a mineral o i l i s d e t e r m i n e d p r i m a r i l y by i t s hydrocarbon composition. 'Ihe hydrocarbons i n t h e lubr i c a t i n g o i l range o f molecular weight a r e very complex, and t h e i r exact configuration i s not known. However, they may be considered as containing three basic types of h y d r o c a r b o n g r o u p s , namely p a r a f f i n i c , naphthenic, and aromatic. A l l mineral o i l s contain a l l t h r e e of these b a s i c types, but t h e p r o p o r t i o n s vary c o n s i d e r a b l y . High v i s c o s i t y index i s a s s o c i a t e d with a high content of p a r a f f i n i c groups. The pres,ence o f aromatics lowers the v i s c o s i t y index, a s does t h e presence o f naphthenes. I t i s p o s s i h l e t o remove a r o m a t i c s by normal s o l v e n t e x t r a c t i o n p r o c e s s e s , b u t these processes do not e f f e c t i v e l y s e p a r a t e naphthenes from p a r a f f i n s . Thus o i l s from d i f f e r e n t sources d i f f e r not only i n t h e i r v i s c o s i t y index p r i o r t o r e f i n i n g , but they d i f f e r i n t h e e x t e n t of improvement t h a t can be accomplished by r e f i n i n g . ?his p o i n t

EFFECT OF SOLVENT EXTRACTION ON V I (VON FUCHS AND ANDERSON)

I

1

1

I

PENN . R E S I D U E MID-CONTINENT

-

RESIDUE

GULF COASTAL D I S T I L L A T E

FIG.

r

I 2

I

I

4. 6 STAGES OF EXTRACTION

2 I 8

-

The v i s c o s i t y index system h a s s e r v e d a y u s e f u l purpose. However, i t h a s c e r t I n d e f i c i e n c i e s , p a r t i c u l a r l y when a p p l i e d t o o i l s o f h i g h v i s c o s i t y index. Two o i l s , f o r example, b o t h o f 50 c s v i s c o s i t y a t 100°F, b u t having v i s c o s i t i e s o f 10 and 45 c s a t 210°F, would have a v i s c o s i t y i n d e x o f approximately.150. Gross anomalies o f t h i s type do n o t o c c u r i n t h e range o f V I from zero t o one hundred f o r which t h e system was developed, b u t they do c o n s t i t u t e a s e r i o u s p r o b l e m i n some o f t h e c u r r e n t o i l s . Another d i s a d v a n t a g e o f t h e system, throughout t h e v i s c o s i t y index range, is t h a t t h e v i s c o s i t y index o f a b l e n d i s n o t a simple f u n c t i o n o f composition. Tne ASTM has recognized t h e need f o r a b e t t e r v i s c o s i t y - t e m p e r a t u r e system and a cornnittee, j o i n t l y s p o n s o r e d by T e c h n i c a l

Q

Committee B on L u b r i c a t i n g O i l s and Rese a r c h D i v i s i o n V I I on Flow P r o p e r t i e s , b o t h under ASTM Comnittee %2, i s working on t h e problem. The f o l l o w i n g methods a r e b e i n g considered by t h i s comni t t e e . 1. S l o p e o f t h e ASTM v i s c o s i t y t e m p e r a t u r e curve. 2. The V i s c o s i t y - T e m p e r a t u r e I n d e x (Larson and Schwaderer) . 3. The R a t i o n a l V i s c o s i t y Index (Hardiman and Nissen) . 4 . T h e V i s c o s i t y Modulus ( B l o t t a n d Verner) . 5 . The Fundamental V i s c o s i t y Temperatu r e Index ( C o r n e l i s s a n and Wateman). 6 . The Viscosity-Temperature R a t i n g (Ramser) . 7. The V i s c o s i ty-Temperature F u n c t i o n (Wright).

EFFECT OF POLYMER UPON V I (EVANS AND YOUNG)

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10 15 20 CONCENTRATION OF POLYMER, XWt .

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It was pointed out t h a t k i n t h e Walther equation i s only a ' u n i v e r s a l constant' i n an approximate sense. I n o t h e r words, t h e value of k underlying t h e ASTM v i s c o s i t y c h a r t s was s e l e c t e d so a s t o minimize depa r t u r e from l i n e a r i t y with t y p i c a l mineral o i l s . O r i g i n a l l y , t h e c h a r t s were based on a .value o f k = 0 . 8 , b u t t h i s was l a t e r chan.ged t o k = 0 . 6 a s experimental informa t i o n became a v a i l a b l e on a wider v a r i e t y o f petroleum p r o d u c t s . S t i l l l a t e r , & h a t p o r t i o n o f t h e c h a r t s c o v e r i n g very low v i s c o s i t i e s was modified s o a s t o provide a gradual v a r i a t i o n i n value of k. While t h e ASTM V i s c o s i t y C h a r t s a r e adequate f o r many a p p l i c a t i o n s , numerous examples e x i s t with p r e c i s e v i s c o s i t y data o v e r a range o f t e m p e r a t u r e where i t i s necessary t o s e l e c t individual values of k i n t h e Walther equation f o r each s p e c i f i c product. I n such cases, t h e Walther equat-

ion i s o f onconvenient form f o r a n a l y t i c e \ r ' treatment o f experimental d a t a , and t h e r e i s need f o r a simpler form of mathematical expression f o r t h e change o f v i s c o s i t y with temperature, even though i t may r e c p i r e use o f t h r e e a r b i t r a r y constants. The author i n d i c a t e d some o f t h e peculi a r i t i e s o f t h e p r e s e n t Dean and D a v i s system f o r v i s c o s i t y index, and mentioned t h a t t h i s m a t t e r i s c u r r e n t l y under study by ASTM . However, such s t u d i e s a r e largel y concerned with a v i s c o s i t y index system, which would s t i l l r e q u i r e c o n s i d e r a b l e manipulation before t h e a r b i t r a r y v i s c o s i t y index value could he converted t o a tempera t u r e c o e f f i c i e n t a t any given temperature o f i n t e r e s t . Again, t h i s ernpllasizes t h e need f o r an improved mathematical relationship covering the change of v i s c o s i t y with temperature.

EFFECT OF PRESSURE ON VISCOSITY By E.M.Barber,lie Texas Co. The v i s c o s i t y o f l u b r i c a n t s i n c r e a s e s markedly with i n c r e a s i n g pressure. A t t h e pressures existing i n the lubricant film o f hydrodynamic bearings, the v i s c o s i t y o f t h e - l u b r i c a n t may be many t i m e s g r e a t e r than i t s v i s c o s i t y a s measured a t atmospheric pressure. T h i s property o f l u b r i c a n t s undoubtedly has an influence on heari n g performance c h a r a c t e r i s t i c s such a s load-carrying capacity, f r i c t i o n and temperature r i s e . There i s no simple method o f measuring v i s c o s i t y a t high p r e s s u r e . A program of measurement t o d e f i n e t h e pressure-viscosity-temperature p r o p e r t i e s o f a s i n g l e l u b r i c a n t assumes t h e p r o p o r t i o n s o f a research program r a t h e r than o f a r o u t i n e physical property measurement. Consequently pressure-viscosity-temperature d a t a a r e r e l a t i v e l y scarce and the e f f e c t of pressure-viscosity p r o p e r t i e s on b e a r i n g performance i s n o t a s well understood a s may be desirable. ;'- .The falling-body type o f viscometer i s '.-iof t h e most acceptable methods f o r the measurement o f v i s c o s i t y a s a function o f pressure. This type o f equipment was used by P r o f . P . W.Bridgman ( R e f . 1 ) i n h i s pioneering work i n t h i s f i e l d , and i t has been refined and extended i n i t s usefulness i n several pressure-viscosity i n v e s t i g a t i o n s sponsored and/or s u p p o r t e d by t h e A SM E Research Committee on L u b r i c a t i o n (Kef. 2). A typical cross-section of a f a l l i n g body viscometer i s i l l u s t r a t e d by Fig. 1 . A sinker f a l l s v e r t i c a l l y i n a viscometer tube which contains the t e s t f l u i d , and t h e time o f f a l l i s i n t e r p r e t a b l e i n terms o f t h e v i s c o s i t y of t h e f l u i d . To avoid d i f f e r e n t i a l c o m p r e s s i b i l i t y and thermal e f f e c t s , the tube and t h e s i n k e r a r e made o f Symbol

t h e same m a t e r i a l . S i n k e r s o f v a r y i n g weight and o f v a r y i n g c l e a r a n c e r e l a t i v e t o t h e t u b e can be used. The v i s c o m e t e r tube is f i t t e d loosely i n t o the pressure chamber so t h a t i t i s completely surrounded by t h e pressure-transmitting f l u i d . Pressure i s transmitted t o the t e s t f l u i d i n s i d e ' t h e viscometer tube v i a a c o l l a p s i b l e bellows type o f r e s e r v o i r f i t t e d t o one end o f a the viscometer tube. The assembly i s inuners e d i n a temperature c o n t r o l bath. To make a measurement, the assembly i s inverted and the time f o r the sinker t o f a l l i s measured by e l e c t r i c a l signals. Fig. 2 i l l u s t r a t e s the variation of visc o s i t y with pressure and temperature f o r a t y p i c a l paraffin-base mineral o i l o f about 250 SSU a t 100°F and 100 Viscosity Index. Note t h e r e l a t i v e l y l a r g e changes of visc o s i t y w i t h p r e s s u r e , a s f o r example a t 210°F, a p r e s s u r e i n c r e a s e o f 20,000 p s i produces almost a t e n f o l d i n c r e a s e of viscosity. F i g . 3 shows t h e changes o f v i s c o s i t y with p r e s s u r e a t 210°F f o r t h r e e p a r a f f i n base mineral o i l s o f d i f f e r e n t v i s c o s i t y l e v e l . These t h r e e samples A, B and C have S SU v i s c o s i t i e s a t 100°F o f approximately 250, 700 and 2500 seconds, and have approxi m a t e l y 100 V i s c o s i t y I n d e x . The t h r e e c u r v e s a r e s i m i l a r i n t r e n d and g e n e r a l slope, which suggests t h a t l e v e l o f viscosi t y does not g r e a t l y a l t e r t h e t r e n d of v i s c o s i t y with p r e s s u r e f o r o i l s o f comparable molecular type. Fig. 4 shows t h e v a r i a t i o n o f v i s c o s i t y with pressure a t 210°F f o r an assortment o f mineral o i l and s y n t h e t i c type l u b r i c a t i n g f l u i d s which a r e i n t h e same general range o f l e v e l o f v i s c o s i t y a s measured by S S U a t 100°F. Some i d e n t i f i c a t i o n d a t a on t h e f l u i d s o f Fie. 4 a r e given below.

Identification SSU P a r a f f i n i c hiineral O i l Naphthenic Mineral O i l Polybutylene Fluorocarbon Silicone Oil Di ( 2 - E t h y l h e x y l ) P h t h a l a t e D i (2-Ethylhexyl) Sebacate

V.I. 96 23 30 -178 152 13 154

There i s a t e n d e n c y f o r t h e f l u i d s o f h i g h V i s c o s i t y Index, namely t h o s e f l u i d s t h a t undergo t h e l e a s t change o f v i s c o s i t y w i t h t e m p e r a t u r e , a l s o t o show t h e l e a s t change w i t h p r e s s u r e . However, t h i s i s n o t a u n i v e r s a l tendency and t h e r e a r e excepti o n s . Even f o r t h e l i m i t e d s e l e c t i o n o f samples shorn on Fig. 4, n o t e t h e exception i n s a m p l e s F and G which show p a r a l l e l changes o f v i s c o s i t y w i t h p r e s s u r e b u t a i l a r g e d i f f e r e n c e i n V i s c o s i t y Index. The e f f e c t o f t h e s e d i f f e r e n c e s i n t h e pressure-viscosity characteristics of f l u i d s on t h e performance o f b e a r i n g s i s a q u e s t i o n t h a t i s n o t e n t i r e l y resolved. ?he p r e s e n t e v i d e n c e seems t o s u g g e s t however t h a t f l u i d s having a g r e a t e r i n c r e a s e o f v i s c o s i t y with p r e s s u r e w i l l t e n d t o produ c t a somewhat h i g h e r load-carrying capac-

i t y , f r i c t i o n and o p e r a t i n g temperature. I n s t e a d o f a t t e m p t i n g t o draw c o n c l ~ ~ , i o n s from t h e f o r e g o i n g m a t e r i a l , i t seems appropriate t o suggest several areas f o r f u t u r e development t h a t c o u l d a p p r e c i a b l y improve o u r u n d e r s t a n d i n g o f t h e p r e s s u r e v i s c o s i t y problem i n l u b r i c a t i o n . (1) A simple ' r o u t i n e ' type measurement technique t h a t would make pressure-viscosi t y data readily available. ( 2 ) A c o r r e l a t i o n whereby a small number o f measurements c o u l d be used t o p r e d i c t t h e whole p a t t e r n o f a l t h r i c a n t ' s behavior under p r e s s u r e . ( 3 ) A s t u d y o f t h e e f f e c t s , on t h e lubr i c a t i o n performance o f a v a r i e t y o f beari n g s , o f d i f f e r e n c e s i n t h e pressilre-visc o s i t y characteristics of lubricants.

REFERENCES 1 . The E f f e c t o f P r e s s u r e on t h e V i s c o s i t y o f F o r t y - T h r e e P u r e L i q u i d s , by P . W . B r i d g man, P r o c . A m . A c a d . A r t s S c i . , 6 1 , 57 ( 1 9 2 6 ) . 2 . ( a ) V i s c o s i t y and D e n s i t y o f L u b r i c a t i n g F l u i d s f r o m 0 t o 1 5 0 , 0 0 0 PSIG a n d 3 2 t o 4 2 5 ' ~ , by B r a d b u r y , Mark and K l e i n s c h n i d t , ASME T r a n s . , 7 2 , 667 ( 1 9 5 1 ) . ( b ) Progress i n Lubrication Research, F o u r t h R e p o r t o f t h e S p e c i a l R e s e a r c h Comrni t t e e on L u b r i c a t i o n , Appendix N o . 1 . ( c ) E x p e r i m e n t s by R. V. K l e i n s c h m i d t on t h e V i s c o s i t y o f L u b r i c a t i n g O i l s under

H i g h H y d r o s t a t i c P r e s s u r e , ASME T r a n s . , 5 9 , l(1928). 3 . D a t a f o r F i g . 2 , 3 and 4 were s e l e c t e d from V i s c o s i t y a n d D e n s i t y o f O v e r 4 0 Lubr i c a t i n g F l u i d s o f Known C o m p o s i t i o n a t P r e s s u r e s t o 1 5 0 , 0 0 0 PSI and Temperatu t o 4 2 ~ A~ R e~p o .r t o f t h e ASME R e s e a . C o m m i t t e e on L u b r i c a t i o n , o b t a i n a b l e f r o m t h e Research Department, The American Soci e t y o f M e c h a n i c a l E n g i n e e r s , 2 9 West 3 9 t h S t r e e t , New York, N . Y.

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SUMMARIZED D I S C U S S I O N

The p o i n t was r a i s e d t h a t t h e a u t h o r presented curves f o r a number o f o i l s sliowi n g t h e change o f v i s c o s i t y w i t h p r e s s u r e , b u t he d i d n o t p r e s e n t any mathematical r e l a t i o n s a p p l i c a b l e t o t h i s e f f e c t . I t was i n d i c a t e d t h a t B r a d b u r y , Mark and Icleins r h m i d t ( l o c . c i t . ) had p r e s e n t e d a mathematical treatment of t h e A S M E pressureviscosity data, but it is necessary t o consult the original doctoral t h e s i s for t h e v a l u e s o f t h e p a r a m e t e r s f o r t h e vari o u s o i l s . R e f e r e n c e was made t o t h e f a c t t h a t no m a t h e m a t i c a l e x p r e s s i o n f o r t h e p r e s s u r e c o e f f i c i e n t o f v i s c o s i t y can pred i c t t h e f r e e z i n g p o i n t o f t h e o i l under p r e s s u r e , and many c a s e s o f l u b r i c a n t f r e e z i n g were e n c o u n t e r e d i n t h e A S M E work. However, t h i s i s no Inore o f a problem t h a n w i t h many phenomena w h e r e a p h a s e

change o c c u r s . I f v i s c o s i t y i s considered t o be a prope r t y o f a f l u i d , t h e n t h e t e m p e r a t u r e and pressure coefficients a r e inter-related. T h i s l e a d s t o t h e p o s s i b i l i t y o f developi n g a p r e s s u r e - v i s c o s i t y c h a r t analogous t o t h e A S T M temperature-viscosity c h a r t , o r even a c h a r t c o v e r i n g a l l t h r e e v a r i a b l e s . I n t h i s c o n n e c t i o n , i t was i n d i c a t e d t h a t t h e A S T M c h a r t had been found s u i t a h l e i n some c a s e s f o r r e p r o d u c i n g t h e change o f v i s c o s i t y with temperature a t elevated pressures. I n response t o q u e s t i o n s regarding t h e e f f e c t s of v i s c o s i t y i n c r e a s e s under pressu r e i n machine e l e m e n t s , t h e a u t h o r s t a t e d t h a t t h e r e i s c u r r e n t l y under d i s c u s s i o n an A Shl E proposed r e s e a r c h program aimec e v a l u a t i n g t h e magnitude o f t h e s e e f f e c t s . ,

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FIGURE 2

VISCOSITY - PRESSURE -TEMPERATURE CHARACTERISTICS OF A PARAFFIN BASE MINERAL OIL I

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PRESSURE VISCOSITY CHARACTERISTICS AT 2 1 0 ° E FOR T H R E E P A R A F F I N B A S E M I N E R A L OILS r

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PRESSURE VlSCOS I T Y CHARACTERISTICS AT 210°F. FOR A VARIETY OF LUBRICATING FLUIDS I

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E F F E C T O F RATE O F SHEAR ON V I S C O S I T Y By Alan Beerbower, Esso Research & Engineering Cb.

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In comparison w i t h t h e e f f e c t s o f tempe r a t u r e and p r e s s u r e on l u b r i c a n t v i s c o s i t y the e f f e c t s o f s h e a r i n g s t r e s s e s a r e r e l a t ively ill-defined. A great deal of data have been and a r e b e i n g o b t a i n e d i n t h i s f i e l d , b u t have n o t y e t l e d even t o empiri c a l e q u a t i o n s t h a t can be proposed t o cover t h e phenomena i n g e n e r a l . Continuing study i s important, s i n c e t h e e f f e c t s of s h e a r a r e s i g n i f i c a n t and i n some c a s e s a s i m p o r t a n t a s t h o s e o f t h e more commonly studied v a r i a b l e s . H'hi.le n a t u r a l and synthe t i c o i l s above t h e i r pour p o i n t s u s u a l l y show no e f f e c t s , g r e a s e s and polyner-blende d o i l s g i v e a v a r i e t y o f phenomena. The following discussion covers t h e s e a s they apply too l u b r i c a t i o n problems. The s c i e n c e o f Rheology, o r t h e g e n e r a l s t u d y o f t h e flow o f m a t e r i a l s , i s a speci a l i z e d o n e , and i n o r d e r t o d i s c u s s t h e subject a t a l l , i t i s necessary t o define c e r t a i n s p e c i a l terms which a p p e a r c o n s t a n t l y . A p p a r e n t V i s c o s i t y i s d e f i n e d by I