The Ch a i r m a n (Professor A ndrade) said th a t rheology was th e branch of science th a t dealt w ith physical an d stru ctu ral aspects of flow in su b stances o f all kinds. This was w ider th a n th e stu d y of viscosity. A viscous liquid was a substance which would yield under a stress, however small, if left for a sufficiently long tim e. There were substances which were quite capable of sustaining a sm all stress for a practically infinite tim e, b u t if th a t stress was increased beyond a certain value, flow occurred. I t was usual to call these substances plastic. The science of rheology dealt w ith plastic as well as viscous substances. W hereas in ordinary liquids th e flow went on perm anently a t a fixed ra te under a given stress, other substances such as m etals hardened w ith tim e when flowing under stress. These also, however, were included in th e stu d y of rheology.
In the case of certain viscous liquids th e ra te of flow, or, more stric tly speaking, th e velocity gradient, was proportional to th e shearing stress.
These were th e so-called N ew tonian liquids. W ith liquids of another class the velocity gradient did n o t v a ry linearly w ith th e shearing stress, b u t was a more com plicated function, which h ad to be determ ined. In both these cases th e ra te of flow, however, did n o t depend on th e previous h isto ry of the Substance. There were, however, other liquids which, if left alone after flow h ad ceased, changed th eir properties w ith tim e. The thixotropic solutions, in which th e structure, an d hence th e flow properties, was p a r
ticularly susceptible to m echanical tre a tm e n t, were an exam ple of this behaviour.' P articu larly com plicated was th e flow o f solids, especially metals, in which under constant stress th e ra te of flow decreased w ith tim e in a regular m anner. All these classes of flow were th e subject of rheology.
E ven among liquids, then, th ere were various complications, and he would describe oils as am ong th e difficult liquids. H e took as a te st of a simple liquid th e tem p eratu re equation
t j i;1 = Ae'llTv or its simpler form
7] = A eblT
I f a liquid obeyed this law it m eant th a t its molecular sta te of aggregation did n o t change w ith tem perature. Thus th e viscosity of pentane, hexane, and th e other simple paraffins was closely represented by it, b u t th a t of
w ater was n o t, th e reason being t h a t a t lower tem p e ra tu res th e simple be ascribed to response-differences betw een tim epieces ? T his was over and above th e varying tim e-lags due to th e differences betw een th e reflex-action of individuals ^ n d to th e inability of all observers to m ake up th e ir minds
w ith th e same alacrity in a fixed set o f circum stances. In some laboratories had its own justification for existence on theoretical grounds, and he th o u g h t th a t th e paper served its purpose, n o t in finding a use for th e accurate reliable for determ ining viscosity both above and below th a t figure.
IN S T IT U T E O F PE T R O L EU M A N D B R IT IS H R H EO LO G ISTS C L U B . 317
On th e general subject of viscom etry he th o u g h t it m ig h t well be sta te d
IN S T IT U T E OF PE T R O L E U M A N D B R IT IS H R H EO LO G ISTS C L U B . 319
ceivable th a t a concentrated sucrose solution m ay show a v ery slight degree of structural viscosity, an d since th e wall stress was approxim ately doubled in stepping up from one in stru m en t to th e n e x t in th e series, there would be a
The small variations in viscosity results b y different observers m ight well be due to variations in rig id ity of th e calibrating oil due to th erm al history,
enough to a lter th e m echanical conditions of th e tu b e , b u t it m ight affect th e interchange of air an d w ater.
On th e p o in t of th erm al conditions, he sta te d t h a t in th e ir viscosity determ inations th e y h a d n o t carried o u t th e procedure for elim inating hysteresis b y heating th e oil prior to th e determ ination.
D r . A. H . Ni s s a n, in a w ritte n contribution, said t h a t in discussing th e accuracy of viscom etry it was suggested t h a t th e 0-36 per cent, consistent deviation observed betw een th e results of A m erican an d B ritish viscom etry system s m ight be due to th e difference in th e value of g in th e S tates and B ritain.
D etailed considerations show th e v a ria tio n in th e value of g cannot be th e cause of th e observed deviation. I n relative viscom etry we m easure the tim e, t, for a sta n d a rd q u a n tity , Q ,to flow th ro u g h tu b es of sta n d a rd length, L , an d diam eter, D, under a s ta n d a rd head, H , a t c o n stan t tem perature.
Assum ing for th e m om ent kinetic energy correction is negligible— i.e., for tim es of flow exceeding 100 seconds— a n d t h a t th e C ouette correction is a certain num ber of diam eters to be ad d ed to L to render it an effective length L ' , we get for an oil
<‘ >
w here K = co n sta n t
p = density pi = viscosity
or ^ = A V r ...(2) Pi
How ever, we determ ine K 1 b y calibrating th e in stru m en t, in the same laboratory, b y a fluid th e viscosity a n d den sity o f w hich are know n or assum ed. T hus,
-2 =
K ' g h...(2)
p2
or K 1g = K 11= ^ - ... (3)
* ?2i 2
T hus, in equation (2) we in sert K 11 = K xg to ev alu ate —, i.e.
Pi
^ = A 11* ! ... (4) Pi
H ence, — will be independent of th e v a riatio n of th e value of g.
Pi
T aking kinetic energy effects in to account results in ex actly th e same conclusion.
T H E T E S T IN G O F G R E A S E S F O R B A L L -B E A R IN G S . B y S. R . Pe t h r i c k, B.Sc. (see p p. 2 4 8 -2 5 4 ).
This p ap e r was presented b y th e au th o r.
M r. Be l l s a i d t h a t t h e p a p e r w a s m o s t w e l c o m e a t a t i m e w h e n i n c r e a s i n g a t t e n t i o n w a s b e i n g g i v e n t o t h e q u e s t i o n o f d e v e l o p i n g a n d s t a n d a r d i z i n g
lab o rato ry te st m ethods for evaluating th e service perform ance of greases.
The a u th o r h ad perhaps tended to over-simplify th e problem of evaluating greases w hen he suggested th a t such te s t m ethods should be classified under two m ain headings— nam ely, te s t m ethods applicable to greases for use a t
Both in his general classification of these te s t m ethods an d his considera
tion of ap p aren t viscosity, th e a u th o r indicated th a t norm al lime-base perm anent, and th e grease would norm ally more or less rev ert to its original state when th e shear force ceased to be exerted. There were greases which m ight undergo perm anent changes in consistency when ag itated or worked in a bearing, due to breakdow n of th e grease structure, b u t this should not be confused w ith norm al flow characteristics a t various rates of shear.
A part from these considerations, service tests showed t h a t norm al lime- these particular characteristics, as it would, of course, th e ap p aren t viscosity a t both low an d high rate s of shear ?
Of th e various types of stab ility m et w ith in service, w hilst resistance to oxidation and th e tendency of th e grease to separate oil are m ost im portant, the question of agitation or working perm anently breaking down th e grease structure is also of considerable im portance. H e felt th a t th e question of measuring th e tendency of a grease to separate oil under various service conditions is very involved, and doubted if stab ility tests based on capillary action would provide a complete answer. S tab ility tests depending on norm al storage stability, pressure stability, an d h e a t stability. Oil separa
tio n in norm al storage conditions appeared to be caused by a small pressure gradient in th e grease due to its weight. H e suggested th a t oil separation in general was more likely to be due to a slight pressure gradient effect th a n to capillary action.
A A 2
IN S T IT U T E OF PE T R O L EU M A N D B R IT IS H R H EO LO G ISTS C L U B . 3 2 1
Mr. C . E . Mo l d said t h a t th e m ajo rity of tests, drop-point an d th e like,
IN S T IT U T E OE PE T R O L E U M A N D B R IT IS H RH EO LO G ISTS C L U B . 3 2 3 into E nglish. T hey were really very valuable to any petroleum technologist.
Mr. Pe t h r i c k said th a t concerning th e point raised ab o u t th e classifica
tion of greases he was afraid th e bearing m anufacturers were a t fault. Most bearing m anufacturers worked for low speeds. W ith regard to stab ility , more accurately speaking, exam ining grease, are quite inadequate to perm it of its proper evaluation as a lubricant. H e shared th e a u th o r’s u n flattering opinion of th e drop-point and penetration tests, to which he would add th e stab ility or separation test. A p art from th e over-riding considera
w orking conditions, a n d in one operation provided d a ta w hich included th e grease users, or was someone going to prepare tables of equivalents 1 Then he foresaw a v a s t increase In th e tim e required, n o t only for th e carrying too m any instances where, owing to th e deficiencies of greases, cumbersome oil circulation system s have h a d to be installed in otherw ise compact
IN S T IT U T E OF PE T R O L E U M A N D B R IT IS H RH EO LO G ISTS C L U B . 3 2 5 accepted. Complete agreem ent m ust, however, be obtained on th e form ula used by all workers in order to ensure uniform ity. I t therefore follows th a t
1. Increase in tem p era tu re viscosity slope is n o t directly proportional to V.I. function of th e tem p eratu re coefficient of viscosity under all circum stances, and som ething th a t was additive, a b e tte r system would be forthcom ing th a n th e y h a d a t present. There was some d o u b t t h a t these would be achieved in th e near fu tu re, and consequently he th o u g h t th a t even an a rb itra ry value would probably serve th e purpose b e tte r for th e tim e being.
He th o u g h t th e y m ust rely upon th e ir scientists to give th e m ixture law for viscosities, which was n o t as y e t fully understood even for pure liquids.