G S A Shawki

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S. I. E L - S I S I 
Mechanical Engineering Department, 
Assuit University, Assuit, Egypt 
G. S. A. S H A W K I 
Mechanical Engineering Department, 
Cairo University, Cairo, Egypt 
Measurement of Oil-Film Thickness 
Between Disks by Electrical Conductivity 
A special apparatus is designed, to measure the oil-film thickness between two sta-
tionary disks (simulating meshing gear teeth) by applying a-c or d-c potential across the 
film and measuring its resistance for given gap between the disks. 
Experiments show that the treatment of oil with additive (sodium-petroleum sulphonate 
in this case) may well lead to a considerable increase in its electrical conductivity, and to 
a substantial improvement in the independence of the oil-film resistance of the electrical 
history and temperature of the oil. A reliable relationship could eventually be obtained 
between the oil-film resistance and relevant thickness. 
Test results are employed in investigating the behavior of the oil film in a rotating-
disk testing machine developed by the authors for studying the performance of meshing 
gear teeth with adequate supply of lubricant. 
L 
Introduction 
I HE ACCURATE MEASUBEMENT of tllill o i l films has, 
for some time, handicapped quantitative investigations on the per-
formance of lubricating oil films formed between gear teeth. The 
method of electrical conductivity was tried by Cameron [l ]1 and 
Hughes [3] who mentioned in his report that the interpretation of 
electrical resistance in terms of film thickness is not possible, at 
least with ordinary lubricating oils. This may be attributed to 
the high insulating properties of these oils, and to the dependence 
of the breakdown voltage on the previous electrical history of 
the film. 
It is with this picture in mind that the present research has 
been planned and carried out in an attempt to obtain an additive 
which would improve the electrical conductivity of oil, and would 
impart to it certain basic qualities which would render it suitable 
for use in this method of measurement. Only final results of the 
numerous tests made are herein presented. 
The apparatus described in this paper is virtually a length-
measuring device in which oil films may be formed between two 
stationary disks whose distance apart can be finely adjusted and 
precisely measured. The disks are set at a given distance apart 
and the electrical current flowing through the film is measured 
for specified potential applied across it. This would lead to some 
relation between the oil-film resistance and relevant thickness at 
the thinnest section. Such relation would be very useful for in-
terpreting current measurements for the rotating disk testing 
machine [2], 
Description of Apparatus 
The general arrangement of the apparatus is shown in Fig. 1 
and a front view in Fig. 2. 
Two identical disks 1 and 2 are mounted on two horizontal 
spindles 3 and 4 with their axes in same vertical plane. Both 
disks are made of case-hardened mild steel, and measure 6 inches 
in diameter and 2 in. in width. They are electrically insulated 
from their respective spindles by means of perspex bushes. 
The top disk spindle 4 is fixed to a steel frame 5 supported by 
three legs 6 resting on a surface plate. 
The bottom disk spindle 3 is fixed to a steel framework 7 fitted, 
1 Numbers in brackets designate References at end of paper. 
Contributed by the Lubrication Division and presented at the 
Annual Meeting, New York, N. Y., November 30-December 5, 195S, 
o f T H E A M E R I C A N S O C I E T Y OF M E C H A N I C A L E N G I N E E R S . 
NOTE: Statements and opinions advanced in papers are to be 
understood as individual expressions of their authors and not those 
of the Society. Manuscript received at ASME Headquarters, July 11, 
1958. Paper No. 58—A-253. 
at its extreme end, with a micrometer 8 resting against a stand 9. 
It is also fixed to two L-shaped steel pieces 10 pivoted on the main 
frame 5 through two balls symmetrically disposed with respect to 
the bottom disk. These balls are fitted in the L-shaped pieces 
and rest on two vertical flat-topped set screws 11 screwed into the 
main frame. 
It is evident, therefore, that the whole of the bottom disk as-
sembly is virtually a lever with its fulcrum at the two balls and its 
free end at the micrometer. The magnitude of disk displacement 
rendered by leverage has been found, with the help of Johansson 
slip gages inserted between the disks, to be equal to 45.81. 
The bottom disk assembly is made, through two helical ex-
tension springs 12, to rest against two horizontal pointed set 
screws 13 fitted in two steel brackets 14 which are, in turn, fixed 
to two of the apparatus legs. 
The components are so designed that, when the brackets are at 
their highest position in contact with the underneath part of the 
top plate of the main frame, the axes of set screws 13, the points of 
contact of the two balls, and the point of contact of the microme-
ter, all lie in one horizontal plane passing by the line of contact of 
the two disks. This arrangement results in a negligible horizontal 
movement of the bottom disk (with respect to its vertical move-
ment) within the useful 'A-in. range of the micrometer. 
The points of contact of the fulcrum balls and the points of 
contact of set screws 13 are made to lie on one horizontal axis 
about which the whole of the bottom disk assembly may turn. 
A reservoir made of perspex is cemented to the bottom disk to 
provide an oil bath for the gap between the disks. The bath is 
fitted with a small heating element and a drain cock. 
Both direct and alternating potentials can be applied across the 
disks. The d-c circuit, Fig. 3, consists of a 2-volt accumulator 
connected to the terminals of a potentiometer by means of which 
the voltage across the gap may be adjusted. The current is 
measured by a microammeter, and the voltage by a vacuum-tube 
voltmeter. 
The a-c circuit comprises a bridge, one arm of which is formed 
by the oil film, while the other arm has a sensitive nonreactive 
resistance box, the balance point being detected by an oscilloscope. 
The potential applied across the film is measured by a vacuum-
tube voltmeter, and has a maximum value of 2 volts. 
The table carrying the apparatus is mounted on rubber pads to 
isolate it from vibrations that may be transmitted from the 
floor, and measuring instruments are placed on a separate table. 
Test Procedure 
The top disk is first removed to have the surfaces of the disks 
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A.C. CIRCUIT 
Fig. 3 Electric circuits used in the tests 
thoroughly cleaned with acetone, and then fixed again in its 
position. 
The axes of the disks are brought into the same vertical plane 
with the help of a plane surface gage block 15, Fig. 1, and a light 
source, the set screws 13 being adjusted to obtain line contact be-
tween the bot tom disk and the gage block. 
The next step is to adjust the axes of the disks to the parallel 
position. The gage block is therefore carefully removed, and a 
parallel beam of light is projected f rom a collimator toward the 
disks' contact from the micrometer side, the apparatus being en-
closed in a dark space for accuracj ' of observation. The vertical 
set screws 11 supporting the fulcrum balls are adjusted so that the 
width of the band of light seen from the other end of the collimator 
becomes as uniform as possible. The bottom disk is thereafter 
moved very slowly toward the top disk, b y means of the microme-
ter. Should the axes of the disks be parallel, the light, on touch-
ing the disks, would disappear over the whole width of band at the 
same time. 
I t should be noted that the micrometer reading at which the 
Fig. 1 General arrangement of the apparatus 
(1) Bottom disk (9) Stand 
(2) Top disk (10) L-shaped pieces 
(3) Bottom spindle (11) Set screws 
(4) Top spindle (12) Helical extension spring 
(5) Mainframe (13) Set screws 
(6) Supporting legs (14) Brackets 
(7) Framework (15) Plane surface gage block 
(8) Micrometer 
Fig. 2 Front view of the apparatus 
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light disappears is not taken as the zero reading corresponding to 
the touching of the disks, since the same is detected b y the c om-
pletion of the electric circuit. I t has been found that the bo t t om 
disk should be moved a farther distance of about 0.000005 in. 
toward the top disk before the electric circuit m a y be c om-
pleted. This latter distance lies within the range of surface 
roughness of the disks. 
After adjusting the axes of the disks parallel and in the same 
vertical plane, oil is poured into the reservoir, the region of closest 
approach of the disks being submerged in the oil. The extent of 
the film may be varied b y changing the oil level in the bath, and 
the oil temperature b y adjusting the current flow in the heating 
element. 
Either a-c or d-c potential is applied across the gap between the 
disks, respective circuits being used. 
Tests were started with the largest possible gap between the 
disk (0.010 in.) and then the gap was progressively reduced. 
The reverse procedure was avoided as it gave unreliable results; 
the apparent reason seems to be that, with this procedure, oil may 
not completely fill the gap. 
Behavior of Untreated Oil 
"Castrolite 20 A " oil is first used without additive, its behavior 
being obtained for both a-c and d-c potential applied across the 
film. 
Fig. 4 shows that, for a given gap, the oil-film resistance drops 
sharply with increase in applied potential. The oil is also found 
to become temporarily more conductive as the voltage increases; 
this is indicated b y the test points obtained while the voltage was 
reduced to zero and then brought back to its original value. 
T h e variation of oil-film resistance with the gap between the 
disks, Fig. 5, is obtained for a constant a-c potential of 0.05 volt , 
0 0.25 0.5 0.75 1.0 
Applied Potential volt. 
a.c. potential d.c. potential 
Fig. 4 Oil-film resistance versus applied potential for untreated oil. 
(Each curve is obtained for a new sample of oil); oil temperature: 56 F. 
and for two values of the oil temperature; namely, 53 and 160 F 
I t can be seen that the oil-film resistance varies considerably with 
temperature. Moreover, the curves obtained are not very reliable 
since the applied potential may, for relatively small gaps, cause 
breakdown in the oil resistance and hence a change in the electri-
cal behavior of the oil. 
I t is worth-while mentioning that Cameron [1], while ex-
perimenting on a disk-testing machine designed b y Merritt [4], 
obtained a relationship between oil-film resistance and applied 
d-c potential quite similar to that obtained in the present tests, 
also that Hughes [3] has noted the considerable effect of tem-
perature on oil-film resistance. 
I t m a y be concluded, therefore, that the measurement of film 
thickness using untreated oil would lead to unreliable results. 
This is due to the fact that the oil-film resistance, for specific 
operating conditions, depends on its electrical history, that the 
rate of change of resistance with film thickness (if reliable) is very 
high, and that the oil-film resistance is seriously affected b y the 
oil temperature. 
Behavior of Treated Oil 
Castrolite 20 A oil is used with 1, 2, and 4 per cent sodium 
petroleum sulphonate as additive. 
The resistance of treated oil is found to remain constant over 
a certain range of the applied potential, Fig. 6. The value of the 
breakdown voltage is shown to be nearly the same for the various 
values of film thickness investigated with either a-c or d-c po -
tential applied across the gap between the disks. 
I t can be seen readily that, for the same film thickness and 
applied potential, the electrical resistance of treated oil is much 
less than that of the untreated oil, i.e., it becomes more conduc-
Oil-Film Thickness 
Fig. 5 Variation of oil-film resistance with film thickness and oil tem-
perature for untreated oil. (The voltage is brought to zero each time the 
film thickness is reduced.) 
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Variation of film resistance with film thickness for Castrolite 20 A oil with I , 
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Fig. 8 Extended film resistance-thickness characteristic 
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tive. l'vloreover, the temperature seems to have but a negligible 
effect. on the resista,nce of treated oil, Fig. 7. 
It also has been found that" for given film thickness, the re-
sistive behavior of treated oil is almost unaffected by the removal 
and reapplicl~tion of potential, even if it exceeds the breakdown 
value. For the same glLp and applied potential, oil-resistance 
values under d-c potential are noted to be, in general, lower 
than those obt.ained with a-c potential. 
The increase in strength of the addit.ive in the mixture (up to 
4 per cent) is found to lead to a related increase in the vallie of the 
breakdown voltage, i.e., to an extension iu the range of ohil1ie re-
sistance behlLviol' of the oil, a nd n reduction in the mte of change 
of oil resistance with film thickness, Fig. 7. 
Tn the course of tcsts run on the rotating-disk testing machine 
[21, it was found neccssary to extend the film resistance-thickness 
chamcteristic to include film-thickness values up to 0.008 in. 
Such a calibration curve is shown in Fig. 8. 
The effect of film extent on both sides of the liue of nearest ap~ 
proach was investigated in the following manner: 
The bath was first filled wit.h oil to a level much higher than 
that of the intermediate phme between the disks, the eUl'ren t 
passing through the film being measured for given gap and applied 
potential. The oil was thereafter drained till the bath became 
empty. The current then flowing in the short oil film suspended 
between the djsks was found to remain exactly the 5..'tme as that 
measUl'cd before. This would imply that for such thin film the 
curl'eut flows in the region closest to the line of nearest approach. 
An alternalive additive, namely, naphthnlene tet.rnchloride in 
the strength of 1 per cent, was investigated; however, no effect 
011 the electrical behavior of the oil could be detected. On in-
creasing the strength to 1.5 per cent, the additive was found to 
form a layer of precipitate on the disks, and no reliable results 
could be obtained. 
The results of tests conducted on the stationary-disk a pparatus 
described previously, were applied to the running-disk machine [2], 
the lubriclLting oil , C:tstrolite 20 A, being admixed with 4 per cent 
sodium-petl'Oleum su lphonate, a-c or d-c potential (not exceeding 
0.75 and 1.25 volts, respectively) being applied across the gap 
between specimen disks. Fortunately, no difficulty due to pre-
vious electrical history of the oil was encountered in the running 
machine ou account of the cont.inuous repiellislunent of oil t.herein. 
It should be noted th~t the curves relating film thickness to film 
rcsistance a re obLained fol' idle films under no pressure, films 
which do not exact.Jy conform to conditions prevailing in the ro-
tating-disk machine (2]. Test results obtained, however, on this 
latter machine show similar resistive behavior of the oil film as 
that obtained for the stationary disk appamtus. 
Conclusion 
The measurement of oil-fi lm t.hickness by electrical conduc-
tivity method would lead t.o ulll'eliable results should stra igh t 
mineral oil be used. 
The treatment of oil with certain additives is found, however, 
to impart to the oil fundamental favorable feutures which render 
it quite suitable for use with this method of measurement. 
Admixing Castrolite 20 A oil with sodium-petroleum sulphonate 
(in the strength of lip to 4 per cent) greatly increases its electrical 
conductivity and extends its range of ohmic-resistance behavior 
for the same gap nnd applied potential. The additive seems to re-
lieve the oil almost completely from its dependence on the e1ec-
t.rical history and temperature. The resistive behavior of t.reated 
oil, for a given film thickness, is found to be almost unaffected by 
the removal and reapplication of potential, even if it exceeds the 
breakdown value . The additive also makes more gradunl the 
rate of change of resista,nce with film thickness for the same ap-
plied potential. 
16 / MAR CHI 960 
Eventua.lIy, a reliable relationship between the oil-film re-
sistance and the oil-film thickness would emerge and, conse-
quently, the method of electrical conductivity could be applied 
successfully to the measurement of oil-film thickness between 
gear teeth. 
References 
A. Cameron, "Zur Thoorie der Zahnradschmierung," 1950, 
V.D.I. Fuchtagung "Zuhnradforschung," Brunswick, Germany. 
2 S. I. EI..sisi and G. S. A. Shawki, "Performance Charucteristics 
of Luhricating Oil-Film Between Rotating Disks," published in this 
issue, pp. 19- 28. 
3 J . n. Hughes, "A Study of the Oil-Film Formation in Gears by 
Eled·rica l Resi stance Method," JOllrnal 0/ Ap1Ji1'ed Physics, vol. 3, 
1952. 
<1 U. E. i\o[erritt, "Worm Gear Pedormance," Proceedings, 
I.Mcch.E .. vol. 129, 1935. pp. 127- 158. 
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Measurement of oil-film thickness between disks by electrical conductivity.

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Measurement of oil-film thickness between disks by electrical conductivity.

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