- Email: [email protected]
The effect of load on the critical temperature of oil in the lubrication of different materials
\\‘fC.\f<
292
THE
EFFECT IN
THE
OF
LOAD
ON THE
LUBRICATION
CRITICAL OF
TEMPEKATLJRE
DIFFERENT
OF
‘211,
MATERIALS
I<. !vI. bl.2T5-E13\‘Sli\
The critical temperature of an oil film was determined in a modified four-ball machine for hard and relatively soft material at low rates of sliding. Only when both specimens were of hardened steel was the temperature of seizure independent of load. For combinations with copper alloys or polyamide,seizuredepends not only on temperature but also on plastic deformation at the contact area. The permissible contact pressure, at which the critical temperature is still independent of load, has been determined for some combinations of materials.
According
to modern
views, temperature
is an important
factor
which determines
the strength of a lubricant film on a rubbing surface. BLOKE considered that for a given combination of materials, rubbing against each other, there is a constant critical temperature at which a given oil loses its lubricating properties and no longer prevents direct contact between the rubbing surfaces. This critical temperature is almost independent of the load, the speed of sliding and the bulk temperature. However, if the bulk temperature of the oil is sufficiently high, local (contact) temperature, necessary for seizure of the surfaces, is reduced. The work of BOWDEN, TABOR and their collaborators”-” on the strengthof lubricant boundary layers has done much to establish the nature of this phenomenon. In particular, they have shown that the increase in friction and the change in sliding behaviour occurring at a well defined temperature characteristic of the given combination of materials is due to disorientation of the boundary layer of lubricant. Further evidence of the important influence of temperature on the properties of lubricant boundary layers at a metal surface was furnished by testing various oils at the Institute for the Study of Machines, Academy of Sciences of the U.S.S.K.7*8. In these tests a four-ball machine was used, the load being increased by degrees until seizure took place. It was found that as the sliding speed or the oil temperature were increased, seizure occurred with lower loads. The product of the friction value, pressure and sliding velocity (p#v) at the instant preceding seizure is nearly constant (,@J = const). The value (,[email protected]) we call the specific power of friction. The fact that it is a constant (at the maximum loads used) just before seizure, over a wide range of sliding velocities and pressures, may be interpreted to indicate that a critical amount of heat sufficient to cause breakdown of the lubricant fihn has been generated at this point. From the experimental values of the diameter of the contact spot, pressure, friction
293
EFFECTOFLOADONCRITICALTEMPERATUREOFOIL
coefficient and sliding speed, mean temperatures on the rubbing surface at the instant when seizure started were calculated by means of a JAEGER'S formulae. They were approximately the same in agreement with the concept that frictional heating raises the boundary layer to its critical temperature, characteristic of the given oil-metal combination Critical temperature has thus an important practical significance indicating a temperature which must not be exceeded in service. An industrial technique for determining the critical temperature of the lubricant boundary layer was evolved by the author in collaboration with Professor M. M. KHRUSHCHOV~. We also designed a machine (KT-2) for these testslo. The principle of this four-ball machine is shown in Fig. I; Fig. 2 gives a general n=l rev/
mln
Ah
Pig. I. Schematic
view of the four-ball
machine KT-z; (a) test oil; (b) electric heating; dynamometer.
(c) torsion
The load is kept constant (about zoo kg/mmz) and the upper ball has a constant sliding velocity (0.4 mm/set). Such a low rate of sliding practically eliminates any frictional heating of the area of contact. The balls and the test oil which. surrounds them are heated by an external source of heat. The frictional force and the type of sliding (whether smooth or intermittent) are determined. Temperature is measured and controlled by a thermocouple linked to an electron potentiometer. The technique consists in bringing the apparatus to the desired temperature, running the test for one minute, then stopping the machine. The balls are turned about (or replaced by new ones) the oil is changed, then the cycle is repeated at higher temperatures. After each test the diameter of the wear spots on the lower (stationary) balls is measured under a microscope. At the critical temperature there is a sharp rise in friction, accompanied by a change to intermittent (stick-and-slip) motion and by increased wear of the balIs. Up to this temperature the lubricant will generally prevent direct contact between the rubbing surfaces This is shown in Fig. 3, which gives the results view.
WW,4
(t961) 292-299
for several mineral oils. For each of these oils there is a particular critical temperatrirc at which sliding changes from smooth to intermittent and the values of the frictiou coefficient increase sharply. The fluctuations in the coefficient of friction are indicatthd in the graph by vertical lines, and the region of intermittent sliding is shaded. This method can be extended to low temperatures (MATVEEVSKY~~) and is useful for evaluating polar additives for lubricating oils (MATVEEVSKY*).
Fig.
Temperature
Fig. 3. Friction
2. Four-ball
(‘Cl
machine
1iT-r
(cxtcmal
x-icw)
Temperature
(“Cl
value and wear spot diameter as function of the temperature for mineral viscosities at TOOT are: I = LI~“E; 1 = 1.07’E; 3 = r.g”E.
oils whose
Hardened steel balls (ordinarily, chromium steel, 8 mm in diameter), as used for tests in the KT-z machine, have many advantages. Thus we can easily obtain specimens of the same composition and shape, with the same hardness. On the other hand, the four-ball scheme is not well adapted to oil temperature tests with different combinations of test metal pairs, because balls of uniform quality are difficult to obtain
EFFECTOFLOADONCRITICALTEMPERATUREOFOIL
2%
from alloys and plastics. Furthermore, with this geometry of the specimens the calculated pressure at the real areas of contact in the case of plastic materials may be of the order of tens of thousands of kilograms per square centimeter, even at the minimum axial load of the KT-z machine (about z kg). In the case of antif~ction alloys, for instance, such loads will produce considerable plastic deformation at the contact area, and the test results will be greatly distorted. The KT-2 machine was therefore modified (Fig. 4) so that pressures at the real areas of contact were reduced considerably (about two orders of magnitude) while the sliding rate remained low as before (~~ATVEEVSKY~~).
Fig. 4* Sphere-ring arrangement: (I) upper ball; (2) ring specimen; (3) setting of the ringspecimen; (4) cup for oil; (5) clamp to hold the ball; (6) torsion dynamometer.
Temperature C°C1 Fig. 5. Friction as a function of temperature. Vaseline oil containing 0.1 y. stearic acid lubricant: hardened steel rubbing on copper. 0, first; 0, second; x , third set of experiments.
In this machine the rotating ball is made of hardened steel, 12.7 mm in diameter; the lower, stationary specimen consists of a ring of the plastic being tested, on which a rubbing track 0.2 mm wide has been formed by pressing a 12.7 mm diameter steel ball onto it.
\L’hen similar materials art’ tcstcd, tlic rotating ball is rt~placed I-)!~;I lrcmispheric.;~l specimen of the material being tested. \Vith the spccimtsn sizes adoptctl the prt5sur-c on the contact area can be \xried from zg to 600 kg;cm’J. The technicluc~ with tllis arrangement is the same as in the case of four balls, but since \vear on tllcs ring specmen is negligible, changes in thr coefficient of friction and sliding characteristics art’ the criteria for estimating the critical temperaturtb. Figure 5 shows the results obtained for steel rubbing 011 copper, with \-ascline oil containing 0.1% stearic acid as lubricant. A sharp change in friction similar to that in the case of steel on steel, denotes the: critical temperature. Some recent investigations have shown that the critical or failure temperature may vary with the test conditions. CoWLEY, ~LTEE AXI) \vES I ‘3 investigated the effect of temperature on the lubricity of oils, for the case of a loaded hemispherical slider moving to and fro on a plane, the oil being heated in bulk. In these tests with steel rubbing on steel, the failure tempcrature decreased with increasing load and sliding rate. Xew data on the dependence of transition critical temperature in the friction of metals or the load and sliding speed, have been obtained by FEIN, ROWE ANII KRUEZIJ. The rubbing parts in their device were a rotating disc and a slider, while solutions of fatty acids in cetane were used as lubricants. Bulk heating was employed. The tests showed that the transition temperature decreased with increasing load and increased with sliding speed. They gave an experimental curve showing the dependence of the lubricant failure temperature upon the ratio of normal load to sliding speed. The effects of sliding speed on the critical temperature as found by these two groups are contradictor\.. However, it should be noted that in the experiments of FEN et d.lJ the sliding speeds were all considerably higher than those used by C~WLEV cat ~1.1” and by the author. It is therefore possible that FEIN was observing some hydrodynamic effects which would naturally be more effective at higher speeds. On the other hand, both groups of workers agree that an increase in load reduced the transition temperature. In order to ascertain the influence of load on the critical temperature the present author investigated the following friction cases : hardened steel against hardened steel, hardened steel against copper, copper against copper, and hardened steel against plastics. The tests were run in the KT-YZ machine either on the four-ball scheme (when
Specific contact load (kg/cm’) Fig. 6. Critical temperature as a function on the same steel. I.ubricant:
of contact pressure. Hardened steel (LUX 6) rubbing vaseline oil containing O.I”:, stearic acid.
EFFECT OF LOAD ON CRITICAL TEMPERATURE
OF OIL
207
steel was rubbing against steel) or on the sphere-ring scheme (for other rubbing pairs). Vaseline oil of high purity, with 0.1 y0 stearic acid added, was used as a lubricant. Details of the specimens used and contact pressure applied are given below in Table I. TABLE
Test NO.
I
2 3
4
Malerzal
Shape
of rubbing
I
DPH
paws
/ kR/mwG)
Steel lu x 6 Steel IUX 6
Ball 8 mm diam. Ball 8 mm diam.
1000
Steel wx
CopperMO
Ball 12.7 mm diam. Ring specimen 20 x 8 x 3 mm
,000
Copper MO Copper MO
Hemisphere I 2.7 mm Ring specimen 20 x 8
g
x
l’j,500-32,000
75-600
80
3 mm
Steel uX 9 Ball 12.7 mm diam. I’olyamide AK 7 Ring specimen 20 x 8 x 3 mm
80
25-100
IOOD 16
32 -100
The sliding rate at the site of contact was 0.4 mmjsec. Specimens were washed successively in benzine, acetone, ethyl alcohol and anaesthetic grade ether, then immersed in the test oil for 60 minutes to allow the formation of an oriented lubricant layer where applicable. The technique described earlier was used in the tests. Each load was applied two or three times. The results for hardened steel against hardened steel are shown in Fig. 6. The critical temperature over the experimental load range (15,000-32,000 kg/cm2) was constant (140°C). In the hardened steel-copper tests eight different loads were tried; 50, 75, IOO, 250, 300, 400, 500 and 600 kg/cm2 (Fig. 7). In the load range from 75 to 250 kg/cm’ the critical temperature was constant (200°C). At higher loads the critical temperature fell rapidly at first, down to 145°C at 400 kg/cm”, and then only slowly to 130% at a load of 600 kg/cm2.
0 0
100
200
300
Specific contact
400
500
600
w
q
load (kg /cm*)
Fig. 7. Critical temperature as a function of contact pressure. Hardened steel (uX copper (MO). Lubricant: vaseline oil containing o. I o/0 stearic acid.
9) rubbing
on
Wear, 4 (1961) 292-299
With two copper specimens (Fig. 8) increasing the load from 25 to 40 kg/cm* tlitl not alter the critical temperature, which in this case was 90%. Within this range of loads the sliding was smooth. The friction fell slightly as the temperature increased from 20~ to So”C, then rose sharply at the critical temperature, when sliding becamt intermittent (90°C). As the load was increased beyond 40 kg/cm” the critical temperature fell to about 65°C (at 7o kg,‘cm2).
to
i7-------
G -80
7
-
0
E 3 2 60
0
\
---T
g E $40 3 -2
i
I 20
j------i
& 0
I
; 0
20
40 Specific
60 contact
80 load
100 (kg /cm2
120
) cl
1
Fig. 8. Critical temperature as a function of contact pressure. Copper rubbing on copper. Lubricant: vaseline oil containing 0.1 o/o stearic acid.
Specific contact load (kg/cm21 Fig. 9. Critical temperature as function of contact pressure. Hardened steel (w X 9) rubbing on polyamide .%I< 7. Lubricant: vascline oil containing o. I y0 stearic acid.
In the steel-polyamide tests six different loads were used: 32,62, 95, 125, 162 and 180 kg/cm2 (Fig. 9). With loads up to 95 kg/cm” the critical temperature was constant (~zo”C). Increasing the load up to 180 kg/cm2 gradually reduced the critical temperature to ro=j”C. Rriefly, for the loads used, the only case in which the critical temperature is independent of load was when both specimens were hardened steel. These results can be explained by the effect of heavy plastic deformation at the 7~16 showed that a lubricant layer at areas of real contact. Tests made by SEMENO\ the surface of the relatively soft material (aluminium or copper for instance) can break down even at room temperature as a result of heavy plastic deformation. Breakdown occurs because the lubricant is displaced from the contact zone as the surface layers ~‘ear, 4 (1961) 292-299
EFFECT OF LOAD ON CRITICAL TEMPERATURE
OF OIL
299
of the metal are pushed aside in the deformation process and underlying layers, not covered by the lubricant, come to the surface. In sliding friction the possibility of the lubricant layer breaking down, as a result of plastic deformation, is higher still because in addition to the normal force considerable tangential forces arise, which can contribute to the removal of the oil film and so to seizure. When plastic deformations occur only at separate asperities on the contacting surfaces and fail to spread deeper into the metal, failure of the lubricant film is mainly the result of temperature effects. This was the case in the tests with hardened steel against hardened steel, over the whole range of loads used. With other combinations of materials this mechanism held only for a limited load range. When plastic deformation is marked, failure of the lubricant layer may be due to the joint influence of temperature and plastic flow of the material at the contact surface. This was probably the case for hardened steel against copper or polyamide, and copper against copper. Therefore, when determining the critical temperature of an oil film on relatively soft materials rubbing together, the pressure at the contact area should be so chosen that there is no marked plastic deformation. Contact pressures recommended for critical temperature determinations with various material combinations by this technique are given in Table II. TABLE Rubbingmaterials
Permissible contact pressure (kg/cmz)
Hardened steel against hardened steel
up to 30‘000
~_
II
Hardened steel against copper or copper alloys -._
copper (or alloy) agaznst cofiper (or alloy)
100-200
20-40
Hardened steel against p0lyamidz.s (itylon, caplon) __.“~_~
40-80 -----
ACKNOWLEDGEMEXT
should like to express my appreciation to Dr. F. P. BOWDEN, Dr. D. TABOR and Mr. KEITH MCLAREN for their help and advice in preparing this paper for publication. I
REFERENCES 1 H. BLOK, SAE Journal, 44 (5) (1939) 193. 2 F. P. BOWDEN AND L. LEBEN, Phil. Trans. Roy. Sot. (London),
A, 239 (1939)
3 F. P. BOWDEN, J. N. GREGORY AND D. TABOR. Nature, 156 (1945) 97 4 F. P. BOWDEN AND D. TABOR, The Friction and Lubrication of Solids, Clarendon
I. Press, Oxford,
1950. 5 J. W. MENTER AND D. TABOR, Proc. Roy. Sot. ~London~, A 204 (1950) 514. 6 J. V. SANDERS AND D. TABOR, Pvoc. Roy Sot. (London), A 204 (1950) 5.~5. 7 111.M. KHRUSHCHOV AND R. M. MATVEEVSKY, Vest&k Machinostroeniya, I (1954) 12 (in Russian!. s R. M. MATVEEVSKY, A Temperature Method for the Estimatzon of Limitiq Machine Oils Lubrrcity, (in Russian), 1956. 9 J. C. JAEGER, Phil. Msg., 240 (1944). 10 M. M. KHRUSHCHOV, New Machines fov the Investigation of the Wear of MaterLals and Machi~le DefaiZs, (in Russian), 1957. 11 R. M. MATVEEVSKY, VINITI, Topic 32, M-57-88, (in Russian), (1957). 12 R. M. MATVEEVSKY Wear, 2 (1959) 315. 13 C. XV. COWLEY, C. J. ULTEE AND C. IV. WEST, ASLE Trans., I (z) (1958) 281. 14 R. S. FEIN, C. N. ROWE AND C. U’. KRUEZ, ASLE Trams., 2 (1959) (I) 50. 15 A. P. SEMENOV. Doklady Akad. Nauk. S.S.S.R. 86 (2) (1952). 16 A. I?. SEMENOV, The Seizure of Metals, (in Russian), Mash&, 1958; Wear. -f (1961) I.
wear, 4 (1961) 292-299
292
THE
EFFECT IN
THE
OF
LOAD
ON THE
LUBRICATION
CRITICAL OF
TEMPEKATLJRE
DIFFERENT
OF
‘211,
MATERIALS
I<. !vI. bl.2T5-E13\‘Sli\
The critical temperature of an oil film was determined in a modified four-ball machine for hard and relatively soft material at low rates of sliding. Only when both specimens were of hardened steel was the temperature of seizure independent of load. For combinations with copper alloys or polyamide,seizuredepends not only on temperature but also on plastic deformation at the contact area. The permissible contact pressure, at which the critical temperature is still independent of load, has been determined for some combinations of materials.
According
to modern
views, temperature
is an important
factor
which determines
the strength of a lubricant film on a rubbing surface. BLOKE considered that for a given combination of materials, rubbing against each other, there is a constant critical temperature at which a given oil loses its lubricating properties and no longer prevents direct contact between the rubbing surfaces. This critical temperature is almost independent of the load, the speed of sliding and the bulk temperature. However, if the bulk temperature of the oil is sufficiently high, local (contact) temperature, necessary for seizure of the surfaces, is reduced. The work of BOWDEN, TABOR and their collaborators”-” on the strengthof lubricant boundary layers has done much to establish the nature of this phenomenon. In particular, they have shown that the increase in friction and the change in sliding behaviour occurring at a well defined temperature characteristic of the given combination of materials is due to disorientation of the boundary layer of lubricant. Further evidence of the important influence of temperature on the properties of lubricant boundary layers at a metal surface was furnished by testing various oils at the Institute for the Study of Machines, Academy of Sciences of the U.S.S.K.7*8. In these tests a four-ball machine was used, the load being increased by degrees until seizure took place. It was found that as the sliding speed or the oil temperature were increased, seizure occurred with lower loads. The product of the friction value, pressure and sliding velocity (p#v) at the instant preceding seizure is nearly constant (,@J = const). The value (,[email protected]) we call the specific power of friction. The fact that it is a constant (at the maximum loads used) just before seizure, over a wide range of sliding velocities and pressures, may be interpreted to indicate that a critical amount of heat sufficient to cause breakdown of the lubricant fihn has been generated at this point. From the experimental values of the diameter of the contact spot, pressure, friction
293
EFFECTOFLOADONCRITICALTEMPERATUREOFOIL
coefficient and sliding speed, mean temperatures on the rubbing surface at the instant when seizure started were calculated by means of a JAEGER'S formulae. They were approximately the same in agreement with the concept that frictional heating raises the boundary layer to its critical temperature, characteristic of the given oil-metal combination Critical temperature has thus an important practical significance indicating a temperature which must not be exceeded in service. An industrial technique for determining the critical temperature of the lubricant boundary layer was evolved by the author in collaboration with Professor M. M. KHRUSHCHOV~. We also designed a machine (KT-2) for these testslo. The principle of this four-ball machine is shown in Fig. I; Fig. 2 gives a general n=l rev/
mln
Ah
Pig. I. Schematic
view of the four-ball
machine KT-z; (a) test oil; (b) electric heating; dynamometer.
(c) torsion
The load is kept constant (about zoo kg/mmz) and the upper ball has a constant sliding velocity (0.4 mm/set). Such a low rate of sliding practically eliminates any frictional heating of the area of contact. The balls and the test oil which. surrounds them are heated by an external source of heat. The frictional force and the type of sliding (whether smooth or intermittent) are determined. Temperature is measured and controlled by a thermocouple linked to an electron potentiometer. The technique consists in bringing the apparatus to the desired temperature, running the test for one minute, then stopping the machine. The balls are turned about (or replaced by new ones) the oil is changed, then the cycle is repeated at higher temperatures. After each test the diameter of the wear spots on the lower (stationary) balls is measured under a microscope. At the critical temperature there is a sharp rise in friction, accompanied by a change to intermittent (stick-and-slip) motion and by increased wear of the balIs. Up to this temperature the lubricant will generally prevent direct contact between the rubbing surfaces This is shown in Fig. 3, which gives the results view.
WW,4
(t961) 292-299
for several mineral oils. For each of these oils there is a particular critical temperatrirc at which sliding changes from smooth to intermittent and the values of the frictiou coefficient increase sharply. The fluctuations in the coefficient of friction are indicatthd in the graph by vertical lines, and the region of intermittent sliding is shaded. This method can be extended to low temperatures (MATVEEVSKY~~) and is useful for evaluating polar additives for lubricating oils (MATVEEVSKY*).
Fig.
Temperature
Fig. 3. Friction
2. Four-ball
(‘Cl
machine
1iT-r
(cxtcmal
x-icw)
Temperature
(“Cl
value and wear spot diameter as function of the temperature for mineral viscosities at TOOT are: I = LI~“E; 1 = 1.07’E; 3 = r.g”E.
oils whose
Hardened steel balls (ordinarily, chromium steel, 8 mm in diameter), as used for tests in the KT-z machine, have many advantages. Thus we can easily obtain specimens of the same composition and shape, with the same hardness. On the other hand, the four-ball scheme is not well adapted to oil temperature tests with different combinations of test metal pairs, because balls of uniform quality are difficult to obtain
EFFECTOFLOADONCRITICALTEMPERATUREOFOIL
2%
from alloys and plastics. Furthermore, with this geometry of the specimens the calculated pressure at the real areas of contact in the case of plastic materials may be of the order of tens of thousands of kilograms per square centimeter, even at the minimum axial load of the KT-z machine (about z kg). In the case of antif~ction alloys, for instance, such loads will produce considerable plastic deformation at the contact area, and the test results will be greatly distorted. The KT-2 machine was therefore modified (Fig. 4) so that pressures at the real areas of contact were reduced considerably (about two orders of magnitude) while the sliding rate remained low as before (~~ATVEEVSKY~~).
Fig. 4* Sphere-ring arrangement: (I) upper ball; (2) ring specimen; (3) setting of the ringspecimen; (4) cup for oil; (5) clamp to hold the ball; (6) torsion dynamometer.
Temperature C°C1 Fig. 5. Friction as a function of temperature. Vaseline oil containing 0.1 y. stearic acid lubricant: hardened steel rubbing on copper. 0, first; 0, second; x , third set of experiments.
In this machine the rotating ball is made of hardened steel, 12.7 mm in diameter; the lower, stationary specimen consists of a ring of the plastic being tested, on which a rubbing track 0.2 mm wide has been formed by pressing a 12.7 mm diameter steel ball onto it.
\L’hen similar materials art’ tcstcd, tlic rotating ball is rt~placed I-)!~;I lrcmispheric.;~l specimen of the material being tested. \Vith the spccimtsn sizes adoptctl the prt5sur-c on the contact area can be \xried from zg to 600 kg;cm’J. The technicluc~ with tllis arrangement is the same as in the case of four balls, but since \vear on tllcs ring specmen is negligible, changes in thr coefficient of friction and sliding characteristics art’ the criteria for estimating the critical temperaturtb. Figure 5 shows the results obtained for steel rubbing 011 copper, with \-ascline oil containing 0.1% stearic acid as lubricant. A sharp change in friction similar to that in the case of steel on steel, denotes the: critical temperature. Some recent investigations have shown that the critical or failure temperature may vary with the test conditions. CoWLEY, ~LTEE AXI) \vES I ‘3 investigated the effect of temperature on the lubricity of oils, for the case of a loaded hemispherical slider moving to and fro on a plane, the oil being heated in bulk. In these tests with steel rubbing on steel, the failure tempcrature decreased with increasing load and sliding rate. Xew data on the dependence of transition critical temperature in the friction of metals or the load and sliding speed, have been obtained by FEIN, ROWE ANII KRUEZIJ. The rubbing parts in their device were a rotating disc and a slider, while solutions of fatty acids in cetane were used as lubricants. Bulk heating was employed. The tests showed that the transition temperature decreased with increasing load and increased with sliding speed. They gave an experimental curve showing the dependence of the lubricant failure temperature upon the ratio of normal load to sliding speed. The effects of sliding speed on the critical temperature as found by these two groups are contradictor\.. However, it should be noted that in the experiments of FEN et d.lJ the sliding speeds were all considerably higher than those used by C~WLEV cat ~1.1” and by the author. It is therefore possible that FEIN was observing some hydrodynamic effects which would naturally be more effective at higher speeds. On the other hand, both groups of workers agree that an increase in load reduced the transition temperature. In order to ascertain the influence of load on the critical temperature the present author investigated the following friction cases : hardened steel against hardened steel, hardened steel against copper, copper against copper, and hardened steel against plastics. The tests were run in the KT-YZ machine either on the four-ball scheme (when
Specific contact load (kg/cm’) Fig. 6. Critical temperature as a function on the same steel. I.ubricant:
of contact pressure. Hardened steel (LUX 6) rubbing vaseline oil containing O.I”:, stearic acid.
EFFECT OF LOAD ON CRITICAL TEMPERATURE
OF OIL
207
steel was rubbing against steel) or on the sphere-ring scheme (for other rubbing pairs). Vaseline oil of high purity, with 0.1 y0 stearic acid added, was used as a lubricant. Details of the specimens used and contact pressure applied are given below in Table I. TABLE
Test NO.
I
2 3
4
Malerzal
Shape
of rubbing
I
DPH
paws
/ kR/mwG)
Steel lu x 6 Steel IUX 6
Ball 8 mm diam. Ball 8 mm diam.
1000
Steel wx
CopperMO
Ball 12.7 mm diam. Ring specimen 20 x 8 x 3 mm
,000
Copper MO Copper MO
Hemisphere I 2.7 mm Ring specimen 20 x 8
g
x
l’j,500-32,000
75-600
80
3 mm
Steel uX 9 Ball 12.7 mm diam. I’olyamide AK 7 Ring specimen 20 x 8 x 3 mm
80
25-100
IOOD 16
32 -100
The sliding rate at the site of contact was 0.4 mmjsec. Specimens were washed successively in benzine, acetone, ethyl alcohol and anaesthetic grade ether, then immersed in the test oil for 60 minutes to allow the formation of an oriented lubricant layer where applicable. The technique described earlier was used in the tests. Each load was applied two or three times. The results for hardened steel against hardened steel are shown in Fig. 6. The critical temperature over the experimental load range (15,000-32,000 kg/cm2) was constant (140°C). In the hardened steel-copper tests eight different loads were tried; 50, 75, IOO, 250, 300, 400, 500 and 600 kg/cm2 (Fig. 7). In the load range from 75 to 250 kg/cm’ the critical temperature was constant (200°C). At higher loads the critical temperature fell rapidly at first, down to 145°C at 400 kg/cm”, and then only slowly to 130% at a load of 600 kg/cm2.
0 0
100
200
300
Specific contact
400
500
600
w
q
load (kg /cm*)
Fig. 7. Critical temperature as a function of contact pressure. Hardened steel (uX copper (MO). Lubricant: vaseline oil containing o. I o/0 stearic acid.
9) rubbing
on
Wear, 4 (1961) 292-299
With two copper specimens (Fig. 8) increasing the load from 25 to 40 kg/cm* tlitl not alter the critical temperature, which in this case was 90%. Within this range of loads the sliding was smooth. The friction fell slightly as the temperature increased from 20~ to So”C, then rose sharply at the critical temperature, when sliding becamt intermittent (90°C). As the load was increased beyond 40 kg/cm” the critical temperature fell to about 65°C (at 7o kg,‘cm2).
to
i7-------
G -80
7
-
0
E 3 2 60
0
\
---T
g E $40 3 -2
i
I 20
j------i
& 0
I
; 0
20
40 Specific
60 contact
80 load
100 (kg /cm2
120
) cl
1
Fig. 8. Critical temperature as a function of contact pressure. Copper rubbing on copper. Lubricant: vaseline oil containing 0.1 o/o stearic acid.
Specific contact load (kg/cm21 Fig. 9. Critical temperature as function of contact pressure. Hardened steel (w X 9) rubbing on polyamide .%I< 7. Lubricant: vascline oil containing o. I y0 stearic acid.
In the steel-polyamide tests six different loads were used: 32,62, 95, 125, 162 and 180 kg/cm2 (Fig. 9). With loads up to 95 kg/cm” the critical temperature was constant (~zo”C). Increasing the load up to 180 kg/cm2 gradually reduced the critical temperature to ro=j”C. Rriefly, for the loads used, the only case in which the critical temperature is independent of load was when both specimens were hardened steel. These results can be explained by the effect of heavy plastic deformation at the 7~16 showed that a lubricant layer at areas of real contact. Tests made by SEMENO\ the surface of the relatively soft material (aluminium or copper for instance) can break down even at room temperature as a result of heavy plastic deformation. Breakdown occurs because the lubricant is displaced from the contact zone as the surface layers ~‘ear, 4 (1961) 292-299
EFFECT OF LOAD ON CRITICAL TEMPERATURE
OF OIL
299
of the metal are pushed aside in the deformation process and underlying layers, not covered by the lubricant, come to the surface. In sliding friction the possibility of the lubricant layer breaking down, as a result of plastic deformation, is higher still because in addition to the normal force considerable tangential forces arise, which can contribute to the removal of the oil film and so to seizure. When plastic deformations occur only at separate asperities on the contacting surfaces and fail to spread deeper into the metal, failure of the lubricant film is mainly the result of temperature effects. This was the case in the tests with hardened steel against hardened steel, over the whole range of loads used. With other combinations of materials this mechanism held only for a limited load range. When plastic deformation is marked, failure of the lubricant layer may be due to the joint influence of temperature and plastic flow of the material at the contact surface. This was probably the case for hardened steel against copper or polyamide, and copper against copper. Therefore, when determining the critical temperature of an oil film on relatively soft materials rubbing together, the pressure at the contact area should be so chosen that there is no marked plastic deformation. Contact pressures recommended for critical temperature determinations with various material combinations by this technique are given in Table II. TABLE Rubbingmaterials
Permissible contact pressure (kg/cmz)
Hardened steel against hardened steel
up to 30‘000
~_
II
Hardened steel against copper or copper alloys -._
copper (or alloy) agaznst cofiper (or alloy)
100-200
20-40
Hardened steel against p0lyamidz.s (itylon, caplon) __.“~_~
40-80 -----
ACKNOWLEDGEMEXT
should like to express my appreciation to Dr. F. P. BOWDEN, Dr. D. TABOR and Mr. KEITH MCLAREN for their help and advice in preparing this paper for publication. I
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