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Effect of proprietary oil additives on elastohydrodynamic film thickness
Effect of proprietary oil additives on elastohydrodynamic film thickness C. R. Gentle
A study is reported of the effect on a simulated elastohydrodynamic contact of the addition of three proprietary products to the lubricating oil. Chromatic interferometry was used to study changes in lubricant film thickness. Under the conditions employed it was found that Molyslip did not affect film thickness markedly, but there is evidence that solid particles of molybdenum disulphide did enter the contact. STP and Redex both increased the film thickness as a result of increasing the viscosity, but this effect was greatly reduced at high shear rates. No evidence was found for the formation of permanent or semi-permanent surface films, although results have so far only been obtained at room temperature. Keywords: lubricant additives, elastohydrodynamic lubrication, multigrade oils For many years motoring shops have had available a variety of proprietary products which can be added to the oil in the engine, the gearbox, or the differential of automobiles and trucks. The intention is that the additive 'reduces wear, gives smoother running and improves performance'. These claims are not easy to check; partly because they are subjective in that 'smoothness' and 'performance' can be assessed differently by different observers, and partly because any objective assessment would need extensive and varied tests in the field. An individual motorist, or even a truck fleet service manager, has little data on which to formulate an opinion other than the impressive figures published by the firms marketing the product. The situation is further confused by the insistence of oil companies that the additives are not necessary. Modern multigrade oils already contain a cocktail of additives, including the boundary lubricants which are most important in preventing scuffing wear, and so further additives would indeed appear to be superfluous. However, the simple belief persists among motorists that lubrication is somehow better if the viscosity of the lubricant is higher, as evidenced by the resistance to adopting 10 W/40 multigrades in preference to the older 20 W/50 oils. The fact that the proprietary additives in many cases increase the oil viscosity substantially is taken - even by many professional engineers - as sufficient justification for purchasing. The argument is that a more viscous lubricant should generate thicker films to separate the bearing surfaces, and hence the rate of wear is reduced. Clearly this ignores the fact that wear is principally dependent on boundary lubrication, and is therefore controlled chemically, and that thicker oils produce more drag and also circulate less freely through the lubrication system. The purpose of this paper is therefore to report a study of the detailed influence of a range of commercial additives on an elastohydrodynamic (ehd) contact which simulates the gears, cams, tappets etc of an internal combustion engine. The intention was to find out whether raising the lubricant viscosity by using long chain polymer additives produces any better results than simply using an oil of higher viscosity. Department of Mechanical Engineering, Trent Polytechnic, Burton Street, Nottingham NG1 4BU, UK
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Aims The experimental parameter which best measures the efficacy of oils in full fluid film lubrication is the film thickness generated to prevent metal-to-metal contact, leading to wear, friction and noise. During starting and stopping, in particular, it would be beneficial to generate a thicker oil film so that the surfaces are physically separated at a lower speed. Less reliance is then put on the boundary lubrication associated with very thin adhered surface -films. For the additives to make worthwhile contributions to ehd lubrication they must in some way enhance the film thickness. One method of studying the role of the additives is, therefore, an experimental investigation of the films produced in an ehd contact by a commercial multigrade oil mixed with each of several additives. There are several ways in which these additives may increase the film thickness, including: • they may act simply as oil 'thickeners' ie they would increase the viscosity of the oil, resulting in a thicker film • they may enhance the rate at which the viscosity rises with pressure, so that although the viscosity of the oil supplied to the contacts remains largely unchanged, the viscosity inside the ehd contacts is increased because of the enormous pressures there • they may form a semi-solid film, adhering to the contact surfaces, which remains when the surfaces have come to rest and there is no longer any hydrodynamic action. All of these three processes should be detectable from experimental measurements of film thickness against speed, provided that a suitably sensitive technique is adopted. The method chosen here was chromatic interferometry.
E xperimental procedure Optical interferometry of lubricated contacts, particularly ehd contacts, has been developed largely by Cameron ~'2'3 . It relies on simulating a real contact by using a transparent material, such as glass or sapphire, for one of the contacting surfaces. A collimated beam of light is passed at normal incidence through this surface into the contact region. Part
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Gentle - A d d i t i v e effects on ehl film thickness
of the light reflects from the transparent surface, aided by a semi-reflecting surface layer, and part of it traverses the oil film before being reflected from the polished metal surface, back along the same path. The two parts of the beam then recombine but with a path difference due to the film thickness. Observation of the contact through a microscope reveals an interference pattern. For white-light illumination the pattern is composed of many colours, each one representing a certain film thickness which can be determined from a separate calibration experiment. The advantage of using white light in this application is that although the range of thicknesses observed is limited (compared with •monochromatic light) the precision is much greater. At least four colours can be identified readily in each order of interference, against one fringe for monochromatic light.
Figure 3 is only a monochrome picture of the coloured field of view but illustrates that the central region within the horsehoe shaped area comprises the greater part of the contact. The experimental data to be determined are the optical film thickness of the central region, calibrated as explained by Gentle 4 , and the rolling speed of the contact. Since the effect on film thickness of the additives is being studied relative to a base oil, there is no need to convert the optical thickness to actual thickness by considering the refractive index of the oils because the proportion of additive used does not significantly affect this value. The rolling speed, however, must be converted to a speed parameter the variable normally considered in this technique - by
@
The experimental apparatus is shown in the general view of Fig 1 and the detailed diagram of Fig 2. A metallurgical microscope is accurately positioned over a lubricated, polished steel ball driven by a variable speed motor. The rotating ball in turn drives a glass disc located radially and vertically by air bearings. Hence the lubricated condition is for pure rolling (to an accuracy of better than 5%), although provision has been made for introducing sliding at a later date. The ball and its support bearings are mounted at one end of a cantilevered motor plate so that the contact load may be varied by means of a simple weight hanger. The maximum Hertzian pressure in the contact achieved by this system is 0.7 GPa, which is representative of many real situations but cannot reproduce the most heavily loaded gear contacts. This pressure refers to the centre of the contact, which was also the region where the film thickness was measured in the example.
Lighl
source
Disc
~Semi - reflecling
chromiumlayer
Fig 2 Detailed view of the lubricated contact region
e
jLight source Hemispher
1
~/Contilevered
molar~,o~itor
Verticalai
J ~
o ~
Baseplate
J Fig 1 General view of the experimental apparatus
August 1983 Vol 16 No 4
Gentle - A d d i t i v e effects on ehl f i l m thickness
Table 1 Viscosity characteristics of the test mixtures
Fig 3Interference pattern for a typical roiling point contact multiplying by the inlet viscosity rio, since the fundamental relationship in ehd is that: h ¢x( U n o ) a
(1)
Oil*
Oil + STP
Oil + Redex
Oil + Molyslip
Volume % of additive
-
8.3
5.0
6.25
Kinematic viscosity, mm 2/s
302.7
372.3
361.3
292.2
Relative density
0.8826
0.9006
0.8943
0.9128
Dynamic viscosity, Pa.s
0.267
0.335
0.323
0.267
*Castrol G TX
9-
where h is film thickness (m), U is rolling speed (rev/s), 77o is viscosity of oil at inlet (Pa. s) and 'a' is a constant for a particular oil, with a value about 0.7. The first procedure was to blend several oil/additive mixtures and determine their viscosities. The additive packages chosen were STP, Redex and Molyslip (as detailed in Appendix 1). The oil used was Castrol GTX, a widely available multigrade engine oil. The advantage of using a multigrade oil was that the viscosity would not fall rapidly with temperature. Although the experiments were only to be performed at room temperature, the viscosities would not then be dramatically different from those in, say, a gearbox or a differential operating at rather higher temperatures. The volumetric percentage used in each of the three mixtures was that recommended by the manufacturers for maximum effectiveness in crankcases. Only small volumes were needed, both for the viscometry and for the ehd tests, so that mixing volumes were measured using two syringes; 25 ml and 5 ml for the oil and additives respectively. The volume percentages used are listed in Table 1. The next stage was to measure the kinematic viscosity of the mixtures using the method of British Standard BS 188 : 1977. This essentially measured the time for a known volume of liquid to flow through a capillary tube at a closely controlled temperature. In this case the temperature chosen was 22°C and the flow times were about 15 min, considerably more than the recommended minimum of 200 s. An interesting point which was noted during these measurements was that the mixture of oil and Molyslip exhibited a very high surface tension, but this was not pursued. Kinematic viscosity was converted to the dynamic viscosity 77o by measuring the density of each sample. The weights of 10 ml samples of oil were found by a difference method, and the estimated errors were less than 0.25% on density. The results are summarized in Table 1. As will be seen, the immediate effect of adding the STP and the Redex, both of which are extremely thick liquids, is to increase the viscosity of the mixtures. The Molyslip, on the other hand, is a suspension of solid particles of molybdenum disulphide in a fairly light oil. Addition of this actually lowers the kinematic viscosity but raises the density, to leave the dynamic viscosity unchanged.
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o
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/x
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-3
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~
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j
LT- 2
I 0.6
/
I I I 0.8 1.0
I I 2.0 3.0 Boll speed, r/s
I 4.0
I 5.0
I 6.0
Fig 4 Plot of centra'l optical film thickness against rolling speed for multigrade oil (gradient of line --- O.758) After thorough cleaning of the contact areas the ehd film thickness tests were then started. The test mixture was added to the lubricant reservoir and the ball/disc combination was rotated at a low speed and load to spread the lubricant over the two surfaces. The apparatus was left for several hours at the controlled temperature of 22°C -+ ~A°C, to allow time for formation of any soft chemical surface films 5 . The film thickness tests themselves were conducted by increasing the ball speed from rest under full load until the first standard colotir was observed at approximately 1.3 x 10 -7 m film thickness. The speed at this point was measured digitally using an opto-electric device which detected a series of 100 reflective strips located on the circumference of the disc. The speed was then increased further to achieve the other set colours up to a film thickness of about 4.5 x 10 -7 m. The measurements were repeated as the speed was decreased to ensure that the centre of the colour band had genuinely been used and not just one edge. Results were plotted in the conventional manner I,le, using logarithmic axes) in order to check the veracity of Eq (1). A typical graph for the multigrade oil (Fig 4) shows that
G e n t l e - A d d i t i v e e f f e c t s o n e h l f i l m thickness
the results are as expected, with the value of the constant 'a' (the slope) determined by a regression fit to be 0.758. An indication of the error in the readings is given by the spread of the results; it was found that the fifth colour points were consistently below the line, pointing to the error being in the earlier calibration rather than these experiments.
Discussion of results Similar graphs were plotted for the three oil/additive mixtures. The results for the Molyslip mixture were very close to those for the oil itself, but the Redex and the STP mixtures with oil showed a significantly greater film thickness at any given speed. In all three cases the gradient of the straight lines was lower by about 10%. The immediate conclusion is that STP and Redex do increase ehd film thickness. However, since both of these products increased the inlet viscosity of their mixtures, it must be determined whether this effect is any greater than simply using a higher viscosity base oil in the first place. The next stage was to plot the results in terms of the speed parameter (U~7o) to eliminate this viscosity effect. The results are shown in Fig 5, with the experimental points omitted for clarity.
The oil/Molyslip mixture again is close to the multigrade oil line because the viscosities are approximately equal, but the STP and Redex mixtures both produce markedly lower film thicknesses on this 'normalized' basis over the range studied here. Hence, these two mixtures produce lower film thicknesses than a base oil of the same inlet viscosity. There is little point in pursuing the investigation of the oil/ Molyslip mixture any further. The mode of action is intentionally different from that of the other two additives; the aim is to introduce molybdenum disulphide particles into the contact to act, effectively, like a dry lubricant during starting and stopping procedures. This has been observed in previous investigations 5 . The additive has been found to have little effect on film thickness during running. The results for the STP and Redex mixtures do need further examination, however, to establish why the gradients of the lines are significantly lower. A possible explanation is that the additives form a semipermanent surface film on the metallic half of the contact as shown in Fig 6. This film would be so thin as to be transparent, so that the optical measurement would still be of the separation h of the two solid surfaces. However, this would be composed of two portions, the surface film x and the hydrodynamically generated film hehd so that h = x + hehd
/(//":-"
x E v
~ .c E it.
j
'
//~/
f
,
~
Multigrade oil
-----
~""
. ....
500
o,,
(2)
Since only heh d would obey the relationship of Eq (1), then plotting h against the speed parameter would lead to all the points being raised above the theoretical line (for the multigrade oil) by the distance x. On the logarithmic axes this would appear to a greater extent at small values of h and would result in the slope of the line being reduced. This type of behaviour would be evidenced therefore by the line being moved towards the top left of the graph, and with a lower gradient. However, the results obtained clearly show that although the slope is lowered the move is towards the bottom right. Consequently the
sTP
-- Oil + REDEX Oil + MOLYSLIP
I
I I I I I i 500 700 I000 Boll speed (r/s) x Inlet viscosity (mPa.s)
330 f 320
2000
-
~ ~
• Oil plus STP o Oil plus REDEX
Fig 5 Film thickness against speed parameter for multigrade oil and the three mixtures 310I 300
"I
'~290 "E
280I 270 f
0
Fig 6 The effect era surface film on measurement of optical ehd film thickness
196
I
2 Boll speed, r/s
3
Fig 7 Variation of apparent inlet viscosity with ball speed
August 1983 Vol 16 No 4
Gentle - Additive effects on ehl film thickness
effect cannot simply be attributed to the formation of a significant surface film under these conditions. Results such as these show that adding STP and Redex increases the film thickness by a smaller amount than would be achieved simply by using a higher viscosity oil, and that the mixtures do not generate protective surface films of a size which has any effect on the hydrodynamic lubrication of the contacts. The decrease in the gradients of the slopes in Fig 5 is most likely to be attributable to a shear rate effect on the viscosity of the incoming oil. This is difficult to pursue in great detail because the oil is at high pressure and very high shear rates (up to l0 s s -l ), even for the pure rolling case here. The bulk of the oil approaching the contact cannot pass through, because the gap is so small, and must reverse direction, producing two planes of high shear in the inlet, precisely at the point where the contact conditions are governed (as explained in Reference 2). An apparatus for directly studying lubricants at very high pressures and shear rates in a manner reoresentative of ehd contacts has only recently been developed by Bair and Winer 6 . Nevertheless it is worthwhile studying the possibility that the reduced experimental slopes may be caused by the fact that the viscosities of the mixtures, measured under low shear rates, do not remain constant at the much higher shear rates around the contact. Effectively this uses the apparatus as a viscometer, againas explained in Reference 2. If the viscosities fell with increasing ball speed then the experimental lines would drop progressively lower than the ones expected from a constant viscosity assumption; hence, the slopes would be reduced. This can be examined by calculating what fall in low pressure viscosity would account for the lowering of the line predicted by Eq (1). The assumption must be made that where the experimental line for the mixture crosses the line for the base oil on Fig 5, even if only by extrapolation, the effective inlet viscosity of the mixture equals the viscosity of the mixture as determined in the capillary viscometer. The results from this approach are subject to considerable inaccuracies, both experimental and conceptual. In particular the Castrol oil itself will exhibit a variation of viscosity with shear rate as it is almost certainly nonNewtonian. Viscosity estimates for the mixtures must therefore be viewed relative to this changing base line rather than as absolute viscosities. Consequently, in Fig 7, the viscosities of the mixtures are shown in comparison with the low shear viscosity of the Castrol oil, rather than any measured high shear viscosity. Nevertheless the original results of film thickness against speed for the Castrol oil are typical of the results expected from an oil which does not exhibit gross non-Newtonian behaviour and so any variation with shear rate is unlikely to be large when compared with the variations inferred for the mixtures with Redex and STP. The balance of evidence indicates that the viscosities of the mixtures tend to approach the viscosity of the Castrol GTX as the ball speed, and hence the shear rate, increases. It must be admitted, however, that the evidence is largely speculative.
Conclusions The addition of chemicals to mineral oils to improve their boundary lubrication ability has been common practice for many years and it was shown 7 at a very eady stage that this did not have any detectable effect on elastohydrodynamic film thicknesses. Furthermore, viscosity index improvers
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are now also commonly added to reduce the fall in viscosity with temperature. In spite of this, however, the sale of proprietary extra additives has flourished, based on manufacturers' claims of reduced wear etc. These claims may well prove correct in the long run, but the main conclusion of this work is that any benefits are n'ot due to direct enhancement of the oil's elastohydrodynamic lubricating ability. The Molyslip does not increase the viscosity of the oil mixture, does not increase the ehd film thickness and does not produce any detectable surface 'plating' with solid lubricant particles. The Redex and STP both increased the film thickness obtained, but this was not caused by formation of any detectable permanent or semi-permanent surface film. It was attributed to an increase in the viscosity of the mixture compared with the base oil. This viscosity increase was found to be less at higher rotation speeds and the viscosity appeared to revert to the base viscosity at the highest speeds studied. This viscosity rise is useful during starting and stopping of the contact as it increases the film thickness under these conditions, but any advantages may be offset by the increased drag on the engine and the reduced rate of flow around the remainder of the lubricating circuit. The results are at present limited by the lack of a facility to test at high temperatures, and refer only to ehd lubrication, but the initial overall conclusion is that the additives are of only limited usefulness, given that modern lubricating oils already contain a comprehensive range of additives.
References 1. Cameron A. and Gohat R. Theoretical and Experimental Studies of the Oil Film in Lubricated Point Contacts. Proc. Roy. Soc. A, 1966, 291,520 2. Foord C.A., Wedeven L.D., Westlake F.J. and Cameron A.
Optical Elastohydrodynamics.Proc. IMeehE, 1969- 70, 184 I 3. Gentle C.R. and Cameron A. Optical ehd at Extreme Pressures. Nature 1973, 246(5434), 478-479
4. Gentle C.R. Traction in ehd Point Contacts. PhD Thesis, University of London (1971)
5. Gentle C.R. and Day R.S. The Effect of Some Anti-wear Additives on the ehl of a Point Contact. Proc. 1MechE. Tribology Convention, Keele University, 1972
6. Bait S. and Wirier W.O. Some Observations in High Pressure Rheology of Lubricants. J. Lubrication Technology, July 1982, 104,357-364
7. Archard J.F., Hatcher and Kirk Some Experiments upon the Behaviom of Hypoid Oils in Heavily Loaded Contacts. 1"roe. 1MechE, 1963-64, 178(3), 258-263
Appendix 1. Product Information STP is made by:
STP Corporation, 1400 West Commercial Boulevard,
Fort Lauderdale, Florida, USA.
Redex and Molyslip are made by: Lloyds Industries Ltd, Lloyds House,
Wilmslow, Cheshire, UK.
Editors note It is acknowledged that interpretation of these results is open to debate. Letters to the Editor will be welcomed.
197
A study is reported of the effect on a simulated elastohydrodynamic contact of the addition of three proprietary products to the lubricating oil. Chromatic interferometry was used to study changes in lubricant film thickness. Under the conditions employed it was found that Molyslip did not affect film thickness markedly, but there is evidence that solid particles of molybdenum disulphide did enter the contact. STP and Redex both increased the film thickness as a result of increasing the viscosity, but this effect was greatly reduced at high shear rates. No evidence was found for the formation of permanent or semi-permanent surface films, although results have so far only been obtained at room temperature. Keywords: lubricant additives, elastohydrodynamic lubrication, multigrade oils For many years motoring shops have had available a variety of proprietary products which can be added to the oil in the engine, the gearbox, or the differential of automobiles and trucks. The intention is that the additive 'reduces wear, gives smoother running and improves performance'. These claims are not easy to check; partly because they are subjective in that 'smoothness' and 'performance' can be assessed differently by different observers, and partly because any objective assessment would need extensive and varied tests in the field. An individual motorist, or even a truck fleet service manager, has little data on which to formulate an opinion other than the impressive figures published by the firms marketing the product. The situation is further confused by the insistence of oil companies that the additives are not necessary. Modern multigrade oils already contain a cocktail of additives, including the boundary lubricants which are most important in preventing scuffing wear, and so further additives would indeed appear to be superfluous. However, the simple belief persists among motorists that lubrication is somehow better if the viscosity of the lubricant is higher, as evidenced by the resistance to adopting 10 W/40 multigrades in preference to the older 20 W/50 oils. The fact that the proprietary additives in many cases increase the oil viscosity substantially is taken - even by many professional engineers - as sufficient justification for purchasing. The argument is that a more viscous lubricant should generate thicker films to separate the bearing surfaces, and hence the rate of wear is reduced. Clearly this ignores the fact that wear is principally dependent on boundary lubrication, and is therefore controlled chemically, and that thicker oils produce more drag and also circulate less freely through the lubrication system. The purpose of this paper is therefore to report a study of the detailed influence of a range of commercial additives on an elastohydrodynamic (ehd) contact which simulates the gears, cams, tappets etc of an internal combustion engine. The intention was to find out whether raising the lubricant viscosity by using long chain polymer additives produces any better results than simply using an oil of higher viscosity. Department of Mechanical Engineering, Trent Polytechnic, Burton Street, Nottingham NG1 4BU, UK
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Aims The experimental parameter which best measures the efficacy of oils in full fluid film lubrication is the film thickness generated to prevent metal-to-metal contact, leading to wear, friction and noise. During starting and stopping, in particular, it would be beneficial to generate a thicker oil film so that the surfaces are physically separated at a lower speed. Less reliance is then put on the boundary lubrication associated with very thin adhered surface -films. For the additives to make worthwhile contributions to ehd lubrication they must in some way enhance the film thickness. One method of studying the role of the additives is, therefore, an experimental investigation of the films produced in an ehd contact by a commercial multigrade oil mixed with each of several additives. There are several ways in which these additives may increase the film thickness, including: • they may act simply as oil 'thickeners' ie they would increase the viscosity of the oil, resulting in a thicker film • they may enhance the rate at which the viscosity rises with pressure, so that although the viscosity of the oil supplied to the contacts remains largely unchanged, the viscosity inside the ehd contacts is increased because of the enormous pressures there • they may form a semi-solid film, adhering to the contact surfaces, which remains when the surfaces have come to rest and there is no longer any hydrodynamic action. All of these three processes should be detectable from experimental measurements of film thickness against speed, provided that a suitably sensitive technique is adopted. The method chosen here was chromatic interferometry.
E xperimental procedure Optical interferometry of lubricated contacts, particularly ehd contacts, has been developed largely by Cameron ~'2'3 . It relies on simulating a real contact by using a transparent material, such as glass or sapphire, for one of the contacting surfaces. A collimated beam of light is passed at normal incidence through this surface into the contact region. Part
0301-679X/83/040193-05 $03.00 © 1983 Butterworth & Co (Publishers) Ltd
193
Gentle - A d d i t i v e effects on ehl film thickness
of the light reflects from the transparent surface, aided by a semi-reflecting surface layer, and part of it traverses the oil film before being reflected from the polished metal surface, back along the same path. The two parts of the beam then recombine but with a path difference due to the film thickness. Observation of the contact through a microscope reveals an interference pattern. For white-light illumination the pattern is composed of many colours, each one representing a certain film thickness which can be determined from a separate calibration experiment. The advantage of using white light in this application is that although the range of thicknesses observed is limited (compared with •monochromatic light) the precision is much greater. At least four colours can be identified readily in each order of interference, against one fringe for monochromatic light.
Figure 3 is only a monochrome picture of the coloured field of view but illustrates that the central region within the horsehoe shaped area comprises the greater part of the contact. The experimental data to be determined are the optical film thickness of the central region, calibrated as explained by Gentle 4 , and the rolling speed of the contact. Since the effect on film thickness of the additives is being studied relative to a base oil, there is no need to convert the optical thickness to actual thickness by considering the refractive index of the oils because the proportion of additive used does not significantly affect this value. The rolling speed, however, must be converted to a speed parameter the variable normally considered in this technique - by
@
The experimental apparatus is shown in the general view of Fig 1 and the detailed diagram of Fig 2. A metallurgical microscope is accurately positioned over a lubricated, polished steel ball driven by a variable speed motor. The rotating ball in turn drives a glass disc located radially and vertically by air bearings. Hence the lubricated condition is for pure rolling (to an accuracy of better than 5%), although provision has been made for introducing sliding at a later date. The ball and its support bearings are mounted at one end of a cantilevered motor plate so that the contact load may be varied by means of a simple weight hanger. The maximum Hertzian pressure in the contact achieved by this system is 0.7 GPa, which is representative of many real situations but cannot reproduce the most heavily loaded gear contacts. This pressure refers to the centre of the contact, which was also the region where the film thickness was measured in the example.
Lighl
source
Disc
~Semi - reflecling
chromiumlayer
Fig 2 Detailed view of the lubricated contact region
e
jLight source Hemispher
1
~/Contilevered
molar~,o~itor
Verticalai
J ~
o ~
Baseplate
J Fig 1 General view of the experimental apparatus
August 1983 Vol 16 No 4
Gentle - A d d i t i v e effects on ehl f i l m thickness
Table 1 Viscosity characteristics of the test mixtures
Fig 3Interference pattern for a typical roiling point contact multiplying by the inlet viscosity rio, since the fundamental relationship in ehd is that: h ¢x( U n o ) a
(1)
Oil*
Oil + STP
Oil + Redex
Oil + Molyslip
Volume % of additive
-
8.3
5.0
6.25
Kinematic viscosity, mm 2/s
302.7
372.3
361.3
292.2
Relative density
0.8826
0.9006
0.8943
0.9128
Dynamic viscosity, Pa.s
0.267
0.335
0.323
0.267
*Castrol G TX
9-
where h is film thickness (m), U is rolling speed (rev/s), 77o is viscosity of oil at inlet (Pa. s) and 'a' is a constant for a particular oil, with a value about 0.7. The first procedure was to blend several oil/additive mixtures and determine their viscosities. The additive packages chosen were STP, Redex and Molyslip (as detailed in Appendix 1). The oil used was Castrol GTX, a widely available multigrade engine oil. The advantage of using a multigrade oil was that the viscosity would not fall rapidly with temperature. Although the experiments were only to be performed at room temperature, the viscosities would not then be dramatically different from those in, say, a gearbox or a differential operating at rather higher temperatures. The volumetric percentage used in each of the three mixtures was that recommended by the manufacturers for maximum effectiveness in crankcases. Only small volumes were needed, both for the viscometry and for the ehd tests, so that mixing volumes were measured using two syringes; 25 ml and 5 ml for the oil and additives respectively. The volume percentages used are listed in Table 1. The next stage was to measure the kinematic viscosity of the mixtures using the method of British Standard BS 188 : 1977. This essentially measured the time for a known volume of liquid to flow through a capillary tube at a closely controlled temperature. In this case the temperature chosen was 22°C and the flow times were about 15 min, considerably more than the recommended minimum of 200 s. An interesting point which was noted during these measurements was that the mixture of oil and Molyslip exhibited a very high surface tension, but this was not pursued. Kinematic viscosity was converted to the dynamic viscosity 77o by measuring the density of each sample. The weights of 10 ml samples of oil were found by a difference method, and the estimated errors were less than 0.25% on density. The results are summarized in Table 1. As will be seen, the immediate effect of adding the STP and the Redex, both of which are extremely thick liquids, is to increase the viscosity of the mixtures. The Molyslip, on the other hand, is a suspension of solid particles of molybdenum disulphide in a fairly light oil. Addition of this actually lowers the kinematic viscosity but raises the density, to leave the dynamic viscosity unchanged.
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8 7 6
o
j,¢/,
5-
J'
,,,,,pX/
/x
E 4..-c o
5-
-3
X
g
~
Gradient 0 . 7 5 8
j
LT- 2
I 0.6
/
I I I 0.8 1.0
I I 2.0 3.0 Boll speed, r/s
I 4.0
I 5.0
I 6.0
Fig 4 Plot of centra'l optical film thickness against rolling speed for multigrade oil (gradient of line --- O.758) After thorough cleaning of the contact areas the ehd film thickness tests were then started. The test mixture was added to the lubricant reservoir and the ball/disc combination was rotated at a low speed and load to spread the lubricant over the two surfaces. The apparatus was left for several hours at the controlled temperature of 22°C -+ ~A°C, to allow time for formation of any soft chemical surface films 5 . The film thickness tests themselves were conducted by increasing the ball speed from rest under full load until the first standard colotir was observed at approximately 1.3 x 10 -7 m film thickness. The speed at this point was measured digitally using an opto-electric device which detected a series of 100 reflective strips located on the circumference of the disc. The speed was then increased further to achieve the other set colours up to a film thickness of about 4.5 x 10 -7 m. The measurements were repeated as the speed was decreased to ensure that the centre of the colour band had genuinely been used and not just one edge. Results were plotted in the conventional manner I,le, using logarithmic axes) in order to check the veracity of Eq (1). A typical graph for the multigrade oil (Fig 4) shows that
G e n t l e - A d d i t i v e e f f e c t s o n e h l f i l m thickness
the results are as expected, with the value of the constant 'a' (the slope) determined by a regression fit to be 0.758. An indication of the error in the readings is given by the spread of the results; it was found that the fifth colour points were consistently below the line, pointing to the error being in the earlier calibration rather than these experiments.
Discussion of results Similar graphs were plotted for the three oil/additive mixtures. The results for the Molyslip mixture were very close to those for the oil itself, but the Redex and the STP mixtures with oil showed a significantly greater film thickness at any given speed. In all three cases the gradient of the straight lines was lower by about 10%. The immediate conclusion is that STP and Redex do increase ehd film thickness. However, since both of these products increased the inlet viscosity of their mixtures, it must be determined whether this effect is any greater than simply using a higher viscosity base oil in the first place. The next stage was to plot the results in terms of the speed parameter (U~7o) to eliminate this viscosity effect. The results are shown in Fig 5, with the experimental points omitted for clarity.
The oil/Molyslip mixture again is close to the multigrade oil line because the viscosities are approximately equal, but the STP and Redex mixtures both produce markedly lower film thicknesses on this 'normalized' basis over the range studied here. Hence, these two mixtures produce lower film thicknesses than a base oil of the same inlet viscosity. There is little point in pursuing the investigation of the oil/ Molyslip mixture any further. The mode of action is intentionally different from that of the other two additives; the aim is to introduce molybdenum disulphide particles into the contact to act, effectively, like a dry lubricant during starting and stopping procedures. This has been observed in previous investigations 5 . The additive has been found to have little effect on film thickness during running. The results for the STP and Redex mixtures do need further examination, however, to establish why the gradients of the lines are significantly lower. A possible explanation is that the additives form a semipermanent surface film on the metallic half of the contact as shown in Fig 6. This film would be so thin as to be transparent, so that the optical measurement would still be of the separation h of the two solid surfaces. However, this would be composed of two portions, the surface film x and the hydrodynamically generated film hehd so that h = x + hehd
/(//":-"
x E v
~ .c E it.
j
'
//~/
f
,
~
Multigrade oil
-----
~""
. ....
500
o,,
(2)
Since only heh d would obey the relationship of Eq (1), then plotting h against the speed parameter would lead to all the points being raised above the theoretical line (for the multigrade oil) by the distance x. On the logarithmic axes this would appear to a greater extent at small values of h and would result in the slope of the line being reduced. This type of behaviour would be evidenced therefore by the line being moved towards the top left of the graph, and with a lower gradient. However, the results obtained clearly show that although the slope is lowered the move is towards the bottom right. Consequently the
sTP
-- Oil + REDEX Oil + MOLYSLIP
I
I I I I I i 500 700 I000 Boll speed (r/s) x Inlet viscosity (mPa.s)
330 f 320
2000
-
~ ~
• Oil plus STP o Oil plus REDEX
Fig 5 Film thickness against speed parameter for multigrade oil and the three mixtures 310I 300
"I
'~290 "E
280I 270 f
0
Fig 6 The effect era surface film on measurement of optical ehd film thickness
196
I
2 Boll speed, r/s
3
Fig 7 Variation of apparent inlet viscosity with ball speed
August 1983 Vol 16 No 4
Gentle - Additive effects on ehl film thickness
effect cannot simply be attributed to the formation of a significant surface film under these conditions. Results such as these show that adding STP and Redex increases the film thickness by a smaller amount than would be achieved simply by using a higher viscosity oil, and that the mixtures do not generate protective surface films of a size which has any effect on the hydrodynamic lubrication of the contacts. The decrease in the gradients of the slopes in Fig 5 is most likely to be attributable to a shear rate effect on the viscosity of the incoming oil. This is difficult to pursue in great detail because the oil is at high pressure and very high shear rates (up to l0 s s -l ), even for the pure rolling case here. The bulk of the oil approaching the contact cannot pass through, because the gap is so small, and must reverse direction, producing two planes of high shear in the inlet, precisely at the point where the contact conditions are governed (as explained in Reference 2). An apparatus for directly studying lubricants at very high pressures and shear rates in a manner reoresentative of ehd contacts has only recently been developed by Bair and Winer 6 . Nevertheless it is worthwhile studying the possibility that the reduced experimental slopes may be caused by the fact that the viscosities of the mixtures, measured under low shear rates, do not remain constant at the much higher shear rates around the contact. Effectively this uses the apparatus as a viscometer, againas explained in Reference 2. If the viscosities fell with increasing ball speed then the experimental lines would drop progressively lower than the ones expected from a constant viscosity assumption; hence, the slopes would be reduced. This can be examined by calculating what fall in low pressure viscosity would account for the lowering of the line predicted by Eq (1). The assumption must be made that where the experimental line for the mixture crosses the line for the base oil on Fig 5, even if only by extrapolation, the effective inlet viscosity of the mixture equals the viscosity of the mixture as determined in the capillary viscometer. The results from this approach are subject to considerable inaccuracies, both experimental and conceptual. In particular the Castrol oil itself will exhibit a variation of viscosity with shear rate as it is almost certainly nonNewtonian. Viscosity estimates for the mixtures must therefore be viewed relative to this changing base line rather than as absolute viscosities. Consequently, in Fig 7, the viscosities of the mixtures are shown in comparison with the low shear viscosity of the Castrol oil, rather than any measured high shear viscosity. Nevertheless the original results of film thickness against speed for the Castrol oil are typical of the results expected from an oil which does not exhibit gross non-Newtonian behaviour and so any variation with shear rate is unlikely to be large when compared with the variations inferred for the mixtures with Redex and STP. The balance of evidence indicates that the viscosities of the mixtures tend to approach the viscosity of the Castrol GTX as the ball speed, and hence the shear rate, increases. It must be admitted, however, that the evidence is largely speculative.
Conclusions The addition of chemicals to mineral oils to improve their boundary lubrication ability has been common practice for many years and it was shown 7 at a very eady stage that this did not have any detectable effect on elastohydrodynamic film thicknesses. Furthermore, viscosity index improvers
TR I BOLOGY international
are now also commonly added to reduce the fall in viscosity with temperature. In spite of this, however, the sale of proprietary extra additives has flourished, based on manufacturers' claims of reduced wear etc. These claims may well prove correct in the long run, but the main conclusion of this work is that any benefits are n'ot due to direct enhancement of the oil's elastohydrodynamic lubricating ability. The Molyslip does not increase the viscosity of the oil mixture, does not increase the ehd film thickness and does not produce any detectable surface 'plating' with solid lubricant particles. The Redex and STP both increased the film thickness obtained, but this was not caused by formation of any detectable permanent or semi-permanent surface film. It was attributed to an increase in the viscosity of the mixture compared with the base oil. This viscosity increase was found to be less at higher rotation speeds and the viscosity appeared to revert to the base viscosity at the highest speeds studied. This viscosity rise is useful during starting and stopping of the contact as it increases the film thickness under these conditions, but any advantages may be offset by the increased drag on the engine and the reduced rate of flow around the remainder of the lubricating circuit. The results are at present limited by the lack of a facility to test at high temperatures, and refer only to ehd lubrication, but the initial overall conclusion is that the additives are of only limited usefulness, given that modern lubricating oils already contain a comprehensive range of additives.
References 1. Cameron A. and Gohat R. Theoretical and Experimental Studies of the Oil Film in Lubricated Point Contacts. Proc. Roy. Soc. A, 1966, 291,520 2. Foord C.A., Wedeven L.D., Westlake F.J. and Cameron A.
Optical Elastohydrodynamics.Proc. IMeehE, 1969- 70, 184 I 3. Gentle C.R. and Cameron A. Optical ehd at Extreme Pressures. Nature 1973, 246(5434), 478-479
4. Gentle C.R. Traction in ehd Point Contacts. PhD Thesis, University of London (1971)
5. Gentle C.R. and Day R.S. The Effect of Some Anti-wear Additives on the ehl of a Point Contact. Proc. 1MechE. Tribology Convention, Keele University, 1972
6. Bait S. and Wirier W.O. Some Observations in High Pressure Rheology of Lubricants. J. Lubrication Technology, July 1982, 104,357-364
7. Archard J.F., Hatcher and Kirk Some Experiments upon the Behaviom of Hypoid Oils in Heavily Loaded Contacts. 1"roe. 1MechE, 1963-64, 178(3), 258-263
Appendix 1. Product Information STP is made by:
STP Corporation, 1400 West Commercial Boulevard,
Fort Lauderdale, Florida, USA.
Redex and Molyslip are made by: Lloyds Industries Ltd, Lloyds House,
Wilmslow, Cheshire, UK.
Editors note It is acknowledged that interpretation of these results is open to debate. Letters to the Editor will be welcomed.
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