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A new method for the rapid estimation of the antifriction performance of lubricants
187
Wear, 70 (1981) 187 - 195 @ Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
A NEW METHOD FOR THE RAPID ESTIMATION OF THE ANTIFRICTION PERFORMANCE OF LUBRICANTS*
CZESLAW KAJDAS, JOZEF
NITA and KRZYSZTOF
KRAWCZYK
Wyisza Szkota Iniynierska,
26-600 Radom (Poland)
(Received August 17,198O
; in revised form November 2,198O)
Summary A rapid method of measuring the antifriction properties of lubricants over the whole region of friction with a new type of tribometer is described. The tribometer can also be used to investigate chemical reaction effects on a rubbing surface under different frictional conditions.
1. Introduction The principal function of lubricating oils is to minimize energy and wear by interposing fluid or protective layers between rubbing surfaces. The protective layers are formed by physical adsorption or chemical reaction between solids and the surrounding lubricant. Usually, the thickness of the protective layers is small compared with the roughness of the solid surfaces. Efficient lubrication, i.e. the saving of mechanical energy, takes place if the rubbing surfaces are separated under motion and load. The most common tribological parameters which cause failure of lubricant films or protective layers are excessive load and excessive temperature. The influence of excessive temperature is related to the molecular desorption which causes lubricant breakdown. Hence the minimization of friction should be taken into account over the full range of lubrication up to seizure. In practice, in the range of boundary lubrication the property of lubricating oils which allows the minimization of the friction and wear of rubbing surfaces is the lubricity or oiliness. This term has often been abused in the context of antifriction, antiwear and antiseizure properties because it is not clearly defined and has no unit of measurement. A new description of the antifriction property of lubricants is proposed and a new machine is described for measuring this property over the whole region of friction. The examination of liquid lubricants for other tribological purposes is also possible. *Paper presented at the 4th International September 10 - 15,1979.
Conference
on Tribology,
Paisley,
188
2. The model of the antifriction properties of lubricants The friction
force F is determined
by the function
F = CPU’,u, T, s, e)
(1)
where P (N mmP2) is the load per unit projected area, u (m s--l) the sliding velocity, T (K) the temperature, s the state of the rubbing surfaces and e the nature of the environment. If U, T, s and e are all constant the function (1) can be plotted as a curve F, shown in Fig. 1. The straight line F,, in Fig. 1 shows the linear dependence of the friction force on the load P for a rubbing pair with the technically dry friction coefficient f. (the broken line BM is an extrapolation of the line OB). In Fig. 1 there are three regions of lubrication. The first (region I) represents surfaces working under hydrodynamic or elastohydrodynamic conditions. In this region the reduction of friction and wear is most effective. Generally this region is relevant to the viscosity and viscosity-pressure dependence of the lubricating oil. The second region (region II) of mixed and boundary friction shows the antiwear properties of the lubricant. The intensity of wear in this range of lubrication is small (Fig. 1, curve W). If the rubbing surfaces are made of chemically active materials and are immersed
Load
per
unit.
P
Fig. 1. Proposed description of the antifriction property of lubricants.
189
in lubricating oil containing extreme pressure additives the highest rate of chemical reactions takes place in this friction region. The reactions form protective layers. With increasing load, the protective layer is destroyed, causing a higher rate of wear. In the last lubrication region (region III) boundary friction predominates. This region of lubrication indicates the antiseizure properties of the lubricating oil. Under a load Pg (the point of seizure) the lubricant no longer reduces friction between the rubbing pair and the coefficient of friction exceeds fe, i.e. the characteristic coefficient of friction for the rubbing pair without a lubricant. The magnitude of Pg depends on the boundary film in the lubrication region II. From Fig. 1 it can be seen that the area L represents the performance of a lubricant in terms of lubricity in the full range of friction. If all parameters except the load are constant, and when the rate of load increase is fixed, the size of the area L depends only on the physical and physicochemical properties of the lubricant which influence lubricity. The antifriction property of lubricants is then defined by L
=
AfoP, --
2
s pg
(2)
F,(P)dP
0
where A (mm2) is the contact area of the rubbing pair. To calculate the value of L it is necessary to determine
the function
F, =cpr(P)
(3)
This can be achieved with the aid of a new type of tribometer [ 11. The tribometer also enables the determination of the limits of the lubrication regions shown in Fig. 1.
3. Construction
of the tribometer
The tribometer consists of the three general units shown in Fig. 2. The friction unit A, shown in Fig. 3, contains a rubbing pair of rotating test pieces 1 which are ring shaped (22 mm in diameter and 8 mm in height) and are mounted on the shaft of the driving electric motor 2, and two opposed cylindrical specimens 3 bearing on the horizontal rotating ring. The area of rubbing surface of the specimen is about 1 mm2. The specimens are clamped in suitable holders 5. One of the specimens accommodates the temperature gauge 4. The holders 5 can be moved along their own axes but cannot revolve about them and are accommodated in insulating guideways 6 mounted on the rig body 7. The rig body consists of an inner chamber 8 containing the lubricating oil which is to be tested 9 and the temperature gauge 10. Another chamber 11 contains the thermostatic liquid 12. The driving motor 2 is mounted on the rig body 7 by means of a special ball bearing 13 through a spring element 14 on which the strain gauges 15 are mounted. Both of the holders 5 of the specimens 3 project from the insulat-
190
10
21
19
1n
19
11
21
22
Fig. 2. Block scheme of the tribometer. Fig. 3. Diagrammatic representation of the friction unit of the tribometer.
ing guideways 6 and are terminated with insulating inserts 16. Blind holes in the axes of the holders 5 of the inserts accommodate the cylindrically machined end pieces of externally threaded bolts 17 with which the end pieces are screwed into both ends of the half-ring dynamometer 18. A knob 20 is mounted on one of the bolts 17 for manual adjustment of the dynamometer pressure. The second bolt is connected to a worm transmission 21 which is driven by a step motor 22. Strain gauges 19 are attached to the half-ring dynamometer 18 to measure the rubbing load. The temperature gauge 10 is located in the lubricating oil 9. The lubricating oil is introduced to the inner chamber 8 through the filter 23 and is drained through the valve 24. The unit B (Fig. 2) for the adjustment and stabilization of the input of the tribological parameters includes (I) the adjuster and stabilizer for the temperature of the lubricating oil, (II) the automatic load adjuster in the rubbing pair and (III) the adjuster and stabilizer for the sliding velocity of the rubbing pair. The unit C for the measurement and recording of output of the tribological parameters includes equipment for the measurement and recording of (IV) the electrical resistance in the friction zone, (V) the friction force and (VI) the temperature under the friction surface.
4. Experimental procedures There are two types of test. In the first test the rubbing pair operate with the lubricating oil being tested at a constant temperature and under a constant load per unit projected area. The sliding velocity of the rubbing pair is gradually decreased linearly. The environment and the state of the rubbing surfaces remain constant. During this test the temperature under the friction surface, the friction force and the electrical resistance in the friction zone are
191
recorded. The sliding velocity is decreased until the electrical resistance of the friction zone and the friction force are characteristic of technically dry friction (if the load is sufficiently high). Typical results obtained using this operating mode are given in Fig. 4. All experiments were carried out using the standard rubbing pair for this tribometer as described later. In the second type of test the rubbing pair operates at a constant lubricating oil temperature and at a selected sliding velocity. The load per unit projected area of the rubbing pair is gradually increased linearly. All output parameters are measured and recorded as in the first type of test.
Load
per
unit.
P
Fig. 4. The friction force F,, the temperature TP under the rubbing surface and the electrical resistance Rf of the friction zone us. the speed. A standard rubbing pair was lubricated with gear oil A at 293 K in an air atmosphere. The load P was 100 N mme2. Fig. 5. The friction force F,, the temperature TP and the electrical resistance Rf us. the load. A standard rubbing pair was lubricated with gear oil A at 293 K in an air atmosphere. The speed u was 1 m s-l.
192
The load is increased to a maximum value Pp which resulted in technically dry friction. Typical results obtained using this operating mode are given in Fig. 5.
5. Experimental
details
The comparatively simple test specimen configuration allows specimens to be prepared from any material. A rolling bearing ring (steel, EH 15; hardness, 63 HRC) was used as a standard rotating specimen. Standard flat pin specimens were machined from steel St 45 heat treated to a hardness of 35 HRC. Specimens were run in at 293 K in air before each experiment. Running-in of the standard specimens was carried out up to a friction coefficient f. = 0.57 (velocity u = 1 m s-l; load P up to 5 N mme2). The range of speed was 50 - 6000 rev min-’ which corresponded to a sliding velocity of 0.05 - 7 .O m s-I, the rate of linear speed increase by programming was 60 - 2000 rev mine1 and the coefficient An of revolution stabilization was greater than or equal to 1%. ,The range of load per unit projected area of the specimen was 0 - 600 N mme2 and the time to achieve the maximum load by programming was 40 s. Specimens in the chamber could be cooled to 263 K and heated to 473 K. A thermostat kept the selected test temperature constant within f 0.5 K. The temperature rise under the friction surface, at a depth of 0.6 mm, was measured with a thermistor. The temperature could be measured up to 80 K above the temperature of the lubricating oil being tested. The minimum volume of lubricating oil under test was 50 cm3. Each experiment lasted approximately 5 mm.
6. Discussion From Fig. 5 the function in eqn. (3) can be estimated using the tribometer. The simplest way of calculating the value of L according to eqn. (2) is to compute an area with the help of a planimeter. A better and quicker method of calculation of the area L is by means of an integrator connected to the tribometer. The antifriction property of a lubricant expressed by the value L relates to the lubricity of lubricants in the whole range of friction. L also includes the viscosity of the lubricating oil. The value of L should be considered as a specific property of the lubricant depending on the temperature, the sliding velocity, the environment, the type of rubbing material and the condition of the material surface. However, using the proposed method of measuring the antifriction property it is possible to estimate rapidly the tribological performance of the lubricant under investigation. The magnitude of the value of L depends greatly on the phenomena which occur in the region of interaction between sliding metal surfaces,
193
especially in the lubrication region II (Fig. 1). This dependence is illustrated in Fig. 6, which shows the advantage of using the tribometer to examine the chemical reactions taking place on a rubbing surface. It was possible to observe endothermic effects due to bond cleavage of an additive, as shown in Fig. 7. Another advantage of the tribometer is the possibility of investigating
Fig. 6. The friction force F,, the temperature Tp under the rubbing surface and the electrical resistance Rf of the friction zone us. the load. A standard rubbing pair was lubricated with gear oil B at 293 K in an air atmosphere. The speed u was 1 m s-l.
Fig. 7. The friction force Fs and the temperature Tp under the rubbing surface us. the load. A standard rubbing pair was lubricated with white oil containing 0.6 wt.% diferrocenyl disulfide at 293 K in an air atmosphere. The speed u was 1 m s-l.
194
the durability of the load-carrying capacity of the protective layers which are formed by chemical reactions (Figs. 8 and 9). The primary use of the tribometer is for the rapid estimation of the antifriction property of lubricants and for the observation of the effects of chemical reactions on a rubbing surface under different friction conditions. It is also possible to evaluate the rate of wear from the relationship between the height of the pin and time.
Fig. 8. The friction force F,, the temperature T,, under the rubbing surface and the electrical resistance Rf of the friction zone us. the load. A standard rubbing pair was lubricated with white oil containing 0.6% additive C. Results of the fit run at 293 K in an air atmosphere (u = 1 m s-l ) are shown. Fig. 9. The friction force F,, the temperature Tp under the rubbing surface and the electrical resistance Rf of the friction zone us. the load. A standard rubbing pair was lubricated with white oil containing 0.6% additive C. Results of the second run at 293 K in an air atmosphere (V = 1 m s-l) are shown.
196
7, Conclusions The proposed model of the description of the antifriction property of a lubricant can be measured by the new type of tribometer in the whole lubrication range. This property is considered as a specific property of the lubricant which is dependent on the temperature, the sliding velocity, the en~onment, the type of rubbing material and the condition of the surface. The magnitude of the value of the antifriction property of the lubricant (in the meaning of lubricity) depends greatly on chemical reactions which occur in the region of injection between the sliding metal surfaces. Using the proposed method of measuring the antifriction property it is possible to estimate rapidly the tribological performance of the lubricant investigated,
Reference 1 Cz. Kajdas, J. Nita and K. Krawczyk, Tech. Smarownicza
- Trybol., 9 (1978) 86 - 88.
Wear, 70 (1981) 187 - 195 @ Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
A NEW METHOD FOR THE RAPID ESTIMATION OF THE ANTIFRICTION PERFORMANCE OF LUBRICANTS*
CZESLAW KAJDAS, JOZEF
NITA and KRZYSZTOF
KRAWCZYK
Wyisza Szkota Iniynierska,
26-600 Radom (Poland)
(Received August 17,198O
; in revised form November 2,198O)
Summary A rapid method of measuring the antifriction properties of lubricants over the whole region of friction with a new type of tribometer is described. The tribometer can also be used to investigate chemical reaction effects on a rubbing surface under different frictional conditions.
1. Introduction The principal function of lubricating oils is to minimize energy and wear by interposing fluid or protective layers between rubbing surfaces. The protective layers are formed by physical adsorption or chemical reaction between solids and the surrounding lubricant. Usually, the thickness of the protective layers is small compared with the roughness of the solid surfaces. Efficient lubrication, i.e. the saving of mechanical energy, takes place if the rubbing surfaces are separated under motion and load. The most common tribological parameters which cause failure of lubricant films or protective layers are excessive load and excessive temperature. The influence of excessive temperature is related to the molecular desorption which causes lubricant breakdown. Hence the minimization of friction should be taken into account over the full range of lubrication up to seizure. In practice, in the range of boundary lubrication the property of lubricating oils which allows the minimization of the friction and wear of rubbing surfaces is the lubricity or oiliness. This term has often been abused in the context of antifriction, antiwear and antiseizure properties because it is not clearly defined and has no unit of measurement. A new description of the antifriction property of lubricants is proposed and a new machine is described for measuring this property over the whole region of friction. The examination of liquid lubricants for other tribological purposes is also possible. *Paper presented at the 4th International September 10 - 15,1979.
Conference
on Tribology,
Paisley,
188
2. The model of the antifriction properties of lubricants The friction
force F is determined
by the function
F = CPU’,u, T, s, e)
(1)
where P (N mmP2) is the load per unit projected area, u (m s--l) the sliding velocity, T (K) the temperature, s the state of the rubbing surfaces and e the nature of the environment. If U, T, s and e are all constant the function (1) can be plotted as a curve F, shown in Fig. 1. The straight line F,, in Fig. 1 shows the linear dependence of the friction force on the load P for a rubbing pair with the technically dry friction coefficient f. (the broken line BM is an extrapolation of the line OB). In Fig. 1 there are three regions of lubrication. The first (region I) represents surfaces working under hydrodynamic or elastohydrodynamic conditions. In this region the reduction of friction and wear is most effective. Generally this region is relevant to the viscosity and viscosity-pressure dependence of the lubricating oil. The second region (region II) of mixed and boundary friction shows the antiwear properties of the lubricant. The intensity of wear in this range of lubrication is small (Fig. 1, curve W). If the rubbing surfaces are made of chemically active materials and are immersed
Load
per
unit.
P
Fig. 1. Proposed description of the antifriction property of lubricants.
189
in lubricating oil containing extreme pressure additives the highest rate of chemical reactions takes place in this friction region. The reactions form protective layers. With increasing load, the protective layer is destroyed, causing a higher rate of wear. In the last lubrication region (region III) boundary friction predominates. This region of lubrication indicates the antiseizure properties of the lubricating oil. Under a load Pg (the point of seizure) the lubricant no longer reduces friction between the rubbing pair and the coefficient of friction exceeds fe, i.e. the characteristic coefficient of friction for the rubbing pair without a lubricant. The magnitude of Pg depends on the boundary film in the lubrication region II. From Fig. 1 it can be seen that the area L represents the performance of a lubricant in terms of lubricity in the full range of friction. If all parameters except the load are constant, and when the rate of load increase is fixed, the size of the area L depends only on the physical and physicochemical properties of the lubricant which influence lubricity. The antifriction property of lubricants is then defined by L
=
AfoP, --
2
s pg
(2)
F,(P)dP
0
where A (mm2) is the contact area of the rubbing pair. To calculate the value of L it is necessary to determine
the function
F, =cpr(P)
(3)
This can be achieved with the aid of a new type of tribometer [ 11. The tribometer also enables the determination of the limits of the lubrication regions shown in Fig. 1.
3. Construction
of the tribometer
The tribometer consists of the three general units shown in Fig. 2. The friction unit A, shown in Fig. 3, contains a rubbing pair of rotating test pieces 1 which are ring shaped (22 mm in diameter and 8 mm in height) and are mounted on the shaft of the driving electric motor 2, and two opposed cylindrical specimens 3 bearing on the horizontal rotating ring. The area of rubbing surface of the specimen is about 1 mm2. The specimens are clamped in suitable holders 5. One of the specimens accommodates the temperature gauge 4. The holders 5 can be moved along their own axes but cannot revolve about them and are accommodated in insulating guideways 6 mounted on the rig body 7. The rig body consists of an inner chamber 8 containing the lubricating oil which is to be tested 9 and the temperature gauge 10. Another chamber 11 contains the thermostatic liquid 12. The driving motor 2 is mounted on the rig body 7 by means of a special ball bearing 13 through a spring element 14 on which the strain gauges 15 are mounted. Both of the holders 5 of the specimens 3 project from the insulat-
190
10
21
19
1n
19
11
21
22
Fig. 2. Block scheme of the tribometer. Fig. 3. Diagrammatic representation of the friction unit of the tribometer.
ing guideways 6 and are terminated with insulating inserts 16. Blind holes in the axes of the holders 5 of the inserts accommodate the cylindrically machined end pieces of externally threaded bolts 17 with which the end pieces are screwed into both ends of the half-ring dynamometer 18. A knob 20 is mounted on one of the bolts 17 for manual adjustment of the dynamometer pressure. The second bolt is connected to a worm transmission 21 which is driven by a step motor 22. Strain gauges 19 are attached to the half-ring dynamometer 18 to measure the rubbing load. The temperature gauge 10 is located in the lubricating oil 9. The lubricating oil is introduced to the inner chamber 8 through the filter 23 and is drained through the valve 24. The unit B (Fig. 2) for the adjustment and stabilization of the input of the tribological parameters includes (I) the adjuster and stabilizer for the temperature of the lubricating oil, (II) the automatic load adjuster in the rubbing pair and (III) the adjuster and stabilizer for the sliding velocity of the rubbing pair. The unit C for the measurement and recording of output of the tribological parameters includes equipment for the measurement and recording of (IV) the electrical resistance in the friction zone, (V) the friction force and (VI) the temperature under the friction surface.
4. Experimental procedures There are two types of test. In the first test the rubbing pair operate with the lubricating oil being tested at a constant temperature and under a constant load per unit projected area. The sliding velocity of the rubbing pair is gradually decreased linearly. The environment and the state of the rubbing surfaces remain constant. During this test the temperature under the friction surface, the friction force and the electrical resistance in the friction zone are
191
recorded. The sliding velocity is decreased until the electrical resistance of the friction zone and the friction force are characteristic of technically dry friction (if the load is sufficiently high). Typical results obtained using this operating mode are given in Fig. 4. All experiments were carried out using the standard rubbing pair for this tribometer as described later. In the second type of test the rubbing pair operates at a constant lubricating oil temperature and at a selected sliding velocity. The load per unit projected area of the rubbing pair is gradually increased linearly. All output parameters are measured and recorded as in the first type of test.
Load
per
unit.
P
Fig. 4. The friction force F,, the temperature TP under the rubbing surface and the electrical resistance Rf of the friction zone us. the speed. A standard rubbing pair was lubricated with gear oil A at 293 K in an air atmosphere. The load P was 100 N mme2. Fig. 5. The friction force F,, the temperature TP and the electrical resistance Rf us. the load. A standard rubbing pair was lubricated with gear oil A at 293 K in an air atmosphere. The speed u was 1 m s-l.
192
The load is increased to a maximum value Pp which resulted in technically dry friction. Typical results obtained using this operating mode are given in Fig. 5.
5. Experimental
details
The comparatively simple test specimen configuration allows specimens to be prepared from any material. A rolling bearing ring (steel, EH 15; hardness, 63 HRC) was used as a standard rotating specimen. Standard flat pin specimens were machined from steel St 45 heat treated to a hardness of 35 HRC. Specimens were run in at 293 K in air before each experiment. Running-in of the standard specimens was carried out up to a friction coefficient f. = 0.57 (velocity u = 1 m s-l; load P up to 5 N mme2). The range of speed was 50 - 6000 rev min-’ which corresponded to a sliding velocity of 0.05 - 7 .O m s-I, the rate of linear speed increase by programming was 60 - 2000 rev mine1 and the coefficient An of revolution stabilization was greater than or equal to 1%. ,The range of load per unit projected area of the specimen was 0 - 600 N mme2 and the time to achieve the maximum load by programming was 40 s. Specimens in the chamber could be cooled to 263 K and heated to 473 K. A thermostat kept the selected test temperature constant within f 0.5 K. The temperature rise under the friction surface, at a depth of 0.6 mm, was measured with a thermistor. The temperature could be measured up to 80 K above the temperature of the lubricating oil being tested. The minimum volume of lubricating oil under test was 50 cm3. Each experiment lasted approximately 5 mm.
6. Discussion From Fig. 5 the function in eqn. (3) can be estimated using the tribometer. The simplest way of calculating the value of L according to eqn. (2) is to compute an area with the help of a planimeter. A better and quicker method of calculation of the area L is by means of an integrator connected to the tribometer. The antifriction property of a lubricant expressed by the value L relates to the lubricity of lubricants in the whole range of friction. L also includes the viscosity of the lubricating oil. The value of L should be considered as a specific property of the lubricant depending on the temperature, the sliding velocity, the environment, the type of rubbing material and the condition of the material surface. However, using the proposed method of measuring the antifriction property it is possible to estimate rapidly the tribological performance of the lubricant under investigation. The magnitude of the value of L depends greatly on the phenomena which occur in the region of interaction between sliding metal surfaces,
193
especially in the lubrication region II (Fig. 1). This dependence is illustrated in Fig. 6, which shows the advantage of using the tribometer to examine the chemical reactions taking place on a rubbing surface. It was possible to observe endothermic effects due to bond cleavage of an additive, as shown in Fig. 7. Another advantage of the tribometer is the possibility of investigating
Fig. 6. The friction force F,, the temperature Tp under the rubbing surface and the electrical resistance Rf of the friction zone us. the load. A standard rubbing pair was lubricated with gear oil B at 293 K in an air atmosphere. The speed u was 1 m s-l.
Fig. 7. The friction force Fs and the temperature Tp under the rubbing surface us. the load. A standard rubbing pair was lubricated with white oil containing 0.6 wt.% diferrocenyl disulfide at 293 K in an air atmosphere. The speed u was 1 m s-l.
194
the durability of the load-carrying capacity of the protective layers which are formed by chemical reactions (Figs. 8 and 9). The primary use of the tribometer is for the rapid estimation of the antifriction property of lubricants and for the observation of the effects of chemical reactions on a rubbing surface under different friction conditions. It is also possible to evaluate the rate of wear from the relationship between the height of the pin and time.
Fig. 8. The friction force F,, the temperature T,, under the rubbing surface and the electrical resistance Rf of the friction zone us. the load. A standard rubbing pair was lubricated with white oil containing 0.6% additive C. Results of the fit run at 293 K in an air atmosphere (u = 1 m s-l ) are shown. Fig. 9. The friction force F,, the temperature Tp under the rubbing surface and the electrical resistance Rf of the friction zone us. the load. A standard rubbing pair was lubricated with white oil containing 0.6% additive C. Results of the second run at 293 K in an air atmosphere (V = 1 m s-l) are shown.
196
7, Conclusions The proposed model of the description of the antifriction property of a lubricant can be measured by the new type of tribometer in the whole lubrication range. This property is considered as a specific property of the lubricant which is dependent on the temperature, the sliding velocity, the en~onment, the type of rubbing material and the condition of the surface. The magnitude of the value of the antifriction property of the lubricant (in the meaning of lubricity) depends greatly on chemical reactions which occur in the region of injection between the sliding metal surfaces. Using the proposed method of measuring the antifriction property it is possible to estimate rapidly the tribological performance of the lubricant investigated,
Reference 1 Cz. Kajdas, J. Nita and K. Krawczyk, Tech. Smarownicza
- Trybol., 9 (1978) 86 - 88.