MEASUREMENT OF COEFFICIENTS OF FRICTION OF AUTOMOTIVE LUBRICANTS IN PIN AND VEE BLOCK TEST MACHINE

Transcription

1 5 th International Conerence Computational Mechanics and Virtual Engineering COMEC October 2013, Braşov, Romania MEASUREMENT OF COEFFICIENTS OF FRICTION OF AUTOMOTIVE LUBRICANTS IN PIN AND VEE BLOCK TEST MACHINE Venetia S. Sandu Transilvania University, Brasov, ROMANIA, Abstract : The paper deals with experimental mechanics in the ield o luid lubricants, investigating rictional characteristics o automotive engine and gear oils using experimental tests on a pin and vee block test machine (Falex). The test is standardized according to ASTM D3233 and represents a relevant and rapid measurement o coeicients o riction in a tribometer. Three engine oils and three gear oils were tested on the same loading-time procedure being analyzed the coeicients o riction according to SAE class o viscosity, degree o use ( resh or aged) and manuacturer. The calculations at dierent speeds and loads proved that coeicients o riction comply with the Stribeck rule. The research work succeeded to set up a procedure or operation on Falex machine, both or research and educational purposes. Keywords: automotive lubricants, riction coeicient, tribology, pin and vee block test machine, Stribeck curve 1. INTRODUCTION The road vehicles consume in average one-third o uel energy to overcome riction which in case o passenger cars is divided our main contributors: engine (35%), transmissions (15%), tires (35%) and brakes (15%) [1].The identiication o riction loss sources as well as the methods to reduce them has a tremendious eect on global primary resources, especially on uels and lubricants. Friction reduction methods are diicult to apply as operation modes, speeds, loads and working temperatures are variable in broad ranges. The potential activities to reduce riction in engines and transmissions are related to type o lubricants and lubrication regimes, the expected beneits being reducing uel and lubricant consumptions and emissions, wear, costs o maintenace operations and increasing energy eiciency and reliability. Besides the improvement o surace quality (advanced coatings and special surace texturing), an important riction loss source is linked to the tribological characteristics o the lubricants. The objective o this paper is to investigate riction coeicients o engine and transmission lubricants in a special purpose tester and to compare load-carrying and riction reduction capability, the eect o viscosity and aging. 2. TESTING METHODS The accurate measurement o riction reduction in real lie condition is very complicated, time consuming and expensive, so most o the tribological measurements use special purpose tribometers which evaluate coeicients o riction within speciic regimes o lubrication. As a general rule, the lubrication should keep two suraces in relative motion apart, or a minimum wear. A regime o lubrication depends on speciic parameters o the application (load, speed, temperature, geometry o the bearing suraces), but also on luid lubricant viscosity and lubricity. I it is considered as a parameter o the lubrication regime the lubrication oil ilm thickness, our dierent regimes o lubrication can be deined on Stribeck curve which impose dierent demands on lubricants: hydrodynamic lubrication (HL) is the one in which the relative motion o the sliding suraces keeps a continuous luid ilm separating the suraces, elasto-hydrodynamic lubrication (EHL) in which there is a separation oil ilm, but elastic deormation o surace and oil viscosity are important, boundary lubrication ( BL) when suraces are in contact and chemical and physical properties o the ilm are dominant and inally, mixed lubrication ( ML) is the lubrication characterised partly by direct contact o asperities and partly by EHL and BL. 334

2 The chosen tribometer and method should determine lubricant perormance in the same lubrication regime as in real lie and to measure the load-carrying properties and ability to protect against scuing Tribometer The Falex pin and vee block method is used to evaluate lubricant behavior in metal to metal applications, in standardized tests or measurement o extreme pressure [2] and wear properties o luid lubricants [3], wear and load carrying capacity o solid ilm lubricants [4]. The equipment is described in igure 1. A test pin (journal) rotates at a constant speed against two vee blocks, all immersed in 60 ml o lubricant. Figure 1: Falex pin and vee block test machine [2] There are our contact lines between the pin diameter and the vee blocks when a load is applied through a mechanical gauge by means o a ratchet wheel and eccentric arm Lubricants The lubricants supposed to be tested belong to commercial well-known manuacturers and are marked with M1, M2 and M3. Six lubricants were tested, three engine lubricants and three transmission lubricants, our were resh, not used at all, and two were changed according to vehicle manuacturer recommendations. For urther discussions there were measured their densities and viscosities, which are presented in table 1, along with manuacturer code, class o viscosity and condition. Table 1: Lubricants characteristics Lubricant type Class SAE viscosity Density [kg/m 3 ] Viscosity (20ºC)[mm 2 /s] State Engine 5W40M resh Engine 5W40M resh Engine 5W30M * aged, km Transmission 75W80M resh Transmission 75W80M * aged km Transmission 80W90M resh *The viscosity could not be measured with Gibson Jacobs method because the aged lubricant was opaque. The density was measured by weighting a given volume o lubricant in a graduated cylinder, and then by dividing mass to volume. The viscosity was measured [5] on the hydrostatic test bench Cussons using the alling ball viscosimeter Gibson Jacobs, which measures the time in which a alling standardized ball travels a given distance in a graduated cylinder containing the tested lubricant. The test was perormed at the same temperature o the lubricants, at 20 C and kinematic viscosity υ was calculated using equation (1) : 2 d g( δ ρ) F υ= (1) 18vρ 335

3 with d - ball diameter [mm], g - gravitational acceleration [m/s 2 ], δ-sphere density [g/cm 3 ], ρ- lubricant density [g/cm 3 ], F- dimensionless correction actor, v- alling speed o the ball [mm/s] Testing procedure and calculations During the operation o the tribometer an external load is applied on the vee blocks upon the journal, all these parts being immersed in a lubricant bath. Mechanically, the direct load P d, expressed in pounds or Newtons is decomposed in a normal orce F n and a tangential riction orce F. As the pin is driven at constant speed, a riction torque M is produced and the coeicient o riction between pin and vee blocks can be calculated based on equations o orce and torque conservation, as illustrated in igure 2. Applying the torque conservation upon the pin, it yields the ollowing equations (2): d M M O = 0, M 4 F = 0 and F = (2) 2 2d Applying the orce conservation on x direction upon the vee block, it yields the ollowing equations (3): F x = 0, P d + 2 Fn cos 45 = 0, and F n = (3) 2cos 45 Finally, the riction coeicient, µ, is calculated with ormula (4): M o F 2 d M 2cos 45 M µ = = = = (4) F P n d 2d o 2cos 45 with M expressed in inch-pounds and expressed in pounds. Figure 2: Scheme or the calculation o riction coeicient Previous to load measurements, as the vee blocks are not perecty lat, a run in test was perormed adapted rom standard [2] ; the duration was o 15 minutes at the direct load applied o 300 lbs.the assessment o tribological properties is detailed in a program load time considering similar tests perormed in [6] or commercial engine oils,with the coniguration o direct load P applied in time, according to igure 3.The test was repeated three times or each lubricant. d 336

4 Figure 3:Load versus time - testing program 3. RESULTS AND DISCUSSIONS 3.1 Engine lubricants The calculations o the coeicients o riction or engine lubricants were perormed or the same loads and are illustrated in igure 4, having load expressed in pound-orce and coeicient o riction being dimensionless. The measured points were lined using third order polynomial regression. The variation o coeicients o riction with load is similar or two resh oils, the tribological behavior being better or manuacturer M1 than M2. Having a lower viscosity at ambient temperature, the lubricant 5W30, due to aging and contaminants has an increased coeicient o riction at higher loads. 0,08 Coeicient o riction, µ,[-] 0,07 0,06 0,05 0,04 0,03 0,02 0, W40M1 5W40M2 5W30M2 used Poly. (5W40M1) Poly. ( 5W40M2) Poly. (5W30M2 used) 3.2 Transmission lubricants Load [lbs] Figure 4: Coeicient o riction versus load or engine lubricants The coeicients o riction or transmission lubricants were calculated or the same loads as in previous case being illustrated in igure 5. The values are close to those o engine lubricants, in the same range ( ). The coeicients o riction have a small increase with the class o viscosity, the increase o riction being higher or used lubricants, having the same behavior as that o engine lubricants. 337

5 Coeicient o riction, µ [-] 3.3 Failure load test W80M1 75W80M1 used 80W90M3 Poly. (75W80M1) Poly. ( 75W80M1 used) 0 Poly. (80W90M3) Load [lbs] Figure 5: Coeicient o riction versus load or transmission lubricants It is important to measure the ailure load in the test, which is deined as the minimum load at which occurs a welding within test components. As a result the coeicient o riction and riction torque increase sharply. At that moment the pins and vee blocks are broken or damaged, as seen in igure 6. The lower value o the load at which the lubricant withstood represents the limit load carrying. Figure 6: Scued pins and vee-blocks The values o the ailure loads or engine lubricants 5W40M1, 5W40M2 and 5W30M2 were 1500 lbs at a torque o 26 in.lb. 3.4 Discussion on Stribeck curve The tribological properties o luids are widely represented in orm o a graphical dependence known as Stribeck curve [7,8]. The variables are dimensionless, coeicient o riction µ and Stribeck parameter S, the latter being deined according to ormula : v S = η (5) P d with η - dynamic viscosity o the tested lubricant, v - relative speed o the pin and vee blocks, P d - direct applied load reported to unit length o contact lines between pin and vee blocks. For one o the tested lubricants, namely 75W80M1 transmission oil, there were calculated the position o the measurement points on Stribeck curve.the dynamic viscosity was calculated considering the lubricant density and its kinematic viscosity at temperature o the oil in testing cup, the constant rotational speed o the tribometer was turned into tangential speed, by multiplication with the radius o the pin, r. Finally, the Stribeck parameter 338

6 becomes inverse proportional to direct load P d and the correlation with coeicient o riction is illustrated in igure 7. Figure 7: Stribeck curve proile or transmission lubricant That curve expresses the riction variation within all types o lubrication areas, aorementioned described ( BL, ML, EDL and HL).The let dotted line represents the area o BL and ML and the right continuous line represents the area o EDL and HL. For the transmission lubricant, the hydrodynamic lubrication begins in the point o the lowest riction coeicient when the oil ilm supports completely the carrying load. CONCLUSIONS The investigation o engine and transmission lubricants revealed that the values o coeicients o riction are close and or both types, the higher the viscosity class, the higher are the coeicients o riction. More important than the eect o the viscosity is the eect o aging and contamination which increased considerably the coeicients o riction o both lubricants. The coeicients o riction met the proile o Stribeck curve, being identiied the areas o mixed lubrication and hydrodynamic lubrication. The work has also an educational gain, succeeding to implement a procedure or laboratory work in tribology o automotive lubricants, at Transilvania University. ACKNOWLEDGEMENTS The author would like to thank to Master student Andrei Preda or his assistance and support with research acilities. REFERENCES [1] Holmberg K., Andersson P., Erdemir A., Global energy consumption due to riction in passenger cars, Tribology International, Volume 47, 2012, p [2] **** ASTM D (2009), Standard test methods or measurement o extreme pressure properties o luid lubricants (Falex Pin and Vee Block Methods). [3] **** ASTM D (20109), Standard test methods or measurement wear properties o luid lubricants (Falex Pin and Vee Block Methods). [ 4] **** ASTM D (2004), Standard test methods or endurance ( wear) lie and load carrying capacity o solid ilm lubricants (Falex Pin and Vee Block Methods). [5] Gibson W., Jacobs L., The alling sphere viscosimeter, Journal o Chemical Society Transactions, Volume 117,1920, p [6] Industrially Relevant characterization o Adhesion or Wear Resistance by Adapted ASTM methods, [7] Brandao, J.A. et al., Comparative overview o ive gear oils in mixed and boundary ilm lubrication, Tribology International, Vol.47, 2012, p

7 [8] Maru, M. et al., Assessment o the lubricant behaviour o biodiesel uels using Stribeck curves, Fuel Processing Technology, Vol.116, 2013, p