Selection of lubricant characteristics

Back to Picture 6.5-1 it’s time to focus on popup menu located in lower left position of command window. It points to a database in the software where different oils are stored with their own

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features and data. Different oils of software database are identified by I.S.O. 3448 standard, even if automotive and motorsport lubricants are often commercialized with S.A.E. J306 identification.

Following with the topic, it would be important make a clarification about equivalences on different standards. Table 6.5-2 is going to be the key.

In order to perform a good choice about lubricant, It’s suitable have a good knowledge about their composition. According with Ref.[14], modern lubricants are made up of several constituents, starting point is a base oil meshed with proper additives. By the way, lubricants are distinguished on two main families: “mineral oils” and “synthetic oils”.

Basically, mineral oils are used as base oil for manufacturing gear-box oils. Various qualities of mineral oils are distinguished by their V.I., the “viscosity index”. Such dimensionless parameter describes the properties of the base oil depending temperatures. As declared in advance at Chapter 6.5.2, lubrication oil needs to be stable independently from temperature variation. A quality oil needs to be liquid enough at low temperatures, without becoming excessively liquid at high temperatures. Low value of V.I. means large variation of viscosity due to temperature, while an high value means a very tight variation. Values of V.I. for a good mineral oil are approximately around 95÷105. While high-quality mineral oils display a V.I. greater than 150. Synthetic oils are employed where extremes of temperatures, below –20°C and above 140°C are reached.

Characteristics of gear-box oils are strongly affected by “additives” and “packages”, as previously declared at Chapter 6.5.2. Term package refers to a gear-box oil in which additives represent 2÷10%

of the entire volume. Most common additives for gear-box lubricants are:

 E.P. additives for improving high-pressure characteristics.

 Corrosion inhibitors for preventing rust, verdigris and harmful products of oxidation.

 D.D., detergent and dispergent, are additives for removing dirt.

 Friction modifiers which reduce friction and wear.

 V.I. improver to improve high and low temperature performance. (Ref.[14])

Employ of additives enables to create oils for different purposes. Anyway, action of the various additives should be complementary.

Most important feature of gear-box oil is its flow-ability, or viscosity. Such parameter describes the internal friction of a fluid. As declared in advance, lubrication oils are classified on the base of their viscosity and are divided into different viscosity groups. The kinetic nominal viscosity can determined as a function of the surface stress and the sliding speed.

Gearwheels to design are specific for motorsport employ, that means slim thickness of teeth which leads to high surface pressures applied on flanks of teeth. For this reason, oil film needs to be strong enough to sustain high values of applied pressure, without loose the continuity on the stressed area. Proper value for thickness of the lubricant film can be determined applying elasto-hydrodynamic theory. According with Ref.[14], parameters to take into account are:

 Mechanical stress.

 Circumferential speed.

 Effective temperature.

 Kinematic viscosity of the lubricant: high viscosity, high thickness.

Value of film thickness can be adequate when it is larger than the average surface roughness of the tooth flank surface. Alternatively, the lubricant viscosity required to provide a bearing film of sufficient thickness for a given gear system, can be determined when the operating conditions are known. Resistance to scuffing and pitting of gears is improved by viscosity increase.

With reference to Chapter 5.10, which treats verification to pitting, It’s possible to analyze Plot 5.10-2 again. It’s easy to notice that, a good value of lubrication factor, equal or superior to 1, can

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be obtained only through an high value of kinetic viscosity. Anyway It’s necessary to notice that viscosity values are sensibly higher to that employed for gears nowadays.

However, It’s not so useful employ oils with too high values of viscosity, if It is too great negative effects can be exploited. It’s necessary to remind that high viscosity leads to high friction between lubricant and gearwheels. That’s a very dramatic condition for efficiency and temperature.

For these reasons, selection of a lubricant provided of a right value of viscosity is always a compromise between different factors.

By virtue of that, It can be useful knowing behaviour of viscosity depending on temperature.

Basically, viscosity exponentially decreases as temperature increases. By the way, It’s useful examine the log-log Ubbelohde diagram of Plot 6.5-3 which displays three curves: a) is the profile related to mineral based oils, b) is related to synthetic oils based on poly-α-olefins, c) is related to synthetic oils based on polyglycol. From a brief analysis, It’s clear that stability of mineral based oils is not so high, while stability of polyglycol-based oils is considerably better.

Plot 6.5-3: Temperature dependent behaviour of different families of lubricant, Ref.[14].

On the other hand, It can be useful analyze some numeric values. Table 6.5-1 shows behaviour under temperature variation of different types of lubrication oils. Red values represent standard viscosity which identifies the lubricant at 40°C, according to I.S.O. 3448 standard. Moreover, plot can be useful to evaluate spectrum of viscosity regarding different gear-box lubricants.

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Table 6.5-1: Kinetic viscosity of lubrication oils vs. temperature.

Another factor which strongly affects viscosity is pressure. By theory of elasto-hydrodynamic lubrication, It’s known that pressure causes a dramatic increase of viscosity. Its value increases exponentially depending on tooth flank pressure. By virtue of that, It’s clear that an highly stressed gear doesn’t require so high values of viscosity, because the risk is a too high viscosity under operation. By the way, chosen value of viscosity needs to be well balanced. (Ref.[14])

Other two parameters to take into account are temperatures, known as “pour point” and “flash point”. The pour point describes flow properties at low temperatures, It must be, at least, 5 K below the lowest operating temperature. The flash point can be ignored in all, but a few critical high-temperature applications that do not apply in automotive engineering. (Ref.[14])

In order to conclude such dissertation, It’ necessary to consider that gear-box is a complex system, which integrates many components with a wide range of materials. Most critical components are seals. Indeed gaskets must not change in terms of their material characteristics under the influence of gear-box oils. In the specific, seals must not shrink, swell or become brittle. Damage gaskets shouldn’t deposit any material which could lead to impairment of the oil’s characteristics. For this reason, a check on compatibility of seals is always recommended. (Ref.[14])

Unfortunately, all data needed to perform an accurate choice on the lubricant based on numerical verifications aren’t available. Some needs to be provided by supplier, other needs to be obtained by experimental tests. Anyway, this dissertation provides some guidelines useful to perform a first attempt choice. Furthermore, Kisssoft software is provided of algorithm an data necessary to perform all needed verifications on the chosen lubricant oil.

For many seasons, Squadra Corse have been supported by the Italian dealer of Motul, the famous French producer of lubricants and care products for car and motorbikes. By virtue of that, supplier recommended some products of its range, on the base of primary information regarding gear-box project. Four types of lubricant have been proposed, two are synthetic, 75W-90 and 75W-140, two are mineral, 80W-90 and HD 80W-90. On the basis of datasheets included at Appendix 3, Appendix 4, Appendix 5 and Appendix 6, first attempt choice have been performed.

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Table 6.5-2: Correlation of different standards regarding lubricants, Ref.[14].

Lubricant selected is GEAR 300 75W-90, a 100% synthetic oil ester based. Main features, are the very high index of viscosity which implies a wide temperature range between pour point and flash point. It’s necessary underline that pour point It’s not a big issue for S.C.R. project. Large majority of tests and races, first of all, take place in the summer. For this reason, lowest operation temperature is going to coincide with summer environmental temperature. On the other hand, a very large value of flash temperature is an important caution. Compatibility with gaskets is guaranteed. In addition, high finish of gears suggests that choice of lowest value of viscosity in the proposed range, is probably the best solution.

Final step of this process is find a correlation between chosen lubricant and I.S.O. 3448 standard. By Table 6.5-2, It’s clear that 75W corresponds to I.S.O. VG 46, which can be set on the popup menu shown by Picture 6.5-1. Finally, lubrication choice is going to be confirmed at the end of software calculations.