Description:FIELD OF THE INVENTION
This present invention relates to a lubricant composition for resolving issue of brake chatter observed during operation of wet brake tractors in fields where frequent brake operation is carried out.

BACKGROUND OF THE INVENTION
There are continuous efforts worldwide to improve tractors transporting performance by increasing tractor speed. Faster, bigger, more powerful and more manoeuvrable machines are capable of developing ever increasing speeds. Nevertheless, the increase of agricultural vehicles speed also requires efficient braking system that should enable agricultural vehicles to keep the pace with the other fast vehicles participant in road traffic, taking into account traffic safety. Braking technology for these machines of today and tomorrow needs to be equally efficient, low-maintenance, convenient and economical. With the combination of higher-energy level and more rapid deceleration, brake systems with excellent heat dissipation characteristics are required. This requirement is met through wet multiple disc brakes which have similar multiple disc construction but operate in an oil bath. Most of the modern tractors are now equipped with wet brakes. However, complaints about brake chatter found on agricultural and off-highway equipment continue to tarnish the benefits of such stopping devices. The wet brakes used in tractors often produce noise and vibration on engagement. This vibration normally termed as “chatter”, is often attributed to stick-slip. High-performance oil immersed brake (OIB) oils containing the optimized additive chemistries effectively protect clutches, gears and pumps by providing balanced friction, oxidation resistance, anti-wear protection, water tolerance and enhanced performance in temperature extremes and harsh conditions.

US7820599B2 relates to a lubricating oil composition, and more specifically to a lubricating oil composition having a high static friction coefficient of wet clutch, having excellent frictional property in a power transmission mechanism, having excellent fuel saving property, and suitably used for power transmission lubricating oil and engine lubricating oil. A lubricating oil composition characterized by including lubricating oil base oil, an organic molybdenum compound (A) in an Mo content of 100 to 1,000 mass ppm, Zinc dialkyl dithiophosphate (B) in a P content of 0.03 to 0.20 mass %, at least one compound (C) selected from calcium Sulfonate, calcium phenate, and magnesium Sulfonate, wherein a base number obtained through a perchloric acid method is 230 mgKOH/g or more in a Ca and/or Mg content of 0.15 to 0.30 mass %, and an ashless dispersant containing boron or a mixture of an ashless dispersant containing boron and an ashless dispersant containing no boron (D) in a B content of 0.03 mass % or more and an N content of 0.05 mass % or more, in which a mass ratio (P/Mo) of the P content to the Mo content in the composition is 1.5 or more, a mass ratio (CaMg/Mo) of a total content of the Ca and Mg derived from the component (C) to the Mo content in the composition is 3 or more, and a mass ratio (B/N) between Band N derived from the component (D) in the composition is 0.5 or more.

US4379066A relates to a method for reducing oil-immersed disc brake chatter by lubricating the contacting surfaces of oil immersed disc brakes with a composition comprising a hydrocarbon-based lubricant containing certain oil soluble 1,2 or 2, 1-hydroxyalkyl alkanoates containing from 11 to 60 carbon atoms act as appropriate friction modifying agents, which, when added to a lubricating oil, exhibit good anti-chatter characteristics.

GB2087924A relates to a method of reducing brake chatter of an oil-immersed disc brake which comprises incorporating in the lubricating oil composition in contact with the surfaces of the oil-immersed disc brake with at least one hydroxyalkyl alkanoate of 0.5- to 5.0% by weight with 1 to 30 carbon atoms, R' and R" contains from 1 to 18 carbon atoms, 0.5 to 12% by weight of a dispersant polyisobutenyl succinimide of a poly-ethylene polyamine, hydrocarbon soluble detergent, and an extreme pressure additive.

EP2333035A1 relates to a combination of one or more oil-soluble fatty acid esters of a polyhydric alcohol 0.05 to about 2.0 wt % and one or more oil-soluble fatty acid amides 0.05 to about 2.0 wt % act as appropriate friction modifying agents, which when added to a lubricating oil, exhibit good anti-chatter characteristics and gear wear.

US5851962A relates to a lubricant composition for a wet clutch or a wet brake comprising a base oil having added thereto an inorganic phosphorus compound which may contain a Sulphur atom and/or an oxygen atom as its Constituent elements or an amine Salt thereof.

EP1367116A1 relates to certain combination of one or more oil-soluble fatty acid esters of a polyhydric alcohol and one or more oil-soluble fatty acid amides act as appropriate friction-modifying agents, which when added to a lubricating oil, exhibit good anti-chatter characteristics and gear wear. This prior art is mainly for lubricant for use in tractor transmission which is having wet brake or wet clutch.

US3899432 covers the lubricating oil compositions useful both as hydraulic fluids and as lubricants in the transmissions and differentials of heavy machinery such as high-power output tractors. This lubricating oil composition is particularly useful in machinery using wet-type disc brakes physically located within the housing containing the lubricating oil composition.

EP0237804A2 covers the oil composition for manual transmissions which is used to lubricate manual transmissions [manual transmissions of FR (Front engine, Rear drive) automobiles and manual transaxle of FF (Front engine, Front drive) automobiles] of automobiles which include passenger cars, trucks, large-sized cars such as buses, and tractors.

US4181619A covers the Hydraulic fluids containing the reaction product of benzotriazole and tricresyl phosphate are stabilized by the addition thereto of minor amounts of calcium dinonyl naphthalene sulfonate and calcium alkyl phenate. Such fluids and other oleaginous compositions containing the above-described novel additive combination are thus provided with good corrosion control, rust protection and improved cleanliness plus antiwear protection.

IN328288 discloses lubricating oil composition useful as universal tractor transmission oil (UTTO) type lubricant with anti-chatter characteristics, and noise free operation in agricultural tractors equipped with wet brakes. This composition comprises of a base oil, a suitable mix of UTTO additive system, a dispersant olefin copolymer (DOCP), a sulphur phosphorus additive component and a monounsaturated omega-9- fatty acid as a part of additive composition.

JP2010065142A discloses a lubricating oil composition for an iron-based bearing having high rust-preventive capacity as well as excellent lubricity and oxidation stability. Further it discloses that the said composition is obtained by blending a synthetic calcium sulfonate and a synthetic zinc sulfonate with mineral oil-based or synthetic oil-based lubricating base oil and meets the mass ratio criterion of Ca/Zn>1.

The prior art has reported that brake chatter is due to high operating temperature in contact zone, improper friction performance of oil, water ingress during puddling operation, inappropriate oil viscosity, lower film thickness, shearing of oil and carbonaceous deposit formation in oil sump are different factors affecting wet brake oil performance.
There has been a long felt need to provide lubricating oil composition for resolving issue of brake chatter.

SUMMARY OF THE INVENTION
The present invention provides a lubricant composition suitable for tractor wet brake applications (oil immersed brakes application) having unique combination of additives and base oil of suitable viscosity. The invention provides a unique combination of calcium to phosphorus + zinc ratio for achieving the required characteristics which are not reported in the earlier prior art.

In one of the embodiments, the present invention provides a lubricating oil composition for wet brake operations comprising:
a. 20-30% wt/wt of a refined Group I base oil having kinematic viscosity in the range of 9.0 to 12.0 cSt at different temperatures from 90-120ºC;
b. 70-80% wt/wt of a Group II base oil having kinematic viscosity in the range of 8.5 to 11.5 cSt at different temperatures from 90-120ºC;
c. 3-8% wt/wt of a multi-component additive comprising:
a polyisobutylene (PIB) based thickener,
a viscosity modifier unsaturated fatty acid ester,
a sulphurised isobutylene (SIB),
a O,O-dialkyl,S-alkyl thiophosphate, S,S-dialkyl,O-alkyl phosphate, O,O,O-trialkyl phosphate, polyphosphate phenate detergent system, and
a polyacrylate based pour point depressant ; and
d. 0.5-4% wt/wt of a friction modifying additive having an alkylated zinc thiophosphates along with an overbased calcium salt with total base number in the range of 50 - 400 mg of KOH/gm,
wherein all the % wt/wt is based on the total weight of the composition.

In another embodiment, the present invention provides the lubricating oil composition wherein the multi-component additive comprises:
10-35% wt/wt of the O,O-dialkyl,S-alkyl thiophosphate, S,S-dialkyl,O-alkyl phosphate, O,O,O-trialkyl phosphate, polyphosphate phenate detergent system;
0.5-5% wt/wt of the polyisobutylene (PIB) of molecular weight ranging from 400-10500 Dalton;
10-30% wt/wt of sulphurised isobutylene;
1-4% wt/wt of the viscosity modifier of unsaturated fatty acid ester having carbon atoms of C14-C20; and
0.1-2% wt/wt of the polyacrylate based pour point depressant of molecular weight 300-70000 Dalton.

In one of the features of the present invention, the unsaturated fatty acid ester is a mixture of esters of oleic acid and linolenic fatty acids.

In another feature of the present invention, the alkylated zinc thiophosphate comprises alkylated zinc dialkyldithiophosphate (ZnDTP) and Zinc thiophosphate having C1-C14 carbon atoms salts.

In yet another feature of the present invention, the friction modifying additive is a combination of Zinc thiophosphates and calcium salts and is maintained in the mass ratio of Ca, P, Zn elements with Ca/(P+Zn) = 1. In one feature of the present invention, mass ratio of Ca, P, Zn elements with Ca/(P+Zn) > 1. In another feature of the present invention, mass ratio of Ca, P, Zn elements with Ca/(P+Zn) is in the range of 1 to 2.

In yet another feature of the present invention, the calcium salt is selected from calcium sulfonate, calcium phenate and calcium salicylate.

In yet another embodiment, the present invention provides a process for preparing a lubricant oil composition comprising steps of:
(i) preparing a mixture of 20-30% wt/wt of Group I base oil having kinematic viscosity range 9.0-12.0 cSt and 70-80% wt/wt of Group II base oil having kinematic viscosity range of 8.5 to 11.5 cSt;
(ii) adding multi-component additive having polyisobutylene based thickener additive, polymethacrylate based pour point depressant additive, viscosity modifier unsaturated fatty acid ester, sulphurised isobutylene and alkylated phosphates, alkylated thiophosphates, polyphosphates phenate detergent systemto the above base oil mixture in the range of 3-8% wt/wt to obtain a blend; and
(iii) treating the above obtained blend with 0.5-4% wt/wt of friction modifying additive having mixture of zinc thiophosphates and calcium salts with elemental ratio of Ca/(P+Zn) =1.

In one of the features of the present invention, the lubricant composition has unique mass ratio of elements Ca/ (P + Zn) = 1. In one feature of the present invention, mass ratio of Ca, P, Zn elements with Ca/(P+Zn) > 1. In another feature of the present invention, mass ratio of Ca, P, Zn elements with Ca/(P+Zn) is in the range of 1 to 2.

In yet another feature of the present invention, the lubricant composition of the present invention provides high static friction which leads to reduction of brake chatter in frequent braking operations. The lubricant composition provides optimum friction performance throughout the oil life. The lubricant composition provides better high temperature operating suitability in contact zone which leads to longer drain and excellent friction plate performance throughout the operation. Lubricant composition provides the adequate film formation on the surface which leads to reduce the wear and improve the brake plate friction performance.

In yet another feature of the present invention, the lubricant composition of the present invention provides the excellent thermal stability throughout the life which lead to longevity of brake pads in tractor components and lesser number of impurities and negligible carbonaceous material in brake housing.

In yet another feature of the present invention, the lubricant composition provides appropriate friction modification during brake operation leading to superior anti-chatter characteristics and excellent cross wear performance.

BRIEF DESCRIPTION OF FIGURES
Figure 1 depicts friction profile of candidates 1-5.
Figure 2 depicts friction profile of candidates 1 and 4.

DETAILED DESCRIPTION OF THE INVENTION
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments in the specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated composition, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. The composition, methods, and examples provided herein are illustrative only and not intended to be limiting.

The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”.
Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of element or steps but not the exclusion of any other element or step or group of element or steps.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference.

The following terms will be used throughout the specification and will have the following meaning unless otherwise indicated:

Viscosity index (VI) is an arbitrary measure for the change of viscosity with variations in temperature. The lower the VI, the greater the change of viscosity of the oil with temperature and vice versa. It is used to characterize viscosity changes with relation to temperature in lubricating oil.

“Group I base oil” is solvent-refined and consists of minimum 90% v/v saturates having greater than 0.03% wt/wt of sulfur content with a viscosity index range of 80 to 120.

“Group II base oil” is product of hydrocracking process and consists of minimum 90% v/v saturates having maximum 0.03% wt/wt of sulfur content with a viscosity index range of 80 to 120.

Multi-component additive is a suitable additive package known as universal tractor transmission oil (UTTO) additive system, having polyisobutylene based thickener additive, polymethacrylate based pour point depressant additive, viscosity modifier unsaturated fatty acid ester, sulphurised isobutylene and alkylated phosphates, alkylated thiophosphates, polyphosphates phenate detergent system procured from additive manufacturers/suppliers.

The brake chatter is a function of the oil/metal surface interaction, and the oil having an impact on the frictional characteristics of the braking surface. High temperature, improper friction performance, water ingress, inappropriate viscosity, lower film thickness, oil shearing are the main causes of the brake chatter reported in literature. It has been understood that while tractor runs continuously with partial braking, brake plate temperature reaches as high as 300-400 ºC due to continuous operation and loads, leading to the carbon deposit formation in oil. This carbon deposit coats the brake plate immersed in oil causing damage of friction plates in some cases due to inappropriate friction properties of fluid lubricating the oil immersed brakes. These friction properties are defined by the ratio of static to dynamic friction and are commonly reported as a measure of propensity to chatter. Assessment of Brake Chatter can be done by measuring the friction velocity curve and a negative slope of friction velocity curve causes brake chatter. It can be improved by using a fluid which gives a more positive slope of the friction velocity curve. If ratio of static to dynamic friction is greater than 1.0, a negative slope must exist over at least part of friction velocity curve. A robust lubricant composition fortified with suitable base oils and additives plays key role in maintaining the desired friction velocity curve required for smoother operation of oil immersed brakes. This invention describes a lubricant composition fortified with optimized combination of additives with mass unique ratio of elements (Ca/(P + Zn) = 1. This specific combination of additives has potential to resolve the brake chatter issue in oil immersed brake systems. Thus, the present invention provides a lubricant composition as an improvement of the existing prior art which helps to reduce the brake chatter in field.

The present invention provides a lubricating oil composition for wet brake applications comprising suitable viscosity base oil mixture with 20-30% wt/wt of solvent refined Group 1 base oil of kinematic viscosity greater than 11.0 cSt at 100ºC and 70-80% wt/wt of hydrocracked Group II base oil with kinematic viscosity greater than 10.00 cSt at 100ºC is studied.

The lubricating oil composition comprises 3-8% wt/wt of a multi-component additive system. comprising Polyisobutylene (PIB), unsaturated fatty acid ester (mixture of Olyl and Linolenyl C18 carbon atom fatty esters etc.), Sulphurised Isobutylene (SIB) and O, O-dialkyl,S-alkyl thiphosphate, S,S-dialkyl,O-alkyl phosphate, O,O,O-trialkyl phosphate, Polyphosphate and Phenate detergent system.

The composition when treated with additional quantity of friction modifying additive (in the range of (0.5wt % to 4wt %) resulted in mass ratio of elements Ca/(P + Zn) = 1.

Above composition when tested in field in oil immersed brake systems (wet brake tractors) resulted in superior wet brake performance due to better friction modifying properties.

Examples
The present disclosure with reference to the accompanying examples describes the present invention. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. It is understood that the examples are provided for the purpose of illustrating the invention only and are not intended to limit the scope of the invention in any way.

Example 1: Preparation of novel lubricant oil composition
Novel lubricant composition was obtained by preparing a mixture of 20-30% wt/wt of Group I base oil having kinematic viscosity range 9.0-12.0 cSt and 70-80% wt/wt of Group II base oil having kinematic viscosity range of 8.5-11.5 cSt. Adding multi-component additive having polyisobutylene based thickener additive, polymethacrylate based pour point depressant additive, viscosity modifier unsaturated fatty acid ester, sulphurised isobutylene and alkylated phosphates, alkylated thiophosphates, polyphosphates phenate detergent system to the above base oil mixture in the range of 3-8% wt/wt to obtain a blend; and further, treating the above obtained blend with 0.5-4% wt/wt of friction modifying additive having mixture of zinc thiophosphates and calcium salts with elemental ratio of Ca/(P+Zn) in the range of 1 to 2.

Example 2: Product evaluation
Example 2a: Copper Corrosion Characteristics (ASTM D130)
This test method is being used for detection of corrosiveness to copper metal. A polished copper strip is immersed in a specific volume of the sample being tested and heated under conditions of temperature and time that are specific to the class of material being tested. At the end of the heating period, the copper strip is removed, washed and the color and tarnish level assessed against the ASTM Copper Strip Corrosion Standard.
Test condition: Temperature -121? for 3 hr.

Example 2b: Brookfield Viscosity (ASTM D2983)
Brookfield Viscometer is measurement of low shear low temperature viscosity (-10 to -55?) of automotive driveline lubricants as per ASTM D2983. Brookfield Viscosity is an important parameter for assessing fluid condition in gears at low temperature for the proper operation in many mechanical devices and is part of various OEM’s specifications.
Test condition: Temperature -18? for 16 hr.

Example 2c: Pour Point (ASTM D97)
The method prescribed procedure for the determination of pour point of Petroleum products. Pour point is the lowest temperature, expressed as a multiple of 3°C, at which the oil is cooled under prescribed condition. After preliminary heating, the sample is cooled at a specified rate and examined at intervals of 3°C for flow characteristics. The lowest temperature at which movement of the specimen is observed is recorded as the pour point.

Example 2d: Rust-Preventing Characteristics (ASTM D665)
The test method evaluates rust protection ability of the lubricating oils in the presence of water, this test method is commonly used for testing of transmission and hydraulic oils. In this test, a mixture of 300 mL of the candidate oil under test is stirred with 30 mL of de-ionized water or synthetic sea water at a temperature of 60°C for 24 hr with a cylindrical steel test rod completely immersed therein. After test steel rod is inspected for rust formation tendency.

Example 2e: Wear Scar Diameter (WSD) (ASTM D4172B)
Measurement of Wear Scar Diameter as per ASTM D4172B test method is used to determine the relative wear preventive properties of lubricating fluids in sliding contact under the prescribed test conditions. Test conditions used for analysis were 40 kg load, 75°C temperature, 1200 rpm and 1hr duration.

Example 2f: Load carrying capacity of Oils (IP 239)
The IP 239 method is used for evaluation of load carrying, anti-friction and anti wear properties of lubricating oils by means of the four ball machine. The point contact simulation on 4 ball tester gives an idea about the load carrying capacity at 100-800 kg by film breakage, which leads to welding of test balls.

Example 2g: KRL shear stability (CEC L-45-A-99)
The KRL shear test uses a tapered rolling bearing in a cup fitted to a four ball instrument. Load is applied to the bearings as they are rotated at a certain RPM for a specified length of time. The test is typically run for 20 hr. KRL is considered to be one of the most severe shear tests and is used for driveline fluids and gear lubricants. The unit can perform standard methods such as CEC L-45-A-99.
Temperature: 60 ±1?
Speed:1475 ± 25 rpm
Duration: 20 hr
Load: 5000 ±200 N (510 ± 20 kgf)

Example 3: Performance evaluation
Example 3a: Friction performance by MTM
MTM (Mini Traction Machine) was used to measuring the frictional properties of lubricated and unlubricated contacts under a wide range of rolling and sliding conditions. Main application is the fully automated traction mapping of lubricants under conditions commonly found in internal combustion engines. Additional features allow the measurement of anti-wear additive film growth on test specimens, investigation of soft contacts, reciprocating friction and wear measurements. Below test conditions were used for measuring the static to dynamic friction behaviour of candidate oils
Temperature -100?
SRR- 20-80%
Load -60 N
Speed – 0-60 mm2/s

Example 3b: Carbon Residue test (ASTM D4530)
The test method covers the determination of the amount of carbon formed after evaporation under certain conditions and is intended to provide some indication of the relative coke forming tendency. Initially, Oil sample in glass tube is heated at 200°C in a solid bath with air (15L/hr) for different interval. Further, weighed quantity of sample is placed in a glass vial and heated to 500°C under an inert (nitrogen) atmosphere in a controlled manner for a specific time. The sample undergoes coking reactions, and volatiles formed are swept away by the nitrogen. The carbonaceous-type residue remaining is reported as a percent of the original sample as “carbon residue (micro).”

Example 4: Preparation of candidate oils 1 to 5
Lubricating oil compositions having mixed compositions shown in Table 1 (Examples 1 to 5) were prepared, and performances of the lubricating oil compositions were evaluated (Table 2).
Candidates 1-5 were prepared by using a mixture of 20-30% wt/wt of Group I base oil having kinematic viscosity range 9.0-12.0 cSt and 70-80% wt/wt of Group II base oil having kinematic viscosity range of 8.5-11.5 cSt. Adding multi-component additive having polyisobutylene based thickener additive, polymethacrylate based pour point depressant additive, viscosity modifier unsaturated fatty acid ester, sulphurised isobutylene and alkylated phosphates, alkylated thiophosphates, polyphosphates phenate detergent systemto the above base oil mixture in the range of 3-8% wt/wt to obtain a blend; and further, treating the above obtained blend with 0.5-4% wt/wt of friction modifying additive having mixture of zinc thiophosphates and calcium salts with elemental ratio of Ca/(P+Zn) in the range of 0-2.

Table 1: Preparation of candidate oils 1 to 5
S. No Components (wt %) Candidate 1 Candidate 2 Candidate 3 Candidate 4 Candidate 5
1 Base Oil Mixture of solvent refined Group I (20-30%) and hydro cracked Group II base oil (70-80%) Mixture of solvent refined Group I (20-30%) and hydro cracked Group II base oil (70-80%) Mixture of solvent refined Group I (20-30%) and hydro cracked Group II base oil (70-80%) Mixture of solvent refined Group I (20-30%) and hydro cracked Group II base oil (70-80%) Mixture of solvent refined Group I (20-30%) and hydro cracked Group II base oil (70-80%)
2 Multi-component additive 3.0-8.0% 3.0-8.0% 3.0-8.0% 3.0-8.0% 3.0-8.0%
3 Friction Modifying additive
-- 0.5 -2.0% 1.0-3.0% 1.5-4.0% 2.0-4.0%
4 Total 100.0 100.0 100.0 100.0 100.0
5 Ca/(P+Zn) Mass Ratio 0.39 0.55 0.63 1.16 1.28

Table 2: Physicochemical & performance data of candidate oils 1 to 5
S. No Test parameters Test Method Candidate 1 Candidate 2 Candidate 3 Candidate 4 Candidate 5 Reference Oil
a KV @ 100°C, cSt ASTM D-445 11.79 11.84 11.88 12.05 12.03 11.7
b KV @ 40°C, cSt ASTM D-445 87.23 86.82 87.01 88.67 88.55 85.94
c Viscosity Index ASTM D-2270 127 128 128 129 129 129
d CC@121°C, 3hrs ASTM D-130 1a 1a 1a 1a 1a 1a
e Rust test ASTM D-665A Pass Pass Pass Pass Pass Pass
f Pour point, °C ASTM D-97 -27 -27 -30 -27 -27 -33
g Brookfield Viscosity @ -18°C, cP ASTM D-2983 9398 9200 9456 9489 9563 8350
h Wear Scar Dia, mm ASTMD -4172 0.45 0.45 0.45 0.45 0.45 0.45
i Weld Load, kg IP 230 250 250 250 250 250 315
j Viscosity @ 100 deg C(after 20 hr KRL shear), cSt CEC L45-A-99 11.05 11.10 11.18 11.25 11.20 10.12

Screening of candidates
Various candidates were prepared with varying Ca/(P+Zn) mass elemental ratio for the screening. Physicochemical data of all the candidates was generated and further screened for friction behaviour and carbon deposit formation. All the candidates were tested in field tractors for the assessment of drain interval and brake chatter performance. During the trial, various additional parameters were monitored through physicochemical testing, frictional profile assessment of candidate oils, temperature and brake noise measurement.

Example 5: MTM
MTM (Mini Traction Machine), was used to measure the frictional properties of lubricated contacts under a wide range of rolling and sliding conditions. Main application of MTM is traction mapping of lubricants under conditions commonly found in lubricants. Additional features allow the measurement of anti-wear additive film growth on test specimens, investigation of soft contacts, reciprocating friction and wear measurements. Based on the literature, the ratio of static to dynamic coefficient of friction plays a significant role in predicting the noise behaviour. Findings of prior art show if the ratio of static to dynamic coefficient of friction is >1, such friction profile of fluid leads to noise free operation. Further the µ~v graph should show an increasing friction coefficient with speed under simulating conditions. All the candidates were tested for fictional behaviour through MTM and it has been observed that the candidates which have ratio of Ca/(P+Zn) = 1 show positive slope of static to dynamic friction with speed Figure 1) leading to reduced noise characteristics. Further, it had also been observed that candidates 1 to 3 have shown negative slope of friction whereas candidates 4 and 5 have shown positive slope of friction.

Example 6: Carbon residue test (ASTM D4530):
The test method covers the determination of the amount of carbon deposit formation after evaporation under certain conditions and is intended to provide some indication of the relative coke forming tendency. Weighed quantity of sample is placed in a glass vial and heated to 500°C under an inert (nitrogen) atmosphere in a controlled manner for a specific period of time. The sample undergoes coking reactions, and volatiles formed are swept away by the nitrogen. The carbonaceous-type residue remaining is reported as a percent of the original sample as “carbon residue (micro).” Carbon residue is observed less in candidate with Ca/(P+Zn) = 1 (candidate 4) (Table 3).
Table 3: Carbon deposit formation of candidate oils
Sr. No Sample test interval in hrs Candidate 1 with Ca/(P+Zn) < 1 Candidate 4
with Ca/(P+Zn) = 1
CCR (%) CCR (%)
1 0 0.89 0.92
2 6 1.95 1.63
3 12 3.63 2.74
4 18 4.75 3.45
5 24 5.66 4.23

Example 7: Field performance of Tractor trial
Candidate oils 1 & 4 charged in 45 HP tractors for field performance trial and brake chatter performance up to 2000 hrs (Table 4) was evaluated. Wear and brake chatter have been monitored for six tractors with varying mass elemental ratio of Ca/(P+Zn). It has been observed that candidates 4 having mass elemental ratio Ca/(P+Zn) = 1 have shown positive friction curve (figure 2), low wear (Fe & Cu) (Table 5), and further exhibited excellent noise handling capability. Candidate 1 having mass elemental ratio Ca/(P+Zn)<1 have shown negative friction curve(figure 2), high wear (Fe & Cu) (Table 5), and further resulted high brake chatter. In candidate 1 with Ca/(P+Zn) <1 other elements such as large silica particles and high wear particles were found.
Average noise (dB), oil sump temperature, left brake and right brake temperatures of tractors 1 & 4 tested with candidate oils 1 and 4, respectively. Lower noise level and oil sump temperature, significant lower brake temperature observed in Candidate 4 which has Ca/(P+Zn) =1 (Table 6). Whereas, the oil temperature, brake temperature and brake noise level found to be higher for candidate 1 having Ca/(P+Zn) <1.
Further, used oil samples tested through SEM and TEM analysis for examining type of impurities in oil samples which were collected from tractors/ candidate 1 with mass elemental ratio (Ca/(P+Zn)<1 and candidate 4 (Ca/(P+Zn)=1. Less agglomeration of particles by SEM analysis and only few impurities were observed by EDS in candidate 4 with mass elemental ratio Ca/(P+Zn)=1. Amorphous black carbon was also observed in noisy tractors in Candidate 1 which may be due to more oil deterioration in these tractors.

Table 4: Tractor running hours in field
S. No Oils /Tractor number Tractor Running Hrs Remarks
1 Candidate 1 with Ca/(P+Zn) < 1
Tractor 1 280 Noise observed
Tractor 2 475 Noise observed
Tractor 3 356 Noise observed
2 Candidate 4 with Ca/(P+Zn) =1
Tractor 4 1856 No Noise observed
Tractor 5 1956 No Noise observed
Tractor 6 2016 No Noise observed

Table 5: Wear Data of Used oils in Tractors 1-6
S. No Used oil parameters Iron, ppm Copper, ppm
1 Candidate 1 with Ca/(P+Zn) < 1
Tractor 1 467 80
Tractor 2 287 82
Tractor 3 324 69
2 Candidate 4 with Ca/(P+Zn) = 1
Tractor 4 116 11
Tractor 5 138 19
Tractor 6 161 31

Table 6: Noise level and temperature measurement of tractors
S. No Brake noise or chatter Candidate 1 with Ca/(P+Zn) < 1 Candidate 4
with Ca/(P+Zn) = 1
1 Average Noise (dB) 111 83
2 Max Sump temperature, ? 89 72
3 Left side Brake temp (outside), ? 118 78
4 Right side Brake temp (outside), ? 116 73 , Claims:1. A lubricating oil composition for wet brake operations comprising:
a. 20-30% wt/wt of a refined Group I base oil having kinematic viscosity in the range of 9.0 to 12.0 cSt at different temperatures from 90-120ºC;
b. 70-80% wt/wt of a Group II base oil having kinematic viscosity in the range of 8.5 to 11.5 cSt at different temperature from 90-120ºC;
c. 3-8% wt/wt of a multi-component additive comprising:
a polyisobutylene (PIB) based thickener,
a viscosity modifier unsaturated fatty acid ester,
a sulphurised isobutylene (SIB),
a O,O-dialkyl,S-alkyl thiophosphate, S,S-dialkyl,O-alkyl phosphate, O,O,O-trialkyl phosphate, polyphosphate phenate detergent system, and
a polyacrylate based pour point depressant; and
d. 0.5-4% wt/wt of a friction modifying additive having an alkylated zinc thiophosphate along with an overbased calcium salt with total base number in the range of 50-400 mg of KOH/gm,
wherein all the % wt/wt is based on the total weight of the composition.
2. The lubricating oil composition as claimed in claim 1, wherein the multi-component additive comprises:
10-35% by wt/wt of the O,O-dialkyl,S-alkyl thiophosphate, S,S-dialkyl,O-alkyl phosphate, O,O,O-trialkyl phosphate, polyphosphate phenate detergent system;
0.5-5% wt/wt of the polyisobutylene (PIB) of molecular weight ranging from 400-10500 Dalton;
10-30% wt/wt of sulphurised isobutylene;
1-4% wt/wt of the viscosity modifier of unsaturated fatty acid ester having carbon atoms of C14-C20; and
0.1-2% wt/wt of the polyacrylate based pour point depressant of molecular weight 300-70000 Dalton.
3. The lubricating oil composition as claimed in claim 1 or 2, wherein the unsaturated fatty acid ester is a mixture of esters of oleic acid and linolenic fatty acids.
4. The lubricating oil composition as claimed in claim 1, wherein the alkylated zinc thiophosphate comprises alkylated zinc dialkyldithiophosphate (ZnDTP) and Zinc thiophosphate having C1-C14 carbon atoms salts.
5. The lubricating oil composition as claimed in claim 1, wherein the friction modifying additive is a combination of Zinc thiophosphates and calcium salts and is maintained in the mass ratio of Ca, P, Zn elements with Ca/(P+Zn) = 1.
6. The lubricating oil composition as claimed in claim 1, wherein the calcium salt is selected from calcium sulfonate, calcium phenate and calcium salicylate.
7. A process for preparing a lubricant oil composition comprising steps of:
(i) preparing a mixture of 20-30% wt/wt of Group I base oil having kinematic viscosity range 9.0-12.0 cSt and 70-80% wt/wt of Group II base oil having kinematic viscosity range of 8.5 to 11.5 cSt;
(ii) adding multi-component additive having polyisobutylene based thickener, polymethacrylate based pour point depressant additive, viscosity modifier unsaturated fatty acid ester, sulphurised isobutylene and alkylated phosphates, alkylated thiophosphates, polyphosphates phenate detergent system to the above base oil mixture in the range of 3-8% wt/wt to obtain a blend; and
(iii) treating the above obtained blend with 0.5-4% wt/wt of friction modifying additive having mixture of zinc thiophosphates and calcium salts with elemental ratio of Ca/(P+Zn) = 1.