1. TRIBOLOGICAL PROPERTIES OF BIODEGRADABLE
UNIVERSAL TRACTOR TRANSMISSION OIL
Mile STOJILKOVIĆ,
Mirjana KOLB1
NIS GAZPROMNEFT, Serbia,

2. Introduction
The annual consumption  of lubricants in the world is around 38 millions tons, and is projected to growing with a CAGR of 2,5% between and 2019 .
It is estimated that about 50-60% of all the oil used ends up in the environment with no control.
Lubricants, by nature, can be mainly divided into three product segments including mineral, synthetic and bio-based lubricants. Petroleum based lubricants, which are the leading type of base oil used in industry, are poorly degradable and represent an enviromental hazard when released.
The bio-lubricant industry is growing based on these pressures, environmental concerns and sustainability.
Owing to their unique chemical, mechanical and eco-frendly properties, bio-based lubricants are finding applications across a most of industries including, agriculture, shipping, forestry and railways.
Bio-lube consumption thousand tones
4,000
4,000
Series
3,500
3,000
2,500
2,500
2,000
1,550
1,500
950
1,000
500

3. Biolube composition
Bio-lubricants may be defined generally as materials that are based on biodegradable and renewable base stocks.
Bio-lubricants do not have to be composed entirely of vegetable oil base stocks. They can be products derived from renewable oils, such as the fatty acids from fats and oils, reacted with synthetic alcohols or polyols to produce esters that can be considered bio-lubricants. Also, the natural vegetable oils can be treated to produce a modified product that is still biodegradable an renewable. Most bio-lubricants are composed of natural or synthetic esters or mixtures, and additives.
Vegetable oils (palm oil, castor oil, sunflower oil, soybean oil, rapeseed and canola) accounted for over 80% of globally produced bio-lubricants.
The main constituents of vegetable oil (typically more unsaturated) and animal fats (typically more saturated) are triglyceride.
We tested three types of vegetable oils (soybean, rapeseed and sunflower oil). In all of them fatty acids with a chain length C18 are dominant, but their chemical composition are different - the proportion of saturated, monounsaturated and polyunsaturated acids is different.
A triglyceride is an ester derived from one molecule of glycerol and three fatty acids. The three fatty acids are usually differ one of another, in different length of carbon chain and the level of saturation

4. Biolube properties
No toxicity and biodegradability are importantand properties of vegetable oils but the most important are functional properties as defined by the specifications and the requirements of the equipment manufacturers.
Functional – working characteristics of the lubricant are given their physicochemical, rheological and tribological properties.
Advantages - excellent lubricating properties, extremely high viscosity index and very low volatility
Disadvantages -Bio-lubricants based on vegetable oils are very sensitive to high and low temperature- poor pour point and oxidation and thermal stability due to the presence of poly-unsaturated products in the natural oil to compete with products based on mineral oils. Rapeseed oil has lower content of unsaturated products and better oxidation stability compared to soybean and sunflower oil. Improving oxidation stability is achieved by adding antioxidants.

5. Biolubes application in agriculture machinery
Agricultural machinery is the first choice for using biodegradable lube because the machinery is used directly in the environment where the lubes can easily come in contact with soil, water and plants
The universal tractor transmission oil (UTTO) is highly functional oil used in the agricultural machinery such as tractors, combines etc. The main functions of the UTTO oils are: lubrication of gearbox, rear axle and gears, power transfer and hydraulic system lubrication providing
adequate cooling and friction wet brake
The aim of this paper is to examine the tribological properties of the universal tractor transmission oils „UTTO“ based on three different vegetable oils in relation to a referent mineral UTTO oil. Tested oils have same additives and different base. s.
The universal tractor transmission oil ( UTTO) is highly functional oil used in the agricultural machinery such as tractors, combines etc. The main functions of the UTTO oils are:
- lubrication of gearbox, rear axle and
gears, power transfer and hydraulic
system lubrication providing
adequate cooling and friction wet
brakes

6. Experimental work
The tests were performed on four samples of oil, three different UTTO oils based on vegetable oil and one based on mineral oil (Table 2)
Tribological tests for all four samples were conducted on tribometer TPD-93, type "block on disc"
Sample
Base oil
Code
Biodegradable UTTO oil
Rapeseed oil (cold-pressed, degummed RE
Soybean oil (cold-pressed, degummed) SO
Sunflower (cold-pressed, degummed) SU
Mineral base UTTO oil
Mineral oil (API Group I) MN
Tribometer TPD-93, type „block on disk“
Materials of block and disc - annealed alloy steel 16MnCr5 (Č4320) having the hardness of 35 HRC
Sliding speed in the contact zone- v = 0.8 m / s
Contact duration (duration of the tests of each sample) is T = 60 minutes
Normal external loading (Fn) changed during test
Fn1 = 30 N t1 = 15 min
Fn2 = 100 N t2 = 15 min
Fn3 = 300 N t3 = 30 min
Type of lubrication: boundary

7. Results of measurement
The basic indicator of tribological properties of materials is the change of coefficient friction during the contact time.
The results obtained during tests contain information about coefficients of friction, the force of friction, width and depth of wear scar, geometry of the contact
They show that after an initial increase up to a maximum value, the friction coefficient is gradually decreasing in each phase of the load. At a later stage of testing its value stabilizes around a constant value.
The time period in which the value of the coefficient of friction has intensive change is 0 to 200 seconds (depending on the type of oil) and is specified as the breaking-in phase.
Breaking-in phase characterized by significant changes in the topography of the block and the disc, due to topography differences from the transition of the technological to the exploitation phase.
To obtain a clearer picture of changing a coefficient of friction at certain time intervals, the values of the coefficient of friction are recorded every 2 seconds. This gives the information on the dynamics of the process that takes place on the tribometer.

8. Results of measurement- changes the friction coefficient of
the tested oils in function of testing time and variation of loads
Diagrams represented changes the friction coefficient of the tested oils in function of testing time and variation of loads. They show that after an initial increase up to a maximum value, the friction coefficient is gradually decreasing in each phase of the load. At a later stage of testing its value stabilizes around a constant value.
Change the coefficient of friction in oil samples SU
Change the coefficient of friction in oil samples RE
The diagrams show that the coefficient of friction of tested samples based on vegetable oils is lower than the one of reference mineral UTTO oil, especially at higher loads
Change the coefficient of friction in oil samples SO
Change the coefficient of friction in oil samples MN

9. depth of wear scar mm) (μm)
Results of mesurements of testing samples- wear
Bondary lubrication
width of wear scar mm
depth of wear scar mm) (μm)
1,460
15,232
1,513
16,359
1,345
12,926
1,585
17,954
Wear scar on the blocks in tribological tests can be seen in the photographs
Measured width of the scar wear on block
Wear scar on the block is the largest in sample of mineral UTTO oil –Sample MN and the lowest in the sample biodegradable UTTO oils based on sunflower oil -Sample SU.
Narrow wear scar in samples RE, MA and SU indicate good lubricity of vegetable oils.
It shows excellent antiwear properties of vegetable oils even at maximum load.

10. Conclusion
There is no uniform definition for bio-lubricants, which may be biodegradable, non-toxic, bio-based, or combinations thereof. It is generally accepted that bio-lubricants must fulfill the OECD 3012 test for biodegradability.
Most bio-lubricants are composed of natural or synthetic esters or mixtures, and additives
Bio-lubes based on vegetable oils have excellent lubricating properties, and poor oxidation stability. Characteristics of vegetable oils can be improved by the addition of suitable additives.
The basic indicators of tribological properties of materials are: the change of coefficient friction during the contact time, the force of friction, width and depth of wear scar, geometry of the contact. The best ways to determine all-important tribological characteristics is tribometer TPD-93, type "block on the disk.”
The coefficient of friction is very dependent on the type of oil. Generally oils based on vegetable oils have lower coefficient of friction than the one mineral UTTO oil, especially at higher loads. The best results, the lowest value of the coefficient of friction, shows UTTO oil based on rapeseed oil
Wear scar on the block is the largest in referent mineral UTTO oil and the lowest in the sample based on sunflower oil. Narrow wear scar indicate good lubricity of vegetable oils.
Bio-tractols based on vegetable oils have excellent anti-wear properties even at maximum load.