Applied Mechanics and Materials Vol. 315 (2013) pp 936-940
Online available since 2013/Apr/10 at www.scientific.net
© (2013) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.315.936
Friction Characteristics of RBD Palm Olein using Four-Ball Tribotester
S.Syahrullail1, a, J.Y.Wira2,b, W.B.Wan Nik3,c and W.N.Fawwaz1
1
Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor,
Malaysia.
2
3
Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia Kuala
Lumpur, Jalan Semarak, 54100 Kuala Lumpur, Malaysia.
Faculty of Maritime Studies and Marine Science, Universiti Malaysia Terengganu, 21030 Kuala
Terengganu, Malaysia.
a
b
c
[email protected], [email protected], [email protected]
Keywords: Palm oil, fourball tribotester, paraffin, friction coefficient, surface roughness.
Abstract. In this study, the effect of load on the tribological performance of RBD palm olein is
investigated using a four-ball wear tester according to the standard test of ASTM D4172. Tests were
conducted with 40, 60, 80, 100 and 120 kg normal loads. The experimental temperature and
rotational speed were held constant at 75 ºC and 1200 rpm, respectively. The test duration was 60
minutes in all cases. For each load, the tribological properties of RBD palm olein were compared
with the properties of additive-free paraffinic mineral oil. The results focused on the frictional
torque, wear scar diameter, friction coefficient and the flash temperature. Following the completion
of the wear test experiments, the ball wear condition and lubricant properties were observed. These
results show that RBD palm olein has a lower coefficient of friction than paraffinic mineral oil;
however showed a high oxidation effect under high temperature work conditions.
Introduction
Currently, vegetable oil-based lubricants have started to replace the mineral-based oils for
industrial lubrication. This trend has occurred because mineral oil lubricants are not readily
biodegradable and are toxic. Global environmental awareness has encouraged the production of
environmentally-friendly lubricants. The production and use of non-toxic, biodegradable lubricants
has become a major issue, especially when the lubricant involved will come into contact with soil,
crops or ground water. Biodegradability is the ability of a substance to be decomposed by the action
of bacteria into CO2, water and mineral compounds. There are several factors that affect the
biodegradability of a substance, including the molecular structure, chemical properties and
environmental conditions [1]. Additional beneficial properties of vegetable oil, such as a high
viscosity index, good lubricity, high flash point and low evaporative loss, have also made it
preferable for use instead of mineral oil-based lubricants [2]. Therefore, there has been major
interest in the development of many types of lubricants, including greases and hydraulic fluids, that
are based on vegetable oils, such as a rapeseed oil, castor oil and palm oil. These oils all have
excellent lubricating properties, load carrying capacities, and corrosion protection properties in
comparison with mineral oil [3].
A few decades ago, large quantities of palm oil were consumed by railway companies who used
it almost exclusively for greasing the axle boxes of the railway carriages [4]. Palm oil has several
advantages over mineral oil. Palm oil is comparatively inexpensive, readily available,
biodegradable, environmental-friendly and renewable [5]. Furthermore, the production of mineralbased lubricants, such as those obtained from petroleum, uses more energy and generates additional
pollution during the refinement process than the corresponding process for vegetable oils. In this
study, the tribological performance of RBD palm olein, which is a refined palm oil product, was
investigated using a four-ball tester. The same testing procedure was repeated using additive-free
paraffinic mineral oil as a control sample, and the results of the two tests were compared. For all
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Applied Mechanics and Materials Vol. 315
937
tests, the initial temperature was set at 75 ºC and the rotational speed was 1200 rpm. The normal
loads were varied from 40 to 120 kg at 20 kg intervals. The results show that RBD palm olein has a
lower coefficient of friction than the additive-free paraffinic mineral oil.
Experimental Procedure
Apparatus. The four-ball wear machine was used in accordance with the standard testing procedure
ASTM D4172. The testing apparatus consists of a mobile ball bearing that is rotated in contact with
three fixed ball bearings which are immersed in the test lubricant, as shown in Fig. 1. The load was
varied and speed was constant at 1200 rpm. The test oil was heated to 75 °C within 3 degrees
throughout the experiment. The duration of the experiment was 60 minutes for all tests. The test ball
is 12.7 mm in diameter. The test balls are composed of chrome alloy steel, are made from AISI E52100, are Grade 25 Extra Polished and have a Rockwell C hardness of 64 to 66.
Collet
Ball bearing
Oil cup
Thermocouple
Applied force
(upward)
Fig. 1 Schematic diagram of four-ball tester apparatus.
Lubricants. The test lubricants were refined bleached deodorised (RBD) palm olein and
additive-free paraffinic mineral oil (written as paraffinic mineral oil). The paraffinic mineral oil was
used as a control lubricant for direct comparison. RBD palm olein and paraffinic mineral oil have
densities of 915.5 and 871.5 kg/m3, respectively. At 75 °C, RBD palm olein and paraffinic mineral
oil have viscosities of 19 and 16 cSt, respectively. Palm olein is the liquid fraction that is obtained
by the fractionation of palm oil after crystallisation at a controlled temperature. Standard grade RBD
palm olein, which was defined as a standard material obtained through processes set forth in the
Malaysian Standard MS 816:1991, was used [6].
Wear Scar Diameter. The wear scar diameter of each of the three bottom test balls was
measured to determine the lubricity performance of the test lubricant. In general, the larger the wear
scar diameter, the more severe the wear. The wear scar was evaluated by a computer running CCD
software and from the captured photomicrograph. Using this process, the wear scar diameter was
determined for each of the three fixed balls.
Results and Discussion
Temperature of Test Lubricants. A thermocouple was attached to the four-ball tester to record the
temperature changes throughout the duration of the experiment. The temperature changes were
plotted as shown in Fig. 2. In these experiments, the normal load was varied from 40 to 120 kg, by
20 kg increments. From this figure, the temperature for both test lubricants increased with the
incremental increases of the normal load. The paraffinic mineral oil showed higher temperature
increases with the increases in the load compared to the RBD palm olein. For paraffinic mineral oil,
the lubricant temperature started to increase gradually under 80 kg experimental load conditions.
For RBD palm olein, however, the temperature rise did not begin until the 100 kg load level was
reached. The maximum temperature was detected at 120 kg experimental conditions, and was
recorded at 115 and 90 °C for paraffinic mineral oil and RBD palm olein, respectively. As the
normal load was increased incrementally, the friction between the test balls increased, resulting in
an increase in the heat generated.
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120
Paraffinic mineral oil
RBD palm olein
o
Temperature, C
110
100
90
80
70
60
40
60
80
100
120
Normal load, kg
Fig. 2 Test lubricant temperatures for various normal loads.
Wear Scar Diameter. Fig. 3 shows the effect of load on the measured wear scar diameter for
both RBD palm olein and paraffinic mineral oil. From this figure, it is very obvious that the wear
scar diameter increases gradually with incremental increases in the normal load. The temperature
increase contributed to a decrease in the test lubricant viscosity [7]. Low viscosity lubricants tend to
create only a thin film. Increasing temperature causes the lubrication film to become less stable and
eventually to break down. As a result, the metal-to-metal contact area will increase [8] and produce
an increase in the wear scar diameter under high pressure conditions. The wear scar will also
increase due to the removal of the metallic soap film which occurs at high load [2]. For RBD palm
olein, the condition between the test balls falls in the mixed lubrication regime consisting of a thin
lubricant film with adsorption of fatty acids from the palm oil playing the role of maintaining the
thin lubricant. As a result, less metal-to-metal contact occurred which resulted in a smaller wear scar
diameter [9]. These results show that the RBD palm olein has the ability to reduce wear relative to
mineral oil due to its higher proportion of long chain saturated fatty acids [10]. However, at 40 and
60 kg normal loads, the wear scar diameter for RBD palm olein is slightly larger compared to the
wear scar in paraffinic mineral oil. This behaviour is likely due to chemical attack on the surface of
the balls by the fatty acids present in the vegetable oil. The metallic soap film is rubbed away during
sliding, producing an increase in wear due to the absence of the non-reactive detergents [11].
Wear scar diameter (µm)
4500
4000
3500
Paraffinic mineral oil
RBD palm olein
3000
2500
2000
1500
1000
500
0
40
60
80
100
120
Normal load, kg
Fig. 3 Wear scar diameter for various normal loads.
Coefficient of Friction. The friction coefficient was calculated according to IP-239, and is
expressed as follows:
T
µ = 0.22248
(1)
W
where T is the frictional torque in kg.mm and W is the applied load in kg [12]. The frictional torque
data were recorded by a computer and the friction coefficient was calculated. From Fig. 4, the
coefficient of friction for paraffinic mineral oil is observed to be higher compared to that for RBD
palm olein. The coefficient of friction for the contact between balls lubricated with paraffinic
mineral oil increased with the incremental increase of the normal loads. The increase in the
coefficient of friction was gradual for 40, 60 and 80 kg loads and began to fluctuate slightly between
0.10 and 0.11 with increases in load beyond 80 kg for the paraffinic mineral oil. For experimental
Applied Mechanics and Materials Vol. 315
939
conditions using RBD palm olein, the incremental increases of the load did not noticeably increase
the coefficient of friction. For all normal load conditions, the coefficient of friction tested using
RBD palm olein fluctuated between 0.05 and 0.07. This behaviour is related to the existence of the
fatty acids in the RBD palm olein; these fatty acids help to maintain the lubricant layer, giving a
lower coefficient of friction compared to the paraffinic mineral oil [13].
Coefficient of friction
0.14
Paraffinic mineral oil
RBD palm olein
0.12
0.1
0.08
0.06
0.04
0.02
0
40
60
80
100
120
Normal load, kg
Fig. 4 Coefficient of friction for various normal loads.
Flash Temperature Parameter. A flash temperature parameter (FTP) was calculated for all of
the experimental conditions according to Eq. 2. In this equation, W is the normal load in kilograms
and d is the mean wear scar diameter in millimetres at the particular load. A detailed explanation of
the parameter is given by Lane [14].
W
FTP = 1.4
(2)
d
High values for the flash temperature parameter indicate that the lubricant shows good
performance with a reduced possibility of lubricant breakdown [15]. Fig. 5 shows that, at 40 and 60
kg loads, paraffinic mineral oil has a higher flash temperature parameter compared to RBD palm
olein, implying that the lubricant layer of RBD palm olein has a higher possibility of breakdown.
This result is in good agreement with the wear scar diameter measurements, where RBD palm olein
has a higher wear scar diameter compared to the paraffinic mineral oil at 40 and 60 kg. However, at
normal loads of 80 to 120 kg, the flash temperature parameter for paraffinic mineral oil is
significantly reduced. The maximum flash temperature parameter for RBD palm olein and
paraffinic mineral oil occurred at 80 and 60 kg, respectively.
Flash temp. parameter
140
Paraffinic mineral oil
RBD palm olein
120
100
80
60
40
20
0
40
60
80
100
120
Normal load, kg
Fig. 5 Flash temperature parameter for various normal loads.
Conclusion
RBD palm olein has better performance properties in terms of friction reduction (coefficient of
friction) and wear resistance (anti-wear properties). RBD palm olein also performed better when
evaluating lubricant film breakdown at high pressure (high normal load). RBD palm olein showed
poor thermal stability compared to paraffinic mineral oil. However, this feature did not influence the
lubricity performance of RBD palm olein which out-performed paraffinic mineral oil in this regard.
940
Mechanical & Manufacturing Engineering
Acknowledgement
The authors wish to thank the Faculty of Mechanical Engineering at the Universiti Teknologi
Malaysia for their support and assistance during this study. The authors also wish to thank the
Research University Grant (GUP) from the Universiti Teknologi Malaysia, the Ministry of Higher
Education (FRGS-MOHE) and the Ministry of Science, Technology and Innovation (eScience –
MOSTI) of Malaysia for their financial support.
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Mechanical & Manufacturing Engineering
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Friction Characteristics of RBD Palm Olein Using Four-Ball Tribotester
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