Grease Degradation Alternate Test Method and Storage Interval for Roller Bearings: An Empirical Research
K. K. Nithia1*, M. Y. Noordin1
1 School of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Johor Darul Takzim, Malaysia
*Corresponding Author: [email protected] Accepted: 15 February 2022 | Published: 1 March 2022
DOI:https://doi.org/10.55057/ijarti.2022.4.1.6
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Abstract: Pandemic caused parts into prolong storage conditions. Empirical experiment was carried out to determine the viscoelastic behaviour and consistency of unworked grease from inside a wheel roller bearing. The grease samples intervals were 9, 10, 11 and 24 months from stored wheel roller bearings. Fresh grease is also tested as a controlled sample. A rheometer amplitude oscillation test was performed. The grease was tested for contaminants such as oxidation, nitration, soot, sulfation, and water using FTIR. Rheology test exhibited changes in viscoelastic behaviour. Grease consistency changed from grade 2 to grade 1. This confirms that the grease tends to soften and degrade over time. FTIR results revealed that there is no significant contaminant for unworked and packed grease in the wheel. Storage and maintenance interval derived from this study.
Keywords: Grease, Lubrication, Degradation, Roller bearings
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1. Introduction
Covid 19 pandemic, aircrafts are grounded due to low utilization globally. As a result, aircrafts were kept in prolong storage and parts such as an inside wheel roller bearing, which was previously used continuously are now being rarely used. Therefore, high inventory storage parts are now inducted into prolong storage in the warehouse. For safety reasons, there is a need to establish whether the lengthen storage period affects the usability of the aircraft parts.
Aircraft bearings are made for high rotational speed and will experience extreme operating temperatures. The temperature in the bearing ranges from negative 54℃ to positive 177℃. The aircraft wheel is a vital component for manoeuvre such taking off, landing and taxiing. The wheel bearing takes up high load during taxiing, taking off and landing. Thus, commercial aircraft wheel bearings are greased with MIL-PRF-81322G and NLGI grade 2 with work penetration of 265-320 tenth of millimetre. The grease lubricates and reduces the surface friction between the cone and cup. In addition, the grease also creates a thin film between the roller and sliding surface as a load carrying capacity. The motivation for this research was based on an industry observation that showed the wheel roller bearing grease became thinner, and experienced colour changed. These observations were apparent when the wheel was stored in a static state for more than 9 months. The objective is to carry out an empirical study on an in-service grease to establish changes in mechanical properties. The mechanical properties investigated are the viscoelastic behaviour of the grease, oil bleed from the thickened grease and material content analysis for the colour change. Past research works that motivated for this
research were Couronne et al. 2000 concluded that rheometry as an alternate method for standard techniques such as cone penetration. Suetsugu et al. 2013 concluded that loss modulus has an excellent correlation to determine penetration of grease. Alan Gurt and M.Khonsari in 2021 where by the findings were that rheometer may be used to evaluate the change in consistency with the same grease type and not to compare with different types of grease.
Rheometer can be used to assess relative penetration for used and fresh grease. In that research also concluded that cross over stress has a strong law of correction to cone penetration. In their research the correlation between cross over stress and cone penetration registered R2 of 0.9527.
The consistency test for in-service grease was unable to be performed by conventional method since the in-service grease sample mass was very small. An alternate method using a rheometer was recommended from the conventional penetration test as per ASTM D217 and ASTM D1403 [1]. The alternate measurement on rheometer stated ‘cross over stress’ at room temperature [2]. This research hypothesized that the wheel bearing grease NLGI grade 2 changes in the consistency when stored in a static state for 9 months and more. The study was not comparing any grease by brands. The research investigates the changes in the grease's mechanical stability based on the off-wing wheel prolong storage. The prolonged storage of the aircraft wheel is defined as the off-wing wheel positioned vertically and static for more than six (6) months. The storage condition was in a storage room with ambient temperature and humidity.
2. Material and Methods
The material that was used in this study was aircraft grease NLGI grade 2. The brand name of the grease will be classified. The research used several apparatus including the use of MCR 301 Anton Paar rheometer, Perkin Elmer FTIR-spectrum 400 spectrometer, commercial digital weighing scale, wheel bearing removal tools and standard laboratory apparatus. The in-service grease was taken from aircraft wheel bearings stored at ambient environment shaded storeroom. Nevertheless, the aircraft wheels were covered with canvas to avoid dust particle contaminants and stored indoors away from sunlight. The average ambient temperature was between 32℃ to 34℃ while the average humidity is between 70% to 80%. The in-service grease samples were those stored for 10 months, 11 months and 24 months. These samples were unworked used grease, but the wheels had never been installed on-wing and were not operational. The in-service grease samples were compared with fresh grease sample taken from the grease container. Hence, the fresh grease is undisturbed, and the viscoelastic properties will be the benchmark in this research.
The samples were labelled as S0 for fresh undisturbed grease, S1 for unworked 24 months grease, S2 for unworked 9 months grease, S3 for unworked10 months grease, S4 and S5 for unworked 11 months grease. The S0 fresh grease was taken from the reservoir drum and the date of manufacture was October 2018. The samples were packed in a plastic bag and were kept at room temperature.
The samples S0, S1, S2, S3 and S4 were tested for Storage Modulus (G’), Loss Modulus (G”), yield stress (τy) and cross overstress (τco). The test used was the sweep amplitude oscillation test at a frequency of 1Hz. The oscillatory test is a method to determine the usefulness of the grease in rheology perspective [3]. The first advantage of oscillatory over rotational test is that the samples are less likely to fracture [4]. The second advantage is that oscillatory test results are negligible to variables of gap and surface roughness [5-6]. The Anton Paar MCR 301 rheometer equipment was used to conduct the experiment. The geometry was a parallel plate with a diameter of 25mm. The gap for the geometry was 1mm. The oscillation was at 1Hz
(1mRad per sec). The test interval was set at 17 intervals with a strain rate (ϔ) of 0.01% till 100%. The test plate temperature was 25℃. Pre-shear was not performed since sample quantities were very small. Samples were trimmed using a spatula. After each test was performed, the surfaces were cleaned using laboratory ethanol.
The Fourier Transform InfraRed (FTIR) spectrometer model Spectrum 400 Perkin Elmer was used to test grease samples. The samples, S0 fresh grease, S1 24 months’ grease and S5 11 months’ grease were taken for spectrometry. The test infrared ranges were from 4000 cm-1 till 400 cm-1 wavenumber. The laboratory technician performed the FTIR test. This test was done to determine fingerprint element, water element and oxidation element. These 3 elements comparison will determine the chemical stability [7] changes over a period of 11 months to 24 months.
3. Theory/Calculation
Grease is a lubricant used when there is no possibility of splash or immersed type of lubrication.
It is commonly a remote type of lubrication with the least maintenance servicing requirement.
Grease can act as a lubricant reservoir by storing the base oil inside the thickener structure and prevent leakage of oil from the bearing. Grease consists of 3 basic elements, which are base oil (85%), thickener (10%) and additives (5%) [8]. Moreover, grease is categorized as soap type or non-soap type. The common soap type thickeners are Lithium 12 – Hydroxy stearate and calcium stearate. The soap is a reaction product from organic fatty carboxylic acid and alkali metal, forming an organic salt. The non-soap thickeners are commonly polyurea, fumed silica, fluoropolymer and clay. The clay thickeners are bentonite and hectorite. The clay particles are dispersed into base oil to form the grease. The clay particles need to be activated with polar material. The clay thickener grease has no melting point; thus, it is suitable for high operating temperatures. The reaction process is known as saponification. The base oil also has been categorized as synthetic and non-synthetic oil. The additive is normally an anti-oxidant, anti- corrosion or anti-wear agents. The additives will be consumed during operation of the grease over the life cycle [9]. Almost 90% of the roller bearings are lubricated with grease [10]. Grease is unique by mechanical properties of flow and deformation compared to lubricating oil. The definition of grease by Lugt (2009) [11] is “a solid to semi-fluid product or dispersion of a thickening agent in a liquid lubricant. Other ingredients imparting special properties may also be included”. Grease behaves as a Non-Newtonian fluid, where the viscosity changes with the rate of strain. Thus, viscosity cannot be used as a parameter due to non-linear behaviour. Thus, the grease behaves as elastic as solid-like and viscous as liquid-like, which is called viscoelastic behaviour described by Maxwell model [12]. Maxwell describes viscoelastic phenomena in a spring and viscous dampener series that provide a time base shear stress response. The flow of Non-Newtonian time-dependent fluid is broken into two (2) characteristics of the apparent viscosity. The first time-dependent fluid flow characteristic is rheopectic. Rheopectic means that the material becomes more viscous with an increase in applied shear over time. An excellent example of a rheopectic material flow is Latex. The second fluid flow characteristic is thixotropic. Thixotropic means that the material experience is more fluidic with an increase in applied shear over time. Commonly grease is characterized as thixotropic.
The solid-like part is the thickener that kept the oil inside the thickener network structure by capillary forces and Van der Waals [13]. The molecules of the thickener network structure have a complex bonding by forces of ionic and Van der Waals [14-15]. At the microstructure level, the thickeners appear like strings of fibres. These fibres have a length of 10µm to 100µm [16].
There is a correlation between grease consistency and thickener fibre length to diameter ratio [17].
The primary grease usability indicator is the consistency of the grease. The consistency is the resistance to the deformation. The consistency is measured as working penetration in tenth of millimetre. The National Lubrication & Grease Institute (NLGI) developed a consistency number by grade 000 till 6. The smaller the consistency number in NLGI (2017), the higher the penetration value (see Table 1) is.
Table 1: NLGI grease consistency grade (NLGI 2017)
Grease Consistency Grade NLGI (2017)
Work Penetration Cone Penetration (CP) at 25℃
in tenth millimetre
000 444 - 475
00 400 – 430
0 355 – 385
1 310 – 340
2 265 – 295
3 220 – 250
4 175 – 205
5 130 – 160
6 85 – 115
Therefore, cone penetration in tenth millimetre was the measurement of the consistency of the grease. Hence, by applying equation 1 (Gurt and Khonsari, 2019) [18], an equivalent cone penetration was obtained to cross-referred with NLGI (2017) table (see Table 1).
Cone Penetration = 838.489 𝜏𝑐𝑜−0.169 Equation 1 [18]
4. Results and Discussions
Based on amplitude oscillatory rheometer test at 1-hertz frequency for trimmed grease samples.
The test was performed at 25℃ and with a geometry of parallel plate of 25mm diameter and gap of 1mm. The oscillatory rheometer test was to measure grease viscoelastic property. The test was empirical, and only a single test per sample was conducted due to sample mass limitation. The samples were only sufficient for a single test. However, to reduce rheometer error of loading history, the grease was applied at the bottom plate and the top plate [19]. Figure 1 shows the Storage Modulus G’ in Pascal of the samples. The curve is flat and constant at the beginning of the low strain rate, known as the low viscoelastic (VLE) region. At this region, the grease would resist the deformation when low stress was applied. The point where the curve declines at the bottom was where the transition happened from solid like to liquid like phenomena. Here, the value of the Storage Modulus reduces, and the Loss Modulus, G”
increases (see Figure 2). The point of balance of solid-like and liquid-like is when G’ equals to G” (see Figure 3). This point is known as flow point, and the stress is known as cross-over stress denoted as τco. Beyond τco, the viscoelastic behaviour became non-linear. The rheometer test aimed to obtain the cross-over stress of the grease samples. All the grease samples cross- over stress results were tabulated in Table 2.
Figure 1: Storage Modulus of Grease Samples Result
Figure 2: Loss Modulus of Grease Samples Result 0.00E+00
5.00E+03 1.00E+04 1.50E+04 2.00E+04 2.50E+04 3.00E+04 3.50E+04 4.00E+04
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Modulus, G' (Pa)
Interval
Storage Modulus, G' Grease Samples (Unworked)
S0 Storage Modulus [Pa] S1 Storage Modulus [Pa]
S2 Storage Modulus [Pa] S3 Storage Modulus [Pa]
S4 Storage Modulus [Pa]
0.00E+00 1.00E+03 2.00E+03 3.00E+03 4.00E+03 5.00E+03 6.00E+03 7.00E+03
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Modulus, G" (Pa)
Interval
Loss Modulus, G" Grease Samples (Unworked)
S0 Loss Modulus [Pa] S1 Loss Modulus [Pa] S2 Loss Modulus [Pa]
S3 Loss Modulus [Pa] S4 Loss Modulus [Pa]
Figure 3: G’ and G” rheometer test result on sample S0
Figure 4: G’ and G” rheometer test result of sample S1
Applying τco into the Equation 1, the equivalent cone penetration was established on all the tested grease samples. The unworked and undisturbed fresh grease on the cone penetration equivalent was 288 tenth-millimetre. The grease specification for this brand grease is NLGI grade 2 with cone penetration range 265-320 tenth millimetre in accordance with MIL-PRF- 81322G. However, based on this empirical experiment, we can see the cone penetration value increases with the ageing of the unworked grease. Hence, the consistency number changed
0.00E+00 5.00E+03 1.00E+04 1.50E+04 2.00E+04 2.50E+04 3.00E+04 3.50E+04 4.00E+04
Modulus, G , Pa
Shear Stress, Pa
G' and G'' Fresh Grease-S0(unworked) - Amplitude Oscillatory Test at 1Hz
Storage Modulus [Pa] Loss Modulus (Pa)
0.00E+00 1.00E+03 2.00E+03 3.00E+03 4.00E+03 5.00E+03 6.00E+03 7.00E+03 8.00E+03 9.00E+03 1.00E+04
Modulus, G, Pa
Shear Stress, Pa
G' and G'' Grease-S1(unworked 24Months) - Amplitude Oscillatory Test at 1Hz
Storage Modulus [Pa] Loss Modulus [Pa]
Flow point
Flow point
from NLGI grade 2 to NLGI grade 1. Thus, the physical changes on the grease were softening and viscous.
Table 2: Viscoelastic Results of Grease Samples Grease
Samples
Storage Modulus max, G' [Pa]
Shear Stress, yield, 𝝉𝒚, [Pa]
Cross Over Stress, 𝝉𝒄𝒐, when G' = G", [Pa]
Cone Penetration Equivalent (Gurt and Khonsari, 2019) (Tenth-millimeter)
Consistency Number (NLGI, 2017)
Fresh (S0) 36000 36.4 555 288 2
9 Months (S2)
11800 21.1 279 324 1
10 Months (S3)
7400 23.8 207 341 1
11 Months (S4)
5150 16.4 160 355 0
24 Months (S1)
8880 15.9 274 325 1
The three samples with intervals of unworked grease of 11 months and 24 months, and reference of fresh undisturbed and unworked grease were tested with FTIR. The results were merely coherent when comparing grease samples of S0, S5 and S1 (see Figure 4). The FTIR spectrum by the transmittance of infrared ray that passed through the sample showed that the higher the percentage of transmittance meant the higher the infrared ray passed through the sample with the least absorption. The contaminant spectrum [20], such as oxidation, nitration, soot, sulfation and water presence transmittance value were checked. Table 3 shows the contaminant spectrum range in wavenumber and the corresponding transmittance value in %.
The transmittance values were extremely high, showing that the infrared ray passed through the samples more than 90%. Figure 5 shows the graphical view of the transmittance of the contaminant range. Nevertheless, to deduce the contaminant's absorbance can only be computed using Equation 2 by converting transmittance to absorbance. This equation is known as Beer-Lambert Equation [21].
𝐴𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 = 2 − log10(%𝑇) Equation 2
%T is transmittance in percentage. Referring to Table 4 and Figure 6, the absorption levels were very small in all 3 grease samples. Only sample 24 months (S1) has a relatively higher water contaminant compared to 11 months and fresh samples. This could support the oxidation mechanism in lubrication that will eventually lead to ketones, acids, alcohols, and water [22].
Overall, it can be deduced that no apparent contamination is present up to 24 months from the unworked grease samples. Fingerprint Spectrum of the sample's spectrum shows 400 to 1700 wavenumber. The grease is made of Silicone (Si-O-C) and polysulfide S-S stretch). High likely the thickener is made of Clay (Hectorite).
Figure 4: Grease Samples FTIR Results
Table 3: FTIR Transmittance spectrum on contaminants
Contaminant range (wavenumber)
Fresh grease S0
transmittance (%)
11 Months grease S5
transmittance (%)
24 Months grease S1
transmittance (%) Oxidation 1700 cm-1 93.8132 93.3781 93.9872 Nitration 1630 cm-1 98.746 98.7921 98.198
Soot 2000 cm-1 98.299 97.7291 97.8712
Sulfation 1150 cm-1 90.5837 91.4264 90.101
Water 3400 cm-1 98.6403 99.5464 97.5604
Table 4: FTIR absorbance on contaminants
Contaminant range (wavenumber)
Fresh grease S0
Absorbance (AU)
11 Months grease S5
absorbance (AU)
24 Months grease S1
absorbance (AU) Oxidation 1700 cm-1 0.02774 0.02975 0.02693 Nitration 1630 cm-1 0.00548 0.00528 0.00790
Soot 2000 cm-1 0.00745 0.00998 0.00935
Sulfation 1150 cm-1 0.04295 0.03893 0.04527
Water 3400 cm-1 0.00595 0.00197 0.01073
0 20 40 60 80 100 120 140
4000 3898 3796 3694 3592 3490 3388 3286 3184 3082 2980 2878 2776 2674 2572 2470 2368 2266 2164 2062 1960 1858 1756 1654 1552 1450 1348 1246 1144 1042 940 838 736 634 532
Transmittance spectrum [%]
Wavenumber [cm-1]
FTIR Grease Samples S0(Fresh), S5(11Months) and S1(24Months)
S5 S1 S0
Figure 5: Grease Samples FTIR contaminant spectrum region with transmittance value
Figure 6: Grease Samples FTIR contaminant spectrum region with absorbance value
5. Conclusion
This empirical experiment was carried out on unworked grease that was packed in wheel roller bearings kept in storage. The stored wheel bearings were stationary. By visual observation, the grease inside the roller bearing was found thinning, and a pool of base oil was separated from the grease pack. Further, observation showed the grease softening and became liquid-like. The experiment results from Table 2 showed the consistency number changed from grade 2 to grade 1 and an increase in cone penetration value over time. This supports the visual observation of grease thinning and softening and base oil separation. The oil separation from the static packed grease was due to the internal strain of the grease layers over time. The grease contaminant was verified with FTIR for chemical stability for storage over period of 24 months. The results from Table 3 and Table 4 indicate that there is no significant contaminant of oxidation, nitration, soot, sulfation and water. This is possibly due to the right environment control of the storage
93.8132 98.746 98.299 90.5837 98.6403
93.3781 98.7921 97.7291 91.4264 99.5464
93.9872 98.198 97.8712 90.101 97.5604
O X I D A T I O N 1 7 0 0 C M - 1
N I T R A T I O N 1 6 3 0 C M - 1
S O O T 2 0 0 0 C M - 1 S U L F A T I O N 1 1 5 0 C M - 1
W A T E R 3 4 0 0 C M - 1
FT I R G R E A S E C O N TA M IN AT I O N A N A LY S I S ( T R A N S M I T TA N C E - WAV E N U M B E R )
Fresh grease S0 transmittance 11Months grease S5 transmittance 24Months grease S1 transmittance
0.02774 0.00548
0.00745
0.04295 0.00595
0.02975 0.00528
0.00998
0.03893 0.00197
0.02693 0.00790
0.00935
0.04527 0.01073
0.00000 0.00500 0.01000 0.01500 0.02000 0.02500 0.03000 0.03500 0.04000 0.04500 0.05000 Oxidation 1700cm-1
Nitration 1630cm-1 Soot 2000cm-1 Sulfation 1150cm-1 Water 3400cm-1
Grease contaminant FTIR
24Months grease S1 absorbance (AU) 11Months grease S5 absorbance (AU) Fresh grease S0 Absorbance (AU)
room. Therefore, based on this empirical experiment, it is suggested that for unworked and stored wheels to repeat application of grease at every interval of 12 months.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The author would like to thank the laboratory technicians at University Malaya for the access and assistance at rheology and FTIR laboratories.
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