Comparative Studies on Low Noise Greases Operating under High Temperature Oxidation Conditions

Xiong Chunhua; Mi Hongying; Feng Qiang; Wu Baojie

(1. Beijing POL Research Institute, Beijing 102300; 2. Tianjin Branch, Lubricant Company, SINOPEC Corp., Tianjin 300480)

Comparative Studies on Low Noise Greases Operating under High Temperature Oxidation Conditions

Xiong Chunhua1; Mi Hongying1; Feng Qiang2; Wu Baojie2

(1. Beijing POL Research Institute, Beijing 102300; 2. Tianjin Branch, Lubricant Company, SINOPEC Corp., Tianjin 300480)

Oxidation induction time (OIT) testing by differential scanning calorimetry (DSC) was used to evaluate the oxidation resistance of lubricating greases. Under the high temperature condition, bearing noise was detected when grease passed the initial stable stage of oxidation. The chemical and physical structure of grease samples before and after high temperature oxidation were also analyzed by FT-IR spectrometry and scanning electron microscopy (SEM), then the effects of oxidation at high temperature on bearing noise were investigated. It is found out that for lithium greases, oxidation of base oil and thickener is the main reason responsible for the increasing bearing noise. As regards the polyurea greases, the change of fiber microstructure at high temperature is the main reason for the increasing bearing noise.

high temperature; bearing noise; oxidation; lubricating grease

1 Introduction

Bearing noise performance of lubricating grease is affected by many factors, among which the design of bearing, precision level, waviness and surface roughness of bearing have a significant impact on bearing noise. However, the low-noise performance of grease cannot be ignored[1]. For grease, its cleanliness has a crucial impact on lownoise performance of grease[2-4]. Besides that factor, the base oil type, viscosity, and the fiber structure of thickener all affect the low-noise performance of grease greatly[5]. Due to differences among different bearing operating conditions, there are different failure modes. Among those different failure modes, grease deterioration is mainly caused by two factors. One is physical shearing, and the other is chemical oxidation[6]. The long-term use of grease produces friction and high temperature, which can lead to oxidative degradation of grease. Lithium grease can cause the increase of bearing noise after high-temperature oxidation, as carbonyl substances are produced during the process of oxidation[7]. However, there are very few studies about bearing noise of polyurea greases under hightemperature operating conditions. Therefore, this paper intends to make a comparative study between bearing noise of the polyurea grease and the lithium grease after being subjected to high-temperature oxidation. In this paper, the oxidation induction time (OIT) testing by differential scanning calorimetry (DSC) was used to evaluate the oxidation resistance of grease. Under the high-temperature condition, bearing noise was detected when grease passed its initial stable oxidation stage, and the chemical and physical structure of grease samples was also analyzed by infrared spectroscopy (IR) and scanning electron microscopy (SEM), with the effects of hightemperature oxidation on bearing noise investigated.

2 Experimental

2.1 Preparation of test samples

The sample of lithium grease using 12-hydroxystearate lithium soap thickened mineral oil was labeled as the Grease A. At 80 ℃, the lithium hydroxide solution was added to the base oil containing 12-hydroxystearic acid for reaction, and the grease kettle temperature was raised to the specified high value and was then cooled down prior to addition of additives. Finally, the test samples were prepared after milling and dispersing by a three-roll mill machine.

The sample of polyurea grease was labeled as the GreaseB, which was prepared by adopting the polyurea thickened mineral oil and a certain ester oil. The mineral oil used for Grease B and Grease A was the same. Fatty amine reacted on MDI in the base oil, and the grease kettle temperature was raised to the specified high value and then was cooled down prior to addition of additives. Finally the test samples were prepared after milling and dispersing by using a three-roll mill machine. The composition and penetration of Grease A and Grease B are shown in Table 1.

Table 1 Composition of grease samples

2.2 Test methods

2.2.1 Oxidation stability test

The oxidation induction time (OIT) testing was performed by differential scanning calorimetry (DSC) to evaluate the oxidation resistance of grease. Oxidation induction time (OIT) was measured by using a small amount of sample paved thinly in an aluminum crucible. At a certain temperature, the sample was purged with an oxygen flow used as an oxidation agent. This oxidation mode was a thin film oxidation, similar to grease oxidation mode in the bearing raceways. The test equipment was DSC204 (differential scanning calorimeter), manufactured by Netzsch GmbH. Test conditions were as follows: The test sample was 5±0.5 mg in mass and the test temperature was 220 ℃, with the oxygen flow rate equating to 50 mL/min.

2.2.2 Noise performance test

The grease bearing noise in the process of high-temperature test was evaluated by the bearing vibration (acceleration) measuring method. The test equipment was a S0910 III type bearing vibration measuring instrument. The vibration speed of spindle of the measuring instrument was equal to 1 500 ± 30 r/min under an axial load of 40 N.

2.2.3 High-temperature test

Grease samples to be tested were filled into the Bearing 6202 which had been previously cleaned and dried by solvent naphtha. The volume of grease made up about 40% of bearing capacity. The bearing seals were not reinstalled after being greased. All bearing noise performance was measured by the S0910Ⅲ type bearing vibration measuring instrument and the initial vibration values were recorded. Then the bearing was placed inside an oven filled with air at 150 ℃ to conduct static oxidation test. During the baking process, the bearing was taken out periodically to be cooled down to room temperature. The bearing noise test was conducted on the S0910Ⅲ type bearing vibration measuring instrument with the vibration values recorded. The time required for the bearing noise test was about 20 minutes.

2.2.4 Analysis by infrared spectroscopy

After going through high-temperature oxidation tests, grease samples were analyzed by IR spectroscopy. The changes in absorption peaks of functional groups can be obtained after IR analyses, and then the oxidation degree of the grease samples was known. The infrared spectrometer used in the experiments was a Perkin Elmer’s FT-IR Spectrum One Spectrometer

2.2.5 Scanning electron microscopy

The scanning electron microscope was a Japanese Hitachi’s Model S-3400N instrument with a magnification of 15 000 times, which was used to examine thickener’s fiber changes before and after the oxidation tests.

3 Results and Discussion

3.1 Oxidation resistance test

A differential scanning calorimeter was used to test the oxidation induction time (OIT) of two grease samples. The test results are shown in Figure 1. The results showed that the oxidation induction time of Grease A and Grease B was 73.6 min and 110.1 min, respectively. The oxidation resistance of the polyurea Grease B was far better than that of the lithium Grease A.

Figure 1 OIT of grease at 220 ℃

3.2 Bearing noise test under high-temperature oxidation conditions

Under high-temperature oxidation conditions, the test results of bearing noise versus oxidation time are shown in Figure 2. The test results showed that the bearing vibration value of the lithium Grease A increased to about 36 dB in the early high-temperature oxidation period, and the value increased slowly with the increase in the hightemperature oxidation duration. The grease noise level was good and the bearing ran smoothly in the 260-hour oxidation test. But after 260 hours the bearing vibration value increased rapidly, and the bearing stopped working soon. The bearing vibration value of the polyurea Grease B increased to about 45 dB in the early high-temperature oxidation period, and the value increased slowly with an increasing high-temperature oxidation time. After the 140-hour oxidation test, the bearing vibration value reached 50 dB and then stabilized at between 50—52 dB. The grease lubrication performance was good and the bearing operated normally before 782 hours of oxidation. After the comparative study was made, it can be found that the bearing noise level of the polyurea grease was signif icantly better than that of the lithium grease under hightemperature oxidation conditions within a definite period. But in the process for testing the polyurea grease lubricated bearing, a sudden increase of bearing noise did not occur.

Figure 2 Changes in vibration of different greases in the bearing 6202 with oxidation time

3.3 Infrared spectrometric analysis of grease samples

During the high-temperature oxidation process, infrared spectra of grease samples operating in bearing were determined periodically. They were compared with the infrared spectra of grease samples that did not undergo the hightemperature oxidation process. By comparing the changes in infrared absorption peaks of grease samples before and after oxidation process, the changes in the chemical composition of grease samples could be detected. The oxidized grease samples generated the substances containing hydroxyl, carbonyl and carbon-oxygen single bonds, which would lead to changes of absorption peaks close to the wavenumber of 3 500—3 200 cm-1, 1 800—1 650 cm-1and 1 100 cm-1in their IR spectra.

Figure 3 Change of IR spectrums of grease A during oxidation at high temperature

The IR spectra of the grease A before and after high-temperature oxidation are shown in Figure 3. The asymmetric and symmetric stretching vibration absorption peaks corresponding to thickener of the grease A did not change distinctly at 1 580 cm-1and 1 560 cm-1within 213-hour oxidation at high temperature, while the characteristic absorption peaks of base oil also did not change obviously at 1 464 cm-1, 1 377 cm-1and 720 cm-1, respectively. After high-temperature oxidation process, there was an absorption peak identified at 1 330 cm-1of grease sample which was a symmetric stretching vibration absorption peak[8]ofaromatic nitro compound generated from degradation of antioxidant in the grease sample. In addition, after being oxidized at high temperature for 260 hours, the absorption peaks of asymmetric and symmetric stretching vibration of the grease A disappeared, and the absorption peaks of base oil at 1 464 cm-1, 1 377 cm-1and 720 cm-1apparently weakened. The absorption peak of associate hydrogen bonds moved to 3 500 cm-1, and there was an absorption peak of carbonyl group at 1 720 cm-1, which indicated that after being oxidized at high temperature for 260 hours the gelling agent and base oil of grease sample had been obviously oxidized to form oxidized products containing hydroxyl and carbonyl groups, ultimately resulting in a significant increase of bearing noise.

The IR spectra of the grease B before and after hightemperature oxidation are shown in Figure 4. It was obvious that within 260-hour oxidation at high temperature, the stretching vibration absorption peaks at 3 300 cm-1and bending vibration absorption peaks at 1 580 cm-1and 1 570 cm-1of N—H bonds in the thickener of the grease B did not change distinctly, while the stretching vibration absorption peak at 1 633 cm-1did not change obviously either. There was no apparent change for the characteristic absorption peak at 1 745 cm-1and other characteristic absorption peaks at 1 464 cm-1, 1 377 cm-1and 720 cm-1of ester oil in base oil. The above phenomenon indicated that during the 160-hour oxidation tests at high temperature the thickener and base oil of the grease B had not changed obviously, but after being oxidized for 428 hours there was an obvious change relating to the stretching vibration absorption of N—H bonds in the thickener, since the absorption peak at this position was weakening gradually, and the bending vibration peaks of N—H bonds at 1 580 cm-1and 1 570 cm-1were weakened too, whereas the stretching vibration absorption peak corresponding to C=O bonds of the thickener was weakened apparently too. But there was no obvious change in the characteristic absorption peak of base oil which indicated that there was no distinct oxidation of base oil at high temperature.

Figure 4 Changes in IR spectra of the grease B during oxidation at high temperature

3.4 Analysis of fibrous structure of thickener

Figure 5 Fibrous structure of thickener of the grease A before and after high-temperature oxidation

Before and after high-temperature oxidation, stereoscan photographs of fibrous structure of the grease A and the grease B are shown in Figure 5 and Figure 6, respectively. Before being oxidized at high temperature, the fibrous structure of thickener in the grease A represented an interwoven structure, and with the extension of oxidationtime at high temperature, the thickener fiber became large and the fiber surface got fuzzy. After being oxidized for 260 hours, the thickener fiber all disappeared and there was a layer of finely oxidized sediment in the photograph, which indicated that the thickener had been damaged badly, and the above phenomenon was consistent with the results of IR spectroscopy in Figure 3.

Figure 6 Fibrous structure of thickener in the grease B before and after high-temperature oxidation

The thickener of the polyurea grease B showed a fibrous structure prior to testing. After high-temperature oxidation for 23 hours, there was a distinct change in the fibrous structure of the grease B, and the shape of thickener became large pellets. After the high-temperature oxidation for 428 hours the thickener was still in the pelletized form.

3.5 Discussion

The oxidation induction time (OIT) of grease tested by PDSC was used to evaluate the oxidation resistance of grease samples. Compared with data of OIT shown in Figure 1, it is identified that the polyurea grease B had better oxidation resistance as compared to the lithium grease A. During oxidation of the lithium grease A, the substance containing carbonyl group could be detected by IR spectrometry. The base oil of the grease sample would gradually generate oxidation products containing carbonyl groups[9]. The polymerization of the carbonyl compounds resulted in an increase of viscosity of base oil to form oxidation products such as colloid, asphaltenes, carbonaceous film, etc. During oxidation, the terminal of carboxylate in lithium 12-hydroxyl stearate was oxidized to inorganic compounds such as lithium carbonate, and the alkyl group was oxidized to alcohol, aldehyde, ketone, and acid that were dissolved in the base oil. It can be seen from the changes in the IR spectra of the lithium grease A during oxidation at high temperature as shown in Figure 3 that the absorption peak of lithium 12-hydroxyl stearate was gradually disappearing in the late oxidation stage, as evidenced by the changes in the photographs of the grease A in Figure 5. After 260-hour high-temperature oxidation, the stereoscan photographs of the lithium grease A indicated that the fibrous structure of thickener all disappeared, with the remainder composed of only inorganic compounds of lithium in a fine granular state. The fine granular solid oxidation products were of micrometerscale in size, which existed in the oxidized grease in the form of solid mechanical impurities to cause grease noise. Upon comparing Figure 2 with Figure 3, it can be seen that after being oxidized for 213 hours the thickener and base oil of the grease A displayed no change, and there was no obvious change in bearing noise. With the oxidation duration increased further to 260 hours the characteristic absorption peak of thickener would disappear, and the characteristic absorption peak of base oil was weakening distinctly, but the content of oxidation products increased along with a sharp increase in the bearing noise. Therefore, at the beginning and during the stable stage of oxidation at high temperature, the thickener and base oilof the lithium grease had not been oxidized apparently, which would not increase the bearing noise. But at the final stage of high-temperature oxidation the thickener and the base oil would be oxidized obviously. In the course of oxidation, the grease generated hydroxyl group containing substances firstly, and then they were oxidized further to form carbonyl compounds. In the course of oxidation, the oxidative hydroxyl products had little effect on bearing noise, whereas the oxidative carbonyl products could enhance bearing noise significantly[7]. Once generated, the carbonyl oxidative products of grease would build up on the surface of steel balls and raceway to form colloid, asphaltenes and carbonaceous film and could result in shape change of steel balls and raceway, which made precision machined connecting surface rough to increase the vibration and noise of bearing operation. The deposition of colloid, asphaltenes and carbonaceous film generated by oxidative chemical degradation of grease thickener on the surface of steel balls and raceway could lead to increased vibration and noise of the lithium grease A after experiencing 260 hours of high-temperature oxidation as shown in Figure 2. With the oxidation degree of grease samples becoming increasingly severe with an increasing temperature, the oxidative products would gradually lose liquidity, so that the inside and outside raceway, cage and steel balls of bearing would be stuck together, resulting in a jammed bearing and lubrication failure.

As shown in Figure 2 and Figure 4, after being oxidized for 260 hours, the thickener and base oil of the grease B did not show changes, and the bearing noise remained relatively constant without obvious changes. When the grease B was further subjected to oxidation for 428 hours, the stretching vibration absorption peak of N—H bonds of the thickener was gradually weakening, while the bending vibration absorption peaks of N—H bonds at 1 580 cm-1and 1 570 cm-1were weakening gradually as well. In addition, the stretching vibration absorption peak corresponding to C=O groups of the polyurea thickener decreased apparently, but the characteristic absorption peaks of base oil showed no change, which indicated that the structure of thickener had changed significantly. Within the whole time range of oxidation at high temperature, there was no obvious change in the characteristic absorption peak of base oil in the grease B, meanwhile there were no distinct IR absorption peaks of oxidative products identified like those formed after oxidation of the grease A, which indicated that the polyurea grease B possessed better oxidation resistance than the lithium grease A, and the oxidation induction time of two samples had provided testing data to support this exposition. After the grease B sample was oxidized for 782 hours, the absorption peak of thickener was weakening apparently, but the absorption peak of base oil had no change yet, which indicated that the base oil did not generate oxidative products containing hydroxyl and carbonyl radicals which generally occurred after high-temperature oxidation.

The structure of thickener in the ployurea grease B had changed apparently after oxidation at high temperature. As shown in Figure 6, before the high-temperature oxidation process the thickener structure was a kind of interwoven fibrous structure and after high-temperature oxidation it took a globular form, and concurrently the size of thickener increased a lot, while after undergoing 428-hours oxidation testing the structure of thickener had not altered apparently. Compared with tests on grease bearing noise as shown in Figure 2, the bearing noise of the polyurea grease B increased rapidly to about 48 dB and remained constant basically, which indicated that the structural change of thickener was the key factor that can affect bearing noise of the ployurea grease B, but oxidation was not so decisive for the ployurea grease B. However the mechanism and effect of high-temperature oxidation on the thickener appearance of the polyurea grease B were not certain which needed to be studied further.

4 Conclusions

a) The mechanisms and effects of high temperature oxidation on bearing noise of the lithium base grease and the polyurea grease were different. At high temperature, the oxidation products of the lithium base grease had a significant effect on grease bearing noise, and for the case of the polyurea grease its bearing noise was mainly affected by structural change of ployurea thickener at high temperature.

b) The lithium base grease at high temperature had relatively low bearing noise at first, but it had insufficient durability. Once the lithium base grease was oxidized, its bearing noise increased dramatically. The polyurea grease at high temperature had a relatively high bearing noiseinitially, and as the oxidation of polyuria grease was intensified, its fiber structure became larger and when it was finally fully oxidized, the fibrous structure of the polyurea grease thickener gradually took a globular form.

[1] Matsubara K, Dong D M, Endo T. Low noise greases for bearings [J]. Spokesman, 2008, 72(6): 25-34

[2] Miller H. Noise characteristics of rolling bearing greases [J]. Lubrication Engineering, 2000, 56(10): 36-41

[3] Samman N, Johnson M. How clean is your grease? [J]. Spokesman, 2004, 68(1): 12-18

[4] Ramamurthy S, Krousgrill C M, Sadeghi F. Vibration in grease lubricated bearing systems [J]. Tribology Transactions, 2000, 43(3): 403-410

[5] Liu Q L, Wu B J, Li H F. Influence of grease performance on bearing noise [J]. Bearing, 2001(1): 23-26. (in Chinese)

[6] Cann P M, Webster M N, Doner J P. Grease degradation in R0F bearing tests [J]. Tribology Transactions, 2007, 50(2): 187-197

[7] Feng Q, Wu B J, Li X L. Investigation on the effects of oxidative degradation on noise life of lithium greases [J]. Bearing, 2013(5): 21-25. (in Chinese)

[8] Weng S F. Fourier Transform Infrared Spectrometry[M]. Beijing: Chemical Industry Press, 2005 (in Chinese)

[9] Moehle W E, Cobb T W, Schneller E R, et al. Utilizing the TEOST MHT to evaluate fundamental oxidation processes in low-phosphorus engine oils [J]. Tribology Transactions, 2007, 50(1): 96-103

Received date: 2014-10-23; Accepted date: 2014-10-31.

Xiong Chunhua, Telephone: +86-13910335165; E-mail: xch@263.net.cn.