Gear Lubrication – Stopping Micropitting by Using the Right Lubricant

Different high-performance gear oils were examined on an FZG back-to-back gear test rig to determine if changeover to these high-performance gear oils could stop micropitting formation which occurred with other industrial gear oils. The test results showed that high-performance gear oils using advanced additive technologies can react at the surface of the tooth flanks after an oil change and stop further micropittingformation. The micropitting area stagnates.

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Micropitting formation is often reported to occur in field applications even though industrial gear oils with a high micropitting load-carrying capacity are used. Such oils offer good micropitting protection determined in the load stage test of the micropitting test according to FVA 54/7 [5], but show a low endurance micropitting performance. Therefore, the possibility of stopping micropitting formation, which occurred with these gear oils, was investigated by using high-performance gear oils.

Due to advanced additive technologies, these high-performance gear oils show a high micropitting load-carrying capacity in the micropitting load stage test as well as a high endurance micropitting performance.

Micropitting Gear Failure
Micropitting is a type of fatigue failure occurring on hardened tooth flanks of highly loaded gears. Figure 1 shows typical limits of the load-carrying capacity for case hardened gears according to Niemann [7]. This failure consists of very small cracks and pores on the surface of tooth flanks. Micropitting looks greyish and causes material loss and a change in the profile form of the tooth flanks, which can lead to pitting and breakdown of the gears. A typical micropitting gear failure of an industrial gearbox is shown in Figure 2. In this case, misalignment was the reason for micropitting formation..

The formation of micropitting depends on different influences. Besides material, surface roughness, and geometry of the tooth flanks, it is the lubricant and the operating conditions which have a major influence on micropitting formation. See Table 1.

In modern gearboxes, the gears are often highly loaded and run under conditions of mixed lubrication. In this case, the tooth flanks of the mating gears are not fully separated by the lubricant film and the additives of the lubricant have to protect the tooth flanks against micropitting formation.

Micopitting Load-Carrying Capacity
The micropitting load-carrying capacity of gears can be calculated according to ISO TR 15144-1 [4], where the influence of lubricant, operating conditions, and surface roughness is considered with the specific lubricant film thickness.

For this purpose, the specific lubricant film thickness of a practical gear is compared with a minimum required specific lubricant film thickness. The latter is the specific film thickness where no micropitting risk is given for a lubricant and can be determined by performing a micropitting test according to FVA 54/7 [5].

Standard Micropitting Test
The micropitting test according to FVA 54/7 [5] consists of a load stage test and an endurance test. Test gears type C-GF run at a circumferential speed of 8.3 m/s and a lubricant temperature of 90°C or 60°C. The load and the test periods are varied.

In the load stage test, the load is increased stepwise from load stage LS 5 to load stage LS 10 with a running time of 16 h per load stage. After the load stage test, an endurance test with a running time of 80 h in load stage LS 8 and 5 x 80 h in load stage LS 10 is  performed. The pinion torque and the corresponding Hertzian pressure of the different load stages are given in Table 2.

At the end of the load stage test and the endurance test with the first test gears, the load stage test is repeated with new test gears to check repeatability. After each test period, the test gears are disassembled and the profile of the tested flanks is measured using a 3D measurement system. In the load stage test, the failure criterion has been reached if the mean profile form deviation due to micropitting exceeds the limiting value of 7.5 µm. The load stage in which the failure criterion is reached is called failure load stage. An overview regarding the classification of test results obtained in the micropitting test is given in Table 3.

Lubricants with a high micropitting load-carrying capacity reach the failure criterion of a profile form deviation of 7.5 µm due to micropitting in load stage ≥ LS 10 of the load stage test. Examples for the evaluation of the micropitting test are given in Figure 3 and Figure 4.
In the endurance test, a stagnation of micropitting formation compared with the micropitting area at the end of the load stage test is preferred but not required.

For high-performance gear oil on the basis of polyglycol with a high micropitting performance, a typical test result is given in Figure 5 showing the profile form deviation due to micropitting. The profile form deviation of the pinion is below the failure criterion for the whole load stage test. In the endurance test, the profile form deviation stagnates compared with the step test.

Modification of the Standard Micropitting Test
The aim of research is whether changing from oils with low endurance micropitting performance to oil with high endurance micropitting performance can stop micropitting formation. Therefore the standard micropitting test according to FVA 54/7 [5] was modified. For oil with a high micropitting load-carrying capacity, but low endurance performance, a load stage test and an endurance test are performed with the first test gears. After the repeated load stage test with new test gears, an oil change is made. The subsequent endurance test is conducted to find out whether a product with a high micropitting load-carrying capacity in the load stage test as well as a high endurance performance can stop micropitting formation. The oil temperature is set to 60ºC compared to an oil temperature of 90ºC, the lubricant film thickness in the gear mesh is higher, but the formation of a reaction layer on the tooth flank surfaces is more difficult. The latter is more critical regarding the formation of micropitting.

Test Equipment
The test runs conducted to determine whether micropitting can be stopped by using high-performance gear oils were performed on a FZG back-to-back gear test rig [3]. The schematic setup of the FZG back-to-back gear test rig is shown in Figure 6.

The FZG back-to-back gear test rig utilizes a re-circulating power loop principle, also known as a foursquare configuration, in order to provide a fixed torque (load) to a pair of test gears. The test gearbox and drive gearbox are connected with two torsion shafts. One shaft is divided into two parts and contains a load coupling used to apply the torque (load) through the use of weights hung on the loading arm. A separate oil aggregate contains heating and cooling elements to control the oil temperature as required by the operating test conditions. Before the oil is injected into the gear mesh of the test gears, it is filtered with a 10µm filter.

In order to investigate if micropitting can be stopped, test gears type C-GF of the standard micropitting test according to FVA 54/7 [5] are used. The geometrical data and manufacturing details of the test gears type C-GF are shown in Table 4 and Table 5.

Lubricants Tested
A specialty lubricant manufacturer’s goal is to supply industrial gear oils on the basis of mineral oil, polyalphaolefin, or polyglycol showing a high micropitting load-carrying capacity of failure load stage ≥ LS 10 in the load stage test as well as a stagnation of micropitting formation in the endurance test according to FVA 54/7 [5] by selection of advanced additive technologies. See also Figure 5. These industrial gear oils show excellent micropitting protection not only at an oil temperature of 90°C, but also at a lower oil temperature of 60°C, and the advanced additive technologies can react on the surface of the tooth flanks and protect them against micropitting formation.

Additionally, the tests questioned whether micropitting formation occurred with other gear oils and whether changing to high-performance gear oils can stop micropitting. The oil data of the tested gear oils are shown in Table 6 and Table 7. Both the high-performance gear oils and the reference oil are of ISO VG 320 and possess a high micropitting load-carrying capacity according to FVA 54/7. All tested gear oils are specified according to DIN 51517 [2], which includes the minimum requirements for industrial gear oils and is similar to AGMA 9005 [1]. For the reference oil, the micropitting load-carrying capacity was only tested at an oil temperature of 60°C because at this oil temperature, it was to be investigated if micropitting formation can be stopped. See also Figure 7, Figure 8 Figure 9.

Test Results
For reference oil with a high micropitting load-carrying capacity, but low endurance performance, a load stage test and an endurance test were performed with the first test gears according to FVA 54/7 [5]. The reference oil always showed the test result GFT-high at the end of the load stage test. But in the subsequent endurance test, micropitting formation increased. The endurance test was finished due to pitting formation. Then the load stage test was repeated for the reference oil using new test gears. All repeated test runs of the reference oil showed a good repeatability compared with the first load stage test of this oil. See Figure 7, Figure 8, and Figure 9.

After the repeated load stage test for the reference oil, an oil change to high-performance gear oil on basis of mineral oil with a high micropitting load-carrying capacity as well as a high endurance micropitting performance was conducted. This oil change to a high-performance mineral gear oil using advanced additive technologies stopped micropitting formation compared with the reference oil. This shows that these advanced additive technologies can react at the surface of the tooth flanks after an oil change and build up a new improved reaction layer. Further micropitting formation was stopped and the micropitting area stagnated. See Figure 7.

The oil change to high-performance gear oil on the basis of polyalphaolefin with a high micropitting load-carrying capacity as well as a high endurance micropitting performance can also stop the micropitting formation compared with the reference oil. The reason for the stagnation of the micropitting areas are, again, the advanced additive technologies in this high-performance polyalphaolefin gear oil. Even after an oil change, these advanced additive technologies can react at the surface of the tooth flanks and build up a new improved reaction layer. See Figure 8.

Finally, also high-performance gear oil on the basis of polyglycol with a high micropitting load-carrying capacity as well as a high endurance micropitting performance can stop micropitting formation after an oil change compared with the reference oil due to the advanced additive technologies. See Figure 9.

All endurance tests for the high-performance gear oils on the basis of mineral oil, polyalphaolefin, and polyglycol were stopped after the same running time as the reference oil but without any pitting formation.

Conclusions
High-performance gear oils on the basis of mineral oil, polyalphaolefin, or polyglycol show a high micropitting load-carrying capacity of failure load stage ≥ LS 10 in the load stage test as well as a stagnation of micropitting formation in the endurance test according to FVA 54/7 [5] by selection of advanced additive technologies.

The new test results showed that high-performance gear oils using advanced additive technologies can react at the surface of the tooth flanks after an oil change and stop further micropitting formation. The micropitting area, which occurred with other gear oils, stagnates after changeover to high-performance gear oils. Therefore, these high-performance gear oils are strongly recommended to stop micropitting formation in field applications.   

References
1) AGMA 9005-E02: Industrial gear lubrication, 2002
2) DIN 51517-3: Schmierstoffe – Schmieröle – Teil 3: Schmieröle CLP; Minderstanforderungen, 2009
3) ISO 14635-1: Gears – FZG test procedures – Part 1: FZG test method A/8.3/90 for relative scuffing load-carrying capacity of oils, 2000.
4) ISO TR 15144-1: Calculation of micropitting load capacity of cylindrical spur and helical gears – Part 1: Introduction and basic principles, 2010
5) FVA 54/7: Test procedure for the investigation of the micro-pitting capacity of gear lubricants, FVA information sheet, Forschungsvereinigung Antriebstechnik e.V., 1993
6) Gregorius H.: Verzahnungsschäden an Getrieben – Ursachen, Schadensbilder, Abhilfe, Tribologie-Kolloquium, TZN Technologiezentrum Niederrhein, 2006
7) Niemann G.: Maschinenelemente, Band 2: Getriebe allgemein, Zahnradgetriebe – Grundlagen, Stirnradgetriebe, Springer Verlag, 2003

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is Head of Application Engineering Marketing and Technology with  KLÜBER LUBRICATION MÜNCHEN KG Geisenhausenerstraße. Contact him by phone at +49-89-78 76 713, or email hermann.siebert@klueber.com.
graduated from the Technical University of Munich (TUM) in 2003 with a master’s degree in mechanical engineering. Then he worked as a research assistant at the Institute for Machine Elements and Gear Research Centre (FZG) of the Technical University of Munich (TUM) from 2003 to 2010. In 2011 he obtained his PhD in mechanical engineering, with a thesis on “Gear load-carrying capacity for lubrication with semifluid gear greases”. In 2010, he joined Klüber Lubrication in Munich. He is working in the company’s marketing and application engineering department, where he is responsible for Klüber’s industrial gear oils. He is a member of different working groups within the Research Association for Power Transmission Engineering (FVA), the German Society for Petroleum and Coal – Science and Technology (DGMK), and the Standards Committee Mechanical Engineering (NAM).