In practice, industrial gears are often operated with lower oil temperatures than would normally be generated in a fully-loaded gearbox. Lower temperatures prevail, for instance, while a gearbox is being taken back into use after prolonged standstill, i.e. during the time it takes for the oil to heat up from ambient temperature to service temperature.
Similarly, when a gearbox is being operated below its full load capacity, with reduced speed, or with frequent stop-and-go, the operating temperature of the oil will be lower than it would be under full load. Such applications require gear oils that reliably protect gears and rolling bearings against damage not only at full-load operating temperatures, but also at lower ones.
The oil temperature influences the bulk temperature of the gears and bearings as well as the integral contact temperature [3], [10]. Besides the roughness of the mating surfaces and the applied load, the contact temperature is strongly influenced by the circumferential speed. At a lower speed, which mostly occurs at the output of a gearbox, the oil temperature, the bulk temperature of gears and bearings, and the contact temperature approximate each other more closely. Naturally, a reduction of the oil temperature leads to a substantial increase of oil viscosity during operation and hence to the formation of a thicker lubricant film in the contact zone. Lubricant film thickness calculations conducted for typical gearbox applications, however, show that even at oil temperatures as low as 40°C, the gearbox will still operate under mixed or boundary lubrication conditions depending on the other operating conditions. This means that the formation of a gear oil reaction layer on the component surfaces is vital to protect the friction bodies against damage.
Typical Gear Failures
Figure 1 shows typical limits of the load-carrying capacity for case hardened gears according to Niemann [11]. In the range of slow and medium circumferential speeds, the micropitting risk is increased, compared to very low circumferential speeds where the risk of slow speed wear prevails.
Micropitting Failure of Gears
Micropitting is a type of fatigue failure occurring on hardened tooth flanks of highly loaded gears [11]. 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 gear box 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 also 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.
Wear Failure of Gears
Wear is an abrasive material removal occurring on the tooth flanks of gears. This failure proceeds continuously and causes material loss and a change in the profile form of the tooth flanks, which can lead to breakdown of the gears. Typical wear on the tooth flanks of an industrial gear is shown in Figure 3.
Also the wear behavior depends on different influences. Besides surface hardness, material, and geometry of the tooth flanks, it is again the lubricant and the operating conditions which have a major influence on wear behavior. See also Table 2.
Failures of Rolling Bearings
Gear damage is also often caused by high rolling bearing wear or premature fatigue of rolling bearings [14]. Failures based on material or production mistakes are very seldom. But type, condition, and quantity of lubricant has a main influence on bearing failures as well as hard or liquid contaminations [13]. Figure 4 shows the typical failures on a rolling bearing. The additives contained in gear oils may have a decisive effect on rolling bearing damage.
Test Equipment
Investigation of micropitting load-carrying capacity and wear behavior of gears. The test runs conducted to determine the micropitting load-carrying capacity and the wear behavior of gear oils on gears were performed on a FZG back-to-back gear test rig [6]. The setup schematic of the FZG back-to-back gear test rig is shown in Figure 5. The FZG back-to-back gear test rig utilizes a recirculating power loop principle, also known as a four-square 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. Heating and cooling elements are used to control the oil temperature as required by the operating test conditions.
In order to investigate the micropitting load-carrying capacity, test gears type C-GF of the standard micropitting test according to FVA 54/7 [8] are used. The slow speed wear behavior is investigated by using test gears type C-PT according to DGMK 377-01 [2]. The geometrical data and manufacturing details of the test gears type C-GF and type C-PT are shown in Table 3 and Table 4.
Investigation of the Wear Behavior of Rolling Bearings
The test runs conducted to determine the wear behavior of gear oils on rolling bearings were performed on an FE8 bearing test rig [5] required for lubricating oils CLP according to DIN 51517-3 [4]. The schematic setup of the FE8 bearing test rig is shown in Figure 6.
The wear behavior of rolling bearings is investigated with test bearings type D according to DIN 51819-3 [5]. The geometrical data and manufacturing details of the test bearings type D (81212 according to DIN 722) are given in Table 5.
Test Procedures
To determine the performance capacities of gear oils with regard to the prevention of micropitting and wear, they are today subjected to standardized tests under critical lubricating conditions and temperatures as are commonly encountered in practice. The micropitting resistance in gears is analyzed in the FZG micropitting test according to FVA 54/7 [8] at an oil temperature of ϑoil = 90°C. Meanwhile it has become increasingly common to conduct micropitting tests also at a reduced oil temperature of ϑoil = 60°C in order to determine if micropitting can be reliably prevented also at these lower temperatures, which are encountered, for example, in wind turbine gearboxes. The slow speed wear behavior of gears is analyzed in the FZG wear test according to DGMK 377-01 [2] at oil temperatures of ϑoil = 90°C and 120°C. The influence of temperature is thus taken into account, albeit on a rather high level. The wear behavior of rolling bearings is examined in the FE8 wear test according to DIN 51819-3 [5] at an oil temperature of ϑoil = 80°C.
FZG Micropitting Test (GF-C/8.3/90 or GF-C/8.3/60)
The micropitting load-carrying capacity of gears can be calculated according to ISO TR 15144-1 [7], 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 [8].
The micropitting test GF-C/8.3/90 or GF-C/8.3/60 according to FVA 54/7 consists of a load stage test and an endurance test performed on a FZG back-to-back gear test rig [6]. Test gears type C-GF run at a circumferential speed of vt = 8.3 m/s and a lubricant temperature of ϑoil = 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 6. In load stage LS 10 the test gears are highly loaded. The endurance limit of the material is about σHlim = 1400 N/mm².
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 once 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 7. 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 (GFT-high). Examples of the evaluation of the micropitting test are given in Figure 7 and Figure 8. 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 a high-performance gear oil on the basis of polyglycol with a high micropitting performance, a test result is given exemplarily in Figure 9 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 (GFT-high). In the endurance test, the profile form deviation stagnates compared with the step test.
FZG Slow Speed Wear Test (C/0.05:0.57/90:120/12)
The results of the FZG slow speed test according to DGMK 377-01 [2] can be used for relative ranking of gear oils to a reference oil and in addition, specific wear rates clT can be derived for inclusion in the wear calculation method developed by Plewe [12]. The FZG slow speed wear test C/0.05:0.57/90:120/12 according to DGMK 377-01 determines the wear characteristics of gear oils at two different temperatures under mixed and boundary lubrication conditions [2]. With an additional test part the influence of circumferential speed can be investigated. See Table 8.
Test gears type C-PT run at a very low circumferential speed of vt = 0.05 m/s. The load applied is load stage LS 12 which is equivalent to a pinion torque of T1 = 378.2 Nm. This corresponds to a Hertzian pressure of pc = 1853 N/mm² in the gear contact. The oil temperature is ϑoil = 90°C during test part 1 of 2 x 20 hours and ϑoil = 120°C during test part 2 of 2 x 20 hours. In the optional test part 3 of 1 x 40 hours a higher circumferential speed of vt = 0.57 m/s is run at an oil temperature of ϑoil = 90°C.
The test is run on a modified FZG back-to-back gear test rig according to ISO 14635-1 [6] using an additional reducer gearbox after the drive motor in order to run very low speeds. After each test interval the pinion and the wheel are disassembled and weighed separately.
For a high-performance gear oil on the basis of polyglycol a test result of the FZG slow speed wear test is given exemplarily in Figure 10 showing a very low wear behavior. The sum wear of pinion + wheel is below the failure criterion of 40 mg (wear category low) not only for the individual test part of 40 h but also for the whole test procedure of 120 h.
FE8 Wear Test (D 7.5/80-80)
The influence of gear oils on the wear behavior of rolling bearings is examined in the FE8 wear test D 7.5/80-80 according to DIN 51819-3 [5] using the FE8 bearing test rig. In this test, axial cylinder roller bearings type D are subjected to a speed of n = 7.5 min-1 with an axial force of Fa = 80 kN at a steady-state temperature of 80°C over a period of 80 h. With a C/P < 2 a very high load is applied. Wear is then determined gravimetrically. Gear oils in the FE8 wear test according to DIN 51819-3 have to show a roller wear of <= 30 mg according to DIN 51517-3. Currently, the cage wear has to be reported only but the limit of a cage wear of <= 200 mg of the former version of DIN 51517-3 [4] is still on everybody’s mind. See Table 9.
Tested Lubricants
Industrial gears are typically lubricated with gear oils based on mineral oil, PAO or polyglycol. To prevent gearbox breakdown, the gear oils of today are expected to protect gears and rolling bearings against damage such as wear or micropitting even under critical lubricating conditions. Critical lubricating conditions are encountered in particular in gearboxes operating at low speeds, high loads or high temperatures where mixed or boundary lubrication prevails, i.e. where the friction bodies are not fully separated by a lubricant film. In such applications it is vital that additives contained in the lubricant provide effective protection against gear and rolling bearing damage. Depending on the type of additives, however, they require a certain temperature in the contact zone to react at the surface of gears or rolling bearings, leading to the formation of a protective layer.
The tested gear oils (GEM 1 N, GEM 4 N, and GH 6) are on the basis of mineral oil, polyalphaolefin, or polyglycol showing high wear protection as well as a high micropitting load-carrying capacity. These high-performance gear oils show a very low wear behavior of gears determined in the FZG slow speed wear test according to DGMK 377-01 [2] at oil temperatures of 90°C and 120°C. The micropitting load-carrying capacity of gears tested in the FZG micropitting test according to FVA 54/7 [8] at an already reduced oil temperature of 60°C is high with a failure load stage >= LS 10 in the load stage test and a stagnation of micropitting formation in the endurance test by selection of advanced additive technologies. The wear behavior of rolling bearings examined in the FE8 wear test according to DIN 51819-3 [5] at an oil temperature of 80°C is also very low. All tested gear oils are of ISO VG 220 and are specified according to DIN 51517-3 [4], which includes the minimum requirements for industrial gear oils and is similar to AGMA 9005 [1]. The oil data of the tested gear oils are shown in Figure 10.
Modification Of The Standard Test Procedures
For CLP oils to fulfill the minimum requirements for gear oils according to DIN 51517-3 [4], neither the FZG micropitting test according to FVA 54/7 [8] nor the FZG slow speed wear test according to DGMK 377-01 [2] is required. Only the FE8 wear test is required for CLP oils with an oil temperature of ϑoil = 80°C. But for today’s high-performance gear oils, the gear box OEMs also require the FZG micropitting test performed at an oil temperature of ϑoil = 90°C or 60°C and the FZG slow speed wear test performed at an oil temperature of ϑoil = 90°C and 120°C.
To find out if the high-performance gear oils of today are able to reliably protect gears and rolling bearings in gearboxes against damage also at lower oil temperatures, the standardized test methods mentioned above had to be modified. For the FZG micropitting test according to FVA 54/7, the FZG slow speed wear test according to DGMK 377-01, and the FE8 wear test according to DIN 51819-3 the oil temperature is reduced permanently to 40°C. All other test conditions as load and speed are unchanged. The modified of the micropitting test GF-C/8.3/40 consists of a load stage test and an endurance test performed on a FZG back-to-back gear test rig. Test gears type C-GF run at a circumferential speed of vt = 8.3 m/s and a lubricant temperature of ϑoil = 40°C. The load and the test periods are varied. See Table 10.
The wear behavior is determined in the modified FZG slow speed wear test C/0.05:0.57/40:40/12. Test gears type C-PT run at a circumferential speed of vt = 0.05 m/s and are loaded with load stage LS 12. The oil temperature is ϑoil = 40°C during test part 1 of 2 x 20 hours and ϑoil = 40°C during test part 2 of 2 x 20 hours. In the optional test part 3 of 1 x 40 hours a higher circumferential speed of vt = 0.57 m/s is run at an oil temperature of ϑoil = 40°C.
In the modified FE8 wear test D 7.5/80-40, axial cylinder roller bearings type D are subjected to a speed of n = 7.5 min-1 with an axial force of Fa = 80 kN at a steady-state temperature of 40°C over a period of 80 h using the FE8 bearing test rig.
Test Results
The aim of this research was to determine whether a high performance gear oil can prevent micropitting formation and wear even at lower oil temperatures. It has been shown that the advanced additive technologies used in today’s high-performance gear oils are capable of inducing the required reactions on the surfaces of gears and bearings also at 40°C, thus providing reliable damage protection even under these operating conditions. See Figure 11, Figure 12, and Figure 13.
For high-performance gear oils (GEM 1 N, GEM 4 N, and GH 6) based on mineral oil, polyalphaolefin, and polyglycol, a FZG micropitting test at an oil temperature of ϑoil = 60°C was performed showing a high micropitting load-carrying capacity of GFT-high for all these gear oils being expected. Additionally, the FZG micropitting tests at the reduced oil temperature of ϑoil = 40°C reached a high micropitting load-carrying capacity of GFT-high for these gear oils. This shows that the advanced additive technologies can react at the surface of the tooth flanks after also at low oil temperatures and build up a reaction layer. Micropitting formation can be prevented even under these operating conditions. See Figure 11.
Not only at the oil temperatures of ϑoil = 90°C and 120°C of the FZG slow speed wear test, the tested high-performance gear oils (GEM 1 N, GEM 4 N, and GH 6) based on mineral oil, polyalphaolefin, and polyglycol can show their very low wear behavior with a sum wear on pinion + wheel of less then 20 mg but also in the modified FZG slow speed wear test with a reduced oil temperature of ϑoil = 40°C. The reason for this are again the advanced additive technologies in these high-performance gear oils. Even at reduced oil temperatures, these advanced additive technologies can react at the surface of the tooth flanks and build up a reaction layer preventing wear failure. See Figure 12.
Finally, also in the FE8 wear test the tested high-performance gear oils (GEM 1 N, GEM 4 N, and GH 6) based on mineral oil, polyalphaolefin, and polyglycol showed their optimum wear protection not only at an oil temperature of ϑoil = 80°C, but also at a reduced oil temperature of ϑoil = 40°C. This is due to the advanced additive technologies. See Figure 13.
Conclusions
Different high-performance gear oils on the basis of mineral oil, polyalphaolefin, or polyglycol were examined on an FZG back-to-back gear test rig as well as on an FE8 bearing test rig to determine if these high-performance gear oils can prevent micropitting formation and abrasive wear even at low oil temperatures. The standardized tests methods were modified with regard to the oil temperature which was set permanently to 40°C.
The test results showed that the advanced additive technologies used in today’s high-performance gear oils can react at the surface of the tooth flanks even at low oil temperatures.
Even under such operating conditions gear failures in field applications can be prevented by using high-performance gear oils.
References
[1] AGMA 9005-E02: Industrial gear lubrication, 2002
[2] DGMK 377-01: Method to assess the wear characteristics of lubricants – FZG test method C/0.05/90:120/12, 1997
[3] DIN 3990-4: Tragfähigkeitsberechnung von Stirnrädern – Berechnung der Fresstragfähigkeit, 1987
[4] DIN 51517-3: Lubricants – Lubricating oils – Part 3: Lubricating oils CLP; Minimum requirements, 2011
[5] DIN 51819-3: Testing of lubricants – Mechanical-dynamic testing in the roller bearing test apparatus FE8 – Part 3: Test method for lubricating oils, axial cylindrical roller bearing, 2005
[6] 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.
[7] ISO TR 15144-1: Calculation of micropitting load capacity of cylindrical spur and helical gears – Part 1: Introduction and basic principles, 2010
[8] FVA 54/7: Test procedure for the investigation of the micro-pitting capacity of gear lubricants, FVA information sheet, Forschungsvereinigung Antriebstechnik e.V., 1993
[9] Gregorius H.: Verzahnungsschäden an Getrieben – Ursachen, Schadensbilder, Abhilfe, Tribologie-Kolloquium, TZN Technologiezentrum Niederrhein, 2006
[10] Michaelis K.: Die Integraltemperatur zur Beurteilung der Fresstragfähigkeit von Stirnradgetrieben, Dissertation, TU München, 1987
[11] Niemann G.: Maschinenelemente, Band 2: Getriebe allgemein, Zahnradgetriebe – Grundlagen, Stirnradgetriebe, Springer Verlag, 2003
[12] Plewe H-J.: Untersuchungen über den Abriebverschleiß von geschmierten, langsam laufenden Zahnrädern, Dissertation, TU München, 1980
[13] Schaeffler/FAG: Wälzlagerschäden – Schadenserkennung und Begutachtung gelaufener Wälzlager, 2004
[14] Wahler, M.: Schmierung bestimmt die Lebensdauer, Antriebspraxis, 01/2006
