Aus Wissenschaft und Forschung 24 Tribologie + Schmierungstechnik · 68. Jahrgang · 5/ 2021 DOI 10.24053/ TuS-2021-0028 Test and evaluation method for greases in grease-sealing rotary shaft seals Susanne Hahn, Simon Feldmeth, Frank Bauer* Eingereicht: 2.9.2021 Nach Begutachtung angenommen: 30.11.2021 Dieser Beitrag wurde im Rahmen der 62. Tribologie-Fachtagung 2021 der Gesellschaft für Tribologie (GfT) eingereicht. Fettgeschmierte Dichtsysteme zeigen im Vergleich zu ölgeschmierten Dichtsystemen deutlich häufiger Mangelschmierung. Verschiedene Fette zeigen dabei ein unterschiedliches Schmiervermögen im Dichtsystem. Zur Untersuchung dieses Schmiervermögens wurde eine Prüf- und Bewertungsmethodik entwickelt, mit der die Schmierfähigkeit von Fetten bewertet und miteinander verglichen werden kann. Die Test- und Bewertungsmethode besteht aus einem Prüflauf und einer anschließenden Analyse der Testkomponenten. Der Prüflauf umfasst ein 24-stündiges Drehzahlkollektiv und wird auf einem Prüfstand mit einem realen Dichtsystem durchgeführt. Für die Bewertungsmethode werden acht Kriterien analysiert, die für die Bewertung der Mangelschmierung als geeignet eingestuft wurden. Zu den Kriterien gehören sowohl während des Prüflaufs gemessenen Daten, wie Reibung und Temperatur, als auch Kriterien, die bei der anschließenden Untersuchung aller Testkomponenten geprüft werden. Alle Kriterien werden für die einzelnen Prüfläufe bewertet und zu einer Gesamtnote zusammengefasst, die einen direkten Vergleich verschiedener Fette ermöglicht. Die entwickelte Test- und Bewertungsmethode zeigt erhebliche Unterschiede zwischen 23 untersuchten Fetten auf. Sie ist demnach geeignet, die Schmierfähigkeit von Fetten in Dichtsystemen in kurzer Zeit zu analysieren und zu vergleichen. Durch die schnelle Prüfung und gezielte Auswertung der relevanten Kriterien können Schmierfette wesentlich kosten- und ressourceneffizienter als bisher getestet werden. Schlüsselwörter Fett, Radial-Wellendichtring, Fettabdichtung, Mangelschmierung, Prüf- und Bewertungsmethodik, Reibmoment, Fettauswahl Grease-lubricated sealing systems show starved lubrication much more frequently than oil lubricated sealing systems. Different greases provide different lubricity to the sealing system. A test and evaluation method was developed that allows to assess the lubricity of different greases and to compare them with each other. The test and evaluation method consists of a test run and a subsequent analysis of the test components. The test run comprises a 24-hour speed collective and is performed on a test rig using real sealing system components. For the evaluation method, eight criteria are analysed, which are rated suitable for evaluating starved lubrication. The criteria include parameters measured during the test run, such as friction torque and temperature, as well as parameters inspected in the subsequent examination of all test components. All criteria are assessed and combined to an overall score, that allows to directly compare different greases with each other. The test and evaluation method developed reveals significant differences between 23 greases examined. Thus, the lubricity of greases in sealing systems can be analysed and compared much more quickly than in the past. Thanks to the rapid testing and focused evaluation of the relevant criteria, greases can be tested much more costand resource-efficiently than before. Keywords grease, rotary shaft seal, sealing of grease, starved lubrication, test and evaluation method, friction torque, selection of grease Kurzfassung Abstract * Susanne Hahn, geb. Jaekel, M.Sc. Orcid-ID: https: / / orcid.org/ 0000-0002-1891-0286 Dipl.-Ing. Simon Feldmeth Orcid-ID: https: / / orcid.org/ 0000-0003-0018-0710 PD Dr.-Ing. Frank Bauer Orcid-ID: https: / / orcid.org/ 0000-0001-7799-7628 University of Stuttgart, Institute of Machine Components (IMA) Pfaffenwaldring 9, 70569 Stuttgart, Germany TuS_5_2021.qxp_TuS_5_2021 10.12.21 11: 05 Seite 24 Introduction Grease lubrication is often preferred to oil lubrication, because the design effort is lower and a lifetime lubrication can be realised. The sealing of grease on the other hand causes various problems compared to oil sealing. Sealing systems with grease lubrication show starved lubrication much more frequently than oil lubricated systems [1, 2]. Starved lubrication can damage the sealing edge and thus cause a failure of the sealing system and a leakage of the grease. This grease is no longer available for lubricating the machine parts, which can lead to the failure of entire assemblies. Outside the sealing system, wear and leaking grease reveal the damage to the users. The reasons for starved lubrication are multiple and in the focus of current research [3]. Starved lubrication is rather a phenomenon than a parameter that can be evaluated directly. In the field of rolling bearings the lubrication can (e.g.) be estimated by means of the film thickness of the greases [4, 5]. This concept cannot be applied to rotary shaft seals for several reasons. First, the film thickness of rotary shaft seals cannot be measured in operation. Second, the lubrication conditions of seals differ fundamentally from the lubrication conditions in rolling bearings. This concerns e.g.: • the geometry: non-conforming contact (balls) versus conforming contact (angular shape of the sealing edge) • the materials: hard contact (steel on steel) versus soft contact (elastomer on steel) • the replenishment: rollover versus constant position of circumferential sealing edge • the load: Hertzian pressure up to several thousand MPa [6] versus up to a few MPa [7] As starved lubrication in grease-sealing rotary shaft seals cannot be measured directly, it must be studied indirectly through other criteria that indicate the lubrication condition. Narten [8] analysed the influence of the components and environment of a greasesealing system. Test runs were evaluated by means of the temperature of the components of the sealing system, the friction torque, the wear track on the shaft, the seal and the change in the radial load. Sommer [9] analysed the influence of different thickeners and thickener concentrations on grease-lubricated sealing systems. Thereby, he evaluated the friction and the temperature of the sealing system as well as the wear track and the sealing edge. This analysation is well suitable to compare a limited amount of different sealing systems. To analyse and compare the lubrication condition of many different sealing systems, a direct qualitative comparison of different measurement parameters is no more practical. Instead, the most relevant criteria must be identified and the evaluation of the single criteria must be combined in one quantitative and therefore comparable indicator. If the values for this evaluation are gathered with a standardises test run, the lubrication condition of different sealing systems can directly be compared with each other, and the method can be used to select the best lubricating sealing system out of a given selection. This paper presents such a test and evaluation method, that was developed as part of the ‘Fettmangel’-project [3]. The paper focuses specifically on the development of the test and evaluation method. The results obtained by this method were also correlated with various grease properties, e.g. NLGI grade. This correlation is the main topic of a further paper [10]. The test and evaluation method was developed and applied to compare the lubricity of 23 different greases in a sealing system. All greases are commercially available greases with different thickeners, base oils and additives. They have NLGI grades from below the definition (< 000) up to 2-3. In the tests, rotary shaft seals of the form BAU4X2 50x65x8 of the company Freudenberg made of nitrile butadiene rubber (72 NBR 902) were used with needle bearing inner rings of type INA IR45X50X35-EGS as counterfaces. The inner rings have plunge-ground surfaces and their lead-freeness was verified before the tests as described in [11-14]. The rotary shaft seals were named with a continuous number in form of ‘FM123’. As each test run was conducted with a new seal, this number is used to identify the test runs. Test devices The test runs were carried out on a friction torque test rig in Figure 1. The test rig has two independent test units Aus Wissenschaft und Forschung 25 Tribologie + Schmierungstechnik · 68. Jahrgang · 5/ 2021 DOI 10.24053/ TuS-2021-0028 Figure 1: Friction torque test rig TuS_5_2021.qxp_TuS_5_2021 10.12.21 11: 05 Seite 25 that a test run lasts 24 hours. The test runs are carried out without tempering the test rig. Before the test run, approximate 1 ml of the test grease is applied to the shaft by means of a syringe. While the shaft is rotating slowly (Figure 3, left) the grease is put on, until the shaft is completely surrounded by a grease ring (Figure 3, center). The seal is then pushed over this ring as shown in Figure 3 (right), so that the ring is located on the fluid side of the sealing edge. Evaluation criteria Starved lubrication in rotary shaft seals cannot be measured directly but must be assessed indirectly via other criteria. In order to realise a robust evaluation, eight criteria, that proved themselves relevant in the assessment of sealing systems in the past [3], were evaluated for the test runs with the 23 greases. These criteria are described in Table 1 and base on measurements during the test run, a microscopic analysis of the seal and an evaluation of the wear track after the test run. The level of the friction torque and temperature as well as the change in the radial load and the visual impression of the sealing edge do not correlate with the other criteria, therefore these criteria are not further discussed in this work but displayed in [3]. As there are quantitative and qualitative criteria, all criteria are assessed with grades from 1 (very good) to 5 (very bad) so that they can be compared directly with each other. This also allows to finally average all criteria for each test run, so that the test runs can directly be compared with each other. In the following, the evaluation of the single criteria is illustrated on the example of a good and a bad result for each criterion. Figure 4 shows the measurement of the temperature on the air side of the sealing contact for two sealing systems. While Grease 7/ FM048 displays a regular record Aus Wissenschaft und Forschung 26 Tribologie + Schmierungstechnik · 68. Jahrgang · 5/ 2021 DOI 10.24053/ TuS-2021-0028 whose shafts rotate in opposite directions. The counterface is mounted on a shaft adapter and driven by a servo motor via a belt. The seal in the seal holder is mounted in an aerostatic bearing. Therefore, the friction torque of the sealing system can be evaluated via a load cell on which the seal holder rests. A pyrometer measures the temperature on the air side of the seal, close to the sealing contact. After the test run, the sealing systems are examined for different kinds of damages. The seals are analysed uncleaned and cleaned with a Keyence VHX-1000 digital microscope, using the IMA-Sealobserver ® [15] that not only allows a focused view of the whole sealing lip but also a quantitative measurement of the wear width [16]. The depth and wear width of the wear track on the counterface were investigated with a HOMMEL T8000 roughness measuring device [17]. Test procedure Figure 2 shows the test cycle used for the test runs. The test cycle consists of seven one-hour sections each with ascending circumferential velocities of 0.15...5.5 m/ s and a subsequent standstill period of 1 h. The circumferential velocities were selected on the basis of tests where only wear and no thermal overload was observed [9]. This eight-hour test cycle is repeated three times so 0.15 0.2 0.25 0.5 1.625 2.75 5.5 0 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 Circumferential velocity [m/ s] Time [h] Figure 2: Test cycle, the eight-hour test cycle is repeated three times during one 24-hour test run Figure 3: Application of the grease ring (left), complete grease ring (center), sliding the seal over the grease ring (right) TuS_5_2021.qxp_TuS_5_2021 10.12.21 11: 05 Seite 26 with similar cycles, Grease 17/ FM064 reveals a fluctuating temperature that differs a lot between the single cycles. Figure 5 shows two friction torque curves. The friction torque of Grease 2/ FM035 (top) proceeds as expected: After a short descend of the friction torque at the beginning, it shows only little fluctuation during the first cycle, whereas the second and third cycle have a very smooth friction torque that strongly resembles the first. When the circumferential velocity is increased, the friction torque first rises for a short time until the system has adapted itself on the new conditions. Grease 19/ FM042 (bottom) in contrast shows strong fluctuation with big differences between the single cycles, which indicates changing lubrication conditions during the test run. Aus Wissenschaft und Forschung 27 Tribologie + Schmierungstechnik · 68. Jahrgang · 5/ 2021 DOI 10.24053/ TuS-2021-0028 Figure 4: Temperature of test runs Criteria Quantitative Qualitative Grade Assessment scale Measurements during the test run Temperature fluctuation x 1 Only small fluctuation, test cycles are similar 5 Strong fluctuation, test cycles differ strongly Friction torque fluctuation x 1 Only small fluctuation, test cycles are similar 5 Strong fluctuation, test cycles differ strongly, outliers Microscopic analysis after the test run Abrasion particles x 1 No abrasion visible on the air side 5 Severe abrasion, sealing lip is not visible under abrasion Grease: visual impression x 1 No discolouration of the grease and no particles are visible in the grease 5 Strong discolouration of the grease, the grease is interspersed with particles, the particles form a solid deposit on the sealing edge Sealing edge: max. wear width x 1 Lowest maximum wear width ( ) 5 Highest maximum wear width ( ) Evaluation of the wear track after the test run Wear track: visual impression x 1 Wear track barely visible without aids 5 Wear track broad and strongly discoloured Wear track: depth x 1 Wear track not recognisable in tactile measurement 2 Wear track recognisable in tactile measurement (roughness smoothing) 3 Minimum measured value ( ) 5 Maximum measured value ( ) Wear track: width x 1 Wear track not recognisable in tactile measurement 2 Wear track recognisable in tactile measurement (roughness smoothing) 3 Minimum measured value ( ) 5 Maximum measured value ( ) Table 1: Evaluated criteria Grease 17 FM064 Criterion Grade Temperature fluctuation 5.0 0 1 2 3 4 5 6 30 40 50 60 70 80 90 100 110 0: 00: 00 8: 00: 00 16: 00: 00 24: 00: 00 Circumferential velocity [m/ s] Temperature [°C] Time [hh: mm: ss] Grease 7 / FM048 Grease 17 / FM064 Circumferential velocity Cycle 1 Cycle 2 Cycle 3 Grease 7 FM048 Criterion Grade Temperature fluctuation 1.0 TuS_5_2021.qxp_TuS_5_2021 10.12.21 11: 05 Seite 27 terspersed with particles. It is not clear, if these particles origin from abrasion or/ and from carbonised grease components. A solid deposit layer developed close to the sealing edge. Figure 7 shows the cleaned sealing edge of two systems with a magnification of 200 x. While Grease 2/ FM035 (top) shows only a small sealing edge with a maximum wear width of 0.20 mm, Grease 19/ FM042 (bottom) is broadly worn on a maximum width of 0.71 mm. Aus Wissenschaft und Forschung 28 Tribologie + Schmierungstechnik · 68. Jahrgang · 5/ 2021 DOI 10.24053/ TuS-2021-0028 Figure 6 shows an optical microscope image at 50x magnification of the sealing edge after the test run in an uncleaned condition. While Grease 2/ FM035 shows no abrasion particles on the air side, a thick layer of abrasion particles on the air side of Grease 19/ FM042 covers parts of the sealing lip. Grease 2/ FM035 (top) has not visibly changed across the test and shows neither discolouration nor particles in the grease, whereas Grease 19/ FM042 (bottom) is in- Figure 5: Friction torque of test runs Criterion Grade Friction torque fluctuation 1.5 Criterion Grade Friction torque fluctuation 4.0 Grease 2 FM035 Grease 19 FM042 Figure 6: Visual impression of the grease after the test run Sealing edge after test run (Magnification 50x) New grease Fluid side Air side Fluid side Air side Abrasion particles Sealing edge Sealing edge Grease Grease 1.00 mm 1.00 mm Grease 2 FM035 Criterion Grade Abrasion particles 1.0 Grease: visual impression 1.0 Grease 19 FM042 Criterion Grade Abrasion particles 4.0 Grease: visual impression 4.0 TuS_5_2021.qxp_TuS_5_2021 10.12.21 11: 05 Seite 28 Figure 8 shows a counterface after the test run (top) and a tactile measurement of its surface (bottom). The wear track of Grease 15/ FM061 is barely visible on the counterface and not recognisable in the tactile measurement. Grease 13/ FM053 instead shows a broad and discoloured wear track that is clearly evident in the tactile measurement with a depth of 7.30 µm and width of 0.921 mm. Evaluation of the tested greases Each of the 23 greases was tested in at least one test run in each rotational direction, and the eight criteria were analysed for each test run. The eight grades were then averaged for all test runs, Figure 9. The individual tests (identified by the name of the seal e.g. FM123) were listed in ascending order of mean value. Good grades are highlighted in green whereas bad grades are highlighted in red. The overview over all tests (Figure 9, right) shows significant differences between the single tests. Tests with a good arithmetic mean usually show good results over most criteria whereas greases with a bad arithmetic mean are rated bad in most criteria. As the criteria were evaluated separately from each other, the level of the criteria can differ although the trend of the evaluation remains similar. This is especially visible on the evaluation of criteria 6 (wear track: visual impression) which is rated with rather bad grades. Nonetheless, the rating of the visual impression follows the trend of the Aus Wissenschaft und Forschung 29 Tribologie + Schmierungstechnik · 68. Jahrgang · 5/ 2021 DOI 10.24053/ TuS-2021-0028 Figure 7: Wear width of the sealing edge after the test run Sealing edge after test run, cleaned (Magnification 200x) Fluid side Air side Fluid side Air side Sealing edge Sealing edge 0,20 mm 1.00 mm 0.71 mm 0.20 mm 1.00 mm Figure 8: Evaluation of the wear track on the counterface after the test run: visual impression (top), tactile measurement (depth and width, bottom) 0 10 20 30 mm µm 0 5 10 Grease 13 Grease 13 Grease 15 Grease 15 Grease 19 FM042 Criterion Grade Sealing edge: max. wear width 4.1 Grease 2 FM035 Criterion Grade Sealing edge: max. wear width 1.3 Grease 15 FM061 Wear track Criterion Grade Visual impression 1.0 Depth 1.0 Width 1.0 Grease 13 FM053 Wear track Criterion Grade Visual impression 5.0 Depth 4.9 Width 5.0 TuS_5_2021.qxp_TuS_5_2021 10.12.21 11: 05 Seite 29 sible with a 24-hour test run and the presented test and evaluation method. Nonetheless, at least 2 tests should be run for each grease. Summary and conclusion Starved lubrication is a common problem with grease lubricated rotary shaft seals. As the lubrication condition in seals can’t be measured or evaluated directly with one parameter, an alternative evaluation is needed to compare different greases with each other. Therefore, a test and evaluation method was presented, that helps to evaluate the lubricity of greases and allows to compare different greases with each other. The 24-hour test run and the test components are evaluated on the basis of 8 criteria. A rating of the criteria according to a standardised system allows the direct comparison between qualitative and quantitative criteria and also between the different greases using an overall grease score N. This arithmetic mean of all criteria of all tests per grease provides a Aus Wissenschaft und Forschung 30 Tribologie + Schmierungstechnik · 68. Jahrgang · 5/ 2021 DOI 10.24053/ TuS-2021-0028 other criteria and the rating of the visual impression of the wear track tends to get worse for a rising arithmetic mean. Tests that were carried out with the same grease usually show similar results, so the results are reproducible. To assess the lubricity of the greases, the arithmetic means of all single grades of all tests with each grease were averaged to the overall grease score N. Figure 10 shows the greases sorted by ascending N. The error bars indicate the lowest and the highest arithmetic mean of the individual test runs. Most greases show reproducible results with only a small fluctuation between the tests, resulting in differences of maximum 0.19 in N. Exceptions are the greases 10, 4 and 17, whose scores differ by 1.4 to 2.9, so that an assessment of starved lubrication is difficult. However, there are clear differences between well lubricating greases with an overall grease score between 1 and 2 and increasingly starved lubricating greases with an overall grease score above 3.5. So, an identification of starved lubricating greases seems pos- Temperature fluctuation Friction torque fluctuation Abrasion particles Grease: visual impression Sealing edge: wear width Wear track: visual impression Wear track: depth Wear track: width Arithmetic mean of grades Seal Grease 1 2 3 4 5 6 7 8 FM063 Grease 16 2.0 2.5 2.0 1.5 1.4 2.0 2.0 2.0 1.9 FM011 Grease 8 3.0 1.8 2.0 1.0 1.1 3.0 2.0 2.0 2.0 FM039 Grease 10 2.5 1.8 3.0 3.0 1.2 3.5 3.0 3.2 2.7 FM017 Grease 10 2.5 2.0 3.0 3.0 1.2 3.5 3.1 3.5 2.7 FM065 Grease 17 4.5 5.0 2.5 2.5 1.8 4.0 2.0 2.0 3.0 FM037 Grease 4 3.5 4.0 3.5 2.0 2.6 3.5 3.2 3.6 3.2 Coloration 5.0 1.0 3.0 ... ... Figure 9: Summary of the grades of all tests 1.1 1.2 1.2 1.3 1.3 1.4 1.4 1.4 1.4 1.4 1.4 1.5 1.5 1.8 1.8 1.9 2.0 3.6 3.8 3.9 4.1 4.5 4.6 1 2 3 4 5 Grease 2** Grease 6 Grease 11 Grease 3 Grease 9 Grease 22 Grease 7 Grease 15 Grease 21 Grease 14 Grease 23 Grease 1 Grease 18 Grease 20 Grease 8** Grease 16 Grease 10** Grease 4** Grease 17 Grease 5* Grease 19 Grease 12 Grease 13 Overall grease score N [-] Min Max Mean * 3 tests averaged instead of 2 ** 4 tests averaged instead of 2 Figure 10: Assessment of the tested greases TuS_5_2021.qxp_TuS_5_2021 10.12.21 11: 05 Seite 30 quantitative characteristic of the lubricity of the grease. The evaluation shows considerable differences between the greases and thus allows to differ between well and badly lubricated sealing systems. The presented test and evaluation method allows to analyse and compare the lubricity of greases in sealing systems much faster than it was previously possible. Thanks to the rapid testing and focused evaluation of the relevant criteria, lubricating greases can be tested much more costand resource-efficiently than before. Some of the criteria, e.g. abrasion and condition of the grease or depth and width of the wear track, are not totally independent of each other [18]. Nonetheless, the arithmetic mean value as indicator of the lubrication condition of a sealing system bases on enough different criteria, so that the result is robust against measurement errors or individual outliers. Still, one approach could be to reduce the number of criteria assessed by reducing directly related criteria. It also might be possible to reduce the testing effort with only a minor effect on the overall grease score by dispensing with expensive measurements such as friction torque. Acknowledgements The IGF project 19930 N/ 1 of the Forschungskuratorium Maschinenbau e.V. (FKM) was funded by the AiF as a support of the Industrielle Gemeinschaftsforschung (IGF, Industrial Collective Research) by the Federal Ministry for Economic Affairs and Energy (BMWi) on the basis of a decision by the German Bundestag. References [1] Baart, P.; Lugt, P. M.; Prakash, B.: Review of the lubrication, sealing, and pumping mechanisms in oiland greaselubricated radial lip seals. In: Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2009, 223 (3), pp. 347-358. https: / / doi.org/ 10.1243/ 13506501JET473 [2] Narten, M.: Abdichtung von fließfettgeschmierten Getrieben mit Radialwellendichtungen - Reibungsminderung durch Makrostrukturierung der Dichtungsgegenlauffläche. Ph.D. thesis, University of Stuttgart, 2014, ISBN 978-3-936100-53-2. [3] Jaekel, S.; Feldmeth, S.; Bauer, F.: Fettmangel. Vermeidung von Mangelschmierung bei fettabdichtenden Radial-Wellendichtungen. 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