Aus der Praxis für die Praxis 1 Introduction High equipment temperatures can cause several problems in lubricated systems, for example: - lifetime reduction of components and lubricants - maintenance time and costs - high energy losses - need of extra cooling equipment - safety problems Lubricants can have a big effect on the temperature behaviour of a system. An appropriate viscosity should be chosen to ensure hydrodynamic lubrication between moving parts. Choosing a lower viscosity than needed results in mixed friction, wear and heat, whereas choosing a higher viscosity results in higher fluid friction, which also increases heat. 52 Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 * Michael Möller Stefan Mooren Dipl.-Ing. Michael Plew BP Europa SE/ Castrol Industrial, Technical Support, Mönchengladbach, Germany Temperature reduction using PD additives M. Möller, S. Mooren, M. Plew* Hohe Temperaturen haben einen Einfluss auf die Lebensdauer von Maschinenelementen wie zum Beispiel Lagern, Dichtungen oder Schmierstoffen. Spezielle Schmierstoffadditive können helfen, die Temperatur zu reduzieren. Zudem werden neben der Temperaturreduzierung oft auch Energieeinspareffekte in diesen Systemen festgestellt. PD-Additive (Plastische Deformations Additive) können die Temperatur in Aggregaten wie zum Beispiel hochbelasteten Getrieben oder Wälzlagern senken. Die Wirkung dieser Additive an belasteten Oberflächen reduziert die Rauheit der gegeneinander wirkenden Elemente, daraus resultierend erreichen diese schneller und öfter eine hydrodynamische Schmierung bei Variation von Geschwindigkeit und Belastung. Systeme mit höherer Rauheit dagegen werden öfter Mischreibung oder gar Grenzreibung erfahren und mehr Verschleiß sowie höhere Temperaturen erzeugen. Die Temperaturreduzierung von geschmierten Systemen wird oft mit spezifischen Feldtestungen oder aber auf sehr speziellen Prüfständen nachgewiesen. So erzeugte Testresultate sind zumeist nicht mit anderen Ergebnissen vergleichbar. Anhand von Untersuchungen unterschiedlicher Schmierstoffe und Schmierstoffadditivierungen auf dem FE8 Wälzlagerprüfstand, sowie Auswertungen diverser FZG Fresslastuntersuchungen (A/ 8,3/ 90 und A/ 16,6/ 90), wird dargestellt, dass solche Ergebnisse ebenfalls auf standardisierten Prüfmaschinen nachvollziehbar sind. Dieser Beitrag zeigt die Temperaturreduzierung anhand von Ergebnissen, die sowohl auf einem speziellen Kundenprüfstand als auch auf standardisierten Prüfständen ermittelt wurden. Schlüsselwörter PD-Additive, plastische Deformation, MPR-Test, FZG Test, FE8 Test, Temperaturreduzierung, Energieeffizienz High temperatures have an effect on the lifetime of equipment like bearings, seals or lubricants. Special lubricant additives can help to reduce the temperature of lubricated systems. In addition to lower temperatures, increased energy efficiency effects are often seen in these systems. PD-additives (plastic deformation additives) can lower the temperature in equipment like high loaded gears or roller bearings. The impact of this temperature reduction has often been proven by tests conducted by customers as well as on very special test rigs. This test results are often not comparable with other results. This paper will show the temperature reduction effected by PD additives taking some common test rigs as FZG [1] and FE8 [2] test machine as well some results of customer tests. Keywords PD-Additive, plastic deformation additives, MPR- Test, FZG Test, FE8 Test, Temperature reduction, Energy efficiency Kurzfassung Abstract T+S_2_16 05.02.16 14: 24 Seite 52 Aus der Praxis für die Praxis Systems which are subject to boundary or mixed friction for extended periods of time and reach hydrodynamic states less often or for shorter periods of time than other systems will create more energy loss, resulting in higher system temperatures. 2.2 Temperature reduction by PD additives As described briefly under Point 2, PD additives have a significant effect on surface roughness under high loads and increased temperatures. The roughness will be reduced without creating additional wear. In other words, the peaks of the surface roughness will be shifted into the neighbouring valleys. Graphic 2 shows the surface of a test specimen of a MPR test rig (a micropitting screening test) after lubrication with a PD containing gear oil. The surface roughness was reduced from Ra = 200 nm to Ra = 98 nm during the test run whereas other tested fluids (also PAO CLP [4] gear lubricants with high micropitting resistance) have shown an increase of surface roughness up to Ra ≈ 450 nm. By reducing surface roughness, a hydrodynamic regime will be built up faster and more stable. Having higher roughness in a system it is more critical to reach again mixed friction when vary the load or the speed of the system. Having partly more mixed friction the temperature will increase. Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 53 Graphic 1: Stribeck curve Graphic 2: surface roughness of a MPR test rig specimen lubricated with PD gear oil High loads and shock loads may also result in mixed or boundary friction, also increasing friction, wear and heat. By using lubricants with PD additives, the temperatures of high and shock loaded equipment can drop significantly. 2 Function of PD additives PD additives take effect on the surface of the moving metal parts like gear teeth or rollers and boards of bearings. At first, they build up an additive film on the surface of the parts similar to common anti-wear and EP additives. When subjected to high loads and/ or temperatures, the second phase of the PD reaction gets started and PD additives partly diffuse into the top layer of the surface. When load and stress increase further, these PD additive containing layers become plastically deformed. Peaks of the surface roughness will be shifted into neighbouring valleys. Surface roughness is reduced, resulting in easier formation of a hydrodynamic lubrication film in combination with reduced stress on local surfaces and roughness peaks. 2.1 Temperature behaviour Persistent system temperature depends on several different things, like: - distribution of stress and stressed areas - materials and their friction coefficients - surface roughness of friction partners - movement patterns - environmental temperatures - lubrication of the system o grease lubrication o oil splash lubrication o circular lubrication - effects of cooling systems, if applicable - operation mode o continuous operation o part time operation - lubrication/ friction condition (see graphic 1) o mixed or boundary friction o EHD T+S_2_16 05.02.16 14: 24 Seite 53 Aus der Praxis für die Praxis Wear will also be reduced, because the system is working more continuously in hydrodynamic lubrication states. 3 Gear manufacturer test rig experience In applications of the underground coal mining industry, lubricating conditions are very tough. Workers as well as equipment have to deal with high environmental temperatures, high humidity, a lot of dirt and dust as well as hard rocks and particles. Some highly loaded coal cutting shearer loaders working under such conditions were unable to achieve the expected lifetime. Oil temperatures had risen too high with the standard CLP 320 gear oil which was used at that time. As result the gears of those machines experienced several breakdowns. Installations of new gears in underground mines are very difficult, resulting in increased downtimes of the shearer loaders and poor productivity. In cooperation with the manufacture of the shearer loaders and their gears, Gebr. Eickhoff Maschinenfabrik und Eisengießerei GmbH, tests with other fluids were decided. On Eickhoff´s test rig in Bochum, the PD containing fluid Castrol Tribol 1710/ 320 has shown exciting results. When testing the standard CLP 320 gear oil, temperatures about 100 °C up to 105 °C were measured, whereas tests with Castrol Tribol 1710/ 320 resulted in temperatures of only about 80 °C up to 85 °C, a drop of round about 20 °C. Because of this test result the equipment manufacture, Gebr. Eickhoff Maschinenfabrik und Eisengießerei GmbH, and coal mining companies like Deutsche Steinkohle AG changed their shearer loader gear oils from Standard CLP 320 to Castrol Tribol 1710/ 320. Using Castrol Tribol 1710/ 320, the shearer loaders also proved to have lower oil temperatures and confirmed the test rig results. This led to less frequent service intervals and prolonged equipment lifetime. 4 Roller bearing test using a FE8 test rig [2] The FE8 test rig [2], specified in DIN 51819-1 [2], is used to determine the anti-wear and EP behaviour of lubricating greases and oils in roller bearings under mixed friction conditions. DIN 51819-3 [3] describes the test procedure for lubricating oils. An axial cylindrical roller bearing gets loaded with an axial force of 80 kN and runs at 7.5 rpm at an oil temperature of 80 °C. The bearing is lubricated by an oil circulation system with 0.1 liter per minute. The oil tank has a volume of 4 liters. During the test run the friction torque is measured. After the test the bearing’s roller wear and cage wear are measured. 4.1 Modified FE8 Test 75/ 100-70 To measure a temperature difference between 2 lubrication oils the DIN 51819-3 [3] procedure was inappropriate, so a modified procedure was chosen. The load was increased to 100 kN and the heating temperature of the circulating oil was reduced to 70 °C. The parameters are comparable to a test procedure, which is also often used by Schaeffler AG to test the bearing wear caused by fluids. In addition to the common setup, a second oil tank was installed, including valves to switch over from one oil tank to the other „on the fly“ during the test run. This has been decided on in order to ensure that there is no influence caused by replacing the test bearings and make influences on friction force and temperature directly comparable. Photo 1 shows the test rig used in our test facility in Mönchengladbach, including both mounted oil tanks. The first oil type was running for 100 h before switching to the second oil type (Castrol oil with PD additives), also running for 100 h initially. Within those 100 h, both oils reached reliable steady values. During the whole test run, friction torque and temperature gradient were recorded. 4.2 Results of FE8 Test 75/ 100-70 We tested two different lubricants in the modified procedure. Both candidates are based on poly-alpha-olefin (PAO), have the same viscosity (ISO VG 320) and fulfil requirements of DIN 51517-3 [5] for CLP [4] gear oils. During the test run of Lube 1, a sulphur-phosphor formulated product and Lube 2, a PD formulated product, differences in friction torque and temperature behaviour were observed, as seen in Graphic 3. The bearing temperatures measured with Lube 1 had risen up to 80 °C and the friction torque stabilized at about 35 Nm. Since the oil temperature exceeded 70 °C, the heater did not contribute due to the high oil temperature. 54 Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 Photo 1: modified FE8 test rig, BP-GLT T+S_2_16 05.02.16 14: 24 Seite 54 Aus der Praxis für die Praxis After switching to Lube 2, the heater had to turn on periodically to ensure that the oil temperature does not fall below the specified 70 °C. Therefore the temperature might have dropped even more, if the heater had been switched off. Nevertheless, a constant temperature difference of about 10 °C was observed during this test. In addition, the friction torque was significantly lower, levelling out at about 30 Nm, which is 5 Nm less compared to Lube 1. 5 FZG rig test [1] The FZG test rig is a well-known lubricant gear test rig described with several test procedures in different standards and specifications. In DIN 51517-3 [5], minimum requirements for CLP type lubricants [4], it is listed as one of the required wear tests. A CLP [4] gear lubricant according to this standard has to achieve a minimum failure load stage of 12 using procedure FZG A/ 8.3/ 90 described in DIN ISO 14635 [1]. This method measures the scuffing load capacity of oils. Photo 2 shows a FZG test rig in our Mönchengladbach based Technology. Most common CLP gear lubricants with ISO VG 220 or ISO VG 320 are tested with a failure load stage of > 12 in this test procedure. 5.1 Test procedure FZG A/ 8.3/ 90 and A/ 16.6/ 90 [1] When reaching load stage 5, the oil has to be preheated to 90 °C ± 0.3 °C before starting the test run. The test has to run for 21700 revolutions, which takes approximately 15 minutes in each load stage. Before starting the next load stage, the test wheel has to be checked regarding scuffing marks, see Photo 3. In most cases the temperature rises during the test run. We have monitored the oil temperature at the end of every load stage. Before starting the next load stage, the oil has to be cooled down to 90 °C ± 0.3°C again, so every load stage from 5 to 12 (or higher) starts at the same temperature. Other procedures like the A/ 16.6/ 90 [1] with double speed compared to A/ 8.3/ 90 [1] are also common. In this procedure, the tests also start with 90 °C oil temperature in each load stage (beginning with load stage 5). Because load stages 13 and 14 were not measured in every test run or by every lubricant in our evaluation, they were not considered in this paper. Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 55 Graphic 3: results of modified FE8 test 75/ 100-70, BP-GLT Photo 2: FZG test rig, BP GLT Photo 3: Test wheel with scuffing marks, BP GLT 5.2 Results of FZG tests, lubricants with S/ P additives As described, we measured the temperature at the end of each load stage. Typically the end temperatures of each stage should rise with increasing loads. This behaviour could be confirmed in each test run. All lubricants used in the following tests had a viscosity-grade of ISO VG 220 or ISO VG 320 to avoid significant influence of viscosity. T+S_2_16 05.02.16 14: 24 Seite 55 Aus der Praxis für die Praxis At first, we compared the temperatures of mineral-oil based CLP 220 products containing sulphur-phosphor additives (S/ P or -PS-). Two different products were tested. Lube 2 (M-PS-gr-220) has a high resistance against micropitting [6]. Graphic 4 shows an end temperature of about 140 °C in stage 12, sometimes a little bit higher. Findings with S-P products: Conducting double speed tests with 16.6 m/ s, the end temperatures can rise up to 7 °C higher compared to 8.3 m/ s tests (see Graphic 4). Tests of synthetic PAO based CLP gear lubes with sulphur-phosphor additives resulted in temperatures of 135 °C up to 140 °C (see Graphic 5). Using PAO based products instead of mineral oil based products, the temperatures in higher load stages can be up to 7 °C less (see Graphic 6). All in all, end temperatures in load stage 12 of approximately 140 °C are possible and typical when using ISO VG 220 sulphur-phosphor gear lubricants based on mineral oil or PAO. 5.3 Results of FZG tests, lubricants with PD additives As next step we looked into FZG tests [1] of different lubricants containing PD-additives. Graphic 7 shows a comparison of a mineral-oil based CLP 220 and the semi synthetic (mineral-oil and PAO mix) PD 320 lubricant used nowadays in shearer loaders (see point 3 - customer rig test). In load stage 12, the semi synthetic PD 320 reached an end temperature of 122 °C compared to 140 °C measured when using the mineral sulphur-phosphor based product. This results in an overall temperature difference of 18 °C. This temperature difference compares to the differences found under point 3. The question came up, if used oils in the field of the same product type can achieve similar results. Therefor a field sample was taken and a FZG A/ 8.3/ 90 [1] test run initiated. In load stage 12, this resulted in an oil temperature of 126 °C, a little higher than the freshoil sample, but still 14 °C lower compared to the mineral based sulfur-phos- 56 Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 Graphic 7: FZG mineral S-P and mineral-PAO PD Graphic 4: FZG temperatures of S/ P mineral-oils Graphic 5: test runs with 16.6 m/ s of S/ P oils with different base fluids Graphic 6: temperature differences of PAO and mineral oil based S/ P lubricants T+S_2_16 05.02.16 14: 24 Seite 56 Aus der Praxis für die Praxis phor based product (see Graphic 8). The temperature reduction effects of PD additives are long-term effects and will also be available after several months of field operation. When using PAO based lubricants formulated with PD-additives, we achieved end temperatures of round about 120 °C with the FZG A/ 8.3/ 90 [1] parameters Graphic 9 shows a comparison of one of these PAO PD products and a mineral oil based sulphur-phosphor product. The viscosity of the used synthetic product (ISO VG 320) is higher compared to the mineral-oil based one (ISO VG 220). At 90 °C, the mineral-oil based product has a viscosity of about 25 mm 2 / s, while the PAO based one has about 43 mm 2 / s. Nevertheless, the temperature difference at the end of the FZG test is round about 20 °C in favor of the PAO based PD product. Different PAO PD products were also run with the A/ 16.6/ 90 procedure [1]. Temperatures of round about 130 °C were measured at the end of the test runs. Used oil samples from filed operations resulted in similar temperature behaviour as fresh oil samples. 5.4 Results of FZG tests The FZG test [1] shows significant temperature effects of different additive technologies when comparing the end temperatures of the higher load stages. 6 Conclusion The conducted tests show the impact on temperature behaviour in loaded systems of lubricants with different additive systems. Three different test rigs have proven this impact, of which two are common test rigs and used in many lubrication research centres or laboratories. When using PD-additives, temperatures in highly loaded equipment like roller bearings or closed gears can be reduced. This can have significant benefits regarding the life time of the lubricated and temperature affected parts of the machines. Energy efficiency benefits can also be seen in many applications when using PD-additives. Photos [1-3] BP Global Lubes technology (GLT), Mönchengladbach References [1] DIN ISO 14635-1; Zahnräder - FZG-Prüfverfahren - Teil 1: FZG-Prüfverfahren A/ 8,3/ 90 zur Bestimmung der relativen Fresstragfähigkeit von Schmierölen (ISO 14635- 1: 2000); Published: 2006-05; Beuth-Verlag [2] DIN 51819-1; Prüfung von Schmierstoffen - Mechanischdynamische Prüfung auf dem Wälzlagerschmierstoff-Prüfgerät FE8-Teil 1: Allgemeine Arbeitsgrundlagen; Published: 1999-12; Beuth-Verlag [3] DIN 51819-3; Prüfung von Schmierstoffen - Mechanischdynamische Prüfung auf dem Wälzlagerschmierstoff-Prüfgerät FE8-Teil 3: Verfahren für Schmieröl, einzusetzende Prüflager, Axialzylinderrollenlager; Published: 2005-03; Beuth-Verlag [4] DIN 51502; Schmierstoffe und verwandte Stoffe; Kurzbezeichnung der Schmierstoffe und Kennzeichnung der Schmierstoffbehälter, Schmiergeräte und Schmierstellen; Published: 1990-08; Beuth-Verlag [5] DIN 51517-3; Schmierstoffe - Schmieröle - Teil 3: Schmieröle CLP, Mindestanforderungen; Published: 2014-02; Beuth-Verlag [6] FVA Informationsblatt 54/ I-IV, Testverfahren zur Untersuchung des Schmierstoffes auf due Entstehung von Grauflecken bei Zahnrädern Tribologie + Schmierungstechnik 63. Jahrgang 2/ 2016 57 Graphic 8: FZG mineral-PAO PD new and used Graphic 9: FZG mineral S-P and PAO PD T+S_2_16 05.02.16 14: 24 Seite 57