Aus der Praxis für die Praxis 1 Introduction One of the major designs in steel rolling mills is the chock Bearing ** type, where bearings are housed inside a special housing called a chock, typically four raw radial tapered bearings are used on the drive side and four raw tapered plus a thrust bearing on the free side. Grease is by far the most common form of bearing lubrication; in fact about 90 % of all rolling element bearings are greaselubricated [2]. Due to steel treatment requirement during rolling process, and to avoid over expansion of the steel rollers due to the extreme temperatures of steel during hot rolling (1050-1200 °C), water is used lavishly for cooling (see Figure 1), but during this process, water attacks grease 64 Tribologie + Schmierungstechnik 63. Jahrgang 3/ 2016 Synthetic Complex Lithium Grease for increased Steel Rolling Mills Productivity & Reliability M. A. Abdel Shafy, G. Delafargue* Figure 2: Diluted Grease Steel Rolling Mills typically use conventional mineral lithium greases for their mill stands’ chock bearings. In this application, large quantities of grease need to be pumped into the bearings to counter the severe water contamination and high temperatures typically experienced, with the aim to increase bearings reliability and avoid unnecessary bearing failures, and hence maintain the mill productivity. This paper is intended to describe and discuss a project conducted in a Steel Rolling Mill in Egypt, to replace conventional mineral lithium grease with advanced synthetic complex lithium grease (Mobilith SHC 460). This paper will document the results of reduction in grease consumption and improved bearings operating temperatures, reflecting better protection of the bearings and less failure rates. The overall result was decreased maintenance costs. The paper will also present lab test results, together with grease consumption calculations, bearings temperature records using thermoghraphic imaging, comparative bearings failure rates, and photos showing bearings condition. Improved Productivity, Reliability and Sustainability are hence expected to be achieved in Steel Rolling Mills by using advanced synthetic complex lithium grease. Keywords Synthetic Greases, Lithium Complex, Steel Mills, Productivity, Extended Regreasing, Grease Softening, Water Washout, Chock Bearings Abstract * Bsc. Mohamoud A. Abdel Shafy ExxonMobil Egypt (S.A.E.) 1097 Cornich El Nil, Garden City, Cairo, Egypt ZIP Code 22111 Gilles Delafargue ESSO S.A.F, France ** [chock] noun: a wedge or block of wood, metal, or the like, for filling in a space, holding an object steady, etc. chock bearing: a bearing supporting the end of a rolling mill [1] (which besides lubricating bearings, act as sealant for water and other environmental contamination ingress) inside the chock bearings. As a way to prevent rust and hence bearing failure due to water ingress, fresh grease is pumped into chock bearings (Centralized systems) to replenish the diluted grease inside with fresh grease to protect the bearings (see Figure 2), or in Figure 1: Water Attack to Chock Bearings T+S_3_16 05.04.16 09: 02 Seite 64 Aus der Praxis für die Praxis case of manual greasing, the whole rolling set has to be dismantled to perform the regreasing manually (usually in a roll shop). This of course results in increased grease consumption, waste grease, and lost productivity. 2 Synthetic Complex Lithium Grease Solution In 2012 a field trial to replace conventional mineral lithium grease with synthetic complex lithium grease (Mobilith SHC 460) was carried out in a centralized greasing system steel rolling mill in Egypt. This was done in an attempt to increase the re-greasing intervals, and resulted in decreased grease consumption, and better protection of bearings, for increased bearings reliability and increased steel mill productivity. 3 Benefit Driving Factors The two main driving factors for the decreased greasing and improved bearing protection were: water wash-out characteristics inherited from complex lithium soap, and high temperature characteristics inherited from both synthetic oil base and complex lithium soap. 3.1 Water Wash-out Characteristics Good water tolerance helps maintain consistency of grease, which can lead to grease consumption decrease, bearing life increase, and reduces corrosion related failures. Water wash-out test [ASTM D1264]: A standard test bearing is packed with 4 grams of the grease to be tested. The bearing is rotated at 600 rpm in the water spray chamber at 80 °C for one hour, 300 ml of water per minute are sprayed at the bearing assembly. The percent weight loss of the grease carried away with the water is reported. This test is a relative measure of grease’s ability to resist removal by water (see Figure 3). When compared to conventional mineral lithium grease, synthetic complex lithium grease offers 3 times better water washout performance (as shown in Figure 4) resulting in less amount of grease washed out by water, and in turn decrease of grease consumption and extension of re-greasing intervals . 3.2 High Temperature Characteristics Higher operating temperature (150 ˚C) of synthetic complex lithium grease compared to 130 ˚C of conventional mineral lithium grease, combined with the huge difference in dropping points (265 ˚C vs. 190 ˚C) also contributed to the extension of re-greasing intervals leading to reduced grease consumption (see Figure 5). 4 Field Trial Results Trial was arranged on two phases; Phase 1 was to select individual rolling mill stands and disconnect them from the centralized greasing system that is running on conventional mineral lithium grease, and start greasing them manually using the synthetic complex lithium grease to measure the change in grease consumption and bearing protection effects. Phase 2 was to document the use of synthetic complex lithium grease in the centralized greasing system of Tribologie + Schmierungstechnik 63. Jahrgang 3/ 2016 65 Figure 4: Water Wash-out Results Figure 5: High Temperature Characteristics Figure 3: Water Wash-out Test T+S_3_16 05.04.16 09: 02 Seite 65 Aus der Praxis für die Praxis the whole mill to measure the overall effects and benefits. Following are the results measured on the mill’s bearings running temperatures, grease consumption, grease stability and bearing protection. 4.1 Chock Bearings Reduced Running Temperatures Before changing the grease type, some reference thermoghraphic images were captured for further comparison purposes. Results evidenced a decrease in the running temperatures of the mill’s bearings (both high load low speed bearings in the roughing stage, as well as low load high speed bearings in the finishing stage) as shown in following Figures 6-9 (59,4 °C versus 76,1 °C and 63,3 °C versus 74,4 °C respectively) . This documents the longer grease life and better protecting oil film thickness for longer bearings life. But we should bear in mind that the practical method to measure temperature of a bearing is by reading the temperature outside of the housing, so it is important to remember that the temperature at the bearing housing surface is 10 °F to 15 °F (about 5 °C to 8 °C) lower than bearing temperature [3]. From the housing temperatures shown in the original greasing pattern, which is in the range of 75 °C (167 °F) and which is actually translated to 177-182 °F (80 °C to 83 °C) actual bearing temperature, will lead to a shortened bearing life and grease life. When the lubricant is exposed to constant operating temperatures above 70 °C, the calculated values for re-lubricating intervals are halved for each 15 °C (27 °F) increase in operating temperature [4], this effect was shown on the actual consumptions as detailed in the following subsection 4.2. 4.2 Reduced Chock Bearings Grease Consumption Actual consumption of conventional mineral lithium grease per stand (4 chock bearings) was 8.7 kg grease per day, i. e. 69.6 Kg each 8 days , plus the initial filling of all 4 chock bearings in the roll shop with 10 kg grease once each 8 days (summing up to 79.6 kg per stand for each rolling campaign). In Phase 1 of the trial (where synthetic complex lithium grease was applied manually) and based on reference greasing volumes, grease consumption was gradually reduced in volume. At the same time a starvation protocol was followed of greasing once every 2 days, then 4, then 6 and finally every 8 days (once every full rolling campaign), this was intended to measure grease’s ability to withstand a similar condition of greasing system failure without being noticed by the operators. 66 Tribologie + Schmierungstechnik 63. Jahrgang 3/ 2016 Figure 6: Vertical Roughing Stand Bearing with conventional mineral lithium grease Figure 7: Same Vertical Roughing Stand Bearing in Figure 6 but with s ynthetic complex lithium grease Figure 8: Horizontal Finishing Stand Bearing with conventional mineral lithium grease Figure 9: Same Horizontal Finishing Stand Bearing in Figure 8 but with synthetic complex lithium grease T+S_3_16 05.04.16 09: 02 Seite 66 Aus der Praxis für die Praxis During this phase, daily regreasing amount was reduced to 0.25 kg grease per stand (summing up to 12 kg per stand for each rolling campaign; 8 x 0,25 Kg for the 4 chock bearings + 10 kg initial filling). Comparison between grease consumption of both types is illustrated in (Figure 10) below. Also during this phase, systematic grease starvation regime was followed in the first finishing stage stands for its lower temperature than the final stage stands in the examples discussed before, to have more safety margins during the starvation phase, this regime was done by greasing the bearing subject to the trial every 48 hours (completely no grease every other day) then extended to greasing once every 96 hours, then once every 144 hours, and finally no regreasing was done for during the full rolling campaign (8 days = 192 hours). During this starvation regime, temperatures were kept under control indicating proper bearing protection with minimum grease amount (see Figures 11-14). 4.3 Grease Stability under Water Attack During the trial, grease samples were collected to measure water ingress and grease stability under such conditions, the Synthetic Complex Lithium Grease has shown almost no change in consistency even under extreme high water contamination (up to 56 % water) as shown in (Figure 15). Tribologie + Schmierungstechnik 63. Jahrgang 3/ 2016 67 Figure 11: Finishing Stand at Day 0 of Rolling Campaign Figure 12: Finishing Stand at Day 2 of Rolling Campaign (48 hours with no regreasing) Figure 13: Finishing Stand at Day 6 of Rolling Campaign (144 hours with no regreasing) Figure 14: Finishing Stand at Day 8 of Rolling Campaign (196 hours with no regreasing) Figure 10: Comparison of Grease Consumption per Stand per 8-day Rolling Campaign Figure 15: Grease Penetration Change % from Reference T+S_3_16 05.04.16 09: 02 Seite 67 Aus der Praxis für die Praxis Meanwhile, roll shop operators reported that, at cleaning inspection and re-greasing after each rolling campaign, the synthetic complex lithium grease remaining inside the chock bearings was harder to remove than the conventional mineral lithium grease (see Figure 16 for physical appearance of synthetic complex lithium grease and compare to diluted conventional mineral lithium grease in Figure 2). 4.4 Bearing Protection During the whole trial Phase 1 described above, and Phase 2 (when synthetic complex lithium grease replaced conventional mineral lithium grease in the steel mill centralized greasing system) customer had reported no bearing failure related to grease, and physical examination of bearings at the roll shop showed bearings are in excellent condition even during the extreme grease starvation regime described in subsection 4.2 earlier. See (Figure 17). 5 Conclusion Replacing conventional mineral lithium grease with synthetic complex lithium grease in steel rolling mills chock bearings, may enable steel mills achieve grease consumption reduction by up to 84 %, and improved bearings operating temperatures by 5-15 °C (9-27 °F) on average, reflecting better protection of the bearings and less failure rates. In addition, reduced bearings running temperatures, means potentially less energy consumption, yet more investigation needs to be done on quantifying this effect. Furthermore, significant potential for reduced waste grease by more than 80 % adds to the improved green foot print of steel mills. In conclusion, using advanced synthetic complex lithium grease will result to decreased maintenance costs, Improved Productivity, Reliability and Sustainability in Steel Rolling Mills. References [1] Dictionary.com: Definition of chock, Online Dictionary [2] ROLAND GRUBER: Tech Report Maximizing Bearing Service Life, Industrial Lubrication, Published: Winter 2008, Page 7 [3] NSK Americas: Roller Bearing Life in High Temperatures, TechTalk 2, Published: 2010, Page 2 [4] ROLAND GRUBER: Tech Report Maximizing Bearing Service Life, Industrial Lubrication, Published: Winter 2008, Page 8 68 Tribologie + Schmierungstechnik 63. Jahrgang 3/ 2016 Figure 16: Synthetic Complex Lithium Grease Physical Appearance at Roll Shop Figure 17: Bearings at Excellent Condition Umzug oder Adressenänderung? Bitte T+S nicht vergessen! Wenn Sie umziehen oder Ihre Adresse sich aus sonstigen Gründen ändert, benachrichtigen Sie bitte auch den expert verlag. expert@expertverlag.de | Tel: (07159) 9265-0 | Fax (07159) 9265-20 T+S erreicht Sie dann ohne Verzögerung und ohne unnötigen Aufwand. Danke, dass Sie daran denken. T+S_3_16 05.04.16 09: 02 Seite 68