This article will discuss the findings from our field trials regarding synthetic grease high temperature performance and consumption.
Field Trial Parameters
A steel rolling mill in Egypt that was consuming high levels of conventional mineral lithium grease in chock bearings of steel rolling mills. To address this issue, it was recommended that the mills converted to synthetic grease. The findings suggested that converting chock bearings to a synthetic grease solution can help to reduce grease consumption, better protect bearings, and increase bearing reliability in steel mills.
This trial was arranged in two phases.
In phase 1, individual rolling mill stands were selcted and disconnected from the centralised greasing system running on conventional grease. The mill stands were then manually greased these using the synthetic complex lithium grease and the change in grease consumption and bearing protection effects were measured. Once complete, the entire mill was converted to the synthetic complex lithium grease and the centralized greasing system for phase 2, in which the overall effects and benefits of the synthetic product on grease consumption and protection were measured.
Reduced Running Temperature
Before changing the type of grease, thermographic images were captured for comparison. The below images illustrate the operating temperature outside the bearing hose when operating with conventional mineral lithium grease.
Figure 1: Vertical roughing stand bearing with conventional mineral lithium grease.
The temperature outside the bearing housing for a bearing filled with conventional grease was approximately 76.1°C. (Bear in mind that the temperature at the bearing housing surface was likely about 5°C to 8°C lower than the actual bearing temperature.)
This high operating temperature is problematic for the mill, given that when the lubricant is subject to constant operating temperatures above 70°C, grease life is dramatically reduced. Generally, relubrication intervals reduce by half for each 15°C increase in operating temperature.
Now, take a look at the image below. After converting operations to synthetic grease, new thermographic images were taken and compared the results. As you can see, the temperature outside of the same bearing housing dropped to 59.4°C, suggesting that the mill would experience a decrease in grease consumption rate when all operations convert to the synthetic product.
Figure 2: Same vertical roughing stand bearing pictured in figure 1, but with synthetic complex lithium grease.
Diminished Grease Consumption
In phase 1 of the trial, grease consumption was tracked and compared the levels to reference volumes. The assessment showed that grease consumption gradually reduced in volume. During this phase, the daily regreasing amount was reduced by 0.25 kg grease per stand. The results are illustrated in the comparison chart below:
Figure 3: Comparison of grease consumption per stand per 8-day rolling campaign.
At the same time, a starvation protocol was undertaken- greasing once every 2 days, then every 4, then every 6, and finally every 8 days- to measure the grease’s ability to withstand a similar condition of greasing system failure without being noticed by the operators. Through this experiment, operating temperatures were tracked with thermodynamic imaging and determined that temperatures remained under control, indicating proper bearing protection with minimum grease levels.
Figure 4: Finishing stand day 8 of rolling campaign (196 hours without regreasing)
Grease stability under water attack
Throughout the trial, samples of grease were collected to measure water ingress and grease stability under such conditions. The assessment revealed that the Synthetic Complex Lithium Grease resists any change in consistency, even under extreme high water contamination (up to 56 percent water).
Figure 5: Grease penetration change % from reference.
Additionally, during cleaning inspection and re-greasing, roll shop operators reported that the synthetic complex lithium grease remained inside the chock bearings and was harder to remove than the conventional mineral lithium grease.
Improved Bearing Protection
Throughout the field trial, the mill did not experience a bearing failure related to grease and physical examination of bearings at the roll show were in excellent condition, even during the extreme grease starvation regime.
Figure 6: Bearings in excellent condition.
This trial illustrates, steel mill operators can achieve dramatic reductions in grease consumption, extended regreasing intervals, and improved bearing protection by converting chock bearings in rolling mills to synthetic grease. In fact, operators may achieve up to 84 percent reduction in grease consumption and improved bearing operating temperatures of 5-15°C (9-27°F) on average, reflecting better protection of bearings and reduced rates of failure.
"Proof of Performance: Synthetic Grease for High Temperature Steel Mill" by, Mahmoud A. Abdel Shafy used with permission of the Mobil SHCTM Club (all rights reserved.)