One of the main disadvantages of using grease on oil is that the grease tends to clump and dry out. We all know that over time, oil will naturally drain from the grease thickener. We saw clear evidence when the oil formed puddles in the uneven grease bag in the new grease bucket. We also saw oil dripping from stored grease boxes and pre-packaged bearings on shelves in the storage room.
What's more serious is that over time, hardened oil drainage by-products will be produced in the cavities of the running bearings and gears. This usually takes several years to develop, and in other cases, this situation may reach catastrophic levels in just a few weeks. Failure can spread itself. As the grease dries, friction increases, causing heat to concentrate in the bearing.
The increased heat continues to force the grease to dry more quickly and completely. Over time, the grease can reach the consistency state from hard putty to sandstone, depending on the type of thickener and degradation conditions.
Often, the most serious violations that cause the grease to dry out are aided by other reasons that work synergistically. Let us first review the main reasons for the drying of grease. Once the cause is determined, we will have the necessary information to prescribe the steps to properly lubricate the bearing.
Pollution. Severe pollution caused by dust, dirt, fly ash, and similar dry powder contaminants can thicken the grease, just like the introduction of a gelling agent. A large part of it is pollution caused by maintenance. This is when dust and particles come into contact with the grease when the grease is distributed in the bearing. In order to reduce this situation, please consider using a single-point lubricator. This keeps the grease in a closed container until it is dispensed, thereby preventing the entry of particles.
Incompatible grease. Accidental mixing of grease can lead to accelerated degumming and oil separation. There are many classic examples, including the collision of organoclay grease and soap thickening grease.
High temperature volatility. Greases made from low-viscosity base oils have the highest risk of volatilization. At a sufficiently high temperature, the oil will boil out of the thickener matrix, causing the grease to harden over time (lower oil to thickener ratio and higher base oil viscosity).
Figure 1. This grease image shows the extreme thickening caused by the premature oxidation of the base oil in the grease by copper. The high concentration of copper catalytic metal in grease comes from wear debris.
The base oil is oxidized. Oxidized oil has the physical properties of roof tar and even coal. The same changes occur in grease. The grease image in Figure 1 appears at room temperature because the high concentration of copper catalytic metal in the grease comes from wear debris. Copper prematurely oxidizes the base oil in the grease, resulting in extreme thickening.
The heat is out of control. Excessive grease in the bearing, mechanical conditions (misalignment, excessive preload, etc.) and starvation can cause excessive operating temperatures. This is usually related to excessive lubrication, leading to high heat generation in the agitation. In this case, when the dropping point is reached, the bearing begins to leak, the grease dries up, and eventually malfunctions. Figure 2 shows an example of this type of bearing. Equipment such as UE Systems OnTrak SmartLube can help you avoid this problem by lubricating the bearings with the right amount of grease at the right time. Ultrasonic technology continuously measures the friction in the bearing-when grease is needed, the operator only needs to receive a change on the cloud-based system and can choose to lubricate the bearing with the push of a button.
Figure 2. This bearing shows the result of high operating temperatures usually associated with overlubrication. When the dropping point is reached, the bearing begins to deflate, the grease dries out and eventually malfunctions.
Wring dry mechanically. Certain types of rolling bearings are more prone to oil separation. Spherical roller bearings are such an example. When grease is excessively rolled by bearings or gears, the oil can be quickly separated from the thickener.
This is called mechanical wringing. The oil is squeezed like a wet sponge. Some thickeners have good reversibility, while others do not. When the oil is quickly re-absorbed into the thickener after the load is relaxed, the grease is reversible. Aluminum complex is a soap-based grease with good reversibility.
Squeeze in still water. Grease under constant pressure can be hydrostatically separated. It's like water flowing through a sand filter. The sand stays still, but the water flows freely through the sand grains under pressure (screening).
Some grease products are specially formulated to resist extrusion through the use of special thickeners, polar base oils, VI improvers, thickeners, and higher concentrations of thickeners. Some single-point automatic lubricators (spring type) expose the grease to a constant pressure, which can cause seepage and separation from the thickener. The oil is actually squeezed out from the thickener. However, UE Systems OnTrak SmartLube aims to avoid these problems by distributing grease only when bearing friction occurs.
Vibration and centrifugal force. Grease exposed to prolonged vibration and/or centrifugal force is also known to separate prematurely. If the wrong grease is used, the high-speed mechanical coupling can spin dry some grease products in a short time. Among them, the significant difference in specific gravity between base oil and thickener can significantly increase centrifugal separation.
The cake lock failure is like a micro deadlock. As the thickener dries, it becomes immobile and blocks the flow path or even mechanical movement. The oil may flow through the thickener (static squeeze), but the thickener has nowhere to go. As more grease enters and grease comes out, the more clogging becomes. Eventually, the rigid bonding conditions developed from a hard high-density cake.
The pie lock state will cause the aforementioned thermal offset and bearing failure. Bearings lubricated with a centralized or single-point automatic lubricator may be the most susceptible to caking and locking.
When soft grease is injected into the bearing, but only the oil flows out from the exhaust port, it indicates that there is a cake lock failure. The thickener is stuck in the bearing cavity. In centralized lubrication systems, filter cake clogging may also occur in the pipeline between the pump and the dose injector.
To prevent caking, choose a suitable low-risk grease formulation and ensure that the grease flow rate is high enough to avoid static conditions that cause separation and clogging. Some long-fiber grease thickeners (for example, simple sodium soap) are particularly prone to grease flow stability problems.
Standardized testing of fluidity and pumpability facilitates the screening of greases with a high risk of caking. Frequent movement of grease can also keep the flow path fresh and prevent clogging. However, there may still be some unavoidable dry grease areas in adjacent side cavities.
As mentioned earlier, in full-loss applications (one pass), the quality of the lubricant flowing from the bearing is vigilantly monitored. It may be helpful to sample grease from the collection pan for consistency, oil content analysis, oxidation and contamination.
Use a laboratory with experience in analyzing used grease samples because the technology is very different from the standard ASTM grease performance test method. Now it is also possible to use tools to extract grease samples directly from the running bearing cavity.
In addition, facts have proven that investing in technology to monitor the health of bearings is of great value to factories all over the world. Condition monitoring systems can not only help prevent grease from drying out, but it can also save you time and money by reducing unplanned downtime.
Finally, always get feedback from the refurbishment shop about what they see inside the faulty bearing. Any problematic grease appearance (hardness, color, contamination, oxidation, etc.) should be sampled for analysis. Allow failure to be a teacher of continuous improvement.
Read more about grease monitoring and maintenance:
Why check bearing grease emissions
Use grinding gauges for online grease analysis
Why shouldn't grease cleanliness be ignored
Jim Fitch is the founder and CEO of Noria Corporation and has extensive experience in lubrication, oil analysis, and mechanical failure investigation. He has persuaded...