Causes of Grease Dry-Out, and how to fix it

Over time, average grease has a tendenecy to cake and dry out. This is because oil has a natural tendency to drain out of the grease thickener, and this process is only expidited when the grease is subjected to heavy loads. As grease becomes dry, frictional forces escalate, causing concentrated heat within the bearing. The elevated heat continues to force more rapid and complete drying of the grease. In time, lower quality grease can reach a state of consistency ranging from hard putty to sandstone, depending on the thickener type and degradation conditions. There are many causes for grease dry-out, and below is a list outlining the most common causes. Often times, two or more of these causes work in conert with each other to impede the grease from doing its job. Contamination. Gross contamination by dust, dirt, fly ash and similar dry-power contaminants can thicken the grease just like the introduction of a gelling agent. Incompatible Grease. Accidental mixing of grease can lead to accelerated de-gelling and oil separation. For example, organic clay grease clashes with soap-thickened grease, and a mixture of a calcium sulfonate grease and a lithium based grease will cause problems if mixed in bearings. High-temperature Volatility. The lower the viscosity of the base oils used to make the grease, the higher the risk of volatilization. At sufficiently high temperatures, oil can boil out of the thickener matrix, causing hardening of the grease over time. Base-oil Oxidation. Oxidized oil can take on the physical characteristics of roofing tar or even coal. The same transformation can occur with grease. The grease image in Figure 1 occurred at room temperature due to the high concentration of cupric catalytic metals in the grease from wear debris. The copper prematurely oxidized the base oil in the grease, causing extreme thickening. Thermal Run-away. Too much grease in a bearing, mechanical conditions (misalignment, excess preload, etc.), and starvation can lead to high running temperatures. This is most commonly associated with over-greasing, causing high heat from churning. In such cases, the dropping point is reached, the bearing begins to bleed, the grease runs dry and eventually failure occurs. Figure 2 shows an example of such a bearing. Mechanical Wring-out. Some types of rolling-element bearings are more prone to oil separation. Spherical roller bearings would be such an example. When grease is over-rolled by the bearing or gears, the oil can be rapidly separated from the thickener. This is referred to as mechanical wring-out. The oil is squeezed like a wet sponge. Some thickeners have good reversibility, while others do not. A grease is reversible when oil will quickly reabsorb into the thickener after the load is relaxed. Aluminum complex is a soap-based grease with good reversibility. Hydrostatic Extrusion. Grease under constant pressure can separate by hydrostatic forces. It’s like water flowing through a sand filter. The sand stays put, but the water flows freely under pressure through the grains of sand (sieving). Some grease products are formulated specifically to resist extrusion by using special thickeners, polar base oils, VI improvers, tackifiers and higher thickener concentrations. Some single-point auto-lubers (spring type) expose grease to constant pressure, which causes bleed and separation from the thickener. The oil is literally being squeezed out of the thickener. Vibration and Centrifugal Forces. Grease exposed to prolonged vibration and/or centrifugal forces is also known to separate prematurely. High-speed mechanical couplings can spin some grease products dry in a short period of time if the wrong grease is used. Among other things, significant differences in the specific gravity between the base oil and thickener can sharply increase centrifugal separation. Condition monitoring is key to early detection. It is always important to get feedback from rebuild shops on what they are seeing inside failed bearings. Any appearance of grease that is questionable (hardness, color, contamination, oxidation, etc.) should be sampled for analysis. Allow failure to be a teacher for continuous improvement. TOP 1 Hi-Temp Grease is always the right grease for the job! This lithium complex grease has been time-tested and time-approved, in the field and in the lab, to protect and lubricate applications like automotive wheel bearings, automotive chassis, conveyor bearings, backhoe hinge pins, and bearings subject to shock loads. TOP 1 Hi-Temp Grease is formulated with synthetic blend base oil and selected additives to attain excellent oxidation stability and protect against rust and extreme pressure loads. Conventional lithium greases can have 4-Ball wear scars of up to 0.7mm and yield drop points of 175-180°C. However, TOP 1 Hi-Temp Grease has a 4-Ball wear scar of only 0.45mm and yields an exceptional drop point of 288°C. Because of these performance capabilities, it can be assured that TOP 1 Hi-Temp Grease can withstand heavy loads with its extreme pressure additives and will not cake-out if used properly.