Choice of lubricating grease

Most roller bearings are lubricated with grease. Lubricating grease is functional. It can be used in places where lubricating oil is not suitable or cannot be used. It can reduce the wear of moving parts, and play a role of sealing and shock absorption. Grease can also keep the solid lubricant in suspension. The wrong choice of lubricant accounts for about 40% of all machine failures, and most other failures are caused by lack of lubrication. In terms of bearings, approximately 80% of failures are related to lubrication or contamination.

Purpose and characteristics of grease

　　 Grease is a mixture of base oil, thickener and additives. The base oil can be mineral or synthetic. Although sensitive industries such as pharmaceuticals or food and extreme temperature applications mainly rely on expensive synthetic base oils, mineral oil is sufficient for wood pellet equipment applications. The thickener produces a semi-fluid structure, so as an oil stabilizer, it is equivalent to a sponge that holds the oil in place. Thickeners are mainly metal soaps, including lithium and aluminum. Other common thickeners are polyurea, sodium, clay and calcium. At present, most thickeners are composed of metal soaps and complexing agents. The most widely used is lithium complex thickener, which is a combination of traditional lithium soap and low molecular weight organic acid. Non-soap thickeners such as bentonite are becoming more and more popular in high temperature and special applications, but the poor base oil oxidation and water compatibility of clay thickeners have caused some limitations.

Composition of grease

　　Additive package composition and percentage usually determine the quality of grease. Additives enhance the desired performance, suppress existing poor performance, and add new characteristics. The most common additives include antioxidants and rust inhibitors, extreme pressure (EP) and anti-wear or friction reducers. The number of additives determines the service life (GSL) of the grease. GSL refers to the time until the performance of the lubricating oil disappears and needs to be replenished during use. Environmental conditions such as operating temperature, load, and speed will affect GSL. In the manufacture of wood pellets, the environmental conditions are very harsh. High load, sawdust, humidity and vibration put a lot of stress on the equipment. Grease helps the main bearings and roller bearings to withstand this harsh environment.

　　 In addition to functional properties, grease also has certain physical properties, such as viscosity, that enable it to perform its role. Viscosity defines the thickness of the oil and its resistance to flow and shear. High-viscosity oils have relatively large internal friction (van der Waals forces) molecules. The importance of choosing the right viscosity cannot be ignored. A thick initial film may not be dragged across the loading area, while a too thin film will tear quickly, resulting in metal-to-metal contact. To help withstand operating pressure, greases are consistent, which means they can resist deformation caused by applied external forces. The measure of consistency is called penetration. Penetration depends on whether the consistency is modified through processing or operation. ASTM D 217 and D 1403 are standard test methods for cone penetration of unprocessed and processed grease. To measure penetration, a cone of a given weight sinks into the grease for 5 seconds at a standard temperature of 25 degrees Celsius (77 

degrees Fahrenheit).

　　 Grease dripping point is another indicator for choosing the correct grease. It is an indicator of heat resistance-grease becomes so easy to flow at this temperature that it drips. The dropping point represents the upper temperature limit at which the grease maintains its structure. However, this is not the maximum temperature at which the grease can be used. In addition, grease is characterized by water resistance. Water resistance means to withstand the influence of water without changing the lubricating ability, mainly to prevent oxidation (corrosion) and lack of lubrication (scouring).

　　 Finally, the pumpability of grease is also very important for applications where lubrication is continuously lost (such as wood particles). Grease should be able to withstand temperature differences during storage and can be pumped through the long pipeline of the (automatic) lubrication system.

Selection of lubricating grease for wood pellet production

　　 is very simple. Sawdust pelletizing is the process of pressing the raw wood chips into cylindrical pellets. However, the pelletizing process is much more complicated. The pelletizing causes high loads and other pressures on equipment (such as roller bearings), and in some pellet mills, the main bearing will also experience lubrication loss. Low-cost, mineral oil-based lithium complex grease with extreme pressure additives is a typical product for this application. The consumption of grease varies greatly between different pellet mills, but high consumption can only be economical when the lubricant is cheap. The total lubrication cost comparison shows that special high-performance greases have obvious benefits. Lubrication performance, or total lubrication cost (TLC), considers many factors: price per pound, consumption, average equipment life, and production loss. Therefore, the price of high-quality grease is relative. Although the price per pound is a major criterion, it is actually secondary. Taking into account the possible volume savings and longer life, high-performance grease quickly became a superior choice. A lubricant that is 15 times more expensive is already a victory when the transaction volume is 20 times lower. As the service life of the bearing increases, this benefit will increase. A significant improvement is that it brings lower production losses and higher system availability. And, there is the icing on the cake to save electricity.

　　 In addition to economic benefits, high-quality grease provides superior emergency operation functions and protects equipment. The content of additives and suitability for heavy-duty applications reduce the effects of lubrication failure. Especially in the case of solid lubricants, the coating protects the sliding surface from wear, even if there is no relubrication for a period of time. This is essential to prevent equipment loss, fires and other safety-related failures. In addition to good cleaning, regular maintenance and employee awareness, good grease practices are the basis for the stable operation of high-volume pellet mills.