Grease is made of three main components – base oil, thickener and additives. The base oil makes up the biggest part of the mix (normally between 80% – 95% by weight of a typical grease). Understanding the basics of grease composition helps to ensure that you choose the best grease “structure” for your specific application.
Callum Ford, National Marketing Manager at Lubrication Engineers (LE) South Africa explains that grease is used to lubricate those applications where an oil cannot be contained. “Most greases are made in huge kettles by mixing oils with various types of base materials called ‘soaps’,” he says. “Although many manufacturers still use only open, single-action kettles, more sophisticated producers now use open, double-action and closed pressure vessels. This means temperatures can be accurately controlled, which helps to produce high-quality greases that meet the required standards and specs.”
As grease is used, the oils in the greases are gradually degraded by temperature and pressure in the workpiece being lubricated. This process will continue until the oil becomes carbonised, unless fresh lubricant is applied periodically. As the oil oxidizes in service, it actually becomes a contaminant. It mixes with the collapsing and degrading base to become “used grease”, which will be squeezed out regularly as fresh grease is added.
Ford notes that although “mineral oil base” greases are predominant, synthetic oil base greases cater for many extreme applications. “These are made with synthetic fluids rather than petroleum and are specifically designed to have high and/or low temperature capabilities and longer service life,” he says.
Most bases are “soap-thickened”. Ford says different materials are used for different reasons. Simple soap options include:
• Calcium: A hydrated lime, alkaline-type material used as a soap base makes a water repellent grease of smooth, buttery texture and is excellent for use under heavy churning action and around wet or highly humid environments up to around 79°C operating temperatures.
• Aluminium: These greases, which are also known as aluminum stearates, are very water resistant and fairly rust resistant. However, their heat resistance (i.e. low dropping point) is no better than the
calcium. They also possess a low shear stability and have gel-forming tendencies. Little of this type of grease has been produced and marketed for the past 30 years.
• Sodium: These greases are fibrous and have a fairly high melting point suitable for anti-friction bearings and other high-speed centrifugal-operating conditions. Often used in heavy-duty applications, this type of grease resists shock-loading and pounding. Although sensitive to water, sodium greases have found their own niche. They can be used in dry conditions at temperatures up to 38°C.
• Lithium 12-Hydroxystearate: The latest and best of the simple soap-based greases to evolve were the lithium 12-hydroxystearates. These greases combine the heat resistance of sodium soap greases with the water resistance of calcium soap greases. They also display excellent mechanical stability and low temperature pumpability.
Complex soap-thickened greases were developed in the 1930s and 1940s to create multipurpose greases. “Ford notes that although “mineral oil base” greases are predominant, synthetic oil base greases cater for many extreme applications. “Complex soap-based greases use specific combinations of additives and oils to maximise performance,” says Ford. “They generally have good mechanical stability, low temperature pumpability and will operate at temperatures which are moderately high, with a higher dropping point than corresponding soap greases.”
Calcium complex greases were the first to be developed, followed by the lithium and barium greases.
There are two non-soap thickened greases that Ford says are significant – clay and ureas. “Clay is of very fine grain size and capable of absorbing large amounts of fluid, and has excellent plasticity. The clay is treated, making it very resistant to heat and adequately resistant to water,” says Ford. “Ureas (or polyurea) greases are made with ashless organic thickeners and have a natural resistance to oxidation. Most polyurea greases are developed for a single specific application.”
Ford says that greases shouldn’t be taken at face value, but put to the test. “Many greases appear the same on paper, but it’s important that they resist water wash-out and pound-out, extend lubrication intervals and meet the user requirements,” he says. “LE greases provide these benefits for several reasons. First, our proprietary additives minimise wear, water washout and pound out. Secondly, we use the finest base oils and most technologically advanced additive systems to produce the finest quality, high-performance greases.”