What Exactly is Grease?
A lubricating grease usually consists of three components which, in carefully coordinated amounts and through a controlled process, give us the final product with the desired properties.
ASTM - American Society for Testing and Materials - has defined lubricating grease as "a solid to semi-liquid product consisting of a thickener in a liquid lubricant, and which may also contain other ingredients with special properties."
Another and simpler way to describe grease is to compare it to a sponge . The thickener is the sponge, and the base oil is the water. The thickener in the grease has the function of creating a matrix that holds the base oil in place in a solid structure and gives the grease its consistency. When the grease is exposed to differences in operating conditions such as temperature, load or shear force, a viscoelastic flow is created in the grease. The hardness or consistency of the grease is usually indicated by a number from the NLGI system. Semi-liquid to (00) to solid (2) is most common.
Lubricating grease is used above all in ball and roller bearings, but also in sliding bearings, gearboxes and open gear transmissions. Today, lubricating grease is also increasingly used in forestry machines, for central lubrication of the bar and chain.
A grease stays in place
A lubricating grease should reduce friction in places where a lubricating oil is worn away. It must also block and seal - for example, keep water out and seal against gas. It simply has to be in place.
A lubricating grease must:
- Provide good lubrication, reduce friction and wear
- Seal, so that dirt, water and contaminants cannot penetrate at the lubrication point
- Protect against corrosion
- Stay at the point of application and do not leak, drip or be ejected
- Be compatible with sealing materials and other component materials with which it comes into contact
- Do not change and harden or weaken by repeated mechanical processing in the bearing during the lubrication interval
Base oils in lubricating grease
Since a lubricating grease consists of 80-90% base oil, the base oil is an important part of the final product. The choice of base oil has a lot to say about the grease's performance. Mineral oils are the type of base oil most commonly used in the formulation of lubricating greases. But by using synthetic base oils, you can get properties that are impossible to achieve with mineral oils.
Synthetic base oils are the best choice when a product with a larger temperature range and better chemical resistance is needed. Synthetic base oils also have better electrical properties.
The use of lubricating greases based on synthetic base oils is increasing, although they are often slightly higher in price. The higher price is offset by longer lubrication intervals, better performance and reduced maintenance costs.
Examples of common synthetic base oils are polyalphaolefin (PAO) and esters. Certain types of synthetic esters are often used in the formulation of biodegradable lubricating greases.
Some advantages of synthetic base oils:
- Low volatility
- Better thermal and oxidative stability
- Higher viscosity index
- Better flowability at low temperature
- Lower internal friction
- Better corrosion protection
- Contains no aromatics
The thickener – the body of the grease
The nature of the fat is largely determined by the type of thickener. The thickener is responsible for the structure that keeps the oil in place in the fat. Properties such as mechanical stability, water resistance, melting point, sealing ability and lubrication interval can all be attributed to the thickener.
Fats are often produced by saponifying fatty acids with metal hydroxides. But there are also soap-free thickeners, including thickeners based on the NOVA technology. Thickeners can generally be divided into two main groups: soap-based and non-soap-based, with the soap-based being the most widespread. The soap-based ones can in turn be divided into different subgroups, depending on their chemical composition.