To allow the smooth operation of two parts moving relative to each other lubrication is required. Grease is a semi-solid lubricant and can be used to prevent wear, noise and heat from being generated from the moving parts. Grease is commonly used for gears, bearings, bushing, wire ropes and chain drives.
Grease provides better sealing and load-carrying properties, resistance to dirt and dust and can work under extreme temperatures. However, grease has poor cooling properties. Because grease remains in a place and does not circulate through the machinery. Therefore for a particular application, you will need a lubricant that works well under the given set of conditions.
Grease is used to lubricate most of the roller bearings. The dm-N factor is a metric used to choose a bearing lubricant that will perform well in given conditions. If the value of dm-N is calculated by multiplying the bearing speed with the average of the outer diameter and bore diameter. If the value of dm-N exceeds the critical viscosity then grease should not be used to lubricate.
Types of grease
Grease is made by mixing base oil, a thickener and additives. The base oil provides lubricating properties and forms 80% – 90% of grease, thickener forms 10% -15 % and additives form 0% – 5% of grease. There are different types of grease and you should select one based on the application. The different types of grease are as follows:
When the application requires a mild load and a temperature under 121° C then general-purpose grease like lithium-based, calcium-based or sodium-based can be used. These greases are sensitive to high temperatures and can soften as their structure collapses. Sodium grease is soluble in water and readily washes out and cannot compete with water-resistant greases.
When the operation conditions require very low or very high temperatures, high loads, shock loading and very low and high speed then a high-performance grease is required. Such greases are lithium or aluminium complexes, calcium sulfonates, polyureas and clay-based greases. They can work under a temperature range of -48° C to 232° C and they have good all-round characteristics.
Lithium complexes have superior thermal stability and can be used as a multipurpose grease. Aluminium complexes have good water and chemical resistance but have a shorter life in high-speed and high-temperature conditions.
Polyurea greases have an outstanding resistance to oxidation because they do not contain any metal soap. Polyurea complex greases provide extreme pressure and wear protection that improve its multipurpose properties.
Organoclay grease has good water resistance but requires additives to prevent oxidation and rust. High-temperature grease is used in applications that require high temperatures and uses sulfonate complex thickeners.
Characteristics of grease
- One of the most important features of grease is its consistency. A grease that is too thick may not fit into areas and a grease that is too fluid may leak out. The consistency of grease depends on the viscosity of the base oil and the amount of thickener used. The measure of the consistency of grease is called ‘Penetration’.
- As the machine starts grease has a resistance to motion which implies high viscosity. But as grease is sheared between the surfaces its resistance to flow reduces. As the rate of shear increases its viscosity decreases.
- Bleeding is a condition where the lubricant separates from the thickener during high temperatures. For example, in the centralised lubrication system when grease is pumped through a pipe, it may encounter resistance to flow and form a plug. The oil will continue to flow and separate from grease and the plugging may get worse.
- The dropping point indicates the heat resistance of grease. At this temperature, grease becomes fluid enough to drip. It indicates the upper-temperature limit at which grease can retain its structure.
- Grease gathers the dirt and dust on the outer surface to protect the machinery from wear. If these contaminations become excessive then they may work out to the surfaces. This will lead to grease containing the abrasive material and cause wear.
- The corrosion and rust resistance is the ability of grease to protect the machinery from chemical attack. These properties can be enhanced by using corrosion and rust inhibitors.
- The mineral oil in grease evaporates at a temperature above 177° C and the grease hardens due to an increased concentration of the thickener. Higher evaporation rates may require frequent relubrication.
- The consistency of grease changes as it is worked or sheared between the moving surfaces. A grease that softens when worked up on is called thixotropic and a grease that hardens when worked up on is called rheopectic.