Grease is a thick, semi-solid lubricant used to reduce friction and wear between two sliding surfaces. It is composed of a base oil, a thickener and additives. By varying the oil, thickener and the additives, you can produce greases for different applications.
However, grease cannot offer lubrication effectively at higher temperatures as it tends to decompose and evaporate. For elevated temperatures, special high-temperature grease has to be used. High-temperature grease can provide lubrication effectively at elevated temperatures where other conventional greases would fail.
The primary element of high-temperature grease is base oil, which provides lubrication. The thickeners decide the consistency and help to keep the oil in place. Additives can be used to improve the performance of grease.
- Usually, synthetic oils are used in high-temperature greases as they have excellent thermal stability. The synthetic oil has low volatility and does not evaporate at high temperatures. Commonly used oils are polyalphaolefin (PAO) and ester-based oils.
- Thickeners are used to thicken the base oil and turn it into a semi-solid state. They help to improve resistance to oxidation and thermal stability. The commonly used thickeners are complex soaps like aluminium complex, lithium complex and calcium sulfonate. Non-soap thickeners like clay, polyurea and silica can also be used.
- In high-temperature greases, additives help to improve the performance at elevated temperatures. Some of the additives used are corrosion inhibitors, anti-oxidants, anti-wear agents and extreme pressure additives.
High-temperature greases are used in applications like metal working, aerospace and mechanical equipment operating at a temperature above 200 °C. It is used in the automotive industry to lubricate the parts operating at high temperatures.
Properties of high-temperature grease
- High-temperature greases have a dropping point of 260 °C, below which they do not lose their consistency. The dropping point is the point at which grease turns into a liquid state.
- At high temperatures, grease can oxidise and form a sludge and this does not allow it to function properly. Additives are used to prevent oxidation and extend the life of grease.
- If a high-temperature application also carries heavy loads, then extreme pressure additives can be added to high-temperature grease to separate the two moving surfaces and reduce wear.
- When anti-wear agents are added to high-temperature grease, you can get an extra layer of protection against friction and wear.
- The high-temperature greases have good mechanical stability and do not lose their consistency at elevated temperatures.
- High-temperature greases can also be used in high-speed applications as they have good shear stability.
- High-temperature grease is also water-resistant and prevents corrosion and improves the life of the machine.
Lubricating bearings using high-temperature grease
Bearings support the rotating or moving parts in machinery. Grease is widely used to lubricate the bearings to reduce friction and wear. As the consistency of grease is good, it stays in place during the operation.
Grease covers the bearing and provides an extra layer of protection against dirt and dust that may enter the bearing. It maintains stable lubrication, which helps to increase the life of the bearings. High-temperature grease for bearings provides effective lubrication and reduces friction and wear. Some of the high-temperature greases are Aluminium complex grease, Moly grease, Lithium complex thickened grease, Silicon grease, Sodium soaps and Synthetic grease.
Different bearings have different requirements and should be lubricated accordingly. If the bearings operate in harsh environments, then you need to lubricate them frequently. For high-speed applications, you will have to lubricate frequently.
High-temperature grease provides continuous and effective lubrication in the bearings. It can dissipate the heat generated due to friction in the bearings. It reduces friction, which minimises the heat generated in the bearings. This helps to lower the temperature at which the bearings operate.
To identify any lubrication issues, you should check for grease degradation and monitor the temperature of the bearings. You should also check for contamination and signs of wear in the bearings.
Conclusion
The main component of high-temperature grease is the base oil and it directly affects the performance. The base oil should possess a high boiling point, a high decomposition temperature and good chemical stability. Synthetic oils are less volatile, more resistant to oxidation and more stable at elevated temperatures. Therefore, synthetic oils are widely used in high-temperature greases.
Thickeners also affect the basic and film-forming properties of high-temperature greases, It is important to choose the right thickeners to enhance the properties of high-temperature greases.
Additives are also an important part of high-temperature greases as they can improve the lubricating properties. The additives chosen should be compatible with the thickener to avoid any damage.