Bearings are crucial components in many machines and devices, enabling smooth and efficient rotational movements. Proper lubrication of bearings is essential for their longevity and performance. However, choosing the right grease can be challenging due to the variety of options available
The goal of bearing lubrication should be to cover the rolling and sliding contact surfaces with a thin layer of grease or oil, sufficient to avoid direct contact between metal surfaces. If done according to the guidelines below, lubrication will be effective, and friction, wear, and heat generation will be reduced. Additionally, proper bearing lubrication will prevent rust and corrosion on its surface and contamination. Ultimately, the bearing's lifespan will be extended
In this article, we will present the best parameters for bearing greases and the criteria to consider when choosing the right bearing grease
Role of Grease in Bearings
Bearing lubrication performs several key functions, including friction reduction, as grease forms a thin layer between moving parts, reducing friction and wear
Another key function of grease is corrosion protection. Grease protects metal surfaces from moisture and chemicals
Heat dissipation is an undervalued role of grease, which helps disperse heat generated from friction and pressure within the bearing
An invisible yet significant role of grease is contamination prevention. Grease creates a protective barrier that shields the bearing from dust, dirt, and other contaminants
All the roles mentioned above prolong the so called fatigue life of the bearing
Types of Bearing Greases
Several types of greases can be used for bearings, including mineral oil based, synthetic oil based, and solid greases. Each has its specific properties and applications
Mineral oil based greases:
Mineral oils are one of the most commonly used types of bearing greases. They are derived from petroleum and have a wide range of applications. These greases are relatively inexpensive and available, and their lubricating properties are sufficient for most standard applications
Advantages: Affordable / Good availability / Sufficient performance for standard applications
Disadvantages: Less resistant to extreme temperatures / Shorter lifespan compared to synthetic greases
Synthetic Oil Based Greases
Synthetic oils are chemically produced, allowing precise control over their properties. These greases are usually more resistant to extreme temperatures and have better lubricating properties compared to mineral oils. The most commonly used in this group are:
Silicone Greases
Have a wide temperature range, resistant to viscosity changes with temperature. They also have good water resistance and can be safely used with most plastics. Not suitable for high loads and speeds
Perfluorinated Greases (PFPE)
Non flammable, oxygen compatible, and highly resistant to many chemicals. Do not react with plastics and elastomers. Many have low vapor pressure and are suitable for vacuum or cleanroom applications, with some capable of withstanding temperatures exceeding 300°C.
Dry Greases
Used where standard greases may cause contamination, e.g., in vacuum environments. The most popular include molybdenum disulfide or tungsten disulfide. Dry greases can be polished or sprayed onto balls and races to ensure smooth operation and higher rotational speeds than unlubricated bearings
Polymer Greases
Consist of synthetic polymer impregnated with lubricating oil, filling most of the bearing's internal space. This type of lubricant is used in sealed bearings operating in dusty environments or where grease leakage cannot be tolerated, e.g., in clean environments and vertical shaft applications. Polymer greases often act as solid greases and are characterized by excellent water resistance, withstand regular washing, and tolerate high vibrations and centrifugal force
Disadvantages: Higher cost / Less availability compared to mineral greases
Solid Greases
Solid greases, such as graphite and molybdenum disulfide, are used in situations where oils are impractical, for example, at high temperatures or in vacuum conditions. They form a durable protective layer on metal surfaces
Advantages: Excellent resistance to high temperatures / No need for relubrication
Disadvantages: Difficulty in application / Limited use in standard conditions
Criteria for Choosing the Best Bearing Grease
Choosing the right grease depends on many factors, including operating conditions, bearing type, and specific application requirements. Here are some key criteria to consider:
Operating Temperature
The operating temperature range is one of the most important factors when choosing grease. Mineral greases may be less effective in extreme temperatures, where synthetic or solid greases may be a better choice
Load
The load that the bearing must withstand also affects the choice of grease. For bearings operating under heavy loads, greases with higher viscosity and better lubricating properties, such as synthetic oils, are more suitable
Rotational Speed
High rotational speeds may require low viscosity greases that minimize friction and heat. Conversely, low speeds may require higher viscosity greases
Working Environment
The working environment, such as the presence of water, dust, or chemicals, also affects the choice of grease. Corrosion and contamination resistant greases are essential in harsh conditions
Lubrication Frequency
Some greases require more frequent application, while others can work for longer periods without re lubrication. In hard to reach or safety critical applications, longer grease life is especially important
Examples of the Best Bearing Greases
In our division of greases, we did not include other characteristics of greases that are also important for proper bearing grease selection. I will not list the names of good greases from various manufacturers, but if I had to choose one well known grease that I rate the highest considering the price to quality ratio, I would choose Molykote 3451. Advantages of the Bearing Grease I Chose: Excellent corrosion protection, high lubrication performance, good water resistance, excellent high temperature resistance, long lifespan, better lubricating properties compared to mineral greases, no need for frequent lubrication, and excellent contamination protection
Summary
Choosing the right bearing grease is crucial for ensuring their longevity and performance. Depending on operating conditions, bearing type, and specific application requirements, different types of greases may be more suitable. In the food industry, widely used greases must meet specific conditions for this type of production
Greases approved for food contact are required in the food and beverage industry and must meet stringent hygiene regulations. For bearings where accidental food contact may occur, H1 approved greases are required, and where no contact occurs, H2 approved greases are used
Mineral oil based greases are affordable and sufficient for most standard applications, while synthetic greases offer better performance in extreme temperatures and heavy loads. Solid greases, although harder to apply, are indispensable in extreme conditions
Improper lubrication or incorrect grease selection are the most common reasons bearings do not operate optimally. The term "inappropriate" should be understood broadly as, for example, incompatibility of the grease base with surfaces and other fluids present in the system (e.g., another grease), or insufficient viscosity. It is also important to remember the amount of grease needed, as there is an optimal amount for which friction values become minimal. Indeed, too much grease can lead to excessive heating during operation, while insufficient or excessive amounts of lubricant can cause higher friction values. Contamination of the grease can also adversely affect bearing performance
In considerations about what is the best bearing grease, we did not discuss issues related to grease viscosity. The general rule in this regard is that low viscosity oils and greases are used where low rolling resistance is required, such as in sensitive instruments, high speed bearings. Despite the fact that greases often provide much greater resistance than oils, many modern low torque greases can produce torque values similar to some oils, especially when using a small amount of grease
