Guide to Selecting the Right Grease for Ball and Roller Bearings

Grease for Ball and Roller Bearings can be standard or a specialized lubricant formulated to ensure the reliable operation and longevity of ball and roller bearings. Lubrication plays a critical role in reducing friction, protecting against wear and corrosion, and maintaining the efficiency of various machinery and equipment across multiple industries. In this article, we will detail considerations for selecting the right grease, applications, various types, and other factors that influence performance and compatibility with bearings. 

Types of Greases for Ball and Roller Bearings

The selection of grease for ball and roller bearings is not a one-size-fits-all decision. It requires an understanding of the specific application. Greases having unique properties can be applied so as to maximize bearing life and performance. Below are some examples of grease types. 

Lithium Grease

Lithium grease is the most commonly used lubricant for ball and roller bearings due to its excellent versatility. It has a good resistance to high temperatures and remains stable under varying conditions, making it suitable for a wide range of applications, from automotive to industrial machinery. Its water resistance also makes it ideal for use in environments where moisture is present.

Calcium Sulfonate Grease

Known for its superior corrosion protection and high load-bearing capacity, calcium sulfonate grease excels in marine applications and heavy-duty industrial settings. This grease type stands out for its ability to perform under extreme pressures and in the presence of water, resisting washout and maintaining its protective qualities.

Polyurea Grease

Polyurea grease offers exceptional high-temperature stability and oxidation resistance, which is critical for high-speed bearings in electric motors and automotive applications. Its long service life and minimal bleeding make it a preferred choice for sealed-for-life bearings, where re-lubrication is not feasible.

Synthetic Grease

Formulated with synthetic oils, this type of grease is tailored for extreme temperature conditions, both high and low. Synthetic grease is particularly beneficial in aerospace, cold storage, and high-temperature industrial applications due to its broad operating temperature range. Its enhanced lubricity and chemical stability ensure reliable performance in the most demanding conditions.

Selecting right Grease for Bearings

Selecting the right grease for a bearing requires addressing several factors to ensure peak performance and longer life.

Specify a lubricator that provides the right quantity of grease all the time. Automatic lubrication  can cycle or sequence to avoid over-lubrication. Too much lubrication can  occur easily when lubricating manually and can lead to bearing failure. Schaeffler has the perfect solution and can be purchased directly from Pacific International Bearing Sales Inc. Contact us at [email protected]

Understanding Grease Consistency and Its Impact

The consistency of grease, essentially its thickness, is quantified using the National Lubricating Grease Institute (NLGI) grading system. This system ranges from 000 (highly fluid) to 6 (extremely stiff). For bearings, the selection often centers around three grades:

The Role of Oil Viscosity

Base oil viscosity is the backbone of grease’s lubricating performance, determining the thickness of the lubricating film under operational conditions. It’s pivotal to select a grease whose base oil viscosity is appropriate for the bearing’s operational speed and load conditions. Adjustments may be necessary:

• For viscosities below 20 mm²/s at 40°C (104°F), consider a slight increase to ensure a robust lubrication film.
• In the 20 to 250 mm²/s range, standard recommendations generally suffice without alterations.
• Above 250 mm²/s, specialized advice may be warranted to tailor grease selection to specific application demands.

Extreme Pressure additives are essential for bearings operating under high load or shock conditions. These additives form a protective layer on the bearing surfaces, preventing metal-to-metal contact and the resultant wear. However, their use must be balanced against the potential for adverse reactions with bearing materials, particularly at elevated temperatures.

Requirements

Beyond the basics, several application-specific considerations may influence grease selection:

Wide Temperature Operation – bearings functioning across a broad temperature range benefit from greases with a mineral base and lithium complex, ensuring consistent performance from sub-zero to elevated temperatures.

High-Speed and High-Temperature Conditions – synthetic base oils are preferred for stability at extreme temperatures and resistance to breakdown, crucial for bearings in demanding applications.

Severe Loads and Vibratory Conditions – greases fortified with EP additives and robust thickeners, like lithium soap, offer enhanced protection against wear and deformation under heavy loads or in vibrating machinery.

Optimal Grease Application Techniques – the right amount of grease is paramount.

Over-greasing can lead to excessive heat buildup due to the churning of grease, while under-greasing leaves bearings vulnerable to premature wear and failure. A general rule of thumb is to fill the bearing’s free volume (the space between the balls and races) to about one-third full for optimal performance. For sealed bearings, manufacturers often pre-fill the correct amount, eliminating the guesswork.

To calculate the optimal amount of grease required for a bearing, the following formula can be applied:

Grease Quantity (g)=Bearing Outside Diameter (mm) + Bearing Width (mm) / 2×Bearing Factor

The bearing factor is determined by the bearing type and its size, which can usually be found in the manufacturer’s catalog or technical guidelines.

The re-lubrication frequency depends on the bearing size, speed, and operating conditions. A common method is to use calculation tools or charts provided by bearing and grease manufacturers, which consider these factors to recommend intervals. Always monitor the bearing’s condition and environmental influences, as these may necessitate adjustments to the re-lubrication schedule.

After the initial grease fill, the re-lubrication intervals depend on various factors, including the bearing size, type, speed, and operating conditions. While specific formulas can vary based on bearing design and application, a typical formula to estimate the re-lubrication interval might look like this:

​

Where:

• f = A factor based on the type of bearing and grease used (consult manufacturer’s guidelines)
• B = A base number from bearing specifications or empirical data
• n  = Rotational speed in rpm
• dm = Mean diameter of the bearing in mm, which is the average of the bearing’s inside (d) and outside (D) diameters ( dm=0.5× (d+D)

Proper grease application involves more than just adding grease. Follow these steps for effective lubrication:

• Ensure the grease gun and area around the bearing are clean to avoid introducing contaminants that can damage the bearing.
• Check that the new grease is compatible with any residual grease in the bearing or housing to prevent degradation of lubricating properties.
• Use a grease gun for precise application, aiming the grease directly into the bearing or onto the grease nipple, if present. For manual application, wear gloves and apply the grease evenly around the bearing elements.
• Slowly rotate the bearing as you apply grease to ensure even distribution within the bearing. This helps in achieving a uniform coating over all rolling elements and races.

Grease Lifespan and Performance

The foundation of maximizing grease lifespan begins with proper storage. Grease should be stored in a cool, dry place away from direct sunlight and extreme temperatures, which can degrade its quality. Containers should be sealed tightly to prevent contamination and oxidation. Ideally, store grease in its original packaging until it’s ready to be used, and always use older stocks first (FIFO – First In, First Out principle).

Monitoring the condition of the grease can help identify contamination, separation, or degradation before they impact bearing performance. Techniques such as visual inspections, sampling, and analysis can provide valuable insights into the grease’s condition and the need for replacement.

Grease lifespan is closely linked to operating temperatures. High temperatures can accelerate the degradation and evaporation of the base oil, reducing the grease’s lubricating properties. Ensuring that machinery operates within recommended temperature ranges can help preserve grease life. In cases where high temperatures are unavoidable, selecting a grease with a high dropping point and good thermal stability is crucial.

Implementing a lubrication analysis program can significantly extend grease life and enhance bearing performance. Regular analysis of grease samples can detect changes in viscosity, contamination levels, and wear particles, providing early warning signs of potential issues. This proactive approach allows for timely interventions, such as cleaning or re-lubrication, to maintain optimal lubrication conditions.

Conclusion

In conclusion, selecting the right grease for ball and roller bearings is a critical maintenance decision that demands careful consideration of bearing type, operating conditions, and the specific features of various grease formulations. By understanding and applying the principles outlined in this guide, you can maximize the life  and performance of your bearings, ensuring smooth operation and reduced downtime.

Consistent maintenance and regular monitoring are a key to the longevity of your lubrication system. Should you require specialized advice, assistance in selecting the perfect grease for your needs, or need to replenish your maintenance supplies, PIB Sales is here to help.

Visit the PIB Sales online shop for a comprehensive selection of top-quality ball and roller bearings or contact us for expert guidance tailored to your unique requirements.