Principles for Choosing and Injecting of Bearing Grease

一、When selecting grease for ball bearings, the primary factors are the working environment, operating temperature, and load conditions.

1. Working Environment

This refers to the minimum and maximum temperatures, air humidity, presence of corrosive gases, or excessive dust in the environment.

• For outdoor use in cold regions, lithium-based low-temperature grease should be chosen,
• In environments with high humidity or water presence, calcium-based grease is appropriate,
• In dry environments with low moisture, sodium-based grease, which is water-soluble, is ideal.

2. Operating Temperature

The first consideration is the minimum temperature of the working environment, and the second is the maximum temperature that may occur during operation.

When working at high temperatures, select high-temperature grease with a higher dropping point. The actual maximum operating temperature should be 10-20°C lower than the grease’s dropping point (for synthetic greases, it should be 20-30°C lower).

3. Load Conditions

For heavy loads ball bearings, choose grease with a lower penetration value. When operating under high pressure, in addition to requiring a low penetration value, the grease should also have a high oil film strength and good extrusion properties.

Methods of Grease Injection for Bearings

There are two types of grease injection tools for rolling bearings: manual grease injectors and pressure grease injectors, commonly known as grease guns. Larger manufacturing, usage, and repair units may use specialized grease injectors with metering devices.

二、Grease Injection Quantity for Bearings

1. Impact of Grease Quantity on Ball Bearing Performance

• For sealed ball bearings, an appropriate amount of grease is already filled during production. 
• For open ball bearings, although sealing devices are installed to prevent grease leakage, a small amount will still gradually decrease due to slight leakage, requiring regular supplementation.
• It has been proven that the amount of grease injected into a bearing significantly affects its operation. Too little grease can result in insufficient lubrication, leading to dry friction, generating high heat, and causing rapid bearing failure, which could severely affect the entire equipment. Excess grease increases rolling resistance, raises the grease temperature, and when it reaches its dropping point, it becomes liquid and leaks out, leading to the same consequences as insufficient grease. Additionally, leaking grease causes waste and environmental pollution.

2. Principle for Grease Injection Quantity

The principle is to use the least amount of grease possible while ensuring adequate lubrication.

Based on practical experience, the following principles are more appropriate:

• For open bearings, the appropriate grease amount should be calculated based on the bearing chamber volume (the internal space between the two bearing covers and the bearing, excluding the bearing balls) and bearing speed (for AC motors, pole numbers can replace speed).
• For bearings with an oil deflector (also known as a grease deflector), increase the initial grease amount slightly, and avoid injecting grease into the bearing cover cavity (this area collects waste grease and does not contribute to lubrication). If grease is not replenished regularly as required, the lubrication effect will decrease, leading to bearing damage due to overheated and dried grease.

3. Precautions When Injecting Grease

The area should be clean, and all tools should be thoroughly cleaned with gasoline. After injecting grease, other parts should be quickly assembled to avoid dust and debris, especially sand and metal shavings, entering the bearing along with the grease.

4. Grease Injection During Operation

For machinery using open bearings, grease should be added to the bearing chamber after a certain period of use. For machines with grease injection systems, this can be done without disassembly; otherwise, the bearing cover should be removed to inject grease directly into the chamber. It is important to use grease of the same type as the original to avoid harmful reactions between different grease compositions, which can reduce or even lose lubrication effectiveness and cause bearing overheating and damage.

Time Interval for Replenishing Grease

• The service life of grease is influenced by various factors, such as the type of grease, bearing speed and temperature, the amount of dust and corrosive gases in the working environment, and the design and effectiveness of sealing devices.
• For sealed or smaller bearings, either the bearing or the grease will determine the lifespan of the bearing, and no mid-life grease replacement is necessary or feasible.
• Based on experience, temperature has a significant effect on the grease replenishment interval: when the temperature (measured on the bearing outer ring) exceeds 40°C, for every additional 15°C, the grease replenishment interval is halved.