Guidelines for adding or replacing bearing grease

After undergoing complex flow patterns during the initial stage, the grease inside rolling bearings settles into a stable distribution. Ultimately, the minimal amount of grease remaining in a fluid state at the friction points performs the primary lubricating function, whilst the grease retained within the outer ring remains stationary and does not directly participate in lubrication. However, the grease retained within the outer ring acts as a seal, preventing the loss of the fluid grease present at the friction points.

Experiments have shown that if the grease inside the outer ring is removed after the bearing has been in operation for 50 hours, bearing wear increases significantly. At the same time, the base oil separated from the stationary grease inside and outside the bearing will, under the influence of factors such as heat and vibration, migrate to the friction surfaces and participate in the lubrication process.

Clearly, there is no need to overfill the bearing with grease. As grease has poor oil film recovery properties, overfilling will actually lead to a deterioration in the bearing’s lubrication condition. Therefore, determining the appropriate amount of grease to fill the bearing is crucial. It is evident that both overfilling and underfilling of grease will cause the bearing temperature to rise, making it impossible to ensure the bearing remains in an optimal operating state.

I. The Hazards of Overfilling with Grease

Causing sealing issues, increasing energy consumption and damaging the grease itself. Rising temperatures during operation can lead to seal failure, whilst also increasing equipment energy consumption and adversely affecting the performance of the grease itself. For example, if a motor is overfilled with grease, it will lead to increased power consumption and, in severe cases, may even cause motor failure.

Exacerbates bearing wear and grease degradation. Taking bearings as an example, excessive grease agitating violently within the bearing cavity increases rotational resistance, leading to higher temperatures and increased energy consumption; in turn, the rise in temperature accelerates grease oxidation (premature degradation, shortening service life) and promotes the accelerated separation of the base oil contained within the grease—it should be noted that the lubricating function of grease relies primarily on the base oil it contains.

Residual thickening agents cause subsequent lubrication failures. Once the base oil has been lost from the grease, thickening agents remain. These agents gradually dry out and harden, preventing newly added grease from reaching the core of the bearing effectively, resulting in poor lubrication and subsequently causing bearing wear or even failure. Furthermore, hardened lumps of grease may break off and be carried into the bearing, further exacerbating the damage.

Damage to Seals and Introduction of External Contaminants: Over-lubrication increases the pressure on seals, causing them to crack and fail. This leads to seal loosening, allowing contaminants and moisture to enter, ultimately damaging the bearing.

**Pressure risks when using a grease gun* When using a grease gun, attention must be paid to the pressure capacity of the seals. Some grease guns can reach pressures of up to 15,000 pounds per square inch (psi), whilst standard lip seals have a pressure capacity of only around 500 psi; therefore, the pressure of the grease gun must be carefully controlled.

Specific hazards to electric motors: The risks of overfilling a motor with grease are similar to those for bearings, but additional problems may arise: excess grease may seep onto the motor windings and harden on their surface, leading to poor heat dissipation.

II. Principles for Grease Filling in Rolling Bearings

(1) Generally, bearings should not be filled to capacity; the filling volume should be 1/2 to 3/4 of the total internal space of the bearing housing; (2) For horizontally mounted bearings, the filling level should occupy 2/3 to 3/4 of the internal cavity space; (3) For vertically mounted bearings, the filling level should be 1/2 of the cavity space (upper side) and 3/4 (lower side); (4) In environments prone to contamination, for low- or medium-speed bearings, all spaces within the bearing and bearing housing should be completely filled with grease; (5) Before greasing high-speed bearings, the bearings must first be immersed in high-quality lubricating oil (typically the base oil of the grease to be used) to prevent bearing seizure during start-up due to insufficient grease on the friction surfaces.

III. How to Fill with Grease Appropriately

Key Measures to Prevent Over-lubrication

Firstly, the appropriate grease filling quantity must be determined; secondly, operational guidelines for grease filling quantities must be established. After adding or replacing grease, close attention must be paid to equipment indicators such as temperature, noise and vibration, whilst monitoring for any signs of oil leakage. For critical equipment, specialised monitoring tools may also be employed.