Bearing solid grease (also known as dry film grease or solid additive-containing grease) is a special lubricant that evenly disperses solid lubricant particles in the base grease.
Its characteristics and its complex impact on bearing noise are explained in detail below.
Characteristics of solid grease
The core characteristics of solid grease come from the solid lubricants it contains. Common ones include: molybdenum disulfide (MoS2), graphite, polytetrafluoroethylene (PTFE), boron nitride, etc.
1. Excellent extreme pressure and wear resistance
Mechanism: Solid particles, especially MoS2 and graphite, have a layered structure. Under extremely high pressure, slippage occurs easily between these layers, forming a strong lubricating film on the contact surface. This film can effectively prevent direct contact between metal surfaces under heavy or impact loads, preventing gluing and wear.
Advantages: Especially suitable for heavy load, low speed, and impact load conditions, which are difficult for traditional liquid greases to handle.
2. Good high temperature stability
Mechanism: Solid lubricants themselves usually have high thermal stability (e.g. MoS2 can withstand about 350°C in air and higher in vacuum). When the base oil evaporates, oxidizes or degrades due to high temperatures, solid particles can still adhere to the metal surface to provide lubrication.
Advantages: Expands the bearing's working ability in high temperature environments.
3. Low volatility and long lasting
Mechanism: Solid particles will not evaporate like base oil. In situations where relubrication is not possible or in vacuum environments, solid grease can provide longer-lasting lubrication.
Advantages: Suitable for maintenance-free, long-life requirements or bearings in vacuum environments (such as space equipment).
4. Wide adaptability
Mechanism: It does not rely on the hydrodynamic effect of the oil film, so it can form an effective lubricating film under low speed conditions. At the same time, it is not sensitive to certain chemicals and can be used in environments where solvents or strong oxidants are present (depending on the specific solid lubricant type).
5. Controllable conductivity/insulation
Mechanism: Graphite has good electrical conductivity, while PTFE and boron nitride are excellent insulators. By selecting different solid additives, the conductive properties of the grease can be designed to meet antistatic or insulating requirements.
Effect of solid grease on bearing noise (double-edged sword effect)
The effect of solid grease on bearing noise is very complex. It cannot simply be said to be "noise increase" or "noise reduction", but depends on the specific working conditions, grease formula and bearing condition.
1. Situations that may increase noise (negative impact)
This is the most common and most talked about side.
Physical stimulation of particulate matter:
Mechanism: Solid lubricant particles are tiny solids. Between the raceways and rolling elements in the precision operation of the bearing, these hard or semi-hard particles can produce additional microscopic shocks and vibrations like "pebbles". When the rolling elements run over these particles, a high-frequency "hissing" or "rustling" sound will be produced, which is manifested as an increase in background noise.
Key factors: particle size, hardness and dispersion. The larger the particle size, the higher the hardness, and the more uneven the dispersion, the more significant the negative impact on noise will be.
Affects the rheology of grease:
Mechanism: The addition of a large amount of solid particles will significantly change the rheological properties of the base fat, usually making it more viscous and "dry". This may lead to poor fluidity of the grease in the bearing cavity. During startup or low-speed operation, the grease cannot be evenly distributed, resulting in local poor lubrication and dry friction, resulting in abnormal noise.
Negative impact on noise (NZ value):
In extremely quiet environments (such as servo motors, precision instruments), bearing noise (non-periodic, irregular noise) is an important indicator. Particles in solid grease can easily cause random and irregular noise, causing the bearing's NZ value (noise criterion) to exceed the standard.
2. Possible noise reduction situations (positive impact)
Under certain conditions, solid grease can actually help reduce noise.
Suppress wear and noise under lean oil lubrication:
Mechanism: After long-term operation, or in a "lean" state where base oil is lost due to high temperature, traditional grease may fail, resulting in direct metal contact and severe friction noise (squealing, vibration). At this time, the solid lubricating film formed by solid grease can continue to work, filling microscopic depressions and reducing metal contact, thus suppressing serious noise caused by poor lubrication.
Filling surface micro-roughness:
Mechanism: Suitable, fine solid particles can fill the tiny scratches or pits on the surface of bearing raceways and rolling elements, playing a "filling" role to make the contact surface smoother, thereby reducing vibration and noise caused by surface roughness.
Damping and vibration reduction effect:
Mechanism: Certain solid lubricants (such as PTFE) and thicker lubricating films have a certain damping effect and can absorb part of the vibration energy, helping to reduce the overall vibration level of the bearing.
Summary and application suggestions
Conclusion
The core advantage of solid grease is its extreme pressure anti-wear and high temperature stability, not its quiet performance. Its effect on bearing noise is essentially a trade-off between the "tribological benefits" of solid particles and the "acoustic interference".
It should be avoided in situations where extreme silence is sought.
Under harsh working conditions (heavy load, high temperature, low speed), in order to protect