During the speed-up process of the unit, the shaft vibration increased abnormally as the speed increased, triggering an oil film oscillation alarm and forcing the unit to slow down. Also, after startup after maintenance, the bearing temperature continues to be high, and the oil film thickness monitoring value is lower than the design threshold, and there is a risk of tile burning.

Have you analyzed the root cause of these failures?

First of all, understand that the rotational speed is the source of power for the formation of dynamic pressure oil film. An increase in rotational speed will enhance the dynamic pressure effect of the lubricating oil, increasing the oil film thickness and bearing capacity. However, when the rotational speed exceeds the critical value, it is easy to induce vortex and oscillation of the oil film, destroying the stability of the oil film.

In addition, the oil temperature directly determines the viscosity of the lubricating oil. An increase in the oil temperature will lead to a decrease in the viscosity, which will reduce the thickness of the oil film and the bearing capacity, but it can improve fluidity and heat dissipation. If the oil temperature is too low, it will be difficult to form an oil film due to excessive viscosity and increase starting resistance.

What factors are related to the formation of oil film?

1.Speed

Mechanism of action: The rotation of the journal brings lubricating oil into the wedge-shaped gap through viscous force, forming a dynamic pressure oil film. The higher the speed, the more oil is brought in, and the oil film pressure and thickness increase.

Risk point: When the rotational speed exceeds the critical range, the oil film is prone to half-speed whirl, which then develops into oil film oscillation, causing severe vibration of the unit.

2.Bearing load

Mechanism of action: When the load increases, the journal eccentricity increases, the wedge gap gradient increases, and the peak oil film pressure increases, but the oil film thickness decreases.

Balance requirements: If the load is too small, the oil film will be too thick and the journal will shake; if the load is too large, the oil film will be too thin, causing dry friction and bearing burnout.

3.Oil viscosity

Mechanism of action: Viscosity is the core indicator of the oil film's load-bearing capacity. The higher the viscosity, the stronger the oil film's load-bearing capacity, but the fluidity and heat dissipation will become worse; the opposite is true when the viscosity decreases.

Influencing factors: Viscosity is mainly determined by the lubricating oil grade and oil temperature.

4.Journal and bearing clearance

Mechanism of action: When the gap is too large, the lubricating oil is easily lost and the oil film thickness is insufficient; when the gap is too small, the oil supply is limited and the heat dissipation capacity is reduced, which can easily lead to overheating of the bearing bush.

Control requirements: The clearance must strictly follow the equipment design standards to ensure the balance between oil film formation and heat dissipation.

5. Bearing and journal dimensions

Mechanism of action: The ovality and taper of the journal and the arc deviation of the bearing pad surface will destroy the uniformity of the wedge gap and lead to abnormal oil film pressure distribution.

Control requirements: During maintenance, the journal and bearing dimensions need to be accurately tested to ensure that the shape and position tolerances comply with specifications.

6. Lubricating oil temperature

Mechanism of action: Every time the oil temperature increases by 10°C, the viscosity of the lubricating oil decreases by approximately 20%~30%, directly changing the oil film thickness and bearing capacity.

Operation control: Conventional unit lubricating oil temperature needs to be controlled between 40~50°C to balance oil film performance and heat dissipation requirements.

7. Lubricating oil pressure

Mechanism of action: Oil pressure is the prerequisite to ensure the continuous supply of lubricating oil. Insufficient oil pressure will lead to a shortage of oil supply and the oil film cannot be stably established; too high oil pressure will aggravate oil leakage and seal leakage.

Linkage control: The oil pressure needs to be adjusted according to the oil temperature. In low temperature environments, the oil pressure can be appropriately increased to ensure oil supply.

8.Bearing oil inlet hole diameter

Mechanism of action: The diameter of the oil inlet hole determines the amount of oil supply. If the hole diameter is too small, it will lead to insufficient oil supply and difficulty in forming an oil film; if the hole diameter is too large, it will cause oil pressure loss and affect the stability of the oil film pressure.

Design matching: The diameter of the oil inlet hole needs to be designed to match the bearing load, speed and other parameters.

(Source: Power Generation Knowledge Exchange)