Comments

  1. Currently, only hydrodynamic lubrication is utilized for journal bearings. Thus, the applied methodology is valid for eccentricity ratios up to 0.9. This means that the maximum distance between the journal and bearing centers should be up to 90% of the radial clearance (radial clearance=RB-RJ). An IMPACT contact element between journal and bearing graphics is implemented to deal with numerical issues that may arise in cases of large eccentricity, or to provide a solution for a static analysis. However, results that are derived from both lubrication and contact should not be regarded as realistic. Contact occurrence and/or large eccentricity may indicate that current journal bearing characteristics should be changed.
  2. The method attribute defines the methodology used to describe the lubrication (hydrodynamic) forces and moments.
    1. The Sommerfeld method includes negative lubricant pressures in the calculation of the forces, while the Gumbel includes only positive. Consequently, the Gumbel method is more accurate because lubricants cannot withstand negative pressures.
    2. The short method should be used when the length-to-bearing diameter ratio is smaller than 0.5. The long method is adequate when the diameter ratio is bigger than 2. Either the short or long method can be used for ratios between 0.5 and 2. The proper choice depends on the application.
    3. The Dynamic Gumbel method considers the continuous variation of the location where the positive pressure exists each time step. Thus, it provides better results in a transient state, but is computationally more expensive. For example, in the picture below, the region of positive pressure is not aligned with the line that connects the two centers as in the Gumbel method.


    Figure 1.
  3. Use the lubrication_force_graphics attribute for a better understanding of the pressure distribution along the journal bearing. The force vectors represent the pressure that acts at the journal.
  4. The requests that are received for a journal bearing are the relative displacement, velocity, and acceleration between the journal bearing centers expressed in the rm and the absolute displacement, velocity, and acceleration of the journal and bearing center expressed in the output_rm. Also, the lubrication forces and moments that act at the journal and the bearing are received. The maximum and minimum pressure and the minimum oil film thickness, along with their radial and axial (for misaligned) positions, are calculated to evaluate the performance of the journal bearing. The axial position is measured with respect to the z axis of the rm, while the radial position is the angle that is measured from the x axis of the rm.
  5. nr and na attributes can be increased if a finer force vector distribution is wanted. Also, an increase may be required for better results of the Maximum/Minimum Pressure and Minimum Oil Film Thickness Requests. For misaligned journal bearings, these values may be increased for obtaining a more accurate solution. Although, increasing nr and na leads to a reduced computational performance in most cases.