Bearing Dynamics

Dynamic analysis of rotor bearings is one of ØDS’s main areas of expertise.

We have developed state-of-the-art computational tools and achieved comprehensive experimental data to fully support our rotor dynamic analysis
capabilities.

• Tilting Pad Bearings
• Rolling Element Bearings
• Magnetic Bearings
• Squeeze Film Damper Bearings
• Sleeve Bearings
• Hydrostatic Bearings
• Vibration Control
• Transfer Function
• Dynamic Stiffness and Damping Coefficients
• Minimum Film Thickness
• Pressure and Temperature
• Cavitation

Bearing Dynamics

Why?
Bearing dynamics is one of the most important aspects of rotor dynamics. Correct modelling of bearing behaviour plays a crucial role in the reliability of rotating machines; erroneous bearing selection can cause severe machine failure.

Fluid Film Bearings
ØDS has developed state-of-the-art tools for calculation of dynamic bearing characteristics. The software for fluid film bearings considers:

• Film rupture in the divergent part of the pads.
• Variation of oil viscosity due to heat dissipated by friction.
• Pad flexibility due to heat gradient over the pads.
• Pivot flexibility.
• Pedestal and foundation flexibility.

For tilting pad bearings, the position of the pads represents additional degrees of freedom. Traditionally, dynamic bearing coefficients have been expressed by condensing these degrees of freedom at the rotational frequency (s = iΩ). This is perfectly adequate for unbalance response calculations. However, rotor dynamic stability analysis requires condensing at the eigenvalue (s = -λ + iω) of the considered mode instead. Therefore, ØDS uses either

• Full transfer function or
• Whirl frequency and log.dec. dependent dynamic coefficients.

The fact that we have in-house software enables us to solve non-trivial problems. For example, statistical information on bearing clearances can be related to rotor dynamic stability margin, and thereby allowing a cost-benefit analysis of recommended bearing modifications.

Rolling Element Bearings
The dependence on preload is among the difficulties in predicting the rotor dynamic properties of rolling element bearings. Housing FEA can therefore be required if a high accuracy is needed.

ØDS has the tools for analysis of elastohydrodynamic lubrication, including the effect of roughness and particles.

Active Bearings
During the last decade, active bearings have been implemented more frequently in high performance turbomachinery in order to control vibrations and reduce friction between rotor and stator.

A well-established control mechanism is active magnetic bearings. These bearings provide a magnetic field that levitates the rotor free from the stator. The active control system counters any dynamic rotor motion by means of an opposite dynamic force from the magnetic field.

Active control can also be used in lubricated bearings either by pressure control or pneumatic control.

Seals
Oil and gas seal dynamics are equally important in rotor dynamic analysis. Furthermore, oil and gas seals will carry part of the static load. ØDS’s expertise therefore includes

• Seals (labyrinth, honeycomb, wet, etc.) and other narrow internal clearances
• Impeller/diffuser interaction

Experience and Resources

ØDS has in-house state-of-the-art software for rotor dynamic and bearing dynamic calculations. This enables us to tailor-make the software to solve nontrivial problems.

ØDS has well-established contacts with leading universities and laboratories around the world. This implies that we can assist in organising both component testing and advanced research at the most qualified institutes.