Hydrodynamic bearings (also known as fluid film bearings) are often deployed as journal bearings. As such, they come in several types, including radial tilting pad bearings, thrust bearings for axial displacement and journal sleeve bearings.
Journal sleeve bearings are typically used in low-speed and low-friction applications while radial tilting pad bearings are used for high-speed applications due to the high amplitude of vibrations.
Thrust bearings, on the other hand, are generally employed for axial displacement in high-speed applications as they contain tilting pads that support high-thrust loading of rotors.
The journal bearing has several functions. It acts as a frictionless support for the rotor while it is rotating. It cools down the rotor by transferring the heat energy generated from the process gas to the rotor and then to the oil by convection, or from the process steam in the case of steam turbines. Another role is to dampen high-amplitude vibrations by means of pivoted tilting pads and an oil stream entering the bearing, which creates an oil film between the rotor and the bearing stationary pads.
The bearing itself consists of the journal region of the rotor and the bearing housing, containing the internal chamber of bearing and pivoted tilting pads. There is also a channel for oil entrance, an oil outlet and a thrust collar in the case of thrust bearings.
Theory of operation
The bearing undergoes a hydrodynamic wedge effect as the rotor spins eccentrically inside the bearing housing at its normal operating speed. The rotor exerts a force on the oil enclosed between the rotor and the pads according to Newton’s second law and in accordance with oil’s incompressibility property.
More in January/February 2013 issue of Turbomachinery International