Prior to 1960, cylindrical or plain fixed bore journal bearings were used as the industry standard for most turbine generators operating in U.S. power stations. These bearings were conservatively designed to support heavy rotors that rarely deviated from base load operation close to nameplate rating. Today, most turbine generators are required to operate in some form of automatic load following central dispatch control mode. This mode of operation necessitates the need for a stable/predictable start up and operation within a wide load range.
A journal bearing performs multiple functions. The obvious function is supporting the static weight of the rotor. A not so obvious function is to stabilize the hydrodynamic forces that develop as a function of multiple variables. The complexity of these hydrodynamic forces increases when the driving forces change to satisfy the need to operate within a wide load range. Most turbine generators installed before 1980 were designed with a comfortable margin of weight and rigidity, which tends to minimize the effects of these forces. Modern
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Rotor-bearing instabilities are commonly referred to as oil whirl or whip. High amplitude subsynchronous vibration is the result this phenomenon, which is primarily stimulated by rotational forces and input driving forces. Rotor dynamic analysis software has proven to be successful in developing solutions to this problem. Installing a bearing designed for the specific conditions is often a cost effective solution.
Fixed bore journal bearing
Many variations of the fixed bore journal bearing have been used through the years to prevent the onset of whirl. The primary objective of these variations was to influence the oil flow patterns to develop specific directional stabilizing forces.
Pressure dam
Pros: Economic, straightforward manufacturing process, good suppression of oil whirl, good damping at critical speeds
Cons: Can become unstable with little warning, accelerated wear in dam area, load direction must be known
Elliptical or lemon bore bearing
Pros: Economic, straightforward manufacturing process, good damping at critical speeds
Cons: Subject to oil whirl at high speed, load direction must be known
Offset half bearing
Pros: Economic, straightforward manufacturing process, excellent suppression of oil whirl at high speeds
Cons: Fair suppression of oil whirl at moderate speeds, load direction must be known
Three and four lobe bearing
Pros: Good suppression of oil whirl, overall good performance
Cons: Increased manufacturing costs and process complexity, subject to whirl at high speeds
Hydrostatic
Pros: Good suppression of oil whirl, wide range of design parameters, reasonable manufacturing costs and process complexity
Cons: Poor damping at critical speeds, high pressure lubricant supply
Tilting pad journal bearing
The tilting pad journal bearing was developed with the intent of having the flexibility to absorb and react to hydrodynamic forces from multiple directions. The basic design consists of individual shoes or pads that are supported on pivot points and free to rock and pitch to follow the motion of the shaft. This relative freedom decreases the destabilizing cross-coupling forces usually seen in fixed bore bearings. The primary parameters that influence the static and dynamic characteristics of a tilt pad bearing are:
Pivot offset
Most tilting pad bearings are designed with a central pivot, which means the pivot supporting each pad is located central to the shape of the pad. In specific situations it is advantageous to offset the pivot toward the trailing edge of the pad to ensure the formation of a converging oil wedge.
Bearing preload
Preload refers to the comparison of pad bore clearance relative to set bore clearance. Due to design of most tilting pad bearings it is possible change this parameter as required. Most lightly loaded bearings are designed with preload.
External loading condition
The orientation of a tilting pad bearing determines where the static load of the shaft is directed. The most common configurations are load on pad (LOP) and load between pad (LBP). This condition can vary anywhere between LOP and LBP depending on the design requirements.
