The Role of a Thrust Bearing in a Turbine

One of the key components in turbines that produce clean energy such as wind, solar and water energy is a simple little part called a thrust bearing. Though it may look simple, this part has a big job to do.

The thrust bearing maintains rotor-to-stator axial position and prevents metal-on-metal contact between the rotor and stationary portion of the shaft collar. It consists of Babbitt metal lined stationary shoes that run against the rotor thrust runner.

Runner

Currently, most hydro turbines are fitted with proximity sensors for detecting wear on the thrust bearing collar. This sensor detects the position of the collar, and if it strays from a preset (and fixed) position, a system trip is initiated.

This sensor is critical to the operation of a power plant, and should be tested regularly for proper positioning. Testing must be done both mechanically and electronically for the sensor to function reliably.

FPL Energy personnel investigated the failure and discovered that the round keys holding the split runner half halves to each other were distressed. One key had ejected, and the other had sheared set screws. This caused preferential wipe, resulting in the displacement of the two half units. The resulting vibration channel was close to shutdown.

Collar

Unlike journal bearings, which take only radial loads on the shaft, thrust bearings are designed to support the axial load that is applied on the rotor. This axial load is transferred to the thrust bearing by a raised collar around the shaft.

Depending on the size of the turbine, gas and steam thrust bearings use either tilting pad hydrodynamic bearings or combined bearing assemblies. In these types of applications, "Directed Lubrication" is often used to minimize power losses, oil flow requirements and pad surface temperatures.

To accurately test the proximity sensor it is necessary to be able to move the sensor toward and away from the collar at a fixed, predetermined distance reliably over a period of time. This is achieved with the aid of a movable piston that carries the sensor, and a fixed piston that is located between these two.

Pads

The pads support the applied thrust and eliminate metal-on-metal contact between the runner and collar. The flat plate design has radial grooves on the running surface that were originally intended to circulate oil through the bearing pot and wash cool oil over the babbitt pads.

These grooves are now a source of unbalanced loads on the pads, which can lead to a chain reaction with high temperature and rub that leads to pad erosion, which in turn causes more severe damage to the pads. The grooves also break the wedge-shaped regions of oil that support the load, causing more uneven pressure distribution and more vibration.

Pockets and other recesses machined into the pad surfaces have significant influence on pressure distribution and rotor sensitivity, affecting the optimal position of the pivot for optimum load capacity. A simplified theoretical model is used to determine the effects of these pockets.

Tilting Shoe

Despite the different routes to alternative energy, equipment that facilitates these sources operates according to the same basic laws of physics as all other machines. This includes the use of thrust bearings.

Gas turbines and steam turbines require tilting pad journal and thrust bearings, often in combined assemblies, with copper chrome or babbitted steel shoes and special directed lubrication features to help reduce power losses and operating temperatures. The axial position of the rotor shaft is extremely critical to the proper operation of these machines, and even a slight variation can cause premature thrust bearing wear, which may result in costly plant shutdowns.

Western Machine Works precision machinists machined the thrust bearings for this hydro power facility, achieving the tight tolerance and 32 RMS finish required. The fabricated thrust bearings were then quality checked for completeness and conformance to specifications prior to shipment.

Self-Equalizing Link

In hydrodynamic thrust bearings, the rotor is supported by a thin film of lubricant between the rotating collar surface and the thrust shoe. This type of bearing saves weight, space and supports more load than a rolling or elastomeric bearing. It requires no maintenance and resists vibrations better than a ball bearing.

Ideally, the thrust collar and bearing surface must be parallel to each other so that each pad experiences the same load. However, misalignment can occur in large power equipment due to thermal deformations or production inaccuracies that deflect the shaft by several tenths of a degree.

To compensate for this, a self-equalizing link is used. It consists of an upper leveling plate that pushes against the lower pads when loaded. This forces each of the pads to tilt slightly upward and equalizes the load across the bearing.