Thrust Bearing

The thrust bearing is placed at the inlet end of the turbine casing as this is the hottest end and hence the most effected by differential turbine/casing expansion. This helps to prevent damage to the glands and also allows the use of reduced clearances, necessary as the specific volume of the steam is at its highest

Standard

A half set of pads are fitted in the aft thrust direction as these are mainly for location only and do not carry any axial loading caused by the passage of the steam.
Oil enters the lower portion of the bearing and passes up via a restriction to ensure the assembly remains flooded with oil

Self Aligning

The high inertia of the spheical carrier reduces the arragements ability to cope with distortions and imperfections. The key prevent sthe rotation of the carrier.

Modern Self Aligning

This design has less inertia and hence is more effective with dealing with distortion.

Mitchell tilting pad bearings are commonly used due to their self aligning properties. The length of the pads is limited due to lubrication problems at the thin end of the oil wedge.
The pads are formed initially as a single ring then machined to requirements. This is why all pads must be changed following failure

Thrust Bearing Clearance

For inital setting up the rotor is centralised by jacking for'd and aft, and the clearance on the ahead side measured. A complete set of pads with carrier are made up to exactly the correct size. The astern size is measured, a running lube oil clearance subtracted and the astern set built up. The whole lot is fitted and final clearances measured.

The retaining ring is split at the horizontal axis. Stop plates at the joints prevent movement of the mitchell pads, one of these stop plates is extended and prevents the retaining ring moving

On the HP turbine normally only a half set of thrust pads are fitted. For the LP turbine with an astern turbine a full set is used. Shils and liners are fitted to set clearance.

There is a tendency for oil to be flung to the periphery under centrifugal action. ence, the orifice is fitted to ensure flooding, also metering flow from main system.

Total clearance = 0.25mm

This may be checked by attaching a finger plate to the casing and jacking the rotor for'd and aft. Poker guages may be used when the turbine is running.

Gun metal or mild steel is used for backing plate. Babbit metal (87% Tin, 8-9% Antimony, 3-4% Copper) for bearing face

An independent thrust collar may be case hardened and fitted using a combination of interference fit, longitudinal key and circumferential retaining ring.