The thrust padThere was a need for a thrust bearing in flour-milling machinery. The moving millstone was driven by a vertical shaft that was supported by some sort of footstep bearing. I have shown an arrangement for a footstep bearing in figure 16-16. The sliding surfaces are just the flat end of the shaft and the face in the base. There can be no wedge action with such an arrangement and lubrication was from a reservoir in the base with a radial groove in the shaft to carry oil to the centre of the shaft

where it might find its way between the two surfaces. Wear must have been inevitable.

When the screw propeller began to be used on ships in about 1850 there was a need for a thrust bearing to transfer the force produced on the propeller, that had been transmitted through the shaft, on to the structure of the ship and, of course, bearings to support this long shaft. For 50 years the thrust bearing was the plain annular bearing shown in figure 16-17 derived from the footstep bearing possibly with several bearing collars on the same shaft. At the end of the 19th century the idea of wedge action was evolved by Navier and others and around 1900 the problem of the thrust bearing was resolved by an Australian called Michell. (He pronounced it Mitchel.) He invented the self-aligning thrust pad.

 In essence his invention was to divide the annular thrust pad into eight separate sectors and allow each sector to pivot about a radial axis. The arrangement is shown in figure 16-18 which is taken from a photograph. The substantial back-plate has radial grooves and the thrust pads have elongated nibs, shown on the inverted pad top left, that fit in these

grooves to act as pivots. The shaft with a plain collar goes through the hole in the middle so that the collar rests on the thrust pads. The pivots on which the pads move are offset to one side so that when lubricant is fed to the rotating shaft it goes under the pads and they rock until they make a small angle and produce a wedge of oil and

Fig 16-18

Fig 16-17

Fig 16-16

high pressures. If the pivots are in the right position the pads are self-aligning. Such a bearing is unidirectional. Michell bearings are very much smaller than plain bearings for the same duty and much more reliable. They appear to be easy to supply with lubricant. Given the very thin films involved these bearings require great accuracy in manufacture. I do not know for sure but presumably these are used in pairs one for “ahead” one for “astern”.Power Plant Pumps: Guidelines for Application and Operation to Maximize Uptime, Availability, and Reliability.