All About Engines

J72 All About Engines action of high-pressure steam. The others are of brass. All are rolled and drawn to the required section and highly polished to reduce friction with the steam. They are thinned at the tips for reasons which have already been given. Two methods are employed to fix them into position in the grooves. Either they are fixed singly or made up in sections, and afterwards tightly wedged in position. On one edge near the outer end is a small slot in which a wire or strip is hard-soldered to keep them the right distance apart. The drum of a modern reaction turbine is shown in Fig. 103 on Plate 13. The drum has the effect of stiffening the shaft, so the problem of whirling does not enter here as it does in the de Laval turbine. The speed is always below the critical speed (see page 156), but danger arose from the difficulty of lubrication. When two rubbing surfaces have a large relative velocity the film of oil tends to break up, and what is called dis- continuity occurs. The method adopted by Sir Charles Parsons in his earlier turbines, and still fol- lowed when the speed is 3,000 or more per minute, is to use several concentric bushes or tubes fed with oil under pressure. If the friction between the shaft and the inner bush increases—as it will increase if rupture of the film occurs—the bush is dragged round with the shaft and overheating is prevented. The arrangement has the additional advantage that vibra- tions due to the shaft being slightly out of balance are effectively damped. The oil films form a cushion.