Fig. 291.— 3-Blade Exhauster.
Frø. 292.— Waller 4-Bi.ade Exhauster.
EXHAUSTING MACHINERY 481
the multiple type is by far the most common in use at the present time. Stated briefly, the advantages which the multiple machine possesses over the two-blade type are as follows:—
(«) For the same size an increased capacity is obtained, amounting to about 30 per cent. with. the three-blade type and 40 per cent, with the four-blade type, as compared with the two-blade machine.
(b) The oscillation is considerably reduced.
(c) The outer cylinder is a true circle, consequently no special borin-g machinery is required.
(d) The whole machine is more easily constructed.
On the other hand, as is often pointed out, the increase in the number of blades is obtained only by reducing the extent of the wearing surfaces, and some engineers state that the three- or four-blade machines have their disadvantages in cases where the back-pressure at the outlet is considerable and where high speed is necessary.
The design of the two-blade machine is such that it is capable of dealing with very high pressures, due to the small number of parts and the method in which the load is taken. Taking the slide in position shown in Fig. 290 it is readily seen that at tlie time of maximum pressure the pin is under its minimum strain, and the reaction due to the load on the exposed part of the slide is transferred to the other side of the driving drum where the maximum of wearing surface can be provided, with. consequent reduced wear. Witli a three- or four-blade machine with. the slide in the same position, the pin is under maximum stress, and the load on the roller face is very heavy, due to the load on the slide and the reaction from the pin.
Moreover, in tlie design of the multiple machine it is often found that very large rollers liave to be used in order to obtain adequate wearing surface at this point. For high speed and high pressure many ehgineers consider the two-blade machine to be ahead of any other, and this type