Typical forms of poppet valves are shown
in Figs. 164, 165 and 166, the last being
quadruple seated and the others double -

seated. All are designed so as to be nearly Fio. 164.— Double
balanced. In order to place the valve in ^d^bonS^t!'**'' '"''"^
position the seatings must have different
diameters. The inside diameter of the larger seating should be
the same as the outside diameter of the smaller seating so that
the unbalanced area exposed to steam pressure will not be larger
than necessai y. When the valve is closed the total area subjected



160 VALVE GEARS

to an unbalanced pressure is only an annulus equal to or slightly
exceeding the combined width of the surfaces in contact. A
email unbalanced pressure is desirable because it tends to keep
the valve steam tight and prevent accidental opening.
Four poppet valves are necessary on a double acting engine,
two for admission and two for exhaust. In the study of slide




FiQ, 165. — Double seated poppet %alve shomng poeitioD in cylinder.
valves it was seen that only one valve was necessary on a double
acting engine and that it could be made to connect each cylinder
port alternately with the steam and exhaust ducts; in other words,
one chamber was connected alternately with two other chambers.
A poppet valve can connect or disconnect only two chambers.
The valve stem is always vertical,

A casting called the vahe basket (see Fig. 165) contains the seats
for the valve and four such castings are inserted in the cylinder
casting. The valve and basket should be of the same material
and cast from the same heat so that both will deform equally
under the influence of heat and in that way prevent warping.
