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VARIOUS MECHANICAL APPLIANCES.
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wheels,"
though such a name is misleading, and entirely loses its significance
when the bucket is given its best form. When a jet of water strikes a
stationary bucket shaped as shown in Fig. 45 or in Fig. 46, as soon as
the motion has become permanent the wedge-shaped portion of the water
shaded with horizontal lines be-^gg, comes practically stationary. We
have actual impact only for a minute interval of time—i.e., while
the wedge is forming. After this the water is simply deflected from
its course, and the bucket becomes almost instantaneously a pressure
bucket.
When
such a bucket is used for a wheel it is plain that this shaded portion
of the water is " carried " and must subsequently escape with nearly
the full velocity of the bucket. Its useful effect is therefore very
small as compared with that of the water actually deflected. No
advantage comes, then, from impact; on the contrary, serious losses
are due to it.
The originally flat bucket (see Fig. 45) has been materially improved :
1st.
By giving it curvature (see Fig. 46). 2d. By filling in the wedge and
making it a part of the bucket. This second improvement brings us to
the " Pelton wheel" (see Fig. 53), which is by no means an " impact "
but distinctly a " pressure " wheel. By filling in the wedge impact is
avoided. The same thing in principle could be accomplished with the
simply curved bucket by having the jet strike one side instead of the
centre (see Fig. 47).
A
prominent distinction between the Hurdy-gurdy wheel and the Partial
Turbine rests in the fact that the former has " open " and the latter "
closed " buckets. When properly constructed the one is no more an "
impact wheel " than the other.
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