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Apart
from the customary fulfilment of check-lists separated
for pre-design and detail construction, every single
runner is put in order as part of a continuously variable
construction series according to it's hydraulic data
and it's special geometric proportions.
A runner is designed by means of an arithmetic programme
as a function of the specific speed of nq, the head
H and a typical measurement e.g. pitch circle
diameter D.
The
sum of all hydraulic and mechanical strength-related
experimental data and experiences have gone into this
parametric arithmetical programme, which gives out
all bucket contours and cutting edge geometries as
parallel sections.
Furthermore,
the calculation programme, which is symbolized in
illustration 1, determines all compressive
loads per unit area and mechanical stresses.
Diverging
interventions into the programme for reasons of adaptation,
e.g. to given turbine case contours for replacement
runners, are also easily possible.
The actual results of the calculation programme are
the special coordinates of the parallel sections and
the broad outlines which go with them.
These coordinates in the form of tables are transformed
into a 3D-network diagram (illustration 2).
This
network diagram is overlied by a functional surface
area programme. Ensuingly, a CNC-processing programme
for the machine tool, which is used for milling the
spatial contours of a Pelton bucket, is drawn up.
The 3D-model (illustration 3) of a bucket as
a basis facilitates both milling the runner blades
themselves as well as producing a drop outline for
the forged buckets and, furthermore, possibly milling
a model runner on a suitable scale for a laboratory
acceptance test.
The
individual buckets are processed on CNC-machines.
The bucket is simply put up with 2 clampings, thereby
making an entire processing of the bucket possible.
This
open accessibility on all sides allows the use of
chiefly short and very stiff tools in the processing
machinery, thereby making high cutting speeds combined
with very high surface qualities realisable in the
best way possible.
In addition, processing can be done with customary
tools and does not require special tools.
Illustration 4 shows the processing of the
pedestal foot and illustration 5 the processing of
the inside of a bucket with an inside width of 490
mm and an untreated single weight of 250 kg.
The
advantages of a small-series production in the production
cycle can be made use of in an optimal way through
electronic data processed and edited programming as
well as mechanical processing of the buckets.
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