4B326d01, silniki parowe
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The MUNCASTER
steam-engine models
EDGAR T. WESTBURY is bringing a modern eye to bear on
some classic models of the past
Continued from 4 April 1957. pages 488 to 490
A
LTHOUGH
the horizontal type
popular until later developments,
and particularly higher steam pres-
sures. made it desirable. However.
Mucaster knew steam-engine practice
better than I ever shall and I would
never dispute his authority over such
details.
A piston valve is nothing more than
a slide-valve having a circular instead
of a flat face, but this alteration in
shape involves characteristics which
may have advantages or limitations
according to circumstances. First of
all, it is capable of controlling ports
all round its circumference instead of
adapted to control steam admission
either on their outer end faces (as
in the case of the flat valve), or the
inner faces, which would normally
control exhaust events. The latter
arrangement, known as “inside ad-
mission,”is generally preferred as it
enables the steam-chest and passage
design to be simplified, though it
makes no difference to efficiency so
long as design is adapted to suit.
It will, of course, be clear that in
this case steam lap must be provided
by reducing the width of the clearance
portion of the valve, corresponding
with the cavity of the flat valve, and
the total length of the valve must be
such that it exactly covers the ports
in the steam-chest, unless exhaust lap
is specified-in other words, normal
“ line for line ” exhaust timing.
Piston valves generally allow the
cylinder steam passages to be made
shorter and more direct, thus im-
proving thermal efficiency by reducing
the dead volume at the ends of the
stroke and also the conducting surface
area of the passages. They do not,
however, provide the same facility for
visual valve timing as the flat valve,
and it is necessary to adjust their
position by exact measurement in
most cases.
The piston valve of the engine shown
in Fig. 26 is of the inside admission
type, the main steam inlet being in
the centre of the steam-chest’and the
exhaust being taken out from two
ports at the extreme ends to a passage
shown in the plan section
BB.
It is
driven by a rod which passes up
through a clearance hole in the centre
for most of the length of the valve,
thus giving a small amount of side
freedom for self-centring in the gland,
but the uvner end is screwed into a
short tapped hole and a locknut. is
provided so that lateral position
adjustment can be obtained.
It should be noted that for an
inside admission valve the eccentric
timing must be adjusted so that it
trails behind the crank instead of
leading it. The angle of advance,
however, is still in the same direction,
so that for a valve with fairly orthodox
lap and lead, calling for 30 deg. angle
MODEL ENGINEER
of engine has always been
favoured for stationary
work, the alternative direct-acting
form of engine having the cylinder
located vertically above- the crank-
shaft has some advantages where
floor space is limited, and is
generally considered more suitable
for running at high speed than the
former type.
It is, of course, more common in
marine practice than stationary work,
but both on land and sea it has been
extensively used for auxiliary purposes
such as driving electrical generators,
ventilating and forced draught fans,
and centrifugal pumps for circulating
water in condensing plant, or dock
drainage.
One of the earliest engines in this
general class was introduced a few
years after the Nasmyth hammer
5-Vertical
stationary
engines
made its appearance,. and because of
its structural similarrty to the latter
machine it was customary to refer
to it as the “ steam-hammer” type.
The salient features of such engines,
an example of which is illustrated in
Fig. 26, include a relatively small
bedplate on which is mounted a
over a limited width of face, and thus
it can give much more rapid and
efficient valve events than a normal
flat valve, though this feature is not
always used to full advantage.
Secondly, it is not pressed hard
against the portface by the steam
pressure, and therefore works with
much less friction, especially where
high working pressure is employed;
this is perhaps its most important
practical advantage.
But because of being pressure-
balanced it is not self-seating, and
unless it is very carefully fitted to the
bore of the steam-chest or liner, it is
liable to leakage, much more so than
the flat valve. Many small piston-
valve engines have been found less
ethcient than those with flat valves
for this reason, especially when wear
has taken place; large engines have
piston rings fitted to the valve to
avoid leakage, but this is hardly
practicable in a model.
Thirdly, piston valves may be
555
symmetrical pair of cast columns,
usually of channel or hollow-box
section, and these in turn support the
cylinder assembly. In outline, the
structure bears a resemblance to that
of a lighthouse, tapering more or
less gracefully from the cylinder head
to the base, to give maximum rigidity
against both dead load and working
stresses.
The inside faces of both the columns
are flat near the top end, and have
machined surfaces which serve as
crosshead guides. The working parts
are generally similar to those of
horizontal engines, except in certain
points of detail which may be in-
fluenced by their disposition and order
of motion.
In one respect, this particular
engine may be regarded as an anacho-
nism, in that while its main structure
follows the “ steam-hammer ” tradi-
tion, it is fitted with a piston valve,
a feature which did not become
18
APRIL 1957
Muncaster
models.
An engine of this type is illustrated
in Fig. 28. It is intended to run in a
clockwise direction, looking at the
end of the shaft as seen in the right-
hand elevation. Note
that
when the
piston is on the up-stroke, the thrust,
which is tending to straighten out the
piston rod and connecting rod link-
age, presses the crosshead against the
column; but on the down-stroke, the
tendency is to increase the angularity
of the linkage and thus the cross-
head is still pressed against the
column.
If the engine rotation is reversed,
the side thrust on both strokes is in
the opposite direction so that the
crosshead will pull away from the
column and bear against the keep
plates, which are of much smaller
surface area than the column face,
besides having to rely on their retaining
studs for security.
of advance, the setting will be 90 - 30
= 60 deg. behind. the crank in the
direction of engine rotation.
Details of the piston valve and the
two short half-liners, which are
pressed into opposite ends of the
steam-chest, are shown in Fig. 27.
Three ports are shown in each of the
half-liners, giving a large total area,
and the sides of the ports are cut
obliquely to minimise ridge formation
on the valve as a result of wear.
Alternatively, a greater number of
round holes may be used,, and per-
sonally I should favour this method.
A groove is turned in the outside of
the liner to form an annular passage
when it is inserted in the steam-chest.
The liners must be accurately located
to give the designed port timing in
conjunction with the valve dimensions.
A stainless steel valve with bronze
liners is recommended.
Above, Fig. 27
:
Details of the
piston valve and ported half-liners
Below, Fig. 26
:
A vertical engine of the symme-
trical doable-column or “steam hammer ” type
MAIN COLUMNS
In order to ensure accuracy in
cylinder location and guide alignment,
I recommend that the columns should
first be machined on the guide faces
and then clamped together for facing
the top and bottom surfaces. When
erecting the columns, they should
first be bolted to the bedplate with a
gauge block between the guides to
locate them the correct distance apart.
To locate the cylinder, the machined
crosshead, or a dummy made to the
same dimensions, may be fitted to the
piston-rod to ensure correct alignment.
If straightforward machining
methods are used accuracy should be
positive, but it is not advisable to
take anything for granted and routine
checks should be made at all stages
of assembly!
SINGLE-COLUMN VERTICAL
ENGINE
The
“ steam-hammer ” type of
engine is suited equally well for
running in either direction, as the
crosshead guides are symmetrical and
of equal bearing area each side; but
this is but rarely called for in stationary
work. Even marine engines do not
often run for very long, periods in
the reverse direction. In such cases,
a lighter but quite adequate form of
structure can be adopted in which only
one cast column is employed, and the
crosshead is of the slipper type, having
its major bearing surface on the
soleplate, which slides on the face of
the column.
MODEL ENGINEER
556
18 APRIL 1957
excessive projection of the shaft at
the governor end. Engines similar to
this have been used extensively for
driving dynamos, though they were
superseded by enclosed engines in
later years.
Brickbat department
To those readers who, despite my
explanations in the March 7 issue,
have chastised me for not giving
complete details with full dimensions
of all these engine designs, I would
like to point out that the drawings
are copied as exactly as possible from
Muncaster’s original published de-
signs,and this is what has been
asked for by many readers over a
period of several years.
Fig. 28
:
A vertical engine of the single
column type,with shaft governor
The descriptive matter is my own,
but if I attempt to amplify the draw-
ings in any way their individuality
will be lost; in any case, the amount
of work involved and the space
occupied would be out of proportion
to the popular appeal, which is
bound to be specialised to some
extent.
The value of Muncaster’s designs
lies in his genius for adapting typical
examples of all kinds of full-size
engines to reproduction in miniature
while retaining true prototype charac-
ter; exact details are of lesser im-
portance, but my previous articles on
steam-engine construction should
make up any deficiencies in this
respect.
To be continued
As the offset support of the cylinder
by a single cast column leaves the
structure somewhat weak to resist
alternate upward and downward
stresses, the opposite side is stayed by
means of a single machined steel
column (sometimes more than one is
used) which, though light in section,
has greater inherent strength than
cast iron. As this is usually somewhat
out of the vertical plane, both its
length and the angle of its seating at
both ends must be carefully adjusted
to hold the cylinder assembly exactly
perpendicular.
GUIDE FOR MAKING
SMALL WIRE SPRINGS
T
below enables a short run of
HE
simple arrangement shown
wire springs to be made without
having recourse to normal spring-
making machinery.
A wooden block,
A,
has a handle
and several holes of different diameter,
C, drilled through into the V-shaped
opening. The wire, Br is then threaded
through the appropriate hole, passed
under a core rod,
E,
and secured in
a chuck carrying the rod. The dia-
meter of the rod is chosen to suit
the internal. diameter of spring re-
quired.
After making the wire taut and
completing a few turns by manipu-
lating the wooden tool, the core
rod is rotated mechanically until the
correct length of spring is reached.
The spring is of the closed variety,
but by driving a pin or nail through
one of the holes,
D?
open springs can
be made with spacing as desired.
The holes, C, will probably wear
out quickly so that the wooden tool
may need freauent replacement.
Digested from
“ A Device for
Forming Small Lots of Wire Springs,”
in “
Wire Industry,” 1956 (October)
page 921. The illustration is reproduced
by courtesy of the publishers.
¦
MODEL ENGINEER
This drawing does not show the
interior details of the cylinder, but
these may be similar to the previous
design, using either a flat slide or a
piston valve. It is fitted with a
governor, mounted directly on the
shaft and acting on the engine
throttle valve. The rather unusual
position of the eccentric, immediately
adjacent to the flywheel, avoids
18
APRIL 1927
557
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