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The MUNCASTER
steam-engine
models - 9
By Edgar T. Westbury
In utility engines, screwed glands may
be easiest to fit, and also to adiust in
the limited space available. But most
full-size engines have flanged and
studded glands, the most common
being the oval type as illustrated in
Fig. 42 (the proportionate dimensions,
to suit various sizes of rods, are given
in the accompanying table).
Very large engines sometimes have
circular-flanged glands, with three,
four or more studs; the measurements,
however, may be much the same, with
the exception of the minor axis g.
The internal bevel at the mouth of the
spigot (shown in dotted lines on this
drawing) acts as an internally inclined
plane to wedge the packing towards
the piston rod; an included angle of
about 120 deg. (or normal drill-point
angle) will be suitable-also for the
counterbored gland recess or neck
bush. This applies to either screwed
or flanged glands.
BEARINGS
Several of the engines in this series
have been shown with pedestal bear-
ings of the split“ plummer block ”
type, for bolting down to a flat base
or entablature. The proportions of
these are shown in Fig. 43, and the
corresponding table.
In full-size practice, the bearing
housings were often fitted with a
rectangular bottom “ brass,” the fitting
of which may be somewhat difficult
in a model.
Unless meticulous
adherence to detail is considered
necessary, a half-round brass may
generally be regarded as suitable, if
prevented from rotating by a peg or-
other means. In many cases, bushings
may be omitted, and the shaft run
direct in the pedestal casting, of cast
iron or bronze.
An alternative type of main bearing,
applicable where it is cast integral
with the engine bedplate, is illustrated
in Fig. 44.
Here the horns of the
housing extend well above the shaft
centre, and are fitted with a keep plate,
both the halt-brasses being rectangu-
lar. The external shape of the cast
keep plate shown in this illustration
gives a much better appearance than
the plain strip of steel bar so commonly
fitted, and is also more rigid.
In many horizontal engines, pedestal
bearings having both the split bushings
and their housing set at an angle are
employed, with the object of avoiding
maximum thrust being taken on the
dividing line; a typical example is
seen in the Unicorn engine.
CONNECTING RODS
Various cross-sections of rods have
been used in steam-engines; the
earliest types had forged rods of
round or rectangular section, but many
classic examples of beam engines later
had cast-iron rods of cruciform-or
MODEL ENGINEER
(Continued from 30
May
1957. pages 778 to 480)
I
Nthe construction of models
plicity or convenience. The discreet
constructor of a true scale model
will take pains to obtain authentic
information about the actual prototype
before he undertakes construction,
and his task consists of making an
accurate copy-at least in visible
details. But in freelance models, not
only may general design and propor-
tions be at fault, but a model which is
beyond criticism in these respects
may be let down by small details.
Perhaps the worst kind of fault is
an anachronism; that is, something
obviously out of period, such as a
“ marine ” type bolted crankhead
bearing on an early nineteenth century
beam engine. Another very serious
fault in freelance models is to represent
them as models of actual prototypes
-ethically, this borders on the
fraudulent-but it is often done
!
How often, for instance, have we seen
alleged models of Stephenson’s
Rocket
which bore only a very sketchy
resemblance to this famous engine.
In Muncaster’s models, the need to
simplify construction never led him
to commit crimes of this nature;
both the general character and detail
work in his designs were true to type
and period though the use of certain
ready-made parts, such as slotted-
head screws, was allowed to a greater
extent than I should consider per-
missible in a model intended for
exhibition. These parts, however,
could be changed without affecting
actual component design.
which have any pretensions to
fidelity in respect of type or
period, correctness is essential-
not
only in the general style and
character of the main structure
but also in the minor. details and
fittings. I have referred to this in
earlier articles, but a further review
of this matter may be desirable,
especially in view of the errors
which are often seen in model
engines entered in exhibitions.
These models may be broadly
classified as follow:
First, the type of engine which I
have described as a “ utility ” engine.
Many constructors wish simply to
build an engine which will do a job,
such as driving the screw propeller or
paddles of a power boat or a stationary
plant of any kind; they are not particu-
larly worried as to whether it conforms
to any particular period or style, and
in many cases the engine works
behind the scenes, where it is not
visible or obtrusive. In such cases the
only things that matter are functional
efficiency and mechanical soundness.
Secondly, there is the freelance type
of model which is intended to be
generally representative of some period
or phase of engine development, but
not an exact copy of a prototype.
Thirdly, the true scale model of
an actual full-size engine, whether
period or contemporary type.
It is, in my opinion, the second
class of model which presents the
worst pitfalls to the unwary con-
structor because certain features in it
must always be optional, and may be
determined to some extent by sim-
GLANDS
Piston and valve rod glands are
very simple components, but their
appearance may make or mar a model.
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ribbed section.
Still later, when
machining facilities were improved,
round and rectangular rods again
became popular, but were now bright-
finished all over. The bearings of
these were usually of the “ gib and
cotter” type, at least at the crank-
head, and often at the crosshead.
Two typical examples of these
bearings are seen in Fig. 45; the first
has a rectangular strap embracing the
top and bottom sides of the rod end,
and bolted in position. The split
bearing is of rectangular shape to fit
the frame thus formed, with side
flanges for location, and the inner
half has a tapered groove in the back
so that it can be secured, or wear
taken up, by the wedge or “ cotter”
which passes through slots in the
strap.
In the second example, the strap is
eliminated, the rod having an open-
ended slot with a gap piece secured
by a bolt. The wedge method is
again employed for adjustment, but
in this case is drawn in by means of a
bolt against a tapered face in the rod
itself. Many other variations of these
bearings were used, but the same
principle of adjustment was applied.
portion of the valve stem is made
larger in diameter than the shank
which passes through the gland, and
a washer or neck bush prevents
packing from jamming in the thread.
The main object of this form of
design is to prevent any likelihood of
the spindle being unscrewed out of the
valve and getting blown into the face
of the operator-not by any means
an impossibility with some types of
stop valves used in the past. These, I
believe, have long been banned on
full-size engines and boilers by Board
of Trade regulations.
Both right-angled and straight-
through types of stop valves are used
on engines, according to convenience,
the latter being more difficult to make
in small sizes, though methods of
doing so have been described several
times in MODEL ENGINEER. In either
type, the entry side should be under
the valve head, so that the gland is not
under pressure when the valve is
closed.
Fig. 43: Standard ” Plummer block ”
type pedestal bearings
the functonal design may be noted;
first, that the actual valve head is
separate from the stem and free to
rotate upon it, being retained in place
by a tangential pin which intersects
the groove in the stem. It may be
made with a spherical curve outside,
and also on the inside thrust face,. as
shown; this gives it a self-aligmng
property, though the “mitred” 90-
deg. head and flat internal thrust
face is more common.
Secondly, the valve seating in the
body is raised, so that metal is available
for re-facing if it becomes necessary,
and grit or scale tends to fall away
from the seating instead of remaining
to become imbedded when the valve
closes. Last, but not least, the threaded
STOP VALVES
In many steam-engine models, real-
ism is marred by the use of crudely-
shaped auxiliary fittings, such as
drain cocks or stop valves, which may
serve their purpose but are quite out
of keeping with the rest of the design.
An example of a stop valve which is
correct both internally and externally
is shown in Fig. 46. It is best adapted
for construction from castings, or at
least one for the body, but could be
fabricated or machined from the solid.
One or two important points about
REVERSING GEAR
It has already been mentioned that
the majority of stationary engines are
made to run in one direction only, as
the need to reverse them simply does
Left, Fig. 44: An
alternative type of
pedestal
bearing,
cast integral with
the engine bedplate
Right,‘ Fig. 45: Al-
ternative types of
” gib and cotter ”
connecting-rod bear-
ings
842
13 JUNE
1957
MODEL ENGINEER
Left, Fig. 46: A stan-
dard type of right-
angled stop valve
for the manual con-
trol of steam supply
STANDAPD GLAND
Right, Fig. 47: Dia-
gram showing the
positions of eccen-
tric in relation to
crank, for forward
and reverse rotation
\”
With this arrangement, an engine
once started in a given direction will
automatically take up its correct
timing so as to continue to run in that
direction.
The “ slip ” eccentric, as it is called,
is very popular for simple model
locomotives and marine engines, and
has also been applied to quite large
engines in conjunction with some
means of disconnecting the slide
valve and operating it by hand .for
manoeuvring. Such engines are not
positively self-starting but this applies
to all single-cylinder engines in any
case, and is not necessarily a practical,
disadvantage. A typical example is
the Trojan marine engine.
There are several types of positive
reversing gears used on steam engines,
most (if not all) of them being de-
signed not only to effect the required
phase shift, but also to vary the
admission cut-off, so as to employ
expansion to best advantage-in other
words, to“notch up,” the notches
being those in the reversing gear
control quadrant. Of these, the earliest
best known, and still most universally
popular, is the so-called Stephenson
link gear, though its title has often
been held in dispute, and is sometimes
ascribed to Howe, who was employed
by George Stephenson.
Muncaster gives an example of a
horizontal engine fitted with this form
of link gear (Fig. 48) which may
serve as a typical illustration of the
general design and proportions of its
working parts.
To be concluded
not arise. Certain types of engines,
however, such as those for hauling,
hoisting or winding, need to be
reversible-and this also applies to
marine engines of all kinds.
In dealing with oscillating engines,
it was seen that rotation can be
reversed simply by changing over steam
and exhaust connections, but this is
not practicable in the normal type of
slide-valve engine, especially if it is
provided with lap and lead for
efficient operation. A piston-valve
engine with no lap or lead, however,
can be reversed in this way, and it is
often done on engines for small
cranes, ash hoists and ships steering
gear, for the sake of simplicity and
the valve gearing through an angle of
180 deg., minus twice the angle of
advance.
This is illustrated in Fig. 47 which
shows the positions of the eccentric,
relative to the crankpin, for both
directions of rotation. The exact
angle, of course, will depend on the
amount of lap and lead employed,
and for an inside-admission piston
valve the angle of advance will be on
the minus side of 90 deg. from the
crankpin in each case.
The simplest method of obtaining
the required phase shift is by using a
single eccentric, not rigidly fitted to
the shaft but capable of a limited
angle of movement, so as to take up
the relative angles shown when
moved to the limit of travel either way.
convenience. Generally, however,
reversing entails shifting the phase of
Fig. 48: A horizontal engine fitted with Stephenson link reversing gear
ADDITIONS TO THE LATHE
Instructions for making centring
holders
chucking accessories; tool
and cutter bars; dividing
simple milling attach-
ments;aids to screwcutting; and
steadying appliances are to be found
inEdgarT. Westbury’s
Lathe Acces-
Priced 3s.6d., postage 3d. (U.S.A.
and Canada $l.00), it can be obtained
from Percival Marshall and Co. Ltd,
19-20, Noel Street, London, W.1.
13 JUNE 1957
843
MODEL ENGINEER
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