8ECFFd01.pdf

The MUNCASTER

Edgar T.

Westbury

steam-engine models

4-Horizontal Stationary Engines

Continued from 21 March 1957. pages 420 to 422

Thus the slightest movement either

way opens one or other of the cylinder

ports to exhaust. Occasionally,

however, engines are timed to give

either positive or negative

exhaust lap, by narrowing or widening

the valve cavity; the latter is the more

common and its object is usually to

avoid excessive “ cushioning ” or

compression, or to eliminate risk of

back pressure in the exhaust system.

The conventional stationary or so-

called “ mill ” engine forms an ex-

cellent exercise in steam-engine con-

struction, and is deservedly popular.

Several examples of these engines were

N DESCRIBING the function of

the slide-valve and the effects

of lap and lead [Fig. 17, March

12], no particular mention was

made of exhaust timing. It would

be a mistake to regard this as

insignificant, but it is generally

satisfactory to allow it to keep in

phase with the steam admission,

as it must inevitably do so in a

simple slide-valve, and it is usual

to make the inside edges, in other

words the width of the valve

cavity, such that they just, and only

just, span the inside edges of the

cylinder ports at mid travel (right-

hand diagram).

Right, Fig. 20: A

cross section of

steam-chest and a

portface, showing

the port dimensions

Below, Fig., 19: A

horizontal mill en-

gine, 1 in. bore x

1 in. stroke, with

bar crosshead slides

488

MODEL ENGINEER

4 APRIL 1957

on a long box-section bedplate, with

machined facings where required, and

it is desirable to machine these in

order to ensure accurate location and

secure mounting of the parts, but in

the absence of proper facilities they

may be filed and scraped, accuracy

being checked by means of a surface

It will be seen that both engines

have an outboard crankshaft bearing,

or “pillow block,” which must be

accurately lined up with the other

bearing mounted on the bedplate;

for this reason the actual foundation

on which the latter is mounted must

also be flat and true. It may be made

from a thick slab of well-seasoned

hardwood, with a cavity cut out to

clear the flywheel.

Above, Fig. 23: Details

of the crank and eccentric

Below,

bore x

24: A horizontal engine, 1-1/2 in.

in. stroke, with a slipper crosshead

similar in major features but differing

in size and certain details. The first,

shown in Fig. 18 [March 21], is

scaled down from a fairly large engine,

12 in. bore x 18 in. stroke, in the

proportion of 1 in. to l ft.

No dimensions are given on the

actual drawing, but Muncaster gives

a table of dimensions worked out to

exact scale. I have taken the liberty

of modifying some of these to give

round fractional figures, as most

constructors would undoubtedly wish

to use standard drills, reamers and

stock materials wherever possible;

but general proportions have been

closely adhered to.

The cylinder dimensions for the

1 in. scale model are 1 in. bore x

1-1/2 in. stroke. Piston rod diameter

3/16 in.,crosshead fin. wide inside

fork, with crosshead pin 3/16 in. dia.

Connecting rod, circular section, fish-

bellied, 3/16in. dia. at the two ends,

swelling to 1/4in. centre; length be-

tween bearing centres 2-5/8 in. Crank-

shaft 7/16 in. dia., with journals reduced

to 3/8in. x 1/2in. bearing length;

crankpin 1/4 in. dia.‘ x 5/16 in. long.

Flywheel 5 in. dia. with rim section

1/2 in. square. Eccentric throw 7/64 in.,

rod circular section, tapered from

1/8 in. to 7/64 in. Valve spindle 7/64 in.

dia.,valve travel 7/32 in. Port

7/16in., exhaust 5/32in. wide. Valve

5/16 in., lead l/100 in. Steam inlet

1/4 in. dia., exhaust outlet 5/16 in.

As, the constructional methods for

this and the second engine are

generally similar, they may be con-

sidered together. In the latter case,

illustrated in Fig. 19, leading dimen-

sions are given on the drawing. Both

engines are intended to be built

from castings, though fabrication of

most of the components is practicable.

The main components are mounted

4 APRIL 1957

dimensions: steam-ports 5/64in. x

travel 7/32in. ,lap 3/64in., cavity

489

MODEL ENGINEER

Left, Fig. 21: Details of the flywheel. Above, Fig. 22: Details of the connect-

ing rod. Below, Fig. 25: An engine of similar dimensions with trunk crosshead

Many constructors, no doubt, would

prefer to make the engine self-

solid in articles on the Unicorn mill

engine.

when nearly all fitters had some

knowledge of smith’s work; but this

seems to be practically a lost art

contained by extending the cast bed-

plate to carry the outboard bearing;

Muncaster suggests forging parts

of this nature, which is very sound

advice-or at least it was in his time,

nowadays. There are, however, many

I have designed most of my engines

Continued on page 515

to avoid the need for extraneous parts

which have to be lined up. A single

bedplate with facings for all essential

parts, machined off to the same level

throughout, greatly facilitates accurate

construction.

The cylinders of both engines are

bolted down to the bedplate, being

provided with cast feet or flanged

brackets, and the bearings, which are

of the split plummer block type, are

similarly fixed. Details of the steam-

chest, flywheel, connecting rod, crank

and eccentric sheave, with its strap,

are given in Figs 20 to 23.

The main difference in the two en-

gines is in the type of crosshead

employed; the first example has die

blocks fitted to the extended ends of

the crosshead pins and working

between girder-shaped guide bars

mounted on pillars fixed to the bed-

plate, while the other has a slipper

type crosshead working on single

guide bars each side. Both types are

well established in full-size practice;

they require very careful lining up in

respect of height and parallelism with

the cylinder axis to ensure smooth

movement of the piston rod.

In this respect they may possibly

be more difficult to fit than the trunk

type of crosshead, in which alignment

is automatic if machining is properly

carried out; on the other hand, they

are sometimes preferred because they

offer facility of adjustment to com-

pensate wear or initial errors.

The crosshead of the first engine is

forked to admit the little-end eye of

the connecting rod, but in the second

arrangement matters are reversed by

forking the end of the rod to span the

slipper crosshead block. This type of

rod may be a little more difficult to

shape than the previous type, but it

looks very nice when properly carried

out and I have given instructions on

how to produce forked rods from the

MODEL ENGINEER

‘4 APRIL 1957

490

Reversing by radio Continued from page 499

FOR SHIP MODELLERS

Oil-bath lubrication is of course

employed, and I recommend the use

of a light oil, such as Shell Vitrea, so

as to keep down losses due to fluid

friction.. It is of course necessary to

fit an au vent or breather to the top

of the box, but I have not shown

this as ‘its position will have to be

arranged so as not to interfere with

any control gear fitted to the cover.

The box should be filled to a depth

of 1/2 in.

Various modifications of the details

of the gearbox may suggest themselves

to readers but the general design, in

which all really essential features are

incorporated, can be guarapteed

reliable and efficient.

One point which should be noted

about reverse gears of this type is

that the gears are only under load

when actually in reverse, which will

normally be no more than a very

small proportion of the total working

time; when running ahead, the gears

run idly and the only losses are those

due to running friction and oil drag,

which can be kept very low; thus

there. is no appreciable reduction in

the performance of the boat.

John N. C. Lewis, in Ship Modeller’s

Logbook, has produced the ideal book

for the enthusiast. He shows the

reader how to make simple, decora-

tive, miniature and scenic models,

including a clipper ship in a bottle

and an Arabian Baggala.

There are details of clench and

carve1 built models, and the text

is enlivened by stories of eighteenth

century smuggling and the work of

the Revenue Cutters.

Obtainable from Percival Marshall

and Co. Ltd, price 12s. 6d., postage

1s. (U.S.A. and Canada $3.00).

Muncaster.

models . . ,

Continued from page 490

crosshead of the slipper type, working

on a slideway machined on the top

surface of the bedplate, with keep

plates secured by three studs or bolts

on each side. A rather unusual

method of fixing the cylinder is em-

ployed, the underside of the casting

being provided with four projections

or feed, into which setscrews or studs

are screwed from the inside of the

bedplate.

In addition, two vertical lugs are

extended upwards from the bedplate

and drilled horizontally to take ex-

tensions of two of the cylinder cover

studs. This undoubtedly gives addit-

ional strength to resist working

stresses, but may be regarded as

“ gilding the lily,” to use a. popular

misquotation.

The height of the cylinder, cross-

head and main bearing centres must

all be exactly the same, and careful

adjustment in machining or fitting will

be necessary to ensure this. A vew

slight error in the height of the shaft

centre would not be harmful; but any

misalignment of cylinder and cross-

head would cause binding of the

working parts, and possibly gland

trouble.

Errors of this kind are often

botched up by makng the parts a

sloppy fit, which certainly enables the

engine to run, with sundry clanks and

groans, but such slovenliness should

never be tolerated in model engineer-

ing.

In the second example, Fig. 25, a

bored“trunk ” guide is employed,

similar to that which I have described

for the Theseus and Perseus engines,

and this enables all uncertainty in the

alignment of cylinder and crosshead to

be eliminated,. so long as machining is

properly carried out. The trunk in

this case is cast integral with the

baseplate, therefore, unless special

machining facilities are available, it

will be necessary to mount the casting

515

on the lathe saddle, packed up to the

exact centre height, for boring the

guide seatings and facing the rear

flange.

It can then be swung round for

boring the main bearing housings,

the caps of which should previously

have been fitted, and facing both

their inner and outer sides. All these

operations can be carried out with

boring bars between ‘centres, and

fitted with suitable cutter bits. The

centre lines of the trunk and main

bearings must be exactly at right

angles to each other.

It will be noted that the bearings

of both engines are of the same type,

also the crankshaft, which may be

either machined from the solid or

built up as indicated in the details

included in Fig. 24. The journals and

crankpin in this case are made a

press fit in the webs, and accuracy

should be checked after assembly,

positive location then being ensured

by fitting pegs or dowels endwise as

shown.

The front end cover of the cylinder

(Fig. 25) is sandwiched between the

flanges of the cylinder and the trunk;

it must be accurately machined on

both sides, including the boring of

the- gland, to avoid introducing align-

ment error. I have described how to

ensure this in connection with previous

engines.

In other respects, the components

of the two engines are identical,

including’ the connecting rod and

crankhead bearing, eccentric sheave,

strap and rod (Fig. 24), and the slide-

valve with its rod and knuckle (shown

in Fig. 25); it will be seen that the

method of attaching the valve to the

rod, to allow free “ floating ” location,

is the same as in the Theseus and

Perseus engines; namely, by reducing

either the width or diameter of the rod

to fit a slot cut in the back of the valve.

To be continued.

MODEL ENGINEER

components in model engines which

could be produced more efficiently

and economically by hand forging

than by any other method, and a

revival of this skill would be well

worthwhile.

The crankhead bearings or “ big

ends ” of both engines are of the solid

box type, with split brasses of rect-

angular section fitted. These were

usually secured by a tapered gib and

cotter, which enabled the bearings to

be adjusted for wear; but Muncaster

shows them simply located in place

by a screw, which passes through a

notch in the rear half-bearing to

prevent its moving sideways. The front

half would presumably have side lips

to engage the sides of the box frame

for the same purpose; removal of the

screw would enable the rear half to

slide back enough to withdraw it over

the collar of the crankpin.

Governor gear is fitted to the first

engine, and could be adapted to the

second- if desired; some further in-

formation on this subject will be

given later.

Two further examules of horizontal

stationary engines are illustrated in

Figs 24 and 25. Each has its own

distinctive features,, but many of the

working parts are sunilar or identical;

the cylinder dimensions are l-1/2in.

bore x l-1/2in. stroke, which may be

considered on the large side by many

constructors, but it would be quite

practicable to reduce the scale to half

these dimensions or even less if

desired.

The‘ first of these engines has a

4 APRIL 1957

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: