E5072d01.pdf

The MUNCASTER

3-Simple

steam-engine

slide-valve

models

EDGAR T. WESTBURY is reviewing some classic models of the

past in the light of modern techniques

Continued from 7 March 1957. pages 337 to 339

engines

N THE ISSUE of February 21, it

was stated that an oscillating

engine can be made to reverse

its direction of rotation simply by

changing over its steam and ex-

haust connections; this applies to

all engines timed to work without

lap or lead, whether single or

double-acting, and with any num-

ber of cylinders, with the exception

of those having flat slide valves.

It is therefore possible to equip

any of these engines with a simple

reversing control, consisting of a four-

way change-over cock or valve, and

such a control is indicated in the plan

view of the double engine, Fig. 11.

Details of the reversing valve are

given in Fig. 12, where it will be seen

to consist of two essential parts, a

stationary portblock, with four ports

and their connecting pipes, and a

rotating valve with interrupted annu-

lar groove just like the portblock

shown in Fig. 9. These parts are

lapped to fit truly together and held

in friction-tight contact by a spring

acting on the centre pivot.

When the valve is turned into such

a position that the blanked portions

close two diametrically opposite ports

in the block, communication to the

cylinders is shut off; this is the “ stop ”

position. By moving it in a clockwise

direction,. however, steam is admitted

to the pipe ‘marked R, and exhaust

connected to L; while moving it the

other way reverses these connections.

I would suggest, in order to improve

the seal of the valve face and make

it less critical in angle of movement,

that the width of the groove stop

should be increased by fitting a peg

of larger diameter, or other means.

A VERTICAL BOILER

The boiler recommended for either

of these engines is shown in section

in the elevation drawing, Fig. 13, and

in plan view, Fig. 14. It is preferably

made in copper, about 3/64 in. thick,

or 18-gauge, for the shell, with end-

plates 1/16 in. thick or 16-gauge, which

can be beaten or spun to the shape

shown. For relatively low pressures,

which should be perfectly satisfactory

for these engines, riveting and soft

soldering will be safe enough, though

as a precaution in case the boiler

should ever run dry the joints of the

inner flue and cross tubes, which

are in contact with the flame, may

with advantage be silver soldered.

The flue is l-1/4in. dia. by 16-gauge,

with cross tubes 3/8 in. dia. by 20-gauge,

and the steam-pipe, which may either

be brazed directly into the top of the

boiler or fitted with a union joint, is

3/16 in. dia.

Whatever other fittings are attached

to the boiler, a safety-valve must

on no account be omitted; this may

be of a simple type, combined with the

filler plug, as shown in Fig. 15. The

boiler should be hydraulic tested to

stand 50 p.s.i. without leakage or

distortion, and the safety-valve spring

(which should be of rustless steel or

phosphor-bronze) adjusted so that it

lifts at 35 lb. Firing may be by

spirit, paraffin or gas, as described in

previous articles.

SIMPLE SLIDE-VALVE ENGINES

The majority of steam-engines,

large and small, are equipped with

slide-valves for the distribution of

steam, and of these, the simpler types

employ an eccentric (which is essen-

tially a form of crank) for direct

operation of the valve in fixed phase

relation to the piston stroke.

To anyone who intends to build

engineering models of any kind, I

consider that a practical under-

standing of the simple slide-valve

engine is essential, and one of the

first essays in construction should be

devoted to producing a working

model of such an engine. Judging by

the many queries received on this

subject, it would seem that the steam-

engine does not receive the attention

it deserves in elementary technical

Right, Fig. 11: Plan r

crankshaft and (at

the bottom) cylin-

der and the rever-

Left, Fig. 12: The

FIG. II

MODEL ENGINEER

education, as many intending con-

structors do not appear to have

mastered its basic principles. I feel

sure, therefore, that more experienced

hands will not grudge a little space

devoted to this important subject.

In my own articles on steam-

engines, I have always recommended

beginners to start right away on

building a slide-valve engine, without

worrying about the conventional oscil-

lating engine,which is generally

regarded as the first stepping stone to

progress in construction.

My reason for this advice is not

simply because it is possible to attain

higher efficiency with the slide-valve

-though this is an undisputed fact

-but because the latter gives facilities

for the observation of valve events,

also for checking and experimenting

with timing; in this way it teaches

the constructor more about engine

functions than is ever possible with

an engine in which the means of

steam distribution are both invisible

and immutable.

When once the principles of the

direct-acting slide-valve have been

mastered, one can, if one so wishes,

go on to study the more complicated

valve gears which enable the ex-

pansion of steam to be controlled or

rotation to be reversed.

Muncaster quite rightly devotes

careful attention to the elementary

principles of slide-valve operation

and gives an isometric section of a

steam-engine cylinder and steam-

chest which I reproduce here (Fig. 16)

to illustrate the essential features.

The piston, P, is shown at about half

stroke, moving in the direction of the

arrows, and the slide valve, V, is in

the appropriate position in relation

to it at this stage.

Steam is admitted under pressure

to the steam chest, C, tilling the space

around the back of the valve, which

has a flat face in contact with a

stationary flat surface in which three

ports open into passages, SS , leading

to the respective ends of the cylinder,

and the central port, E, is in com-

munication with atmosphere, or the

exhaust pipe system.

Both the piston and the slide-valve

are connected mechanically to the

external working parts by rods which

pass through packing glands, FG, to

avoid leakage of steam at the openmgs

in the cylinder and steamchest.

At the position illustrated, steam is

being admitted to the rear or closed

end of the cylinder and forcing the

piston outwards; the valve mean-

while is moving in the opposite

direction, so that it will cut off the

steam supply as the piston nears the

front end of the cylinder. During

this period, the other side of the

piston is in communication, through

the passage, S , and the cavity in the

centre of the valve, with the exhaust

port, E, so that the steam in this space,

which has already done its work, is

displaced by the piston and is free

to escape.

By the time the piston has com-

pleted its outward stroke, the slide-

valve has moved to commence open-

ing the front end of the cylinder to

live steam and the rear end to exhaust,

so that the motive force on the piston

is reversed and it starts on its return

stroke.

An engine of this type is said to be

double-acting, as power is applied to

the piston on both forward and

return strokes; it is the orthodox

arrangement for most types of steam-

engines, though single-acting engines,

in which power is produced on one

side of the piston only, are employed

for certain duties, particularly where

it is desirable to keep the weight of

working parts as low as possible for

the attainment of high speed.

Fig. 16: Section through cylinder

and steam-chest of slide-valve engine

Fig. 14: Sectional plan of boiler,

showing cross-tubes

Below, Fig. 13:

vertical boiler

Section of simple

a twin engine

FIG. 13.

LAP AND LEAD

In early steam engines it was usual

to make the closing faces or lips of

the slide-valve on either side of the

23 t

ports are formed.

The two outer

Right, Fig. 17: Dia-

grams illustrating

“ lap ” and ” lead,”

showing correspond-

ing positions of the

crank and eccentric

(clockwise rotation)

Below, Fig. 15: A

simple safety-valve

FIG. 17

VAWE

J I-LAP

j--

___-

421

MODEL ENGINEER

Incidentally, I may mention that

in my directions, for timing steam-

engines in the past I have occasionally

been criticised for recommending

“ rule of thumb ” methods-in other

words, timing in situ by turning the

crank to dead centres and checking

up on the actual valve position. It

might be considered more “ scientific ”

to deal in exact angles of advance,

and this would be absolutely necessary

in an engine having the eccentrics

machined integral with the crank-

junction with the previous drawing,

central cavity exactly the same width

should make them quite clear.

as the steam ports they controlled.

It will be seen that the lips of the

The valve was timed to move at 90

valve are extended in width (or,

deg. in advance of the crank, so that

strictly speaking, length, in the direc-

the ports commenced to open exactly

tion of travel) so that they overlap

at dead centres and a full piston

the ports, SS , when in mid-travel, as

stroke was occupied both for steam

seen on the right; the extent of the

admission and exhaust.

overlapping is termed “lap.” Such

It was soon found, however, that

a valve, if timed to move at 90 deg.

better working efficiency could be

in front of the crank, would give

obtained by advancing both the

delayed steam opening and thereby

opening and closing points, but not

reduce efficiency; to compensate this,

necessarily in the same ratio. Early

Fig. 18: Horizontal mill engine to 1 in. scale

F IG. 18.

steam opening could be used to

cushion the piston movement, thus

bringing it smoothly to rest at the end

of its stroke and also to ensure that

the maximum effort was available to

start it on its return. By cutting off

the steam supply before the piston

reached the end of its power stroke,

the expansive poroperties of the steam

could be used to complete the stroke,

so that by the time it was released to

exhaust most of its energy had been

usefully expended.

shaft, or otherwise rigidly pre-located.

But in actual practice the measure-

ment of angles on very small engine

components is extremely difficult-

one might easily make an error of

two or three degrees in the location

of a keyway on a small shaft, for

instanceand, moreover, it is not

always advisable to regard valve

setting as being“immutable as the

laws of the Medes and Persians.”

To be continued

WIN A MYFORD

SUPER SEVEN LATHE

Readers are reminded that the

closing date for the MODEL ENGI -

NEER competition, the prize for

which is a Myford Super Seven

lathe, closes on April 5..

the timing of the valve is advanced

by shifting the eccentric so that it

begins to open the steam-port slightly

before the crank reaches dead centre;

the amount of opening at the latter

point, as shown on the left, is known

as “ lead.”

The diagrams above the valve

sections in each case show the relative

positions of crank and eccentric to

produce the required timing; on the

left, the crank is on its dead centre

and the eccentric approximately 120

deg. in advance of it-in the usual

terminology, the“ angle of advance ”

is taken as the amount extra to the

original 90 deg. in front of the crank,

so in this case it would be reckoned as

30 deg. The right-hand diagram shows

the eccentric at mid-stroke, still leading

thecrankbythesameamount,ofcourse.

The means employed to attain this

result are by designing and timing

the valve to produce what is known

as “ lap and lead.” I have been asked

so many times to explain these terms

that I consider it worth while to

reproduce another of Muncaster’s

diagrams (Fig. 17) which, in con-

422

MODEL ENGINEER

21 MARCH 1957

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