EADF2d01.pdf

The MUNCASTER

steam-engine models - 10

Continued from 13 June 1957, pager 841 to 843

EDGAR T. WESTBURY concludes this informative series with

details of reversing gear and notes on Grasshopper engines

A LTHOUGH the Stephenson link

stroke must be made greater than the

required valve travel. Modified types

of links, such as the “launch ” type,

eliminate this discrepancy but involve

their own geometrical problems.

gear is well known to most

readers, some notes on its

basic principles and essential fea-

tures may be helpful to those who

are approaching steam-engine con-

struction for the first time, espec-

ially as it is often the subject of

elementary queries.

Many readers who wish to design

their own engines seem to find some

difficulty in Iaying out the valve gear

arrangement and adjusting it to give

efficient results, despite the amount

of practical advice on these matters

which has been published in MODEL

ENGINEER at various times.

If an error is made in the dimensions

of the rods or links, valve setting will

be impaired. One rod longer than the

other will result in unequal port

openings either in forward or reverse,

or both; an error in the link radius

will alter the settings as the gear is

linked up. Lost motion due to slack

fitting in any of the parts will result

in reduction of valve travel and

retardation of timing. This is but a

brief sketch of the working principles

of reversing gear; some, excellent

articles analysing the details have been

contributed to ME by writers much

better qualified to do so than myself.

‘/8”D i A. ,

This form of reversing gear employs

two separate eccentric sheaves, which

are positively fixed to the engine shaft

in the positions as specified in Fig. 47,

and these are connected by the usual

straps, and rods of exactly equal

length, to a curved slotted link, the

radius of which i’s equal to the distance

from the centre of the eccentric strap

to the eye of the rod. Connection to

the valve rod is by means of a die

There are, of course, a good many

other types of reversing gears besides

the Stephenson link employed on

steam-engines, and some of them are

claimed to have advantages for certain

purposes, but they are generally more

complicated in construction and more

difficult to understand properly. Not

only is the Stephenson gear by far the

most popular type for the class of

engines under discussion, but it is

also one of the most satisfactory in

practice because of its robust con-

struction and the relatively small

number of articulated working parts.

Occasionally, however, rather un-

usual types of steam-engine valve

gears are encountered, and queries

have been received from several

readers on this subject, particularly

in respect of single-eccentric reversing

gears, such as the Ha&worth and

Marshall types. Space does not allow

detailed discussion of these here, but

they are fully explained in the ME

handbook Locomotive Valve Gears by

Lake and Reidinger, and there have

also been quite a number of articles

on valve gears published in ME at

various times.

One of the most uncommon revers-

ing devices I have ever seen was that

fitted to the engine of a drifter in the

Firth of Forth; it had a single

eccentric, the timing of which was

controlled by endwise movement of

a spirally-splined sleeve inside the

CROSSHEAD

block which is a sliding fit in the slot

of the link, and means are provided

of raising or lowering the link so as

to bring either eccentric rod into

SLIMS (2)

operation on the valve rod, or to any

intermediate position.

Thus the rotation of the engine can

be reversed by means of a simple

manual control,. which also serves to

vary the expansion ratio as required.

The control gear (not shown) consists

of a lifting link and a connecting or

“ reach ”rod operated by either a

direct-acting lever or a quick-thread

screw; large engines often have power-

operated reversing gear.

The essential components for the

reversible engine are shown in Fig. 49.

Important points to be noted are that

as the eccentric rods are side by side,

the end forks must be offset in alternate

directions to line up with the link,

but this offset should be as small as

possible to ensure efficient mechanical

action. The type of link shown is the

most common, but it does not enable

the eccentric rod to be lined up exactly

with the valve rod due to the limits

of the slot, and, therefore, the eccentric

_LYJ

\_-

eccentric sheave. As the engine was

old and well worn, there was a good

deal of slack in all the working parts,

which could not be taken up as readily

as in more conventional valve gears,

and it also lacked any provision for

notching up to shorten cut-off.

GRASSHOPPER ENGINES

Several readers have asked me for

some information on the Grass-

hopper type of beam engine, and I am,

27 JUNE 1957

920

MODEL ENGINEER

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27 JUNE 1957

921

MODEL ENGINEER

MUNCASTER

MODELS . . .

therefore, giving a design of an engine

of this type by Muncaster which was

published in ME in 1934.

The special feature of the Grass-

hopper engine is the use of a single-

ended beam anchored from one end,

instead of being pivoted on central

trunnions, as in the orthodox typeof

beam engine. The advantage of this

arrangement is that the working end

of the beam can be connected directly

to the piston rod of the engine, and

in this way it is possible to make the

engine much more compact especially

in the matter of height, as it eliminates

the space taken up by the usual swing-

ing links between the beam and the

piston rod crosshead.

The length of the beam, or lever as

it is called, can also be reduced;

whereas in the normal beam engine

the beam is usually at least three

times the stroke in total length, that

of the Grasshopper is generally about

1-3/4 times the length of the stroke and

its weight is also proportionately less.

The working end of the beam is

guided in a straight line by means of

the usual parallel motiondevice,

suitably modified for thisparticular

purpose, and it is, therefore, necessary

for the anchored end to be capable

of lateral movement. This is provided

for by a swinging link arrangement,

which was usually made in the form

of a cast rectangular frame with

lattice, or cross strutting.

Although the weight of the Grass-

hopper beam is completely unbal-

anced, the fact that it is much lighter

than the orthodox beam enables

engines of this type to be run quite

satisfactorily at higher speeds than the

former type. The compactness of the

general engine structure provided

inherent rigidity, and also simplified

mounting and installation, with very

simple ashlar or brickwork founda-

tions. The engines were, however,

often sunk into the floor, or elevated

to give space for an air pump and

condenser under the bed and a race

was provided to give clearance for

the flywheel.

The air pump was sometimes driven

from the gudgeon pin of the parallel

motion, which was extended to carry

the pump rod clear of the sweep of

the crank pin. In some cases, however,

an air pump having a stroke of about

a third or a quarter of the cylinder

stroke, and of correspondingly greater

diameter, was employed, operated

from a gudgeon pm nearer the

anchored end of the beam.

MODEL ENGINEER

Fig 50: Elevation and plan of Grasshopper engine

It will, of course, be apparent that

the crank throw was considerably

less than the half-stroke of the piston,

but, even so, there was not much

clearance between the cylinder and

the sweep of the crank. For this

reason, gib and cotter crankhead

bearings were sometimes found un-

suitable, and many of these engines

were fitted with bolted split brasses

of the marine type, as in the example

seen in Fig. 50.

The connecting rod was made as

light as practicable, and was, there-

fore, more often made of steel or

wrought iron than of cast iron, as in

many ordinary beam engines.

This particular design is for an

engine of the high-pressure, non-

condensing type, working on about

922

30 p.s.i.,though some of the later

examples of engines worked at con-

siderably higher pressures, as more.

knowledge was available on the design

of boilers and the safe stresses which

could be allowed. Some of these

engines were built up to well after the

beginning of this century.

Among the distinctive features of

the design may be mentioned the

rather unusual method of driving

the vertical shaft of the governor from

a subsidiary or“ follower ” crank

from the overhung crank pin. Al-

though not specified in the design,

this follower crank could also be

used as a means of conveying lubric-

ating oil to the crank pin; this feature

was, however, more characteristic of

Continued on page 943

27 JUNE 1957

they very kindly sent me photographs

of the original and an invitation to

view it in-their showrooms.

Recently I went to the model

engineer:s paradise, the Science Mus-

eum, and there was the model on

show.

If any reader has any drawings on

the Silver Ghost I should be nleased

to contact him.

Surbiton,

Rust in the

continued from page 933

home workshop

’ _ ‘\

lamp with this connection is much

lower, as each burns with a red glow

and does not cause scorching, even

if held against the human skm. In

addition, this arrangement distributes

the heat better than one lamp if the

two lamps are placed some distance

apart.

The only lamp which could safely

be used alone is a 5 w. neon -type

lamp sold as a night light. This

would, in all probability, give out

sufficient heat for the cover for a

small lathe.

A lathe cover to enclose the average

3-1/2in. lathe of the Myford type, with

its motor, countershaft and, possibly,

a small cupboard for valuable tools,

would be about 4 ft long, 2 ft high

and 2 ft wide.The arrangement of two

40 w. lamps in series keeps this size

of enclosure about 8 deg. F higher

than the temperature outside the cover

and should be ample to prevent

condensation and rust. And if a

bag of silica gel is placed inside the

cover, the lathe could be left for

weeks on end without fear of deteriora-

tion (Fig. 2).

It may be felt that leaving a couple

of lamps burning day and night will

be costly. This is not so. The sug-

gested arrangement of two 40 w.

lamps in series will give an electrical

load of 20 w. only, and this will,

consume one unit of electricity in

50 hours. This is a smal1 price to pay

for freedom from worry about the

lathe, as, with electricity at Id. per

unit. the cost of one cigarette will

look after the equipment for a period

of four days. The same cost would

keep a 5 w. lamp burning for 16 days.

There are alternative forms of low

temperature electrical heat. If there

is room, a low-temperature tubular

heater can be used, and the loading

G. R OGERS .

Surrey.

DISPLAYING MODELS

Sir,-This is my model engineering

jubilee year. Of ten models I have

built, seven have won awards at

provincial exhibitions, includmg several

firsts. The other three were simple

models.

If I were asked why I had been

fairly successful I would say it was

because I have always made myself

conversant with the prototype.

With the increasing rate of modern-

isation, future model builders. will

have to rely more and more on

museums, but cannot something be

done to improve the standard of

models shown in museums ? Where

an occasional masterpiece is on view,

it is often as not set between items

of junk. Too frequently the labels

tell more about who presented the

model than about the model itself.

I believe model engineering societies

could also improve the quality of

members work by taking a lesson

from photographic societies and having

meetings devoted to criticism. The

critic is often unpopular with model

engineers.

May I appeal to model steam engine

builders to demonstrate their models

at exhibitions under steam, then

visitors will know that faults are not

being concealed ?

At five Northern model exhibitions

I have shown the only steam driven

models working.

Shiply,

of this is usually 60 w. per ft. Alter-

natively, soil heating or anti-frost

heating cable can be used at a loading

of about 75 w., but for the average

lathe cover this is more than required,

and it entails extra expense in instal-

lation and running costs.

Muncaster models continued from page 922

horizontal direct-acting engines.

The swinging anchor link of this

engine is shorter than on many

Grasshopper engines, being pivoted

from a trunnion on the level of the

entablature frame, which is supported

from two pillars at this end and from

lugs on the cylinder at the other.

More commonly, the anchor linkage

was pivoted from bearings on the bed

plate or engine floor.

The radius rods of the parallel

motion are anchored to vertical

pillars supported from the cylinder,

and stiffened by diagonal struts. A

normal type of slide valve is employed,

operated by means of an eccentric on

the engine shaft, by way of a horizontal

eccentric rod and rocking levers, on

a cross shaft at the left-hand end of

the bed plate. Governing is by means

of a crossed-link centrifugal governor

operating a butterfly throttle valve,

seen immediately below the steam

chest. A plunger feed pump is

provided, operating from a gudgeon

pin near the anchored end of the

lever at approximately l/5 of the

cylinder stroke.

For readers who wish to obtain

further information on Grasshopper

engines, it may be mentioned that

an excellent article, with complete

working drawings, of an Easton and

Amos Grasshopper Engine of 1862

by A. Ebeltoft, a Norwegian reader,

was published in the issue of ME

dated 31 March 1949.

Although there is much more which

could be said about the Muncaster

designs, and I have received many

letters from readers commenting on

them and raising points for discussion,

this series has already run beyond its

allotted span, and there are urgent

demands for articles on other subjects.

I propose, therefore, to take leave of

Muncaster, at least for the time being,

but I assure readers that I shall be

returning to the subject of steam-

engine construction again as soon

as possible.

F RANK D. WOODALL.

Tommy Dodd continued from page 923

and would defy all use of the few

controls to move them ! Should the

engine by chance be in gear, I have

seen two men on the gear lever and

another with a crowbar develop

maximum horse power in order to

shift it. But the language. . . .

trouble with schoolmaster and parents

alike, but they were very happy days.

Incidentally. while on the subject

of single-engine ploughing, a well-

known farming journal published a

picture in the early days of the war

of a somewhat similar engine working

near Rye, Kent. The winding drum

was on a truck alongside the engine

in the field, the crankshaft being

connected by a universal joint to the

winding gear.

Perhaps a reader in that area might

feel interested enough to fill in the gap,

especially about the anchor truck. ¦

943

This engine was taken with my first

camera at Bower Farm, Eynsford, in

1915. It was broken up soon after-

wards, when the first of those huge

picturesque Fowler compounds super-

seded them. The magnetic attraction

of these giants, be it high summer or

deep winter, was the cause of much

27 JUNE 1957

MODEL ENGINEER

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