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The MUNCASTER

steam-engine models - 8

Continued from 16 May 1957, pages 700 to 702

In this instalment EDGAR T. WESTBURY discusses governors and control gear

plicates matters because there is a

definite difference between the action

of a spring and that of gravity, but

in practice governors fitted with

springs work quite well. The governor

illustrated in Fig. 38 has a com-

pression spring fitted on the centre’

shaft, which exerts pressure between

the fixed upper yoke from which the

weight arms are suspended, and the

lower sliding yoke carrying the grooved

collar which operates the control lever.

This design has several features in

common with the governor of the

Tangye engine previously referred to,

but the method of articulating the

links is somewhat different. Instead

of forking the top ends to embrace

the ball, the latter is made hollow

and the link is made ball-ended to

fit inside it; in either case the pivot

pin passes diametrically through the

centre of the ball.This is a very neat

arrangement, but it decreases the

weight of the ball substantially so that

it must be made larger to produce the

same centrifugal effect.

To transmit the movement of the

sliding yoke to the governor lever

with the minimum friction, a horse-

shoe-shaped thrust collar is fitted to

the groove,with extended pivots

engaging the eyes of short arms which

straddle the yoke and are pinned

to the lever shaft. These details are

illustrated in Fig. 39. The governor

shaft runs in a long vertical bearing,

in a bracket mounted on the engine

frame, and is bevel-geared to the

EVERALL of the engine designs

in this series have included

governor gear, which is a

necessary fitting for engines which

have to maintain a fairly constant

speed under varying load conditions

and, in fact, practically any engine

which is not under the constant

supervision of the driver.

Nearly all steam-engine governors

are of the centrifugal type, based on

the original pendulum governor of

James Watt, and, in the earlier engines

at least, were usually made as a self-

contained unit, located at a con-

venient position on the engine to be

driven by belt or gears, and for

connecting up to a throttle control

valve. I have described governor

mechanism in connection with the

Unicorn and Tangye engines as well as

in a separate artical [ MODEL ENGINEER ,

27 October 1955] dealing with a

governor unit suitable for the Vulcan

beam engine or other early types.

Although the basic principle of the

governor is very easy to grasp-

indeed, it is obvious to anyone who

has observed the forces exerted in

rotating masses-a full explanation of

the theoretical considerations involved

in its design would take up far more

space than could be spared here.

Advanced text books on mechanics

usually devote one or two pages of

mathematical formulae to the pheno-

mena of centrifugal force (for example,

see Ganot’s Physics, para. 55), but

I propose to deal only with simple

practical applications of the governor

in the examples illustrated by Mun-

caster.

As first designed by James Watt, the

rotating ball weights of the governor

not only produced the positive operat-

ing effort, under the effect of centri-

fugal force, but also the negative or

restoring effort, under the effect of

gravity; no extra weights or springs

were used, and the vertical shaft

arrangement was essential for its

operation. Muncaster states that in

this form it cannot be recommended

for small engines, and my own ex-

perience supports this view; the reason

of course is that in a small size; the

gravitational effect is reduced to a

much greater extent than the friction

in the working parts.

The action of a governor relying

on the weight of the balls alone to

restore the control on reduction of

speed would be very erratic or

“ sticky.” Some governors, such as

the Porter type, have an additional

moving weight, which slides on the

centre shaft and supplements the

restoring force, but even this is limited

in its effects and has been found

inadequate in small sizes. It is also

necessary in most cases to increase

the positive effort by running small

governors at higher speed than the

full-size types.

The obvious method of increasing

the restoring effort is by fitting some

ind of spring. In theory, this com-

Left, Fig. 39: De-

tails of the link

pivots and thrust

collar for the ver-

tical-shaft governor __

Right, Fig. 40: A

horizontal spring-

loadedgovernor suit-

able for high-speed

engines

778

MODEL ENGINEER

30 MAY 1957

drive shaft which runs in a long

horizontal bearing.

For high-speed engines, such as

those used for driving electrical

generators, it is often found con-

venient to fit the governor directly

on the end of the main shaft, thus

avoiding the necessity for either belt

or gear drive, and making a compact

arrangement. In this case spring

return is an obvious necessity, and it

is also desirable to reduce the number

of working parts in the mechanism,

thus cutting down wear, friction and

any tendency to rattle. The governor

(Fig. 40) fulfils these conditions, as it

entails the use of only two pivoted

joints and a single sliding member,

with a totally-enclosed central com-

pression spring. This type is suitable

for the engine shown in Fig. 28.

It will be seen that as the balls

move outwards under the effect of

centrifugal force, the bell crank levers

to which they are attached press

against a fixed thrust washer, causing

the entire governor assembly to move

bodily to the left. The grooved collar

on the sleeve operates the governor

lever through a thrust collar, as in

the previous example, though this is

not shown. In the drawing, the shaft

extension which carries the governor

is shown as screwed into the end of

the main engine shaft, but there are

practical objections to this arrange-

ment, and I would suggest modi-

fying it.

Apart from the risk of the extension

unscrewing if he shaft ran in a clock-

wise direction or in the event of sudden

acceleration or stopping, there might

be difficulty in ensuring true running

of the extension shaft. It would be

better to make this integral with the

shaft or devise some more positive

scheme of fixing and alignment.

It is hardly necessary to add that

the governor assembly should be

symmetrical and in balance at all

speeds; this is of special importance

not only when it is fitted on a shaft

extension as in this case, but also

where it projects a long way above a

single vertical bearing as in the

previous example.

THROTTLE VALVES

The design of the control gear which

regulates engine speed under the

influence of the governor is of equal

importance to that of the governor

itself. The simplest method of

governing is by means of a throttle

or restricting valve, which may be of

any type so long as it is capable of

being operated with the minimum

effort; it does not need to be capable

of shutting completely, as it is gener-

ally a supplement to the main engine

stop-valve, which should be quite

steamtight when closed.

The type of throttle valve shown in

Fig. 41 is commonly used in full-size

practice and is recommended by

Left, Fig. 38: A

spring-loaded ver-

tical-shaft gover-

nor, bevel-geared

for belt or direct

drive

Right, Fig. 41:

Details of the

sliding piston-

type throttle

valve for gover-

nor control

MODEL ENGINEER

MUNCASTER

MODELS . . .

governor. Sometimes, in order to

obtain sufficient amplitude of move-

ment at the valve rod or spindle from

a relatively small governor movement,

multiplying levers are used in the

connecting mechanism, and the effec-

tive force is, therefore, reduced in

inverse ratio to the increased move-

ment. Thus the governor power must

be adequate to cope with this.

If, however, either the effective

weight or speed of a governor is

increased, it will come into operation

at a lower speed and thus, if the engine

speed is to remain the same, equili-

brium must be restored by increasing

the strength of the return spring or

counterweight. Erratic action or

“ hunting ” (alternate rise and fall of

engine speed) may be caused by

friction in the control gear or by

faulty governor design-this includes

too great a multiplication of lever

movement-so that the control valve

moves over too great a range with a

slight increase of speed. Steadiness of

control is sometimes improved by

fitting a damping device, such as an

air or liquid dashpot. Speed range

can be adjusted by varying the spring

tension, or fitting an external spring

which can be adjusted while running.

ACCURACY OF CONTROL

Although a governor is often

assumed to be capable of keeping the

engine speed exactly constant, this

is not so in practice because the

governor cannot effect any change in

the throttle position until some change

of speed has taken place. There must

obviously be some margin of error in

any type of governor, depending on its

design and control linkage, but more

still on workmanship and elimination

of friction. For most stationary

engines of small size, a margin of

five per cent. deviation from the set

speed is fairly reasonable, but for

special purposes, such as generating

electricity, closer accuracy is necessary,

and the permissible variation may be

less than one per cent. in some cases.

Geared or positively-coupled gover-

nors are preferred for accurate control,

as belt-driven governors may be

liable to variable slip-and their

reliability may be open to question.

Cases have occurred where broken

governor belts have caused serious

accidents, sometimes with loss of life.

Nevertheless belt driven governors

gave good results on steam-engines

for many years.

throttle valve, which results in lower-

ing the working pressure of the steam

before it enters the engine cylinder.

To obtain the best efficiency the steam

should be admitted at full pressure,

but cut off earlier in the piston

stroke so that it can be used ex-

pansively and its energy utilised to the

best effect. This is done in most

large engines by using governors which

vary the timing of the valve gear.

The governors themselves may be of

normal type though they are some-

times of special design; they may

operate on the normal slide or piston

valve through linkage comparable to

that of reversing gear, or on “ trick ”

valves with elaborate porting arrange-

ments. Corliss and drop-valve engines

have trip devices which close the steam

admission valve suddenly at different

cut-off points under governor control.

A very simple and effective ex-

pansion governor is that fitted directly

on the engine shaft-often in the

flywheel-and controlling a movable

eccentric, not only by reducing the

throw but also advancing the timing.

If only the throw and, consequently,

the valve travel were reduced, it

would certainly result in earlier cut-

off, of steam,and, therefore, be

effective in speed control; but it

would be uneconomical because the

admission point would be retarded

or, in other words, lead reduced in

the same proportion. To compensate

for this, therefore, the eccentric is

advanced as its stroke is reduced.

As expansion governors are a

specialised type in which only a

limited number of readers are likely

to be interested, I do not propose to

describe them further. But for the

benefit of those seeking further in-

formation: it may be noted that they

were fully dealt with by Muncaster

in a series of articles in Vol. XXVI

(January to June 1912) of MODEL

ENGINEER.

Muncaster for small engines. It is

similar to that specified for the

Tangye engine, but I have found it

more difficult to employ successfully

than types which have a rotary move-

ment, such as barrel or butterfly valves.

The reason for this is that it takes less

effort to rotate a shaft in a packed

gland than to slide it bodily through

the gland, as in this case.

However, the example illustrated

is certainly capable of working suc-

cessfully on the larger models for

which it is obviously intended, having

a bore diameter of 3/8in. Steam must

be admitted from the left-hand

(horizontal) branch, the vertical pas-

sage being connected to the engine.

It is fitted with a liner having three

ports which open into an annular

passage, so that pressure is even all

round the valve and there is no

tendency to press it against one side

of the liner. The sliding piston should

be a smooth, easy fit in the liner, the

two parts preferably being of dis-

similar metals, such as steel and cast

iron, or brass and hard bronze, to

give good wearing properties. A hole

must be drilled through the piston

to balance the pressure on the upper

surface, otherwise it will be difficult

to move owing to inequalities in this

respect or through the trapping of

steam or warer. Care must be taken to

fit the cover, with its central gland,

in exact concentric register with the

liner, and the piston and rod also

concentric with each other.

To be concluded

The “ power ”of a governor may

be defined as the positive effort which

it is capable of exerting on the control

valve. It must obviously be capable

of overcoming any frictional or other

resistance encountered in the control

gear, and it is desirable to have a

margin of power in hand to ensure

reliable action. The power can be

increased in two ways: by increasing

the weight of the balls, and by in-

creasing the rotational speed. Large

slow-sped engines call for heavy

governor weights unless the governor

is geared up. But in high-speed

engines a governor which appears

much too small may be just as effective.

It should be noted that the force

available to operate the control gear

is influenced by the means by which

motion is transmitted from the

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EXPANSION GOVERNORS

While throttle governing is effective,

and is probably the most satisfactory

method on small engines where rigid

steam economy is not the first con-

sideration, it tends to waste steam

by the wiredrawing effect of the

780

MODEL ENGINEER

30 MAY 1957

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