THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES
______________________________________

Vol. CIX.

No. 2833.

FRIDAY, APRIL 16, 1915.

____________________________________________________________________________________________

The Prevention of Overwinding and Overspeeding in Shafts.*

By G. G. T. POOLE.

Overwinding.

The subject of the prevention of overwinding in shafts
has, owing to the passing of the recent Coal Mines Act,
received much attention and thought from the mining
engineer. Many types of overwinding gears have been
designed, and are now working in conjunction with the
winding engines at various collieries throughout the
country; and, as no useful purpose would be served in
giving individual descriptions of these gears, -the writer
proposes to devote his attention to a description of the
connections between the overwinding gear and the
winding engine, and the points which it is needful to
observe in order that the gear shall be essentially a
safety gear.

It should be clearly understood that the overwinder
itself, when brought into action, merely operates the
gear actuating the brakes and the throttle valve; it
does not actually apply the brakes, neither does it close
the throttle valve. For absolute safety, therefore, it
will be seen that not only must the overwinder be
working satisfactorily, but also the brake gear and the
throttle-valve gear must be absolutely reliable in
working. It is to the consideration of these last-named
connections that the writer intends to devote his paper.

The over winder should be set to work between the
two fixed points, namely, the top and bottom of the pit,
and for any movement of the cages beyond these points

R5 FEET PER SECOND.

1.000 FEET.

Fig. 1.—Representing the Conditions when
Working without the Overspeed Device.

it should act immediately. This will not only prevent
any accident due to an overwind, but will also prevent
the cage from running up into the headgear if the
winding engineman should start his engine in the
wrong direction.

Coming now to the actual conditions when winding,
supposing that A-B in fig. 1 represents the total length
of wind, B-C the distance from the top of the pit to the
headgear, A-F the distance in which to acquire accelera-
tion, and G-B the distance traversed during retardation,
and assuming a uniform rate of acceleration and
retardation, the curve A-D-E-B, representing the
velocity of the cages in the shaft during the whole of
the wind, is obtained. Now in the case of an overwind
when coming to bank the overwinder will come into
action at the point B, causing the steam to be shut off
and the brakes applied, the distance in which the cages
must be brought to rest being B-C.

The same remark will apply to the case where the
winding engineman starts his engine in the wrong
direction; the cages must be brought to rest in the
distance B-C, and where the brakes are capable of
holding the working unbalanced load without slipping,
this distance should be at least 15 ft.

The writer will now consider the case where the
winding engineman, through some cause or other, loses
control of his engine and the cage comes to bank at full
speed, instead of sldwing down at the point E and
coming to bank gradually, Full speed will be main-
tained up to the point H, the over winder will now come
into action, and the brakes will have to be powerful
enough to bring the cages to rest in the distance B-C.
In this case, however, not only must the brakes be
capable of holding the unbalanced load, but they must
also be capable of overcoming the energy of the moving
parts, and strong enough to resist the extra strain

* A paper read before the North of England Institute of
Mining and Mechanical Engineers.

caused by the sudden application of the brakes at full
speed. Thus it will be seen that, in order to avoid
excessive brakes and minimise the risk of an accident
due to extra strains, it is necessary that some device
should be used on the overwinder whereby excessive
speeds when coming to bank may be prevented, and
that the point at which this device should act is just
beyond E, where retardation commences.

Taking the case in which this device is in use, the
conditions will be as shown in fig. 2. E is the point at
which retardation commences, but if this point should
be overrun, then the overspeed device on the overwinder
will come into action at the point J, and the brakes
should be able to bring the cages to rest either at the
point B or at the point G. This. increase in distance
over which the brakes may act will allow of a lighter
brake being used, and will prevent excessive strain.
By taking a numerical example, the foregoing cases
may be more clearly illustrated. The writer will
assume the following conditions :—

Depth of pit = l,OOOft...._______= A-B.

Distance from pit to headgear = 40 ft. = B-C.

Weight of cage and chain ......_____ 4 tons.

Number of decks per cage ......... ...	2

Number of tubs per deck............ 2

Weight of each tub ................ 5 cwt.

Weight of coal per tub.............. 10 cwt.

Maximum speed < f winding ......... 45 ft. per second.

............

Weight of (4|in.) rope, about __..... 201b. per fathom.

Weight of drum (11 ft. in diameter) ... 16 tons.

Weight of each sheave .............. 1 ton.

Then, assuming a uniform rate of acceleration and
retardation, the following is obtained

Rate of acceleration = --------------------- = ^ =___________________

Time of acceleration	6'5

6’9 ft./sec./sec. = a.

Time taken to acquire acceleration = 6’5 sec. =

Space traversed during acceleration = | a ta2 .= | x
6’9 X (6’5)2 = 145 ft., represented by A-F in figs. 1 and 2.

Rate of retardation = ___-------__.___ =	—

.v	Time of retardation 8

5’6 ft./sec./sec.’ = r.

Time taken during retardation =. 8 sec. = tr.

Space traversed during retardation = A rtr 2 =• J x
5’6 x (8)2 = 180 ft. represented by G-B in figs. 1 and 2.

Maximum velocity =• depth __	1,000

i ± i tr 3’25 -f- 15	4

2 + + 2 •

= 44*9 ft./sec., say 45 ft./sec.

Time taken 15 sec. = t.

Space traversed = Velocity X Time = 45 x 15 =
675 ft., represented by F-G in figs. I and 2.

The working unbalanced load is the weight of the
coal plus the weight of the rope, and equals 2 tons of
coal + 3,333 lb. (1,000 ft. of rope) = 7,813 lb.; and if the
coefficient of friction be taken at 0 3, the frictional force
to be exerted by the brakes will be-^?|? lb., or 26,0431b.

VO ■ ■	' '	.	:

Now the energy of the moving parts is the energy of
the cages, &c., in the shaft, and the energy of the drum
and sheaves. The total weight in motion at maximum
speed is as follows -	• .

/	Tons cwt.	-	’

Two cages and chains ___..... 8 0	.

Eight tubs _______________	2 0	.

Coal _____________________	2 . 0

Rope (approximately) ..../:	1 15	‘	1	; .

Total .................. 13 15 = 30,800 lb.

Kinetic energy = ILA.2 =	* 452 foot-pounds,

Zg	64‘4	■

or nearly 970,000 foot-pounds.

The radius of gyration of the drum and sheaves is
approximately 4ft., the velocity at this. radius being
45 X 4

—— ft. per second, say 33 ft. per second. The total
weight rotating at this velocity is as under:—

Tons cwt.

Drum ..................... 16 0
Sheaves..................... 2 0
Rope (approximately) _______	1	10

...

Tetal ...A.___.....„....... 19	10	=	43,680lb.

Kinetic energy = x 33 f00^_p01in^gj or nearly
64’4

740,000 foot-pounds.

The total energy of the moving parts is 970,000 +
740,000 = 1,710,000 foot-pounds. The frictional force
has already been found to be 26,040 lb.; hence the
space passed over in bringing the cages to rest will be
L71°,000ft.	65.5ft/.

26,040

Consequently, if the cages came to bank at full speed,
the brakes would not be powerful enough to stop the
cages in time to prevent them from running into the
headgear, and it would be necessary to have a device for
checking the speed of winding when coming to bank, so
as to prevent an accident. In many winding engines,
however, the brakes are made powerful enough to hold
the maximum unbalanced load—namely, the weight of
one cage and chain and one rope; and taking this case
the maximum unbalanced load would be 4 tons (cage
and chain) + 3,467 1b. (1,040 ft. of rope) = 12,4271b.;
and, by taking a coefficient of friction of ,0’3, the
frictional force would be	lb., or 41,423 lb.

VO

Hence the space passed over in bringing the cages to
,	.-i-i -i 1,710,000 pi

rest will be ’,.	- it., or 41 3 it.

41,423

Thus in this case also the brakes will not be powerful
enough to bring the cages to rest within the 40 ft. from
the top of the pit to the headgear ; consequently, either
the braking force must be increased, or else the device
must be used for preventing excessive speed when
coming to bank.

D. R5 FEET PER SECOND. E. J.

Fig. 2.—Representing the Conditions when
Working with the Ovbrspeed Device.

6.

A

8.' \C.

The particular type of brake which it is advisable to
use will depend upon the conditions of working ; but it
may be mentioned that if a band brake is employed it
should be a complete band, and not half a band, so as to
obtain the full braking effect when the engine is running
in either direction. The brakes may be operated either
by a brake engine or by a weighted lever, and may
be connected to the overwinder either independently
of the driver’s lever, or conjointly with it. If a
brake engine is used to work with the overwinding gear
only, it should be tested frequently so as to ensure its
proper working; and if a weighted lever is used, it
should be connected with an oil dash-pot in order to
relieve the force of application of the weights.

Having dealt with the cases of overwinding, the
writer will now consider the cases of overspeeding.

Overspeeding.

As already mentioned, the speed should be controlled
when coming to bank, but, in addition to this, the speed
of winding should be controlled during the whole length
of .the wind; if the winding engine has no governor for
controlling the speed of the engine, then some means
should te,'provided on the over winder itself, The
arrangement should be such that if the maximum speed
of. winding,- be . exceeded, the device on the oyerwinder
will act on-the throttle valve only, thus shutting off the
steam and .allowing the speed of the. engine to be
reduced; and; when the speed is reduced by a sufficient
amount, the engineman should be able to open the valve
again, without having to stop the engine. This can be
done by making the main valve into a combined trip-
and-throttle-valve, the., tripping arrangement being
connected to the overwinder and the opening arrange-
ment to the engineman’s hand lever. Both of these
arrangements should be independent, and yet work
conjointly with one another. The valve when tripped
would close, and as soon as the speed becomes normal
again the engineman should be able to open the valve