THE COLLIERY GUARDIAN

AND JOURNAL OF THE COAL AND IRON TRADES,

Vol. CV.

FRIDAY, JANUARY 17, 1913

No. 2716.

THE BRAKING OF HIGH-SPEED WINDING
ENGINES.*

By G. K. Chambers.

That this system has not reached its present rather
acute stage previously, is due to the fact that conditions
arise in connection with the braking of electric winders
which are not met with in that of steam winders.

With electric winders power braking usually depends
upon the motive power, a failure of which is by no
means impossible, and may result from any of the
following irregularities:—

(a)	Complete shut down of power station, due to
failure of prime movers or accessories.

(b)	Failure of transformers, switchgear, power line
or cables on the particular circuit to which the hoist
is connected, whether due to lightning or other causes.

(c)	The opening of feeder circuit breakers at the
power station, or in the transmission system, caused
by ■—

(1)	Heavy surges due to lightning discharges.

(2)	Mistake of operator.

(3)	Short circuit on system.

(4} Unusual overload.

(5) Malicious injury to overhead lines.

(d)	The opening of circuit breakers in the mine
sub-station or transformer station, due also to any of
the foregoing causes.

Irregularities (a) and (b) are, of course, very remote,
and need scarcely be considered; but those listed in (c)
and (d) may very easily, and not infrequently do, occur?
especially during the lightning season. Under the
circumstances which are general the momentary opening
of a circuit breaker takes the control of his engine out
of the driver’s hands, and practically the whole respon-
sibility of stopping the winder is thrown upon the
successful operation of a safety device and of the
mechanical brakes, it being impossible under ordinary
circumstances for the driver to apply power braking,
even though the supply has again become available, until
the safety device has been reset, an operation which
takes an appreciable amount of time; his efforts being
limited to increasing the braking pressure due to the dead
weight which has been tripped on to the brake levers
by the emergency device by the addition of his own
weight, in the case of foot brakes, or in the case of, at
all events, one system of air-operated brakes, by the
admission of compressed air to the top side of the brake
engines; efforts which may tend to fire the brake blocks,
and thus do more harm than good.

It follows from this that mechanical brakes on electric
hoists have to perform a function which those on steam
winders are rarely called upon to do, and then probably
only in the case of overwind, when the winding speed is
usually much reduced—a function, in fact, for which the
latter may not necessarily have been designed. The
mechanical braking of electric hoists must therefore be
a more serious problem than that of steam hoists.

In the writer’s opinion, no hoisting plant can be
considered ideal and entirely safe where the operation
of an emergency device throws the whole duty of
stopping the hoist on to the mechanical brakes, and
prevents the simultaneous application of power braking
by the driver. The effect of high speed man winding
may be judged from the fact that there is in operation
on the Rand at present an electrically driven man hoist,
for which a speed of 3,500 ft. per minute has been
authorised, which hoists or lowers the whole of the shift,
consisting of approximately 1,600 men and boys, in

hours, through a vertical depth of 3,200 feet.

The hoisting of loads, whether of rock or men, through
very deep vertical shafts, presents no difficulty as far as
braking is concerned, as a retarding moment can be
obtained throughout the whole of the wind by the
adoption of properly designed cylindro-conical drums
or of tail ropes used in connection with plain cylindrical

* From a paper read before the South African Institution
of Engineers, and published in the Journal.

drums, and the speed of the hoist may either be allowed
to run out of itself or a small amount of power braking
may be applied in order to reduce the period of
retardation, in either case rendering a very slight
application of the mechanical brakes necessary as the
skip or cage approaches the tip or bankin order to bring
it to rest.

When lowering men, however, the condition is entirely
altered, as the moment becomes an accelerating one
throughout the whole of the wind, even though it is
reduced towards the end of the trip on account of
changes in the winding diameters, consequently the
speed of the descending load must be limited, and its
accumulated work and that of all the moving masses
dissipated, or rather converted, by some method of
braking. With long winds in incline shafts from
different levels, the necessity for efficient braking is
still greater, as in many cases the moment is an accele-
rating one at the end of a hoisting trip, whether of rock
or men, and invariably so at the end of a lowering trip,
cylindro-conical drums not being well adapted for
winding from varying levels, and the use of tail ropes
being practically out of the question; this is still further
accentuated by the fact that with such winding
unbalanced operation with the other drum unclutched
is quite frequent, either for changing levels, sinking,
shaft repairs, or other purposes, and consequently only
one mechanical brake is available for a much more
severe duty.

The discussion of braking would be incomplete
without reference to the possibility of runaways of loads
on unclutched drums, due to mistake on the part of the
driver or to faulty clutch mechanism, whether on steam
or electric winding engines, as such accidents have
occasioned considerable loss of life and damage to
property in the past.

Reverting to the consideration of the braking of hoists
for high-speed winding with rope speeds of, say, 3,500 ft.
per minute, it will be found that the peripheral speed of
the brake treads will be in the neighbourhood of 4,000 ft.
per minute, and the question arises as to whether this
is a safe or an excessive speed for mechanical braking,
particularly if the application of the brakes be an
emergency one and the moment at the time of braking
an accelerating one; the possibility of firing and
carbonising the faces of the brake blocks with the
resultant lowering of the co-efficient of friction has, of
course, always been a serious factor, even with compara-
tively low winding speeds, and this possibility is increased
the higher the speeds become.

The following expresses the views of the writer on
this subject:—

(a)	When hoisting rock or men against a retarding
moment, such a peripheral speed for mechanical
braking is allowable.

(b)	When lowering men, under balanced conditions
as far as skips and ropes are concerned, with a constant
accelerating moment, such a speed is allowable for
hand-braking, but approaches the limit for emergency
application.

(c)	When hoisting rock or men with unbalanced
ropes where the weight of the descending load exceeds
that of the ascending load towards the end of the trip,
and the moment is consequently an increasingly
accelerating one, such a speed is not allowable.

(d)	When lowering men with unbalanced ropes, and
consequently with an increasingly accelerating
moment at the end of the trip, such a speed is
excessive.

(e)	When lowering men under entirely unbalanced
conditions with the other drum unclutched, such a
speed is absolutely prohibitive.

If these limits for mechanical braking be accepted, it
follows that if the maximum duty is to be obtained
from a high-speed hoist under all conditions, power
braking must be resorted to in order to bring the speed
within the safe limits of mechanical braking.

The various systems of power braking may be tabu-
lated as follow:—

Non-generative and power consuming :—

(a) Reversal of steam hoist with steam.

(5) Counter current with A.C. hoist.

Re-generative:—

(c)	Ilgner system.

(d)	A.C. load equaliser system.

(e)	Winding at over synchronous speeds with an
A.C. hoist.

(/) Ward-Leonard system.

Self-generative:—

(g)	Reversal of steam hoist without steam.

(h)	Rheostatic braking of Ward-Leonard hoists.

(i)	Direct current excitation of A.C. motor.

(J) Eddy current braking.

The non-generative and power-consuming systems of
power braking are the least economical as far as the
hoist itself is concerned, as the braking effect is obtained
by the reverse application of motive power taken from
the source of supply, i.e., steam from the boilers or
current from the electric power line, and the accumulated
work in the moving masses is dissipated as heat.

(а)	The braking of steam hoists with live steam is
unusual except where the engine exhausts into a con-
denser ; it is, however, sometimes applied at the end of
a trip in order to bring the hoist almost to a stop,
especially with large compound engines, when a small
amount of live steam is by-passed to the low-pressure
engine. It may also be conveniently used under
emergency conditions.

(б)	The application of counter-current is a very
convenient and powerful method of braking A.C. hoists,
but its action ceases immediately with a failure of

I supply, and the whole duty of stopping the hoist is
I thrown upon the mechanical brakes. This disadvantage
I becomes more serious owing to the fact that the careless
application of counter-current by the driver may easily
I cause a failure of supply to the hoist and result in an
emergency application of the brakes, which, given
careful handling, would be uncalled for. This is due
to the fact that by the application of counter-current to
a hoist motor running at full speed the voltage across
the electrodes of the liquid controller becomes double
the normal, and, unless carefully handled, a momentary
rush of current may easily occur which will be sufficiently
heavy to trip the circuit breaker controlling the hoist or
some other circuit breaker in the power system.

Even though the reverse power be carefully applied,
owing to the high voltage across the electrodes in the
controller, it is by no means uncommon for a “ buck ”
to occur, i.e., for an arc to jump across from plate to
plate, with the same results as regards tripping a circuit
breaker and cutting off the supply from the hoist. In
the case of overspeed or runaway, the likelihood of
either of these accidents occurring is accentuated, both
because the voltage across the electrodes becomes still
higher, and because the driver, finding himself in
difficulties, would be apt to apply power braking with
greater strength and rapidity.

A practical objection to lowering unbalanced loads
with counter-current lies in the fact that nearly all the
accumulated work in the descending load has to be
dissipated as heat in the controller, with the result that
there is an excessive rise in the temperature of the
electrolyte, and the possible number of trips per hour
becomes limited on that account.

The fact that the very application of power braking
on this system may nullify itself and by the operation
of a safety device throw the whole duty upon frictional
brakes, with possibly a variable coefficient, is one which,
in the opinion of the writer, cannot be over-emphasised.

The regenerative systems of power braking are of
course the most economical systems so far as the hoist
itself is concerned, as the accumulated work of the
moving masses is returned as useful work to the source
of power supply.