1066

THE COLLIERY GUARDIAN.

Mat 15 1914.

out again to something like its original thickness imme-
diately it passes to the slack or “ idle ” side. This
resilience, due in a measure to spiral activity, and more
pronounced in the three-strand combination than any
other make, may account for the ready recovery from
fatigue, and for the consequent longevity of cotton ropes,
which under favourable conditions will continue to do
good service for over 20 years. But slight compression
of the pliant material is sufficient to fill up the central
gap of a three-strand rope, and when- that is exercised
with equal force from each of the three centres by the
final twisting operation, a trinity of wedges is the result,
acting in unison as the ropes extend and contract in
taking the circuit of the pulleys. When the yarns are
all equal in counts, twist, and tension, each one following
its appointed track without deviation, and when they
are built up in concentric layers of varying length as the
desired thickness is attained, the product is the patent
inter-stranded cotton driving rope.

In order to corroborate German evidences on the
question of higher duties, we must leave the textile
industry with its refined processes, and seek less favour-
able conditions, where more rigorous treatment
accelerates investigation by shortening the period of
endurance. Mr. J. Ingham, of Bolton, who has had

velocity of 3,516 ft. These are attached to electric
motors of 750 brake horse-power, allowing for a normal
load of 1,000 brake horse-power, 26 ropes, 2 in. diameter,
were at first employed to provide against a maximum
of 1,500 brake horse-power. They have since been
reduced to 20, which are calculated at about the normal
power for each drive, leaving the remainder to be carried
through by the momentum of heavy flywheels, much to
the benefit of the drive. The rope friction is declared
not to exceed 21 per cent.

How discounting the power of ropes may lead to mis-
apprehension when estimating comparative values, was
strikingly manifested by calculations made after a
personal inspection of several mill installations in York-
shire, where steel belts have substituted leather or ropes.
In one case a 10 in. leather belt was replaced by 3| in.
steel. This, however, gave a considerable amount of
trouble and had been twice renewed during a run of
about 15 months, a third being in use at the time of the
visit. The cork sheathing on the pulleys had also been

generally about 16 in., say 18 times the rope. Two 1 in.
ropes running on a 20 in. motor pulley were recently
removed after 10 years’ good service, not because they
were worn out, but because of altered condition; while
ropes are regularly run on steam turbine pulleys of 22
diameters.

These and other obtainable data lead to the conclu-
sion that driving force does not decrease pro rata with
diminishing sizes. In the absence of better adjustment
we therefore take refuge in the safe, if empirical, sug-
gestion of fixing upon 1-' in. diameter as the theoretical
dividing line, at and beyond which the old standard of
30 diameters may be observed, and below which a
gradual lessening may. take place, down to, say 18 dia-
meters for | in. rope, leaving the absolute minimum yet
to be decided, always on the understanding that larger
pulleys mean greater advantages, particularly beyond a
velocity of 5,000 ft.

Still another much debated item on the purely

i < ■

Fig. 3.—Colliery Fan driven by Rope from Electric Motor.

Fig. 4.—Leading Types of Rope Pulley Grooves.

I

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A

T

Fig. 5.-—Method of Setting Out a 40 degs.
Groove.

- - U - -

Fig. 6.—Method of Setting Out a 30 degs.
Groove.

considerable experience in rolling mill driving, has com-
piled a list of friction losses from indicators taken upon
an electro-rope driven sheet mill. The rope design load
is set down as 500-horse power, with friction at 3| per
cent. When a peak of 900-horse power is reached the
friction is brought down to 2 per cent., but runs up to
as much as 11'3 per cent, at 150-horse power, or 25 per
cent, of the normal load. The entire plant is driven by
cotton ropes from two pulleys, each 32 ft. diameter,
making 35 revolutions per minute, or a peripheral

stripped. These mishaps were attributed to a very slight
but not permissible fault in lining up the shafts.

When investigating a more recent and much adver-
tised case—where a saving of 60-horse power in about
340, or just over 17 per cent., is said to have been
realised over ropes by the introduction of steel belts—it
transpired that the entire section of the mill thus con-
trolled had been remodelled. The unfortunate habit of
attributing to the transmitting medium itself all the
benefits derivable from the introduction of new methods,
makes that mysterious bourn, known as the unbiassed
opinion, all the more difficult to reach.

Although governed by wheel diameters in both cases,
speed ratio difficulties are usually met by adding more
rope to make up for contact losses. But ropes are at a
disadvantage when used in quantities on small pulleys
less than, say, 9 in. or 10 in. diameter. Short centres
are no impediment to successful rope driving. Many
instances might be quoted where even less clearance is
allowed. When the speeds of both shafts are alike, the
greatest possible pulley contact is available. Ropes are
now applied to a rolling mill drive on which spur gear,
belts, and chains had been successively tried but failed
-to accomplish the task. The rope pulleys are each 4 ft.
3 in. diameter, and the centres being.6 ft. apart, only
allow a rim clearance of 1 ft. 9 in. Long centres with
ropes are, however, very much a matter of pulley
diameters and direction of rotation, particularly on
horizontal drives. The ultimate sag seldom exceeds 74
per cent, of the distance between centres, but 10 per
cent, is generally considered ample for estimating the
depth of a race or for clearing obstructions. Thus,
presuming upon the use of pulleys each 10 ft. diameter,
the centres might be extended 100 ft. before the trailing
and working spans touched each other, i.e., with slack
uppermost. Reversing the direction of rotation obviously
permits longer ranges, reaching in several instances
from 150 to 200 ft. This direction is recommended when
erratic loads or irregular impulses set up oscillations in
the ropes.

Cross driving with ropes is undoubtedly the best
method of conveying power from and to shafts revolving
in opposite directions. Although some curtailment of
life may be expected from unavoidable friction, many
ropes so placed have lasted from seven to 10 years.
Much, however, depends upon the method of attach-
ment. If more than two ropes are employed then suffi-
cient empty groove space must be allowed between each
pair so that they may pass without touching.

A disconcerting problem in connection with rope
driving is that of fixing upon a basis of agreement, as
to the proportionate size of pulleys in relation to ropes.
Flexibility must naturally be the determining factor in
the proposition, although an all-round minimum of 30
diameters has hitherto ruled the reckoning. But the
experience has led to a gradual slackening of the some-
what arbitrary rule. The size of ropes employed on
ring frames has been extended from fin. to 13-16 in.,
and ultimately to Jin., the latter being now the most
popular diameter, because of its greater tenacity and
lasting properties; while driven pulley diameters are

mechanical side of rope transmission awaits careful
consideration, i.e., the construction of rope pulley
grooves. Fig. 4 presents a group of four leading types.
The curved sided form A, though not so much in evi-
dence as the rest, is still insisted upon in some quarters
from a desire to afford easy withdrawal by conforming
in some measure to the contour of the rope. This cur-
vature militates so much against efficiency that it is
generally necessary to increase the diameter until the
groove is completely packed with material, in order to
ensure the required transmission. Flanged grooves
like B are the most commonly used, because of this
narrow pitch, and because of the belief that they give
direction to the ropes. This supposition does not
appear to hold good, for when ropes are inclined to
oscillate they rebound from side to side as shown in
dotted circle, much to their detriment. Then again,
the flanges offer a barrier to any extension upon the
designed size. The weakness of these flanges is mani-
fested by the breakages which too frequently occur
during transit. For all-round good service there is
nothing to beat the flangeless groove, with the driving
angle carried through to the terminals, as in C. An
angle of 45 degs., however, whilst permitting an exten-
sion of J in. in rope diameter, produces a wider pitch
than most engineers care to adopt. No other groove
appears to conform so well to general conditions as the
D type, although more acute impact and delivery are
accomplished without the slightest additional friction,
while the estimated power may be increased by one-
third. This type also retards any tendency to revolve,
which wide angles and curved sides appear to encourage
unless well filled and not too lightly loaded. These use-
less gyrations are not always constant, for the ropes
may fix themselves for a period, then roll over until
their section assumes an irregular polygon, instead of
the more serviceable wedge.

In order that the simplified method of setting out a
40 degs. groove may be better’ understood, an enlarged
diagram is submitted in fig. 5. A circle representing
the required rope diameter is drawn, through which the
vertical and horizontal centre lines are projected. The
chord of the arc A B then becomes the standard of
further measurement, decides the centre for the
rounded base and doubled, points the inverted apex of
the angle, carried .through the points B B and the upper
horizontal lines, where the pitch is indicated. Another
line projected from half A B or its opposite decides the
length of the terminals. It has been urged that every
size of rope has its best driving angle. Whether this
be so or not, there is no appreciable advantage in pro-
viding a multitude of templates to cover the whole
range. Therefore, as the 40 degs. groove gives such
excellent results it may well stand for all sizes above
1 in. diameter, while 30 degs. may be just as usefully
engaged upon this and the smaller sizes in general use.

Thirty deg. grooves (see fig. 6) are set out pretty much
on the same lines, only that B C represents half the
measurement for the apex, while A B still holds good
for the circular base.

It will be observed in both cases that the groove sides