THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXIL

FRIDAY, JULY 14, 1916.

No. 2898.

American Coal

In so far as the American coal mining districts are
concerned, the vertical shaft occurs infrequently or not
at all, except in the anthracite fields, and even there
the seams that are being worked are located at only a
moderate distance below surface. There are probably
very few vertical shafts over 1,700 ft. deep. In the bitu-
minous districts the coal seams are reached and worked
largely by means of inclined approaches and corridors.

Haulage Systems.

In consequence of such conditions, haulage systems
of transportation are much in evidence, and the wire
rope has been an essential factor in the development

Fig. 1.

Fig. 4.

Figs. 1 to 4.—Endless Rope Systems.

Fig. 2.

which has taken place. There are two or three prin-
cipal methods of utilising the steel cable in haulage.
First, there is the endless rope, in which the two ends
of a single rope are spliced together, and the whole
maintained in constant motion in one direction. By
means of suitable clips, cars or trams are secured to the
moving rope, and are thus hauled along. The rate of
speed is ordinarily limited to three or four miles per
hour, which permits an attendant to step on, attend to
the clips, and step off again. The cars are handled
individually or in small groups. In the tail-rope system,
there are two separate ropes, one, known as the head
rope, being secured to the forward end of a train of
cars, whilst the other is attached to the rear of the train,
and is known as the tail rope. The operation of the head
rope hauls the loaded train; the operation of the tail
rope hauls the empty train back again.

Wire Ropes.

The standard wire rope used in both systems has six
strands. Each strand has few wires, say, seven or 19.
The use of locked wire cable does not seem to be
approved in American practice for this or any service
where the rope itself is put in motion. The typical
strand in haulage ropes consists of a central wire, around
which are helically wound six wires, all seven having
the same diameter, and the strands are wound about a
hemp central core. The diameter of the rope is the
diameter of the enveloping cylindrical surface—that is,
it is the longest diagonal of the cross section. If the
wires in the strands are wound in the left-hand direction,
the strands will ordinarily be wound in the right-hand
direction. However, Lang’s lay is also employed—
that is, the system of winding both wires and strands in
the same direction. One-inch rope consisting of six
strands of seven wires, each wound on a hemp centre,
will weigh about 1’58 lb. per linear foot; and this weight
may be regarded as representative of six-strand, hemp
centre rope, whether the number of wires per strand is
greater or not. This gives uniformity in the total metal
cross section. Generally, for this type of rope, the total
metal cross section, and, consequently, the weight per
linear unit of length, vary as the square of the diameter.

Iron v. Steel Wire.

There are four or five qualities of rope in use. First
is the iron rope, made of a fairly pure material contain-
ing but little phosphorus, sulphur, or carbon. The

Mine Haulage.

tensile strength is naturally lower than the steel ropes,
being about 85,0001b. per sq. in. in the case of drawn
wire. Iron rope is the cheapest in price, but is used
only to a limited extent. A 1 in. six by seven iron rope
is listed by a prominent maker at 24c. per ft. The rated
strength—not the proper working load—is given as
30,000 lb. The next higher grade may be taken as
crucible cast steel rope, the material of which is made
by the open hearth precess. A tensile strength of
150,000 to 200,0001b. per sq.in. is claimed by one
responsible manufacturer. A 1 in. six by seven crucible
open hearth steel rope is quoted at 29c. per linear ft.,
and a rated strength of 62,0001b. is assigned. A higher

Fig. 3.

quality of this same steel, the chemical composition
being somewhat different, is stated to have a tensile
strength varying from 180,000 to 220,0001b. per sq.in.
A 1 in. six by seven extra strong crucible open hearth

Fig. 5.

Ta»l rope haulage system

Fig. 6.

Fig. 5 to 8.—Tail

steel rope is listed at 35c. per linear ft., and a rated
strength of 70,0001b. is claimed. The next grade is
plough steel rope, which is a high-grade open hearth
steel, for which a tensile strength of 200,000 to
260,0001b. per sq.in. is claimed. A 1 in. six by seven
plough steel rope, the steel of which is rated at 76,000 lb.,
is listed at 41c. A higher grade of plough steel is also
employed, made from steel for which one manufacturer

claims a tensile strength of 220,000 to 280,0001b. per
sq. in. A 1 in. six by seven extra quality plough steel
rope is listed at 48c. per linear ft., and a rated strength
of 84,0001b. is claimed.

Ropes of the six by seven class are stiff, but they
stand up well against abrasion. Because of the lack of
flexibility of the rope, the diameters of drums and
sheaves must be large, if proper working conditions are
to t>e provided. For an iron rope, the minimum
diameter for the 1 in. six by seven variety is placed by
one American authority at 10| ft.; for the various
qualities of steel ropes, at 7 ft. The distribution of the
metal into a few coarse wires results in minimising the
internal contacts between the wires, and therefore
lessens the internal abrasion during service, and the
internal exposure to corrosion. The external contact
with sheaves and drums is also reduced. While the six
by seven variety is to be regarded as fairly standard for
haulage purposes, there are other and more flexible
styles which may be used, to meet special conditions,
where flexibility has to be taken more into account.

Rope Safety Factor.

A safety factor of 5 may be regarded as representa-
tive of highest grade American practice in respect to six
by seven hemp core ropes used for haulage purposes, but
this safety factor does not include any allowance for
bending stresses. The rated strength of the rope is
first divided by the factor of safety, and then the
quotient reduced by subtracting the allowance for
stresses arising from bending the rope round sheaves
and drums. As these will vary at different points of
the same installation, because of variations in the radius
of curvature of the bends, it is necessary to select the
most disadvantageous case and calculate the stress
from it.

The following formula may be used :—

S = E xl

where S = bending stress per square inch in wires in
the rope; E = 12,000,0001b.; d = diameter of the wire
used in the strands; and D = diameter of drum
measured from centre to centre of rope wound on drum.
The value, 12,000,0001b., is a departure from the prac-
tice which uses Young’s modulus of elasticity
(29,000,000 lb.) for steel. This departure is to be under-
stood as applicable for six-strand rope only. The modi-
fication is backed by the largest manufacturer of wire
rope in the United States, and perhaps in the w’orld.
This corporation justifies it, because of “ data gleaned
from practical experiments, covering a considerable
period of time, and numerous tests,” and because of
“ the fact that the wires of a rope are twisted, and the
stress very much different ” from what it would be if
they were simply wound round in a circular loop.

Fig. 7.

TAIL ROf*E HAULAGE SYSTEM

Fig. 8.

Rope Systems.

Calculated by this formula, the bending stress of a 1 in.
six by seven rope wound on a 7 ft. drum would be
6,400 lb. This is the amount to be deducted after the
rated strength has been divided by five, the factor of
safety. The result is the safe working load, provided
the 7 ft. drum is the minimum bend.

In calculating the stress S2 on the rope due to hauling
up an incline making the angle 0 with the horizontal,