Wire rope is a highly
specialized precision product which is adaptable to many uses and to
varied conditions of operation. To meet the exacting requirements of
different types of service, it is designed and
manufactured in a number of constructions and grades.
COMPLEXITY.
Wire rope is a complex machine,
composed of a number of precise, moving parts which
are designed and manufactured to bear a very definite relationship to one
another. In fact, many wire ropes contain
more moving parts than most mechanisms that fall within the broad general
term of machines. For example a
six-strand rope, consisting of approximately 46 wires per strand, contains a
total of 276 individual wires, all of
which must be able to move with respect to one another if the rope is to
have the necessary flexibility during
operation.
PARTS
GENERAL.
Wire rope is composed of three
parts: wires, strands, and core. The basic unit is the wire.
A predetermined number of wires of proper size are fabricated in a uniform
geometric arrangement of definite pitch or lay to form a strand of required
diameter. The required number of strands are then laid together symmetrically
around a core to form the rope.
CORE TYPE.
In general, wire rope cores are
of three types: fiber, wire strand, and independent wire rope.
Each type of core serves
the basic purpose of affording support to
the strands laid around it.
CORE MATERIAL.
Fiber core applies to any
natural or synthetic fibrous material that is woven into a
rope of its own. Fiber cores are generally made of polypropylene or a hard
fiber such as manila (abaca) or sisal. Strand
cores and independent wire rope cores are composed of wires.
CHOICE OF CORE.
Fiber cores are adequate for
most types of service. Not only do they provide the
necessary foundation, but they also add to the pliability of a wire rope.
There are some installations, however, where
conditions are such that a fiber core is inadequate; in these cases a wire
strand or independent wire rope core is
used. For service where high operating pressures are encountered, where
resistance to heat, additional strength,
or minimum stretch is a prerequisite, either a strand core or an independent
wire rope core is used.
LAYS
GENERAL.
The type of lay describes the
direction of the twist of the wires in a strand and of the strands
in a rope. Strand and rope lays are described in the following paragraphs (Figure
613-1-3).
RIGHT LAY OR RIGHT-HAND HELIX.
Right regular lay or right
lang lay.
The strands in the wire rope
are laid to form a helix about the core
similar to the threads in a right-hand screw. When viewed lengthwise, the
strands are wound helically away from
the observer in a clockwise direction.
LEFT LAY OR LEFT-HAND HELIX.
Left regular lay or
left lang lay.
The strands in the wire rope
are laid to form a helix about the core
similar to the threads in a left-hand screw. When viewed lengthwise, the
strands are wound helically away from
the observer in a counterclockwise direction.
REGULAR LAY.
The wires in the strands and
the strands in the rope are laid in opposite directions.
LANG LAY.
The wires in the strands and
the strands in the rope are laid in the same direction.
PITCH OR LENGTH OF LAY.
The length of a rope lay is
that distance measured parallel to the axis or
centerline of a rope in which a strand makes one complete spiral or turn
around the rope. The length of a strand lay
is the distance measured parallel to the axis or centerline of the strand in
which one wire makes one complete spiral
or turn around the strand.
SIZE
The diameter of a wire rope is
the diameter of the circle which will just enclose all of the strands.
In the case of strands, the diameter is that of
the circle which will just enclose all of the wires. The correct diameter
is the greatest diameter of the rope or strand.
Figure 613-1-5 shows
the correct and incorrect ways of measuring wire
rope.
CONSTRUCTION.
GENERAL.
The design arrangement of the
component parts of the wire rope is called the construction. To
date, nearly 100 different constructions have been manufactured. Figure
613-1-6 shows some of the more typical
wire rope constructions. Construction of wire rope is designated first by
the number of strands and then by the
number of wires in a strand; therefore a 6 by 7 rope has six strands with
seven wires per strand. When wire rope
contains wires of different sizes, the construction is usually designated by
name as well as by number.
SEALE CONSTRUCTION.
Each strand consists of three
rings of wire. The first ring of wires around the
center wire of the strand is of smaller diameter than the center and outer
wires.
WARRINGTON CONSTRUCTION.
Each strand has two layers of
wire about a center wire. The outer
layer consists of wires that are alternately large and small.
FILLER WIRE.
Filler wire has small wires
filling the voids between the rings of wire in the strand. These
small wires are not counted when designating the number of wires in the
strand.
FLATTENED STRAND.
Strands are somewhat triangular
in shape, sometimes formed around a triangular center
wire.
SPRING LAY.
The spring lay is composed of
six main strands laid around a fiber core. Each main strand
consists of three preformed wire strands and three fiber strands laid
alternately around a fiber center. The function
of the fiber parts is to provide a cushion for the wire strands and results
in a rope having great flexibility and
elasticity.
PREFORMED WIRE ROPE
Preformed wire rope is rope
whose wires and strands have been preshaped to conform to the curvature
which they take in the finished rope.
Preforming eliminates the locked up stress and strain existing in
nonpreformed wire rope, prevents the
rope from flying apart when cut or broken, and resists kinking. Preforming
helps to eliminate the tendency of a rope to
rotate about its own axis. Preformed wire rope is more easily spliced
since the strands fit perfectly into place.
However, owing to the permanent helical shape of the strands, the technique
of tucking the ends differs from that of
nonpreformed wire rope. This type of wire rope is designed to give extra
life when used for operating ropes, particularly when used over small
sheaves and when operating with small
safety factors. Preforming is of greatest value when normal failures occur
through fatigue. Preformed wire rope is
of no advantage when used as standing rigging, or in applications where the
chief cause of failure is abrasion or
corrosion.
ZINC-COATED OR WIRE ROPE
In order to protect wire rope
against the action of salt water and other corrosive elements, and to
prevent rust, wire is galvanized (zinc-coated)
or tinned, as the case may require. Wire rope so treated is generally
used for standing rigging. It may also be used
for running rigging and for wheel (steering) ropes on ships.
This type of service does not
cause ropes to wear rapidly. Wire rope subjected to constant bending around
drums and sheaves, such as in hoisting
service, is not usually so treated because the constant flexing of the rope
will cause the protective coating to
peel.
CONDITIONS OF USE
CHARACTERISTICS.
Different types and
constructions of rope have been developed over the years to
meet special conditions of use. Initially, ropes were used for hauling
purposes where flexibility was not a requirement.
Simple ropes, such as the 6 by 7 and 8 by 7, were constructed of rather
large wires. This produced a rope well
qualified to resist abrasion, but not particularly well adapted for service
where flexibility was essential.
In most cases, use a 6 by 7
rope with a relatively high safety factor since its reserve strength is low.
When wire ropes started being used for
hoisting purposes, it became necessary to increase the number of wires,
which for a given diameter of rope means
smaller wires and greater flexibility. In general, flexibility increases
with the number of wires. The Seale,
Warrington, and filler wire types were developed to increase the total
metallic area for a given diameter rope.
The average increase is about 10 percent, though in the Seale construction
this is accomplished at some sacrifice
of flexibility. The problem of rope wear when used over sheaves soon becomes
apparent. The smaller the diameter of the
bearing (outer) wires, the greater the number of those wires which may
break during wear. Accordingly, the flattened
strand type of construction was developed which, for a given total metallic
cross-sectional area, provides a greater bearing surface resulting in more
wear distribution.
STRENGTH.
In addition to being suitable
for a particular service, the type, class, and construction of
a wire rope shall be strong enough to stand the stresses put upon it without
permanent deformation constituting a
hazard to life or property or requiring frequent renewal. The strength of a
wire rope of a given construction depends
upon its size (diameter) and the material from which it is made. For any
particular size and material, the
breaking strength may be obtained from tables given in FED Spec
RR-W-410. As rope gets older, wear and corrosion reduce its strength.
The strength of an old rope is
appreciably less than that of the new one. Because the destruction of life
and property due to failure of a rope is
usually so much greater than the value of the rope, it is a good policy to
use a rope several times stronger than
calculated total stress to ensure against premature failure.
FACTOR OF SAFETY.
The ratio between the breaking
strength of a rope and the total applied load is
the safety factor. Experience has shown that a wire rope will last longer if
the load to which it is subjected in service
never approaches its breaking strength. In other words, a rope which costs
more per foot may cost less per annum
because of the greater service obtained from it. For ordinary hoisting
ropes, a minimum safety factor of five
is normally used. The safety factor usually is increased for ropes running
continuously at high speed over sheaves,
where safety of life is involved, or where deterioration may be expected
because of causes such as unusual
abrasion or poor lubrication. Always replace wire rope with the one
specified for the particular application.
Consult the equipment technical manual, drawing, or the wire
rope list.
