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Wire rope consist of three
basic components, while few in number,
these vary in both complexity and configuration
so as to produce ropes for specific purposes
or characteristics, The three basic components
of a standard wire rope design are:
1) Wires that form the strand,
2) multi-wire strand: laid helically around
a core, and
3) the core
Wire, for
rope, is made in several materials
and types, these include steel, iron, stainless
steel, monel, and bronze. By far,
the most widely used material is
high-carbon steel. This is available
in a variety of grades each of which has
properties related to the basic curve for
steel rope wire. Wire rope manufacturers
select the wire type that is most appropriate
for requirements of the finished product
depending on applicational requirements.
Steel wire
strengths are appropriate to the particular
grade of the wire rope in which they are
used. Grades of wire rope are referred
to as traction steel (TS), mild plow steel
(MPS), plow steel (PS), improved plow steel
(IPS), and extra improved plow steel (EIP).
(These steel grade names originated
at the earliest stages of wire rope development
and have been retained as references to
the strength of a particular size and grade
of rope.) The plow steel strength
curve forms the basis for calculating the
strength of all steel rope wires,
the tensile strength (psi) of any steel
wire grade is not constant-it varies with
the diameter and is highest in the
smallest wires.
The most common finish for steel wire is
'bright" or uncoated. Steel
wires may also be galvanized, i.e., zinc
coated. "Drawn galvanized' wire has
the same strength as bright wire, but wire
"galvanized at finished size"
is usually 10% lower in strength.
In certain applications, "tinned"
wire is used, but it should be noted that
tin provides no sacrificial,
i.e., cathodic, protection for the steel
as does zinc. For other applications, different
coatings are available.
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"Iron"
type wire is actually drawn from low-carbon
steel and has a fairly limited use
except in older elevator installations. When,
however, iron is used for other than
elevator applications, it is most often galvanized.
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Stainless
steel ropes, listed in order of frequency
of use, are made of AISI Types 302/304,
316, and 305. Contrary to general
belief, hard-drawn stainless'. Type 302/304
is magnetic. Type 316 is less magnetic,
and Type 305 has a permeability low enough
to qualify as non-magnetic.
Monet
Metal wire is usually Type 400 and conforms
to Federal Specification QQ-N-281.
Bronze
wire is usually Type A Phosphor Bronze
(CDA #510) although other bronzes are
specified at times.
Strands
are made up of two or more wires, laid
in any one of many specific geometric
arrangements, or in a combination of steel
wires with some other materials such as
natural or synthetic fibers. |
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It
is conceivable that a strand can be made up of
any number of wires, or that a rope can have any
number of strands. The following section, IDENTIFICATION
and CONSTRUCTION, provides a complete description
of wire rope constructions.
The Core is the foundation
of a wire rope, it is made of materials that will
provide proper support for the strands under normal
bending and loading conditions. Core materials
include fibres (hard vegetable or synthetic) or
steel. A steel core consists either of
a strand or an independent wire rope. The three
most commonly used core designations are: fibre
core (FC), independent wire rope core (IWRC),
and wire strand core (WSC) Catalog descriptions
of the various available ropes always include
these abbreviations to identify the core type.
To summarize: a wire rope consists,
in most cases, of three components: Wires, strands,
and a core to these may be added what can be considered
a fourth component: the wire rope's lubricant
- a factor vital to the satisfactory performance
of most operating ropes.
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Wire
rope is identified not only by its component
parts, but also by its construction
i.e., by the way the wires have been laid
to form strands, and by the way the strands
have been laid around the core.
"a''
and "c'' show strands as normally
laid into the rope to right -
in a fashion similar to the threading
in a right-hand bolt. Conversely the "left
lay" rope strands (illustrations
"b" and "d") are laid
in the opposite direction.
The
first two (''a'' and ''b'') show regular
Joy ropes Following these are the types
known as lang lay ropes ( ''c''
and ''d'') - Note that the wires in regular
lay ropes appear to line up with the axis
of the rope, in lang lay rope the wires
form an angle 'with the 'axis is of the
rope. The difference in appearance is
a result of variations in manufacturing
techniques: regular lay ropes are made
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direction
of the wire lay in the strand is opposite to the
direction of the strand lay in the rope; lang
lay ropes are made with both strand lay and rope
lay in the same direction. Finally. ''e''
called alternate lay consist of alternating regular
and lang lay strands.
Of all the types of wire rope
in current use right regular lay (RRL)
is found in the widest range of applications.
Nonetheless, in many equipment applications right
lang lay (RLL) or left lang lay ( LLL) ropes are
required. At present, left lay rope is
infrequently used. As for alternate lay
(R-ALT or L-ALT) ropes, these are only used for
special applications.
Compared to other types. the
superiority of lang lay rope
in certain applications derives from the fact
that when bent over sheaves, its life
span is longer than the others. Stated
in another way. the advantage of lang lay rope
is its greater fatigue resistance.
Yet another claim is made for lang lay ropes:
they are more resistant to abrasion.
Broadly speaking, this is true, but there are
some reservations that should be taken into account.
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It
is important to understand the reasons
for the advantages of lang lay
rope. To begin with, consider
its fatigue and bending properties.
Figure shows, in part, how the lang lay
construction characteristics result in
greater fatigue resistance than
is found in regular lay rope.
Note, how the axis of the wire relates
to the axis of the rope in both cases.
When the regular lay rope is bent, the
same degree of bend is imparted to the
crowns of the outer wires.
Superior
fatigue life in lang lay rope is also
attributable to the longer exposed length
of its outer wires. In the upper photograph
of a regular lay rope, the valley-to-valley
length of individual wires is
about 7/8 "the length of the lang
lay wires in the lower photograph is about
1 1/8" or 30%, longer. Bending
the lang lay rope results in less axial
bending of the outer wires and greater
torsional flexure. These combined
stresses not withstanding; the
lang lay rope displays 15 to 20% superiority
over regular lay when bending is the principal
factor affecting service life,
It's said that lang lay is more
flexible, but flexibility should
not be confused with fatigue resistance.
These two attributes may, under certain
circumstances, bear some relationship,
but they are distinctly separate characteristics.
Flexibility defines the relative
ease with which a rope "flexes"
or bends. Fatigue resistance defines the
ropes ability to endure bending.
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Two
other factors relate to fatigue: they are discussed
here along with abrasion and peening characteristics.
Above Figure illustrates, in
drawings and photographs, the wear pattern in
regular lay vs. lang lay ropes. The drawings (of
a single strand) show the wire direction relative
to the rope axis in both types. Dimension lines
in the upper drawing set off the exposed length
of one wire crown in the regular lay rope. The
lower drawing shows the corresponding four wire
crowns involved in the lang lay rope. The
line a-b shows the relation of the wire crown
to the rope axis. Although there is little
difference in total contact area between rope
and sheave in these two rope types, the forces
and wear on the individual wires are quite different
(Fig)
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The
fact that the wires of regular lay
rope are subject to higher-pressure increases
the rate of wear (abrasion and peening)
of both wire and mating surface of the drum
or sheave. Moreover, this higher pressure
is transmitted to the interior rope structure
and this, in turn, decreases fatigue resistance.
Finally, the worn crown
of the regular lay wire combined with its
shorter exposed length, permits the wire
to spring away from the rope axis. Subsequent
passage on and off a sheave or drum, results
in early fatigue breakage.
A note of
caution: lang lay rope has two important
limitations. First, if either end is not
fixed, it will rotate severely when under
load, and secondly, it is less able to withstand
crushing action on a drum or sheave, than
is regular lay rope. Hence, lang
lay rope should not be operated without
being secured against rotation at both ends;
nor should it be operated over minimum-sized
sheaves or drums under extreme loads. Additionally,
poor drum winding conditions are not well
tolerated by lang lay ropes.
Pre-forming
is a wire rope manufacturing process
wherein the strands and their wires are
formed. during fabrication--to the
helical shape that they will ultimately
assume in the finished rope or strand.
The wire arrangement in the strands
is an important determining factor in the
rope's functional characteristics,
i.e., its ability to meet the operating
conditions to which it will be subjected.
There are many basic strand patterns
around which standard wire ropes are built.
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Wire
ropes are identified by a nomenclature
that is referenced to:
1) The number of strands in the rope.
2) The number (nominal or exact) and arrangement
of wires in each strand, and
3) A descriptive word or letter indicating
the type of construction, i.e., the geometric
arrangement of wires.
At this
point, it would be useful to discuss wire
rope nomenclature in somewhat greater
detail because the subject may generate
some misunderstanding. The reason for
this stems from the practice of referring
to rope either by class or by its specific
construction.
Ropes
are classified by the number of strands
as well as by the number of wires in each
strand, e.g., 6 x 7, 6 x 19, 6 x 37, 8
x 19, 19 x 7, etc. However, these
are nominal classifications that may or
may not reflect the actual construction.
For example. The 6 x 19 class
includes constructions such as 6 x 21
filler wire, 6 x 25 filler wire, and 6
x 26 Warrington Seale. Despite
the fact that none of the three constructions
named have 19 wires, they are designated
as being in the 6 x 19 classification.
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Hence,
a supplier receiving an order for 6 x 19 rope
may assume this to be a class reference, and is,
therefore, legally justified in furnishing any
construction within this category. But, should
the job require the special characteristics of
a 6 x 25 filler wire, and a 6 x 19 Seale is supplied
in its stead, a shorter service life may be expected.
To avoid such misunderstandings,
the safest procedure is to order a specific construction.
In the event that the specific construction is
not known or is in doubt, the rope should
be ordered by class along with a description of
its end use.
Identification of wire rope in
class groups facilitates selection on the basis
of strength and weight / foot since it is customary
domestic industry practice that all ropes (from
a given manufacturer) within a class have the
same nominal strength, weight/foot, and price.
As for other functional characteristics, these
can be obtained by referencing the specific construction
within the class.
Only three wire ropes
under the 6 x 19 classification actually have
19 wires : 6 x 19 two-operation (2-op), 6 x 19
Scale (S), and 6 x 19 Warrington (W). All the
rest have different wire counts. In the 6 x 37
class there is a greater variety of wire constructions.
However, the commonly available constructions
in the 6x 37 class include : 6x 31 Warrington
Seale (WS), 6 x 36 WS, 6 x 41 Seale Filler Wire
(SFW). 6 x 41 WS, 6 x 43 Filler Wire Seale (FWS),
6 x 46 WS, etc. none of which contain exactly
37 wires
While the interior of a strand
is of some significance, its important characteristics
relate to the number and. in consequence, the
size of the outer wires.
Wire rope nomenclature
also defines the following:
Rope Description |
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Length |
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Size
(diameter) |
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Preformed
(pref) or non-preformed (non-pref) |
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Direction
and type of lay |
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Finish |
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Grade
of rope |
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Type
of core |
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If
direction and type of lay are omitted from the
rope description, it is pressured to be right
regular lay Two other assumptions are made by
the supplier:
1) if finish is omitted, this
will be presumed to mean uncoated "bright"
finish, and
2) if no mention is made with
reference to preforming, preforming will be presumed.
(Note that an order for elevator rope must have
an explicit statement since both pref and non-pref
ropes are used extensively.)
As an example, a complete
description would appear thus:
600 ft ¾" 6 x 25 FW pref RLL
Improved Plow Steel IWRC
When a center wire is replaced by a strand, it
is: considered as a single wire, and the rope
classification remains unchanged.
There are, of course, many other
types of wire rope, but they are useful only in
a limited number of applications and, as such,
are sold as specialties. Usually designated according
to their actual construction.
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