The basic unit of wire rope construction is the
individual wire, which may be made of steel, iron, or
other metal in various sizes. The number of wires to a
strand varies, depending on the purpose for which the
rope is intended.
Wire rope is designated by the
number of strands per rope and the number of wires
per strand. Thus, a 1/2-inch 6-by-19 rope will have 6
strands with 19 wires per strand; but it will have the
same outside diameter as a 1/2-inch 6-by-37 wire
rope, which will have 6 strands with 37 wires of much
smaller size per strand. Wire rope made up of a large
number of small wires is flexible, but the small wires
are easily broken, so the wire rope does not resist
external abrasion. Wire rope made up of a smaller
number of larger wires is more resistant to external
abrasion but is less flexible.
The core is the element around which the strands
are laid to form the rope. It may be a hard fiber (such
as manila, hemp, plastic, paper, asbestos, or sisal), a
wire strand, or an independent wire rope. Each type
of core serves the same basic purpose-to support the
strands laid around it.
A fiber core offers the advantage of increased
flexibility. Also, it serves as a cushion to reduce the
effects of sudden strain and acts as a reservoir for the
oil to lubricate the wires and strands to reduce friction
between them. Wire rope with a fiber core is used in
places where flexibility of the rope is important.
A wire strand core not only resists heat more than
a fiber core, but also adds about 15 percent to the
strength of the rope.
On the other hand, the wire
strand makes the rope less flexible than a fiber core.
An independent wire rope core is a separate wire
rope over which the main strands of the row are laid.
It usually consists of six, seven-wire strands laid
around either a fiber core or a wire strand core. This
core strengthens the rope more, provides support
against crushing, and supplies maximum resistance to
Wire rope maybe made by either of two methods.
If the strands or wires are shaped to conform to the
curvature of the finished rope before laying up, the
rope is termed preformed. If they are not shaped
b e f o r e f a b r i c a t i o n , t h e r o p e i s t e r m e d
When cut, preformed wire rope
tends not to unlay, and it is more flexible than
nonpreformed wire rope. Wire nonpreformed wire
rope, twisting produces a stress in the wires; and,
when it is cut or broken, the stress causes the strands
to unlay. In nonpreformed wire, unlaying is rapid
and almost instantaneous, which could cause
serious injury to someone not familiar with it.
The main types of wire rope used by the Navy
consist of 6, 7, 12, 19, 24, or 37 wires in each strand.
Usually, the rope has six strands laid around a fiber or
steel center. Two common types of wire rope,
6-by-19 and 6-by-37 rope, are illustrated in views A
and B of figure 4-6, respectively. The 6-by-19 type of
rope, having 6 strands with 19 wires in each strand, is
commonly used for rough hoisting and skidding work
where abrasion is likely to occur. The 6-by-37 wire
rope, having 6 strands with 37 wires in each strand, is
the most flexible of the standard 6-strand ropes. For
that reason, it is particularly suitable when small
sheaves and drums are to be used, such as on cranes
and similar machinery.
GRADES OF WIRE ROPE
Wire rope is made in a number of different grades.
Three of the most common are mild plow steel, plow
steel, and improved plow steel.
Mild plow steel rope is tough and pliable. It can
stand up under repeated strain and stress, and it has a
tensile strength of from 200,000 to 220,000 pounds
per square inch (psi).
Plow steel wire rope is
unusually tough and strong. It has a tensile strength
(resistance to lengthwise stress) of 220,000 to
240,000 psi. This rope is suitable for hauling,
hoisting, and logging. Improved plow steel rope is
one of the best grades of rope available, and most, if
not all, of the wire rope in your work will probably be
made of this material. It is stronger, tougher, and
more resistant to wear than either plow steel or mild
plow steel. Each square inch of improved plow steel
can withstand a strain of 240,000 to 260,000 psi.
Figure 4-6.Two common types of wire rope.