6 Physical Properties
ELASTIC PROPERTIES OF WIRE ROPE
The following discussion relates to conventional 6- or 8-strand ropes that have
either fiber
or
steel cores; it is not applicable to rotation-resistant ropes since
these constitute a separate case.
Wire rope is an elastic member; it stretches
or
elongates under load. This
stretch derives from two sources:
1)
constructional, and
2) elastic.
In
actuality, there may be a third source
of
stretch-a
result
of
the rope rotating
on its own axis.
Such elongation, which may occur either as a result
of
using a
swivel,
or
from the effect
of
a free-turning load, is brought about
by
the unlaying
of
the rope strands. Because the third source is not a recommeded occurrence, it
is a subject that is beyond the scope
of
this publication. Our discussion will be
limited to
constructional and elastic stretch.
CONSTRUCTIONAL
STRETCH
When a load is applied to wire rope, the helically-laid wires and strands act in
a constricting manner thereby compressing the core and bringing all the rope ele-
ments into closer contact.
The
result is a slight reduction in the rope's diameter
and an accompanying lengthening
of
the rope.
Constructional stretch is influenced by the following factors:
1)
type
of
core (fiber
or
steel)
2) rope construction (6 x
7,6
x 25 FW, 6 x
41
WS, 8 x 19S, etc.)
3)
length
of
lay,
4) material
Ropes with wire strand core (WSC)
or
independent wire rope core (IWRC)
have less constructional stretch than those with fiber core (FC). The reason for
this is that steel cannot compress as much as the fiber can.
Usually, constructional stretch will cease at an early stage in the rope's life.
However, some fiber core ropes,
if
lightly loaded (as in the case
of
elevator
ropes), may display a degree
of
constructional stretch over a considerable portion
of
their life.
A definite value for determining constructional stretch cannot be assigned
since
it
is influenced by several factors. The following table gives some idea
of
the approximate total constructional stretch
as
a percentage
of
rope length.
Rope construction
6 strand FC
6 strand IWRC
8 strand
FC
8 strand IWRC
Approximate Total Constructional Stretch
112%
- 3/4%
114%
-
112%
3/4% - 1%
112%
- 3/4%
Wire Rope Technical Board - Wire Rope Users Manual, Fourth
Edition'
89
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