,"
'"
'",
','
••
< " •
.,'
For certain applications, ropes
may
be
prestretched in order to remove some
of
the constructional stretch. Frequently, this treatment is used on structural
members such
as
bridge rope
and
strand. In some cases, pre stretching
is
applied
to operating ropes where elongation
in
service from constructional stretch
may
present problems, e.g., elevator and sldp hoist
rOPeS.
Whil~
a prestretching technique
has
value, some of the benefit
is
lost
in
reeling
and handling.
DESIGN
FACTORS
The design factor
is
defined
as
the ratio
of
the
minimum breaking force of a
wire rope to the total load it is expected
to
carry. Hence,
the
design factor that is
selected plays
an
important part in determining the rope's service life. Excessive
load,ng, whether continuous or sporadic, will greatly impair a rope's serviceabil-
ity.
Usually, the choice of a certain
wire
rope size
and
grade will
be
based
on
sta-
tic loading and, under static conditions, it is
sufficient for its task.
Figure 66, the
Wire
Rope Relative Service Life Curve, shows how the service
life can be reduced
as
operating loads are increased or can be increased if
the
operating loads are decreased. It should be noted that the relative change is com-
parative
of
one design factor
to
another. For example, a change in
the
designfac-
tor
from
5
to
3 decreases
its
life expectancy index
from
100
to
6O-a
drop
of
40%.
MINIMUM
BREAKING
FORCE
TABLES
The minimum breaking force (called nominal strength in some specifications)
given in tables
19
through 43,
has
been calculated by a consensus, industry-
accepted procedure,
and
manufacturers design wire rope
to
theSe
strengths.
When making design calculations, it should be noted that
the
given figures are
the
static strengths. Designers should base their calculations
on
these strengths.
The
actual breaking force is
the
ultimate load registered on a wire rope sample
during a tension test.
Unless otherwise required, once the load
on
a rope during a
tensile test reaches
the
minimum breaking
forc.e
value, the test
is
successful.
Variables
in
rope testing can only cause a
rC!duction
in the rope breaking strength,
never
an
increase.
Various standards
and
specifications have tensile test method requirements.
These
may
include length
of
sample tested, sample preparation requirements,
speed of tensile machine
and
validity of test results if failure
is
near end termination.
Wire Rope Technical Board - Wire Rope USers Manual, Fourth
Edition'
93
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