BENDING WIRE ROPE OVER SHEAVES AND DRUMS
Sheaves, drums and rollers must be
of
a correct design if optimum service is to be
obtained from both the equipment and the wire rope. Because there are many different
types
of
equipment and many different operating conditions, it
is
difficult to identify the
one specific size
of
sheave or drum most appropriate for every application.
The guideline to follow is this: the most practical design is the one that most
closely accommodates the limiting factors imposed by the equipment, the operating
conditions and the wire rope.
All wire ropes operating over sheaves and drums are subjected to cyclic bending
stresses, thus the rope wires will eventually fatigue. The magnitude
of
these stresses
depends - an other factors being constant - upon the ratio
of
the diameter of the
sheave or drum to the diameter
of
the rope. Frequently, fatigue from cyclic, high-
magnitude bending stress is a principal reason for shortened rope service.
In order for a rope to bend around a sheave, the rope's strands and wires must
move relative to one another. This movement compensates for the difference in
diameter between the underside and the top side
of
the rope, the distance being
greater along the top side than it is on the underside next to the groove. Proper rope
movement (and service) is adversely affected
if
the wires cannot adjust to compen-
sate for this length differential. Also, there can be additional limitations to wire
movement because
of
excessive pressure caused by a sheave groove diameter which
is too small, or by lack
of
rope lubrication. Avoid changing the bending direction
from one sheave to another as this reverse bending further accelerates wire fatigue.
The relationship between sheave diameter and rope diameter is a critical factor
that is used to estimate the rope's fatigue resistance
or
relative service life. It is
expressed in the
Did ratio mentioned earlier in which D is the pitch diameter
of
the
sheave and d is the diameter
of
the rope. Table 9 lists suggested minimum Did val-
ues for various rope constructions. Other values are permitted by various standards
such as those listed in Table 10. Smaller values can affect rope life. Table
11
and
Figure 34 show the effect
of
rope construction and Did on service life.
BREAKING IN A NEW WIRE ROPE
A new wire rope requires careful installation and following all the appropriate
guidelines previously noted. After the rope is installed and the ends secured in the
correct manner, the equipment should be started carefully and then permitted to run
through a cycle
of
operation at very slow speed. During this trial operation, closely
watch all working
parts-sheaves,
drums,
rollers-to
make certain that the rope
runs freely, and without any possible obstructions as it makes its way through the
system.
If
no problems appear in running the rope, the next step should include sev-
eral repititions
of
the normal operational cycle under increasing loads and speeds.
This procedure allows the component parts
of
the new rope to make a gradual
adjustment to the actual operating conditions. Taking the time and effort to perform
these breaking in procedures should result in obtaining the optimum service life
from the wire rope.
Wire Rope Technical Board - Wire Rope Users Manual, Fourth
Edition'
51
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