choosing a "machine," is to determine the potential work load. Stated
iIi
terms
of
wire rope, this means establishing the actual load. To this known dead weight,
there must be added those loads that are caused by abrupt starts (acceleration),
sudden stops (deceleration) , shock loads, high speeds and friction
of
sheave
bearings. Another item in this equation is the loss
of
efficiency that occurs when
the rope bends over sheaves. All
of
these factors must be summed up in order to
determine the true total load.
For any operation, the total load
is
multiplied by a required design factor to
determine the value which the minimum breaking force
of
the rope must meet or
exceed. Standards organizations and regulatory bodies typically establish mini-
mum
design factors. The user must be aware
of
the design factors specified for
their applications and select wire ropes accordingly. (A further discussion
of
Design Factors can be found on p. 93)
2)
Resistance
to
bending fatigue
To describe this, a close analogy can be made with a paper clip. While most
of
us cannot pull a paper clip in two,
if
repeatedly bent back and forth at one point,
it will eventually break. The reason for this is metal fatigue. To some degree, the
same thing happens when a wire rope bends around sheaves, drums, and rollers.
The
sharper-or
more
acute-the
bend, the quicker the fatigue occurs.
Increased rope speed and/or reverse bends may also accelerate fatigue rates.
As for the rope, with all other rope characteristics being equal, the greater the
number
of
outer wires in each strand, the greater the resistance
of
the rope to
bending fatigue.
3)
Resistance
to
vibrational fatigue
Vibration, from whatever source, sends shock waves through the rope. These
waves are a form
of
energy that must be absorbed at some point. This point may
appear at various
places-the
end attachment, the tangent where the rope con-
tacts the sheave,
or
at any other place where the waves are damped and the ener-
gy absorbed.
In the normal operation
of
a machine or hoist, wire ropes develop a wave
action that can be from a low frequency to a sharp, high frequency cycle. A good
example
of
this is found in shaft hoists. When the cage
is
just starting up, the
rope has a very slow swing within the shaft. But, by the time the cage reaches the
top
of
the shaft, the initially low frequency has become a high frequency vibra-
tion. The result is fatigue and eventual breakage
of
the wires at the attachment
point to the cage.
Another type
of
vibrational fatigue is found in operations where there is cyclic
loading. Such loadings would be found, for example, in the boom suspension
system
of
draglines. Here. the energy is absorbed at the end fittings
of
the pen-
dants or at the tangent point where the rope contacts the sheave.
60 • Wire Rope technical Board - Wire Rope Users Manual, Fourth Edition
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