______________________________________________________________________________________
u_
Figure 40. The wire rope industry
refers to this as the X-chart.
It
serves to illustrate the inverse
relationship between abrasion
resistance and resistance to bend-
ing fatigue in a number
of
the
commonly used wire rope con-
structions.
THE "X-CHART" - ABRASION RESISTANCE
VS BENDING FATIGUE RESISTANCE
Two compelling factors that govern most rope selection decisions are: abra-
sion resistance, and resistance to bending fatigue.
Striking a proper balance with
respect to these two important characteristics demands careful consideration. A
graphic presentation
of
this comparison
of
qualities, between the most widely
used rope constructions and others, is given by means
of
the X-chart (Fig. 40).
Referring to this chart when selecting a rope, the mid-point (at the X) comes
closest to an even balance between abrasion resistance and resistance to bending
fatigue. Reading up or down along either leg
of
the
X,
the inverse relationship
becomes more apparent as one quality increases and the other decreases.
The term
flexibility is frequently thought
of
as being synonymous with resis-
tance to bending fatigue.
This is not true. Flexibility refers to the capability
of
flexing
or
bending. While a high degree
of
fatigue resistance may sometimes
accompany the flexibility characteristic, it does not necessarily follow that this is
so. A fiber core rope, for
example,
is more flexible than an IWRC rope. Yet,
when the IWRC rope is bent around sheaves at relatively high loads, it will usu-
ally perform better than the more flexible fiber core rope. The reason for this lies
in the ability
of
IWRC rope to better support the outer strands, retain its round-
ness and maintain freedom
of
internal movement. Under the same conditions, a
fiber core rope will flatten and inhibit free internal adjustment, thereby reducing
fatigue life.
As with all engineering design problems, feasible solutions demand compro-
mise to some degree. At times, it becomes necessary to settle for less than opti-
mum resistance to abrasion in order to obtain maximum fatigue resistance, the
latter being a more important requirement for the given job. A typical example
of
this kind
of
trade-off would be in selecting a highly fatigue resistant rope on an
overhead crane. Conversely, in a haulage installation, a rope with greater resis-
tance to abrasion would be chosen despite the fact that such ropes are markedly
less fatigue resistant. Ultimately, what is sought is an efficient, economical solu-
tion, hence whatever the compromise, it should assist in achieving this goal.
62'
Wire Rope Technical Board - Wire Rope Users Manual, Fourth Edition
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