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to Technical Information Main Page |
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Relative
Service Life
The ability of wire rope to withstand repeated bending work over sheaves
and onto drums is also called the 'fatigue resistance'. This terms does
NOT describe the ability to withstand mechanical damages nor the crush resistance
of the rope.
The fatigue resistance of a rope is not time but cycle dependent. Bending
fatigue is the ability to withstand repeated bending over sheaves and drums
and such ability is depending on factors such as drum diameter, groove dimensions,
rope tension, line speed, rope construction, fluctuation between highest
and lowest loads etc.
For further information on this topic, please contact us for technical assistance
and advice. |
The tables show relative
expected rope service life in relationship to D/d ratios and Design Factors.
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| If the shackle or object
has 2 times the diameter of a 6-strand wire rope sling (D/d 2:1) the
basket sling capacity must be reduced by 40% |
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| If the object lifted
with a 6-strand wire rope sling in a basket hitch is at least 25 x
larger than the sling diameter (D/d 25:1) the basket hitch capacity
need not to be adjusted. |
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Loss
of Strength over
Sheaves and Pins
Rope breaking strength is determined in a standard test wherein fittings
are attached to the ends of the rope and the rope is pulled in a straight
line. If however, the rope passes over a curved surface (such as a sheave
or pin) its strength is decreased. The amount of such reduction will depend
on the severity of the bend as expressed by the D/d ratio.
For example, a rope bent around a pin of its own diameter will have only
50% of the strength attributed to it in the standard test. This is called
'50% efficiency'. Even at D/d ratios of 40, there may be a loss of up to
5%.
At smaller D/d ratios, the loss in strength increases quite rapidly. The
angle of bend needs not to be 180°, 90°, or even 45°; relatively
small bends can cause considerable loss.
The table shown derived from standard test data as published by the 'Wire
Rope Technical Board', is based on static loads only, and is a weighted
average of 458 tests over pins and thimbles on 6x19 and 6x37 class ropes. |
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Why
multi-strand Wire Rope?
The number of outer strands determine the contact area between the rope
and the sheave groove. If this area is increased the points of contact are
multiplied and contact pressures are reduced.
At the same time lateral notching stresses between strands and wires are
reduced, resulting in increased fatigue life.
Extensive tests programs at the University of Stuttgart, Germany, have proven
conclusively that the bending fatigue life of wire rope improves with an
increasing number of outer strands.
Based on that research we have developed Python® High Performance Wire
Rope with 8-, 9-, and 10 outer strands. |
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