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| Rotation
Resistant and Non-Rotating Wire Rope |
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When loaded, every wire rope will
develop torque; that is it has the tendency to
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unlay itself unless both rope ends are secured
against rotation. |
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cause a lower sheave block to rotate and to
spin the line parts together. |
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Rotation resistant ropes can be divided
into 3 categories:
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Spin-Resistant
2-layer
(8 to 10 outer strands) |
Rotation Resistant
2-layer
(11 to 13 outer strands) |
Non-Rotating
3-layer
(14 or more outer strands) |
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Example of a 2-layer
rotation resistant construction
with 12 outer strands.
(19x7) |
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The characteristic of these wire ropes
are that the outer layer is twisted in the opposite direction of their inner
layers. The sometimes confusing issue is that many 8-, 9- and 10 strand
constructions are 2-layer types but their inner strands are NOT twisted
in the opposite direction and therefore these rope are NOT spin-resistant;
plus, for the untrained eye these ropes look very much alike their spin-resistant
variants. These and regular 6-strand ropes will spin violently and unlay
themselves when loaded, with one rope end allowed to spin freely. They may
also develop a significant drop in breaking strength and an even larger
drop in their fatigue life characteristic when used with one end allowed
to rotate.
As already mentioned, to achieve any degree of resisting the tendency of
a rope to spin and unlay under load, all such rope types (other than 4-strand
ones) are constructed with 2 or more layers of opposite twisted strands.
2-layer ropes have a larger tendency to rotate than 3-layer ones (e.g. class
34x7). Furthermore, 2-layer spin-resistant and rotation resistant ropes
will develop only about 55% to 75% of their breaking strength when one end
is allowed to rotate freely. This number increases to between 95% to 100%
for 3-layer non-rotating ropes.
Another important issue is that 2-layer rotation resistant and 2-layer spin-resistant
rope types have shown to break up from the inside. The 8 (e.g. 8x25 spin-resistant)
or 12 outer strands (19x7, 19x19, Compac® 19) are not able to evenly
distribute the radial forces and because of the inherent internal strand
cross overs (which make the rope spin- or rotation resistant) the resultant
severe notching stresses cause the rope core to break up premature (unless
the core is plastic coated, e.g. Python® Multi). Unexpected and sudden
rope failures may be the result. Moreover, 2-layer spin-resistant or rotation
resistant ropes satisfy only low to moderate rotational resistance demands.
3-layer rope constructions (e.g. class 34x7) have many more outer strands
which can much better distribute the radial pressures onto the reverse lay
inner strands. These ropes should be selected for larger mobile- and ALL
tower cranes. |
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