Wear, due to friction on
sheaves, rollers, drums, etc., eventually causes outer wire abrasion.
Before any inspection is made, determine what type of wire rope you have
in service. Many of today's wire ropes are 'compacted', 'calibrated', or
'die formed'. This manufacturing process purposely flattens the outer wires
and for an unexperienced inspector these ropes may appear to be already
abraded when indeed they are brand new. If you are in doubt about what type
of rope you are about to inspect, have a look at a section of the rope which
was not subjected to any abrasive work; e.g. like the safety wraps on the
drum or a section just behind the end connection.
The round outer wires of standard wire rope will become flat on the outside
due to friction when in contact with drums, sheaves, or other abrasive matter
like sand or gravel. This is part of normal service deterioration and in
most crane installations relatively even abrasion will occur. The
rope must be replaced, however, if this wear exceeds 1/3 of the diameter
of the wire.
It is good
practice to compare a section of the rope which was NOT subjected to any
bending work (e.g. the safety wraps, or a short section behind the end fitting)
to the rope section to be inspected.
The same applies when evaluating any possible reduced rope diameter during
service. (See next column)
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When the surface wires are worn
by 1/3 or more of their diameter the rope must be replaced. |
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Abrasion caused by dragging the
rope over a sharp object (steel corner, sharp plate, abrasive surface
etc.) |
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Peening and subsequent wire break
caused by high fleet angle and rope vibration. |
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Rope abrasion caused by normal operating
condition on a high cycle crane. Rope must be retired. |
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As discucssed previously on the 'Measuring
the rope diameter' page and on the 'Break-In-Period'
page, shortly after installation, the wire rope diameter will slightly decrease.
This is normal and is caused by the adjustment of all rope elements when
loaded the first time. To evaluate the diameter reduction, you have to measure
the rope when new, and you also have to measure the rope after the break
in period at a specified load. This gives you a good indication of the magnitude
of the initial diameter reduction in your specific application. The diameter
reading you took after the break in period should now become your 'gauge'.
Do not compare the rope diameter you are about to take with the 'catalogue'
diameter. It may give you a false indication, since wire rope may have a
plus tolerance of up to 4% to 5% over the 'catalogue' diameter.
If you detect a further diameter reduction when measuring the rope under
the same load condition as after the break in period, it is often due to
excessive abrasion of the outside wires, loss of core support, internal
or external corrosion, inner wire failures, and/or inner wire abrasion.
However, there will always be a normal continuous small decrease in diameter
throughout the rope's service life.
Core
deterioration, when it occurs, is revealed by a more rapid reduction in
diameter, and when observed, it is time for removal.
Deciding whether or not a rope is safe is not always a simple matter. A
number of different but interrelated conditions must be evaluated. It would
be dangerously unwise for an inspector to declare a rope 'safe' for continued
service simply because its diameter had not reached a certain minimum diameter
if, at the same time, other observations led to a different conclusion.
Because the removal criteria are much varied for different rope constructions
and types of cores, a table of minimum diameters has been deliberately omitted
from this publication. |
