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One rope lay is the length along
the rope which a single strand requires to make one complete spiral,
or turn, around the core. It is an engineering factor in the design
of a rope, and is carefully controlled during manufacture.
Since there is often some adjustment in rope lay during the initial
break-in stages of a ropes usage, it is recommended that rope lay measurements
should be made after the initial loading, for comparison purposes at
succeeding periodic inspections.
One method for measuring rope lay is with ordinary blank white paper
and a pencil. Firmly hold the paper on the rope and stroke the rope
with the side of the pencil, using a small angle iron as a guide, so
the ropes print appears on the paper.
By drawing a line through one strand of the print, counting off the
number of strands in the rope and then drawing another line on the print
at the place where the same strand appears again, a measurement is established.
Many inspectors have found that a crayon or marking stick and a roll
of calculator tape are ideal for making a print at least three rope
lays long. An average lay length can then be determined.
Changes in length of lay are usually gradual throughout the working
life of a rope. It is important to compare current lay measurements
with previous inspection results to note any sudden changes. An abrupt
change in the pattern can be a signal of an impending problem.
As a rule, if lengthening of lay is noted, with loss of rope diameter,
internal break-up or core destruction should be suspected.
When lengthening of lay is noted, without loss of rope diameter, the
rope is probably unlaying for some reason and further examination should
be made to determine the cause.
Unlaying sometimes results from operating a rope without having both
ends secured to prevent rotation. An end swivel attachment permits such
rotation and unlaying.
Another common cause of unlaying is worn sheaves. When the bottom of
a sheave groove wears, it can restrict normal movement as the rope enters
and leaves the groove. The result can be a build-up of twist which can
change the length of lay.
Whatever the cause, unlaying is an abnormality, and should be noted
for future reference if the immediate cause cannot be determined.
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