Tech Tips
Thimbles & Wire Rope Slings
There are a number of ways to terminate a wire rope sling.
A simple loop eye is the most common and provides a large
sling eye to place on a crane hook. But certain
applications require a thimble to be placed in the sling
eye.
This Tech Tip is not so much about the benefits of having
thimbles in sling eyes, but on the importance of installing
thimbles correctly. Our goal is to help you determine that
only quality products make their way to your job site.
Thimbles provide a couple of important attributes;
1) They protect the wire from contact with edges that may
damage the wires.
2) They can increase the sling’s breaking strength since the
formed sling eye will not be subject to sharp bends under
load.
Wire rope is formed by strands of wires that are laid
helically around a center core. This important
characteristic allows the strands to slide and adjust, but
also affects how a wire rope bends when forming a small eye
required for thimbles.
Notice how the thimble in the sling eye above is tight. The
pressed metal sleeve is close to the base of the sling eye,
and the wires form nicely around the thimble’s small
diameter.
During installation of a thimble it’s important that the
rope’s strands remain in position and in tight contact with
the small diameter thimble. It is equally important that
the rope’s strands stay in position as the pressed metal
sleeve is installed. A few taps from a mallet are often
required to get the sleeve in proper position and to hold
the thimble tight. Strands that are not held in place
during this installation can easily be moved out of
position.
Notice the strands in the thimble sling
eye below. The high strand is a result of the strands not
being held in position when the pressed metal sleeve was
applied. Because the sharp bend causes the strands to be of
different lengths at the base of the eye splice, a strand
can be pushed upward when the sleeve is applied. The result
is the “high strand” you see in the picture below.
Most rigging shops use a horizontal
vice to secure the strands when the metal sleeve is being
applied. The horizontal vice applies pressure on both sides
and the top of the eye at the same time. This three way
pressure enables the swage sleeve to be applied while the
wire rope strands remain tightly held around the thimble.
A horizontal vice is a practical
necessity when wire rope diameters are in the range of
5/8-inch and above. Without this pressure it's difficult to
hold the strands in their position, and just as difficult to
ensure that the thimble is tightly held in the eye.
Many slings have become damaged during
use when a loose thimble starts to peel out of the eye.
When you have the sharp edges of the thimble bearing on the
sling wires the result is cut wires and a possible loss of
load.
Thimble sling eyes should be checked
frequently to ensure the thimbles are tight and not subject
to any movement under load. If you find any slings with
high-stranding in the sling eye the sling should be removed
from service.
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