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Preparation of Papers in Two-Column Format for the Proceedings of the 1997 IEEE International Symposium on Electronics and the Environment Improving Bend-over-Sheave Fatigue in Fiber Ropes


F. Sloan*, R. Nye, and T. Liggett

The Cortland Companies
www.thecortlandcompanies.com
Puget Sound Rope
*currently with Celanese Advanced Materials Inc., Charlotte, NC 28210


Abstract- One of the limitations of synthetic fiber ropes in
industrial uses has been the premature wear of these
materials when subjected to continuous bend-over-sheave
fatigue. While one-way passage over sheaves is not generally
damaging, repeated back-and-forth movement over one part
of the rope, as in heave compensation units, can lead to
damaging heat buildup and unexpected failure modes. This
report details the results of bend-over-sheave fatigue testing
on 18 mm diameter fiber ropes conducted at Cortland Cable
Company. Cycles-to-failure data is presented for the fiber
materials, coatings, and constructions tested to date. Fatigue
life was significantly improved by fiber blending and with
specialty coatings, resulting in a new braided rope design
specifically optimized for bending fatigue (BOB). The BOB
design was found to have a significant CTF advantage over
aramid constructions and steel wire currently in use today.
The importance of test parameters such as sheave design and
cycle rate is highlighted.


INTRODUCTION

Many applications involving rope require the passage of
the rope over sheaves of one kind or another. Synthetic
fiber ropes are not particularly damaged by periodic one-
way passage over these sheaves, however repeated or
continuous back-and-forth cycling over one spot on the
rope can create rapid heat build-up and unforeseen damage
mechanisms. This test program was initiated as a way to
generate a broad array of bending fatigue data on small-
scale 18 mm (3/4) synthetic ropes subjected to continuous
bending over a 230 mm (9) diameter sheave. The purpose
of the test program was to develop a new braided rope
design that could compete with aramid wirelay and steel
wire rope in cyclic bending applications


BACKGROUND

The advantages of synthetic fiber ropes over wire ropes
have been documented extensively, as in [1]. The light
weight, absence of corrosion, and low stiffness drive the
use of fiber ropes in place of wire rope in many
applications in a variety of industries. Recently fiber ropes
made from polyester (PET) and HMPE have begun to
replace large-diameter wire ropes for mooring permanent
and temporary platforms in the petroleum industry [2-3].
The fatigue life of fiber ropes under tension-tension
loading has been the subject of much recent study, e.g. [4],
and the fatigue performance has proven to be as much as an
order of magnitude higher than that of steel wire rope.
However, one application where wire ropes have continued
to outperform synthetics is in cyclic bending fatigue over
sheaves (CBOS). This is one of the most common
applications of wire ropes, in familiar end-uses such as
cranes, elevators, and pulling lines.
Historically fiber ropes were not able to displace wire
rope in over-sheave applications, even if the CBOS fatigue
life had been equivalent. Synthetics could not match the
strength of wire so any replacement would require larger
sheaves and other hardware. In addition, high bearing
pressures and relative motions during bending caused fiber-
to-fiber abrasion that was not apparent from looking at the
exterior of the rope. While also true of wire rope (with the
added complication of corrosion), inspection criteria in
fiber ropes were not as well developed as in wire.
The first application of large synthetic fiber ropes to
bend-over-sheave applications came after the development
of aramid fibers (e.g. Kevlar
®
). These were the first high-
performance synthetic fiber (HPF) ropes that could match
the strength of steel, so that a size-for-size replacement
could be made without increasing the size of winches and
sheaves. Although the CBOS performance of aramid
wirelay ropes did not quite meet the specifications of wire
rope, the advantages of having the synthetic line (most
notably safety, low weight, and corrosion resistance) drove
the end-user away from wire rope. The result was the US
Navys deep-sea salvage lifting lines, all-aramid fiber ropes
in a helical wirelay jacketed construction.
This paper will discuss the development of a new HPF-
based rope product specifically targeted at CBOS
applications. This new technology is based on the use of
fiber blends and specialty coatings. The development
testing also revealed new insights into the effects of test
and material parameters such as material type, design
factor, sheave diameter, test speed, and rope size. CBOS
tests have demonstrated that new products developed using
this technology can meet or exceed the CBOS fatigue
performance of wire ropes. Better fatigue life coupled with
the inherent advantages of weight and environmental
stability should continue to drive the replacement of wire
ropes with synthetics in CBOS applications.


EXPERIMENTAL

Ropes were cycled over mild steel sheaves as shown in
Fig. 1 so that a rope length of one-half a sheave
circumference (in the center of the rope) was subjected to
two bend cycles (straight-bent-straight-bent-straight) per
machine cycle. This area of the rope is called the double-
bend-zone or DBZ, and in this case was approximately

1234 400 mm in length. On either side of the DBZ was a
roughly 360 mm length subjected to only one bend cycle
(straight-bent-straight) per machine cycle the single
bend zone or SBZ.
Sheaves used in these tests had a sheave tread diameter
of 230 mm (9.0) and a sheave groove diameter of 20 mm
(0.79), 5% greater than the nominal rope size. The test
machine was cycled at a rate of 360 machine cycles per
hour. Some experiments were conducted to quantify the
effect of varying sheave tread diameter, sheave groove size,
and cycle rate.
Rope surface temperatures were measured periodically
during the testing by holding a thermal probe against the
rope in the center of the DBZ with the machine stopped.
Temperatures measured using this technique ranged from
35-45
o
C during cycling.
All ropes were 12-strand single braid construction,
designed to precisely meet an 18 mm (0.75) diameter
specification. Actual breaking strengths of these ropes
ranged from 31 to 38 metric tons. The following fiber
materials, coatings, and constructions were tested during
the testing:

Fibers: HMPE
(Dyneema
®
SK-75, Spectra
®
1000,
Plasma
®
), LCP (Vectran
®
HS 1500)
Coatings: Polyurethanes PG, PN, PP, PB, PR
Designs: 12-strand (A) and braided variants B, C, D


DATA TREATMENT

In order to compare the results of testing at different
conditions, a basic theoretical treatment of test parameters
was sought. One normalizing equation comes from the
wire rope industry, and is derived by making a simplistic
assumption about the static bearing stresses on a sheave
under load,


d
D
T
ess
BearingStr 2
(1)

In this case T is the tension in the rope, D is the sheave
diameter, and d is the rope diameter.
By now dividing both sides by the ultimate strength of
the material (U), we get the Drucker-Tachau bearing
pressure ratio [5],


(
)
d
D
U
T
2 (2)

which has been used for many years to normalize wire rope
CBOS test data. By dividing through top and bottom by d
2

we get








d
D
U
d
T
2
2 (3)
Fig. 1. CBOS Test Frame at Cortland Cable

On recognizing that T/d
2
should be linearly related to
the stress in the rope material, and assuming that failure
should occur when the stress exceeds U, then we can
replace part of the equation (removing arbitrary constants)
to get a Normalized Sheave Pressure or NSP, defined as





d
D
MBL
NSP %
(4)

where %MBL is the load in the rope expressed as a
percentage of its minimum rated breaking load. The
inverse of the NSP has also been used by investigators to
normalize CBOS data, e.g. the Life Factor used by
Gibson[6].
Note that because of the inherent non-linearity in rope
structures as well as other factors affecting breaking
strength, the assumptions that lead to Eq. 4 are not strictly
correct. However, the NSP should provide some predictive
capability for a particular rope design (constant size,
material, and construction) over a range of sheave
diameters and stress ranges. This parameter can also
provide quick comparisons between different materials and
constructions.
In order to test the NSP as a normalizing parameter, two
sets of experiments were conducted. In one set two ropes
were subjected to CBOS fatigue loaded to a tension of 9%
of the MBL at a D:d ratio of 24:1. In the second set the
tension was 4.5% of the MBL at a D:d of 12:1. Both had
NS