Samson Technical Bulletin
Published Date: November 2012
Revised: July 2013
Coefficient of Friction
Why Consider CoF?
One of Samson’s top goals is to ensure that the best performing rope product is used for the job. There are
many things to consider when identifying the optimum product. One such consideration is how the rope will
interact with application surfaces. Does the rope need to grip to a surface such as a bitt or winch drum?
Does it need to slide smoothly over a surface, such as a mooring chock or a bullnose? A measurable
way to describe these interactions is in terms of Coefficient of Friction (COF). Without considering the
appropriate COF in the system, the rope life and performance can be dramatically reduced.
How CoF is Defined and
Measured
Static COF is defined as the ratio of shear force
(Fs) to normal force (Fn) between two surfaces at
the moment of impending slip (that is the maximum
shear force prior to slip being initiated). This can be
shown as:
Where μ is
Fn
Fn
the COF and
—
Fs and Fn are
Fs
the shear and
normal forces,
Fs
respectively.
µ=
Surface interation between objects
Typically, COF for flexible rope is measured by
wrapping the rope around a circular drum, pin, or
sheave and displacing one end of the rope under
tension while measuring the tension on the other end
of the rope. In such a test, COF can be correlated to
the two tension loads and to the angle of the wrap
where T1 and
T2 are the
two tension
measurements
and ϴ is
µ = coefficient of friction
T2 the angle of
the wrap, in
T1
radians.
The COF between a rope and a solid surface
depends on a large number of factors both
dependent and independent of the rope design itself.
In addition to the structure and material of the rope,
other factors include the type of surface the rope
is in contact with, the contact pressure between
the two, and the presence of other species at the
interface (i.e. water, oil, etc.).
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The Correct CoF for the
Application
The COF of the rope is influenced, and varies,
by fiber type, coating, and rope construction. In
many applications it is essential that the correct
COF is used. Here are some examples that
highlight this importance:
A Traction Winch System
A traction winch system requires a rope with
a medium to high COF. In the system shown
in the photo below, Samson’s DPX™ fiber
technology provides a high COF in the traction
winch system. This enables the traction
sheaves to grip to the rope surface. A rope with
a lower COF would easily slide on the traction
sheaves, thus making the system inoperable.
Samson Technical Bulletin
continued
Coefficient of Friction
Published Date: November 2012
Revised: July 2013
Tugboat Bullnose
In a standard tugboat application, the mainline is redirected with a
bullnose. As the rope runs through the bullnose, energy is generated
in the form of heat. The amount of heat generated depends on
the level of friction between the surfaces. Samson’s proprietary
Samthane Type E coating used in Saturn-12 (a 12-strand line made
of 100% Dyneema®) keeps this friction and heat generation low,
ultimately adding to the rope’s life.
Mooring, Tugboat, and Barge Bitts
Mooring, tugboat, and barge applications require lines to be held by
bitts. The holding power of rope secured around a bitt depends on
the COF of the line (see photo at left). If a low COF line is used on a
bitt, the line will most likely slip under load with subsequent melting
and strength loss. It is essential that a line with an adequate level of
COF is used on these bitt applications.
These examples show the importance of COF. For help choosing the correct rope for the application or
identifying COF values, please contact the Samson technical sales team via [email protected]
or +1-800-227-7673.
Samson ropes with good COF:
> Control-DPX™
> DPX™-75
> Proton-8
> Q-12 Defender
> Quantum-8
> Quantum-12
> Turbo-EPX
DOCUMENT TB_Coefficient_of_Friction_July2013.pdf
SamsonRope.com | Email [email protected] | Tel +1.360.384.4669
© 2012 Samson Rope Technologies, Inc. All rights reserved. S/WD #108351 11/2012