ESTIMATING TENSION WHEN
PULLING CABLE INTO CONDUIT
When you calculate cable pulling tensions, what
friction coefficient should you use? User responses
vary . . . some answer "0.5" . . . others "0.4," or
"0.35". Who's right?? What coefficient of friction
provides the best tension estimates and correlation
for field planning and optimal cable system design?
To answer this question, we need to understand
more about "coefficient of friction." What exactly is
a "coefficient of friction" (COF). Can we find friction
coefficients in an appropriate reference book?
Let's start with a simple physics class example . . .
a wooden block (say, 5 kgs in weight) on a
horizontal steel plate. Say it takes 2 kgs force (19.6
N) to pull (drag) the block across the plate. The
coefficient of friction (wood on steel) is defined as
the ratio of this "dragging force" (2 kgs) to the
normal force (weight of 5 kg). In this case, the
friction coefficient would be .4. Note that the COF is
a dimensionless number.
Experience tells us that if we replace the wooden
block with a 5 kg rubber block, it will take a greater
force to drag the rubber block (say, 6 kgs force).
The measured coefficient of friction (rubber/steel)
would be 1.2. What's important to note from these
examples is that there is no single coefficient of
friction. The friction coefficient varies with the
Replace the block with cable and the plate with
conduit, and we have cable pulling . . . with a few
complications. Neither the cable nor the conduit is
flat. There may be more than one cable, which can
result in complex rubbing surfaces. Pulls are not
straight, and forces other than gravitational weight
occur at conduit bends. Finally, our Polywater
Pulling Lubricants change and lower the friction
Even with these differences, the friction coefficient
in cable pulling continues to depend on cable jacket