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13
Estimated Pulling Tension
The installer should calculate estimated pulling
tensions for all cables to be pulled, to insure that
the allowable values established in the previous
sections are not exceeded. The principle equa-
tions used for these calculations are as follows:
The estimated pulling tension of one cable in a
straight section of raceway may be calculated
from the following formula that does not consider
changes in elevation:
T = L x W x K
T = Estimated pulling tension in pounds
L = Length of installation in feet
W = Weight of cable in pounds per foot
K = Coefficient of friction
The estimated pulling tension of a cable in an
inclined section of raceway may be calculated
from the following simplified formula, where
prior tension is the tension at the beginning of
the incline and the multiplying factor (M) is tab-
ulated below.
T = L x W x M + (prior tension)
Note, short downward bends may be neglected.
For riser applications, contact the RSCC
Engineering Department.
Multiplying factors (M)
Angle from horizontal in degrees
K
15
30 45 60 75 90
0.1 0.36 0.59 0.78 0.92 0.99 1.00
0.2 0.45 0.67 0.85 0.97 1.02 1.00
0.3 0.55 0.76 0.92 1.02 1.04 1.00
0.4 0.65 0.85 0.99 1.07 1.07 1.00
0.5 0.74 0.93 1.06 1.12 1.10 1.00
To calculate the tension out of a bend, the
following formula may be used:
T = T
1
x F
T = Tension coming out of the bend
in pounds
T
1
= Accumulated tension going into the bend
in pounds
F = Friction factor for various values of coeffi-
cient of friction and bends as shown below.
Friction factors (F)
Angle of bend in degrees
K
15
30 45 60 75 90
0.1 1.03 1.05 1.08 1.11 1.14 1.17
0.2 1.05 1.11 1.17 1.23 1.30 1.37
0.3 1.08 1.17 1.27 1.37 1.48 1.60
0.4 1.11 1.23 1.37 1.52 1.69 1.87
0.5 1.14 1.30 1.48 1.69 1.92 2.19
Note, for large cables where bends close to the
minimum bend radius are contemplated, addi-
tional force may be required to bend the cable.
Estimated Sidewall Pressure
The sidewall pressure acting upon a single
cable at a bend may be estimated from the
following equation:
P = T
R
P = Sidewall pressure on the cable in pounds
per foot
T = Estimated tension out of the bend in pounds
R = Radius of the bend in feet
Back Tension
The force required to pull a cable off the reel is
generally referred to as back tension. This is
normally taken to be zero, since the cable is fed
off the reel. This value may be negative, and
light braking may be applied to control the flow
of cable to avoid feeding at too great a rate. For
downward pulls, considerable braking may be
required.
