Effective Coefficient of Friction
The effective coefficient of friction for a multiple cable pull is a function of the basic coefficient of friction KQ and the weight correction factor Wc.
This increase in friction (K) is caused by an increase in the weight correction factor (Wc). The Wc accounts for the wedging action between the cables and the conduit. The following equation calculates the effective coefficient of friction:
K = KQ Wc
|
Where: |
K = Effective coefficient of friction (dimensionless) KQ = Basic coefficient of friction (dimensionless) Wc = Weight correction factor (dimensionless) |
Research data indicates that at a sidewall bearing pressure (SWBP) of approximately 150 Ib/ft or more, the effective coefficient of friction is significantly reduced. In effect, this means that when the cable is pulled around bends, initially use the higher coefficient of friction to calculate the SWBP. When the SWBP is greater than or equal to 150 Ib/ft, use the lower coefficient of friction.
Table: Basic Coefficients of Friction - KQ
|
Type of Conduit |
Cable Outer Covering Material |
Bends where SWBP is < 150 lb/ft and Straight Pulls |
Bends where SWBP exceeds 150 lb/ft |
|
|
Single Cable Pull |
Multiple Cable Pull |
|||
|
PVC |
XLP, 600V concentric wires PE jacket PVC jacket Neoprene |
.40 .40 .40 .50 .90 |
.60 .40 .45 .60 1.50 |
.15 .20 .15 .30 .15 |
|
Steel |
XLP, 600V concentric wires PE jacket PVC jacket Neoprene |
.60 .50 .50 .65 1.05 |
.65 .50 .50 .65 1.75 |
.25 .30 .25 .30 .25 |
|
Bitumenized Fiber |
XLP, 600V concentric wires PE jacket PVC jacket Neoprene |
.30 .40 .25 .40 .40 |
.65 .40 .60 .45 .55 |
-- -- -- -- -- |
|
Terra-Cotta or Transite |
XLP, 600V concentric wires PE jacket PVC jacket Neoprene |
.70 -- .70 .70 1.00 |
.70 -- .70 .70 1.80 |
-- -- -- -- -- |
|
Fiberglass |
XLP, 600V PE jacket PVC jacket Neoprene |
.23 .25 .39 .53 |
.23 .25 .39 .53 |
.15 .15 .30 .15 |
|
PE |
XLP, 600V concentric wires PE jacket PVC jacket Neoprene CN |
.45 .20 .25 .30 .65 .20 |
.55 .20 .85 .45 .65 |
.15 .20 .15 .30 .15 |