Friction Coefficient in Cable Pulling Sidewall Pressure Limits

Qualitative Description of Sidewall Pressure

In the definition of friction, the normal force that results in the frictional resistance to movement comes from weight of the object (the opposing vector).  In cable pulling through a straight conduit, the normal force is also equal to the weight of the cable.  In a slope, the normal force is the vectored gravitational force or weight.  However, if there is a conduit bend in the pull route, the normal force, sometimes called “sidewall pressure”, is greater than just the weight of the cable.  It includes the radial force of the cable being pulled into the bend or curve of the conduit.  This force is caused by the pulling tension, and increases with increasing pulling tension.

Why Worry About Sidewall Pressure?

Sidewall pressure is a crushing force on the cable.  This kind of force can damage cable. In medium and high voltage cable, excess sidewall pressure is known to affect the insulation accelerating insulation treeing and thus shortening the cable life.  Sidewall pressure can deform metal and foil shields in cable.  In building wire, high sidewall pressure around a bend can tear the cable jacket, exposing the conductor.  Even in fiber optic cables, it is known sidewall pressure can crush the fibers producing micro-crazing and signal loss.

Sidewall Pressure Definition and Limits

Because of this potential damage, cable manufacturers establish sidewall pressure limits for their cables.  These limits depend on cable type and construction.  The limits must be observed for maximum cable performance and life.

Sidewall Pressure is primarily a term and calculation used in the electrical industry.  At the basic level, it is the pulling tension coming out of a bend divided by the radius of the bend.  Typical sidewall pressure limits for electrical cable are 300 to 1000 lbs/ft (4.4 to 14.6 kN/m).

For some cable types, limiting bend radius is the common way to limit sidewall pressure.  Fiber optic cable manufacturers limit bend radius based on cable diameter.  Typical limitations in fiber optic cable pulling are a bend radius greater than 20 to 40 times cable OD.  While determination of sidewall pressure is not typical with fiber optic cable, it is interesting to note that with a maximum pulling tension of 600 lbs (2.7 kN), the radius limits above calculate to sidewall pressure limits of 360 to 720 lbs/ft (5.3 to 10.6 kN/m); in the same range as electrical cable!

Fiber optic cable manufacturers control sidewall force by recommending large radius sheaves.  Power cable recommendations include sidewall pressure limits as well as a typical minimum sheave/roller radius of 1.25 inch (3.17 cm).

Measuring and Controlling Sidewall Pressure

Small differences in friction coefficient have a dramatic impact on tension through bends as noted in article 3 of this series. Since increased friction raises tension, it also raises sidewall pressure. Decreasing the radius of a bend also increases sidewall pressure.  Small radius bends can be a real problem while trying to minimize sidewall pressure.

Sidewall pressure is directly proportional to the tension coming out of the bend and inversely proportional to radius of the bend. It is also depends on the number of cables being pulled, the configuration of those cables, and the resulting weight correction factor.

Sidewall pressure equations:
1 Cable SP = Tout / R
2 Cables SP = (w/2) * Tout / R
3 Cables (Cradled) SP = (3w−2)/3 * Tout / R
3 Cables (Triangular) SP = (w/2) * Tout / R
4 or more Cables SP = (w/2) * Tout / R
Where:
  SP = Sidewall Pressure
  Tout = Tension Coming Out of the Bend (Lbf, Kg., KN)
  R = Bend Section Radius (feet, meters)
  w = Weight Correction Factor (dimensionless)

 

You can see that sidewall pressure is calculated from pulling tension coming out of the bend. It does not contribute to the pulling tension, but results from the pulling tension. It is not additive, and the sidewall pressure is typically different for every bend. Sidewall pressure can be lowered by decreasing the pulling tension or increasing the radius of the bend.  The highest sidewall pressure is not always in the last bend of the conduit run.  Managing sidewall pressure to ensure compliance with cable manufacturer limits is very important.

Sidewall Pressure and COF

Cable friction studies show that higher sidewall pressures have a lower measured COF. An EPRI (Electric Power Research Institute) study from 1984 (Reference 1) suggests the use of two COF values: a higher value for sidewall pressure less than 150 lbs./ft, and a lower value for higher sidewall pressure. Reality is a bit more complicated.

Our studies, especially at lower sidewall pressures (< 20 lbs/ft), generally show a lowering of COF as the sidewall force is increased. To better understand this, look at the graph below.

COF vs Sidewall Pressure

This data is produced by increasing the tail load on a cable pulled through a series of bends, measuring the pulling tension, and then calculating the COF using the pulling equations.  The graph does not imply that the tension goes down as we increase the tail load.  It does not.  But, the tension does not go up as much as we expect with a constant COF.

This is not an intuitive result.  Our studies also indicate that at sidewall pressures above 100 lbs/ft, the curve is relatively horizontal, and notable COF effects with increasing sidewall pressure are gone.  High sidewall bearing pressure cof’s are lower than the cof’s measured in straight section pulls, so using a straight section friction coefficient in calculations is appropriately conservative.

We also can measure differences in lubricants performance under high sidewall pressure.  High quality cable pulling lubricants are formulated to stay wetted between the cable and conduit surfaces even under high sidewall pressure. The discussion of lubricant wetting and coating will be covered in a separate post.

1) D. A. Silver, G.W. Semen, R. A. Bush, G. H. Matthews, “Maximum Safe Pulling Lengths for Solid Dielectric Insulated Cables,” Electric Power Research Institute EL-333, Vols 1 & 2, Final Report, February 1984.