FAQ – Cable Pulling, Lubrication, and Tension

5 Sep, 2022

FAQ – Cable Pulling, Lubrication and Tension

The most frequently asked questions regarding cable pulling and lubrication.

Author: John Fee - Polywater

Tags:  coefficient of friction

What is a cable pulling lubricant?
A cable pulling lubricant is a specialty product designed to reduce the friction of a cable while pulling into a conduit. The friction reduction lowers the force required to pull the cable into the conduit. This reduces the tension (tensile stress) on the cable during installation.

Why use a pulling lubricant?
Pulling lubricants lower tension on cable as it is installed, providing numerous benefits. Lower pulling forces are less likely to physically damage the cable. Lower sidewall pressure, which is the normal pressure on a cable going around a bend can improve installation performance and life. Additionally, lower tension means less equipment wear, improved operational safety, and longer uninterrupted cable runs (fewer splices). These combined benefits result in an improved and lower-cost cable installation.

Is lubricant needed for every cable pull?
For short, straight pulls with lightweight cable, a lubricant may not be needed. For longer pulls, pulls with a greater total bend angle, or pulls with heavy cable, the use of an effective lubricant can significantly lower tension and sidewall pressure.

How do I know how difficult a cable pull will be?
Equations based on the physics of the pull can provide an estimate of tension and sidewall pressure. The primary inputs into the equations are cable weight, conduit system detail (run lengths and bend location and angle), and the friction coefficient of the cable jacket against the conduit wall. The equations calculate pulling tension and sidewall pressure as the cable moves through the conduit.

What is the friction coefficient?
The friction coefficient (or coefficient of friction) is a dimensionless number that is a measure of the frictional resistance to movement of the cable against the conduit wall. A pulling force is required to overcome this frictional force and move the cable. For example, if a 4.5 kg block sitting on a table took 2.25 kgs of force to move it across the table, the coefficient of friction is the ratio of the pulling force to the gravitational force, or 2.25/4.5, so the friction coefficient = 0.5. For plastic and rubber cable jackets against metal or plastic conduit, typical friction coefficients can vary from 0.1 to 1.5.

How does a lower friction coefficient impact tension?
In horizontal straight sections of duct, the pulling tension is directly proportional to the coefficient of friction (COF). So, a reduction of 50% in the COF means a reduction of 50% in the pulling tension. However, in bend sections, the primary normal force comes from the pulling force itself, and the COF is in an exponential factor that multiplies the incoming tension. A reduction of 50% in the COF can mean a reduction of 90% in the tension in 180 degrees of bend.

How do I determine which coefficient of friction to use?
In cable pulling, this coefficient of friction depends not only on the lubricant, but also the cable jacket type and conduit type. All three must be known to determine a meaningful COF. While there is no standard friction measurement method that applies to cable in conduit, our methods at Polywater® measure real cable friction on standard conduit types. Thousands of tests have produced a large friction database. These data are validated by back calculating effective coefficient of friction from actual cable installations. We make this data available in our lubricant technical data sheets or in our tension estimating software, Pull-Planner™

How much lubricant should I use for a pull?

Polywater’s Pull-Planner program determines an appropriate quantity by calculating the volume of a complete coating of the interior of the conduit and/or the exterior of the cable jacket(s). So, the amount of lubricant recommended is proportional to both the length of the pull and the size of the cable/duct. For cable pulling, an excess of lubricant is not a problem, but for cable blowing, you need to avoid lubricant puddling.

Is COF the only important property of a pulling lubricant?
No. The cable pulling environment and the ability and methodology used to apply the lubricant and are both important. While liquid lubricants work neatly in underground pulls by simply pouring into a feeder tube or upturned duct, they cannot be applied to cable going into overhead conduits, thanks to gravity. Lubricants that thicken or freeze at cold temperatures cause problems in cold weather applications. Maintaining friction reduction when pulling though a flooded conduit requires unique lubricant properties.

Are there other factors that raise tension during a cable pull?
Lots of them! Most factors can be managed with good pulling procedures. There are “mathematical” effects from the location of bends in the conduit run. Dirt or sand filled conduit can increase friction considerably, much more than can be reduced by using a lubricant. High temperatures make cable jackets softer and raise their friction. In hot climates this can sometimes be controlled by pulling at cooler times of day. In cable blowing, a compressor cooler is necessary for optimal installation lengths. Read our installation literature and technical papers for best practices on controlling pulling tension.

Can mineral oil or soap be used as pulling lubricants?
While these materials are friction reducers, care must be taken not to use any lubricant that can swell, weaken, or crack the cable jacket. The cable manufacturer can be a source of information on the compatibility of various lubricants with various types of cable jacket. IEEE Standard 1210 covers the testing recommended to establish the compatibility of a lubricant with cable jacket.

Is it okay to use cable lubricant in pre-lubricated cables or conduit?
Polywater lubricants are compatible with both prelubricated cable and prelubricated conduit. Addition of a pulling lubricant often optimizes the performance of these materials, especially in a difficult raceway configuration or particularly long run.

Can I run megger testing on my cable after installation?
Pulling lubricants are conductive. Residue near the splice or conductor should be completely removed before cable is energized or tested. There is no problem with lubricant residue left on the cable jacket.

How does fiber optic cable pulling relate to cable blowing or jetting?
While fiber optic installation methods have significantly different procedures and equipment, they rely on a force to move the cable through the conduit. In pulling, the force is on the front of the cable and comes from a pulling winch. In cable blowing, the force results from the pressure differential between the compressor end and the open end of the conduit. Blowing produces a “localized” force along the entire cable jacket. Pulling and blowing need to overcome the frictional resistance to movement, and when that frictional resistance is reduced using a lubricant, pulling tension will decrease or blowing distance will increase.

Can I use my fiber optic cable pulling lubricant for cable blowing?
Usually not. Pulling lubricants use a water base to carry and spread the lubricant though the conduit. You can typically see the lubricant on the surface of the cable when it emerges from the conduit. However, this type of slippery liquid or gel does not help in cable blowing. While blowing lubricants may be carried by water, the water evaporates quickly due to high velocity air flow. Effective blowing lubricants work dry, and they are designed to reduce friction with a very thin, “molecular” coating thicknesses. Excess lubricant can create too much surface tension or cause air locks, which can interfere with the installation process.

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