Lubricants – Frequently Asked Questions

What is a cable pulling lubricant?

Why use a pulling lubricant?

Is lubricant needed for every cable pull?

How do I know how difficult a cable pull is going to be?

What is the friction coefficient?

How does a lower friction coefficient impact tension?

How do I determine which coefficient of friction to use?

Is coefficient of friction the only important property of a pulling lubricant.

Can mineral oil or soap be used as pulling lubricants?

Is it okay to use cable lubricant in prelubricated cables or conduit?

Can I run megger testing on my cable after installation?

 

 

 

 

 

 

 

What is a cable pulling lubricant?

A cable pulling lubricant is a specialty product designed to reduce the friction of a cable rubbing on 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 when it is installed.

 

Why use a pulling lubricant?

Pulling lubricants lower tension on cable as it is installed, and this has numerous benefits. Lower pulling forces are less likely to physically damage the cable. Lower sidewall pressure (the normal pressure on a cable going around a bend) can improve insulation performance and life. Lower tension means less equipment wear, improved operational safety, and longer uninterrupted cable runs (fewer splices). All of these can 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. But for longer pulls, or pulls with a greater total bend angle and pulls with heavy cable; the use of a effective lubricant can significantly lower tension and sidewall pressure.

 

How do I know how difficult a cable pull is going to 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. As a example, if a 10 lb. block sitting on a table took 5 lbs. of force to move it across the table, then the coefficient of friction is the ratio of the pulling force to the gravitational force, or 5/10 = 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 conduit, 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. But in bend sections, the primary normal force comes from the pulling force itself, and the COF is an exponential factor which multiplies the incoming tension. So, a reduction of 50% in the COF can mean a reduction of as 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 the lubricant, but also the cable jacket type and the conduit type. So, all three must be known to have a meaningful COF. While there is no standard friction measurement method that applies to cable in conduit, our methods at Polywater measure real cable rubbing on standard conduit types. Thousands of tests have produced a large friction database. These data have been validated by back calculating effective coefficient of friction from actual cable installations. These data are available in our lubricant technical data sheets or in our tension estimating software (the Pull-Planner Software).

 

Is coefficient of friction the only important property of a pulling lubricant.

No. The ability to apply the lubricant and the cable pulling environment are equally important. While a liquid lubricant can work neatly in an underground pull by being poured into a feeder tube or upturned duct, a liquid cannot be applied at all to a cable that is going into an overhead conduit. Lubricants that thicken or freeze at cold temperatures can cause problems in cold weather applications. Maintaining friction reduction when pulling though a flooded conduit requires unique lubricant properties.

 

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 prelubricated cables or conduit?

Polywater lubricants are compatible with both prelubricated cables and conduits. Addition of a pulling lubricant will often optimize 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.

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