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Volume IV

OPTIMAL LUBRICANT QUANTITY IN CABLE PULLING

Survey a number of field crews with experience in pulling heavy electrical cable into conduit . . . and ask how much lubricant they would use on a hypothetical 500-foot pull into 4" conduit. You'd be surprised at the large variation in answers you'd receive. They could vary by as much as a factor of ten!

Is there an optimal amount of lubricant for a given pull . . . enough to do the job, but not waste lube by using an excess? Is it possible to use too much lubricant? If there is an optimal amount, how do we determine it, and what factors influence the determination?

Film Thickness Calculations

One theoretical, but easy-to-understand way to estimate lubricant quantity is a "film thickness" approach. For instance, if a 10-mil (.010 ) film of Polywater® Lubricant J provides good friction reduction, we can calculate the quantity of lubricant it takes for such a 10-mil film (or any other film thickness) to completely cover a cable's jacket and/or a conduit's interior wall.

The equation is:

Q = l x D x pi x t x .052     (Equation 1)

Solving this equation for a 10-mil film thickness, a 500-foot length pull, and varying conduit/cable sizes gives the following:

Field Reality

While these calculations are, so far, theoretical, they do bring us to a realistic result. Based on experience, typical field lubricant usage on a routine 500-foot pull in 4" conduit is in the 2-to-5-gallon range. Working from either the nominal 4" ID of the conduit, or the total OD of cables that might be installed in such a conduit [could vary from 2+ inches (single) to 4+ inches (triplex)], we see basic agreement between the calculation and field practice. Looking at it another way, typical practice today is to put an average film thickness of 10 to 15 mils of lubricant on the conduit interior (or cable exterior).

Basis for Recommendations

If you are familiar with American Polywater's applications literature, you may recognize our published recommendation on lubricant amount comes from the above equation, which is further simplified to:

Q = .0015 x l x D     (Equation 2)

This recommendation has historically served end-users well, whether estimating how much lubricant to use for a routine pull or how much to purchase for a large job with many pulls. Recent gamesmanship by other lubricant manufacturers attempting to "cost justify" high cost lubes . . . by modifying the equation . . . are misdirected when one knows how the equation was developed, as well as the realities of field application, where lubricant use is not often measured. The equation is a helpful theoretical calculation--no more and no less.

Lab Research Enlightening

An obvious area for application research is into the film thickness needed for effective lubrication; or, put another way, how does the coefficient of friction vary with lubricant film thickness?

American Polywater has done such research using a "multiple-bend duct" technique for determining friction. In this test, a back-load is put on a cable that has been threaded thru six consecutive 90° conduit bends. The force required to pull the threaded cable is measured. From this pulling force, the coefficient of friction (COF) can be calculated using the following modified pulling equation:

Obviously, the "multiple-bend technique" can be used to measure and study a great number of pulling parameters. Results of some of the other things studied will be covered in future issues of "Technical Talk."

When the "multiple-bend duct" is used for lubricant quantity studies, a measured amount (weight or volume) of lubricant is carefully and evenly applied to the test cable as it is threaded. A series of measurements is made using differing amounts of applied lubricant. The pulling tension is determined and coefficient of friction calculated. Assuming that the lubricant is spread evenly on the cable, a film thickness can also be calculated based on the amount of lubricant, the cable OD, and the conduit length.

Results from such a test are presented below. The cable used had a rubber jacket (type RHH), the conduit was PVC, the back tension was 120 lbs. (sidewall pressures in the 100 to 500 lbs./ft. range), and the lubricant was Polywater® J.

The results are surprising . . . with the Polywater® J Lubricant . . . a film thickness of 1/2 mil has the same coefficient of friction as a 10-mil or a 70-mil film!! Does this mean that we should use 1/10 of the quantity of lubricant recommended in Equation 2?? No . . . because an effective quantity of lubricant is governed by what's needed to get lubricant to all points of cable rub . . . rather than how much must be there . . . but we'll get to that later.

How low can you go?

An obvious question based on the test results above is: What will happen with .1 mil, .01 mil, etc.? Sooner or later logic tells us that the coefficient of friction must rise towards the unlubricated value (greater than .7). Unfortunately, one cannot practically apply quantities of lubricant of under 1/2 mil to a cable . . . because the lubricant can't be rubbed on any thinner . . . so this test can't answer that question.

There is another important thing to be aware of in evaluating this data. The lubricant was Polywater® J, which has extraordinary friction reduction under high shear conditions. Other types of pulling compounds can behave quite differently, so test results for Polywater® J should not be generalized to all pulling lubricants.

Field Experience

Field experience precludes the use of very small quantities of lubricant for difficult pulls. There are a number of documented situations where the use of too little lubricant resulted in high tensions, with lots of resulting problems.

We are not familiar with any situations where "liberal" lubrication caused problems. So it does not seem that use of large quantities of lubricant causes any practical pulling problems (other than the possible mess).

The lab research indicates that we only need to get a thin film of Polywater® at all points of cable/conduit rub for optimal friction reduction. However, the amount of lubricant needed to get such a film is dependent not on the mathematics of coating thickness, but the dynamics of a cable "carrying" lubricant through continuous rubbing over hundreds of feet of conduit.

The "carry" of a lubricant through a conduit depends on the total run length, the number of bends and resultant shear, the absorptivity of the conduit, the presence of dirt or sand, and the nature of the lubricant itself. The field craftspeople handle these variables in a simple, but effective, way . . . for the "tough" pull, they "really lube the cable . . . put on as much as they can . . ."

Lubricant Properties and Application Methods

Certain properties of a lubricant play a major part in its ability to carry through conduit and continue to lubricate. The most important of these properties are the lubricant's viscoelastic nature and drying behavior . . . that is . . . how well does it adhere to the cable. . . does it flow off, drop off, or rub off . . . does a thin film maintain a low friction coefficient after drying for 10 minutes or 20 minutes? The engineering of these characteristics into Polywater® results in its superior field performance . . . as well as . . . somewhat limited aesthetics. That is, the stringiness of Polywater® provides the advantage that once you get it on the cable, it will cling and be pulled far into the conduit.

Sound application methods are also important to ensure that lubricant in sufficient quantity gets to all points of rub. While space does not permit an extensive review of application methodology here, a brief look will demonstrate some of American Polywater's unique concepts. Our Front End Packs™ preload the conduit to get lubricant far into the pull, so the cable front is exposed to lubricant throughout the run. Polywater® lubricant pumps, spreaders, and applicators completely coat cable jacket with lubricant. This not only lowers cable/conduit friction, but also cable/conduit friction in multiplex pulls.

Summary

We believe the final determination of appropriate quantity of pulling lubricant should be left to the field, and should be based on the difficulty of the pull. The formula

Q = .0015 x l x D

is a good starting estimate on quantity. Engineers should consider recommending conduit prelubrication and Front End Pack™ use for long or especially difficult installations.

If you would like more detailed information on American Polywater application recommendations, please call our Customer Service Department toll free at 1-800-328-9384. We'd be happy to send you our application literature.

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