The Basics of Cable Lubrication

Oct 1, 2006 12:00 PM, By Mark Lamendola, Codebookcity.com

Making the proper lubrication selection and application choices reduces tension in the raceway and on the team

Two contractors are swapping stories. The first one says, “The cable got stuck in the middle of a long pull under concrete. Then it broke when we tried to pull it back out.”

The other contractor shakes his head. “We had some bad luck, too,” he says. “Last year, we did a job with more than 1,200 cable runs, and 30% of them failed insulation resistance tests.”

Obviously, situations like these don't make an overwhelmingly positive impression on the customer. When it comes to cable pulls, proper cable lubrication prevents this kind of “bad luck” and the losses that come with it.

Reducing tension When you use the correct lubricant and application method for a given pull, you reduce the friction encountered by the cable, allowing you to keep cable-pulling tension within allowable limits. But it's not just cable-pulling tension you're reducing. Make the right choices, and you'll greatly reduce the possibility of a tense, day-long boondoggle that should have been a one-hour pull.

When cable pulls go wrong, it's nearly always because the cable encounters too much friction within the raceway. Simply pulling harder because a cable pull becomes difficult may make matters worse. Instead of pulling the cable free, you can really jam it in there — making extraction costly.

A better approach is to use proper lubrication to reduce the friction on the cable. The catch is that lubrication is preventive, not corrective. So you must build the lubrication steps into your cable-pulling procedure. You may find that a bit easier to accomplish if you break this effort down into three major steps:

Plan
Determine the allowable tension for a given pull. Determine the type of lubricant needed, how much to use, and how to apply it.
Measure
At a minimum, use a wire-pulling tension gage. Consider a higher-end solution that includes software you can run from a laptop onsite.
Control
Vary the lube feed and cable-pulling force to keep cable tension within the desired limits.

Whether you can implement these steps correctly for a given cable pull depends on what you know about cable-pulling lubricants. We can divide this knowledge into four categories:

Application method.
Incompatibilities.
Performance characteristics.
Application process.

Application method A cable-pulling lubricant will come in one of three forms, which manufacturers refer to as “application methods.” In increasing level of viscosity (thickness), they are: liquids, gels, and waxes.

Photo 1. A Pacific Gas and Electric (PG&E) utility crew member pours liquid lubricant down a vertical section of raceway, prior to a large cable pull.

Liquids This is the application method people most often associate with cable-pulling lubricants. Typically, you pour from a bottle or jug into the raceway as you pull the cable (Photo 1). Liquid lube also comes in a convenient spray bottle.

These lubricants are not all the same. Different formulations work better for some applications than others. There is no “best” lubricant. There is only the right lubricant for the application.

For example, some liquids are “stringier” than others. You may need more stringiness for a given pull. This characteristic helps the lubricant slide into the pull along with the cable. The less straightforward and downhill your pull is, the more stringiness you want. But don't always use the stringiest product, because more stringiness makes pouring a little harder to control.

For some applications, lubricant choices are more critical. For example, if you are pulling communication cables, you should use a slow-drying lubricant specially formulated for that kind of pull. In addition to reducing cable-pulling tension, it addresses some other issues particular to communication cables. For power cables, though, it's not the best choice.

Photo 2. Stick your hand in a bucket of gel, and you can quickly see how it differs from liquid pulling lubricant.

If there isn't a product specifically formulated for your application, one probably isn't necessary. A general-purpose lubricant does an admirable job for a broad range of applications. A specific-purpose lubricant works better for certain applications and helps make the job proceed more smoothly, so use one if it's available.

Use liquid when: You have vertical feeds, such as underground work, or in any application that is conducive to pouring from a jug into a spout.
Avoid liquid in: Overhead pulls, where dripping becomes an issue.

Gels. These have higher viscosity than liquid. You can easily tell a gel from a liquid. Pour a liquid lube onto a horizontal chunk of exposed cable, and it runs down the cable then drips onto the floor. You can't actually pour a gel. If you scoop it out (Photo 2) and slap it on the cable, it will just stay there as a glob. This “globbiness” is what keeps the lubricant from being squeezed away or rubbed off during high-load conditions.
Use gels when: You have overhead loads or slow-moving, high-pulling force loads, and/or in situations where you must minimize dripping.
Avoid gels in: Pulling communication cables. The gel continuously “breaks” in this situation, and can increase pulling tension.

Waxes. These have high viscosity, like gels. The primary difference is waxes are stable at higher temperatures (both storage and application).
Use waxes when: You have hot weather applications, have overhead loads or slow-moving, high pulling force loads, and/or in situations where you must minimize dripping.
Avoid waxes in: Pulling communication cables. The “breaking” of the wax in this situation can increase pulling tension.

Incompatibilities Rule out lubricants that contain materials incompatible with your application or your customer's site policies. For example, you cannot use silicone if the facility has a painting or enameling operation.

A particular jobsite may have other restrictions. Sometimes, the restriction is that a particular brand, if available, must be used. As you plan the job, ask your client for a list of materials that are banned, preferable, or mandated.

Some lubricants are incompatible with some jacket materials. Petroleum-based lubricants, for example, rapidly deteriorate rubber.

Another possible issue is the use of Teflon. You would introduce very little by way of a cable lubricant, but Teflon is currently the subject of controversy. The superior properties of Teflon make it attractive, and many experts see the controversy as being overblown. Because your customer might restrict the use of Teflon, read the site materials policy before ordering supplies.

Performance characteristics So, you've decided on the correct application method and ruled out incompatible materials. Now look at the performance characteristics needed for the pulls you are planning. These include such things as:

Coefficient of friction How slippery it is, but in reverse (a lower number is better). Every combination of cable jacket and raceway results in a different coefficient of friction (COF) — which might not match what's on the label. Other factors can change the COF as well. For critical applications, consider using cable-pulling software with an embedded COF database.

Wetting How much it can spread along the cable. This affects how much of that COF you can actually use. Wherever the lubricant isn't on the cable, it's not lubricating.

Coating The ability to cling to a cable during application. High cling factors allow a lubricant to stay with the cable as it's pulled deep into the raceway.

Resistance to shear How well it stays on under pressure. Pressure on the lubricant can “squeegee” it out of the way, leaving cables to rub on raceway walls.

Cable compatibility Whether it works with the cable. Lubricants aren't universally compatible with all cable types. A lubricant/cable mismatch can cause in-service insulation failure. Overcurrent devices don't necessarily stop such a failure from causing massive damage, for which your firm could be held liable.

Cable manufacturer approval Suitability for the insulation. The manufacturer has tested and approved this lubricant for a given cable type.

Application process Once you've selected the lubricant for a given cable pull, decide on the most effective way to use it. It may seem obvious that pouring from a jug or scooping from a tub is the least expensive way. But what seems obvious isn't necessarily so.

Photo 3. A lubrication pump provides metered lubrication for an easier wiring pull, reduced waste, and easier cleanup.

Pouring or scooping can result in large “dry patches” on the cable, especially in large pulls. It can also lead to excessive use of lubricant, resulting in waste or even slip hazards. One answer to such problems is a lubrication pump (Photo 3). These come in several varieties, ranging from a simple manual push pump you stick in a tub of lube to a standalone, automated system that you wheel to the cable entry point. The higher-end systems allow you to control the rate and pressure of lubrication feed. Many people assume you can't use gel with lubrication pumps. This isn't always true. Just be sure the pump you are using can accept gel.

Determining the best way to reduce cable-pulling tension requires a small investment in planning. Following up on it in the field can keep you from going down the slippery slope of excess tension between cable and raceway — and between you and your customers.

Based at the company's headquarters in Merriam, Kan., Lamendola is a principal with Codebookcity.com.