Making the Right Connections: Duct Puller Selection
February 14, 2012In one form or another, the “duct puller” or “towing head” has been used by trenchless contractors for more than 25 years to install polymer duct and pipe underground. Originally, a simple cap wedged into the duct provided a friction fit for a short pull. While the solution worked, the limited friction engagement of rubber o-rings against the duct wall demanded a better solution in the long term.
As the duct size and length to be installed increased, more advanced methods of connection were developed to ensure a more positive engagement to the duct wall.
Duct pullers that used an expanding “taper lock” were soon developed to provide a firm grasp on the inside diameter of the ductwork. These designs share a similar mechanism: the contracted puller is inserted into the end of the duct, then a conical mandrel on a threaded shaft is pulled forward, driving the expanding shells outward to engage the duct walls.
Sharing the Load
These expanding shells are carefully designed to balance contrary requirements: The duct puller must grip the duct wall firmly, but not so firmly that it causes damage that could rupture the wall!
Instead, the shells provide a toothed engagement into the duct’s inside diameter, while distributing the mandrel’s radial pressure evenly across the shells to minimize the risk of local stress concentrations that could damage the plastic duct.
Different manufacturers use different profiles on their shells: varying the number and size of the teeth or applying different flank angles to create a more or less aggressive tooth engagement. Look for a design that has been proven in design and in practice.
The key is to distribute the radial load evenly across the end of the pipe, so that the duct puller doesn’t superimpose excessive stress on the duct, over and above the stress of the pullback. Expanding shells are designed to match the inside diameter of the duct.
Standard Duct Pullers
Duct pullers are typically sized to meet the traditional Inch Pipe Size nominal dimensions. But the inside diameter of a nominal duct will vary due to the standard dimension ratio (SDR) and nominal pressure (PN) ratings.
Manufacturers compensated for this by designing the conical mandrels and their mating expanding shells so that they can engage a range of SDR (or PN) values for a given nominal duct diameter. Most manufacturers produce duct pullers capable of adjusting their shells to fit SDR 11 to SDR 17. Some manufacturers even produce metric shells specifically sized to match metric duct dimensions. The manufacturer’s catalog description will include a minimum to maximum size range for a given puller.
Look for a duct puller that includes the duct’s inside diameter in this range, and the puller will engage the duct. (Then you can verify that the puller’s Safe Working Load is capable of pulling the duct.)
What’s interesting to note is that the contractor doesn’t technically need to know the nominal duct diameter or SDR value —what’s necessary is the actual inside diameter of the duct. A duct puller may be capable of fitting a thick-walled SDR9 duct if the pullback doesn’t exceed the safe working load; or it may be able to grasp a thin-walled SDR21 duct if the radial stress doesn’t stretch or split the pipe.
To support a thin-walled duct or a duct being pulled near its rated capacity, manufacturers offer duct sleeves: steel collars sized to slide over the outside diameter and support the stress distribution within the duct walls.
The Sealed Duct Puller
A “deluxe” sealed duct puller also has a sleeve, but for a different purpose. Standard duct pullers provide a protective nose cone for the front of the duct, but they can’t exclude mud and water from infiltrating the new ductwork as it’s pulled through the borehole. A sealed duct puller offers a permanent sleeve built onto the faceplate to cover and protect the leading end of the duct.
Look for a sealed duct puller that uses rubber o-rings mounted in the sleeve or faceplate. When the shells engage the inside wall, the o-rings seal against the duct, preventing contaminants from entering the duct.
Sealed duct pullers offer one drawback: now the outside diameter must fit the sleeve, in addition to the inside diameter fitting the puller’s size range. Manufacturers typically offer nominal IPS sizes; metric sizes are also available for international applications.
Maintaining the Equipment
Regardless of which style of duct puller is required for the job, maintaining the equipment is the key to its longevity and function in service. Manufacturers have a readily available parts list online.
Most manufacturers recommend a complete disassembly and greasing after each use. This is the best time to give the puller assembly and components a thorough visual inspection.
Look on-line: These days, manufacturers have instructions and videos posted on their websites and Youtube channels. And of course, your local distributor is always a good resource for first-hand advice.
Inspection of the rubber o-rings is a simple and cheap preventative maintenance operation. Look for cracked or nicked rings and replace them now. Quality o-rings don’t cost a lot and they’ll keep the puller functioning properly.
Inspect the shells of the duct puller. Nicked and dull teeth can’t get a good bite on the ductwork, and the back face should slide smoothly over the conical mandrel. Shells should be stainless steel or plated for wear and corrosion resistance. Usually the steel shells get damaged around the jobsite, so store the duct puller someplace clean and dry between pullbacks.
Duct puller shafts and faceplates should also be inspected for wear or damage. Puller shafts are built for tensile loads and made from high-quality forged eyebolts or alloy steel bars. They should be straight while the cone-shaped mandrels should rotate smoothly on the screw thread.
Faceplates and sleeves are made from steel, aluminum or even hard polyurethane. These leading-edge components can see significant abrasion depending on the soil conditions, especially through the bends. Surface treatments add corrosion resistance and prolong the life of the faceplates.
Spare parts can be ordered through a local distributor and shouldn’t take more than a few days to deliver. When re-assembling the duct puller, grease the puller according to the manufacturer’s instructions, with the recommended lubricant.
Selecting a duct puller is easier than ever with the internet – you can watch videos, get instruction sheets and ask questions, all on-line! Knowing what you want from your equipment will help your distributor find the tools you need, when you need them.
Jon Twidale, P.E., is with DCD Design & Mfg. Ltd.