Learn the primary sources of resistance and the crawler features you should seek to mitigate them
More than 55 percent of motor failures occur because of overheating caused by excessive resistance (Bright Hub Engineering). The two biggest enemies of motors are heat and moisture.
Crawlers are sealed against moisture, but their motors encounter resistance daily and in many forms. Increasingly, crawler manufacturers are focusing on designs that mitigate the damaging effects of resistance, with the goal of enhancing the equipment’s travel range, reliability and cost of ownership.
Common sources of resistance encountered by crawlers include obstacles like flow and debris, friction from cable drag, and strain from daily abuse. These forces reduce range, accelerate wear and increase repair frequency. For manufacturers, designing a reliable crawler means understanding the forces of resistance and actively minimizing their impact. Here we discuss the primary sources of resistance and the crawler features you should seek to mitigate them.
Cable size and weight
A heavier cable isn’t necessarily a stronger cable, but it does create more resistance. That resistance is compounded by distance: the farther a crawler travels, the more cable it must drag. Recently, cable design has focused on reducing cable weight by reducing the number of conductors (accomplished with implementing digital control technology) and by adding high tensile-strength synthetic fibers like Kevlar.
Automatic cable management
Cable tension can cause wear to crawler motors, axles, cable connectors and cable reels. An automated reel helps relieve tension, but the degree to which it does depends on whether it manages cable by tension or slack. During inspection, a tension-triggered reel feeds cable based on how hard the crawler pulls, so the reduction in cable tension is only modest. During withdrawal, a tension-triggered reel cannot manage cable at all; the crawler must be pulled by its cable in freewheel mode, imposing further tension. A slack-sensing reel, by contrast, uses optical sensors to monitor cable slack within the reel. During inspection, it feeds cable before the crawler pulls for it, and during withdrawal it balances tension so as not to pull on the crawler, nor allow excess slack that the crawler could potentially roll over.
Wheelbase and ground clearance
A crawler’s wheel layout impacts both how maneuverable it is and whether it has sufficient ground clearance. A shorter wheelbase is more steerable which, along with ground clearance, helps a crawler avoid obstacles rather than straining against them. A shorter wheelbase combined with an interleaved six-wheel drive helps prevent high-centering, where the crawler bottoms out on an offset or obstacle.
Reduced flat forward-facing surfaces
“Drag coefficient” is a measure of how easily a body moves through fluid substances. Lowering the drag coefficient of a crawler can be accomplished by streamlining the exterior body of the crawler. A flat-faced crawler collects debris like a plow, which adds to the resistance it encounters going down the line. A crawler should instead be designed to cut through water and sewage.
Wheel variety and interchangeability
Wheel traction significantly decreases motor stress, improves range of inspection, and counteracts resistance. Optimizing traction requires changing wheels as pipe conditions dictate. Being able to change wheels with little to no effort will increase the likelihood of operators using the right wheels for the conditions presented in the pipe, which in turn ensures optimum traction.
Avoiding resistance requires the operator to be able see at all times, whether the camera is driving ahead or on its way back. A back-up camera helps you avoid obstacles and driving over your own cable during reverse crawl. However, when choosing a crawler it is also important to note the placement of the back-up camera. The higher a back-up camera sits the less likely its vantage will be obstructed by flow.
Sewer crawler manufacturers are learning more about what features help overcome the harmful effects of resistance. These features are helping to boost travel and reduce downtime and cost of ownership.
Envirosight’s ROVVER X incorporates many of the features discussed here and boasts one of the industry’s highest uptimes and lowest cost of ownership.