Utility equipment requires special non-conductive components and safe hose designs to ensure bucket lifts stay elevated and spark-free.
Contributed by Josh Cosford, Contributing Editor
Some sub-genres of hydraulics are pretty generic, with little in the way of unique requirements due to application, environment or purpose. Industrial hydraulics, by and large, fits into the generally generic mold also used to make white bread and black coffee. Yes, Mr. Injection Molding Engineer, I know your reciprocating screw is special. But when you can configure a power unit online like you’re building your Subway sandwich on Uber Eats, then you’re working within the middle of the bell curve, aren’t you?
Just like the animal kingdom, there are classifications in the field of hydraulic machinery as well. For example, in the Kingdom of mechanica, the Phylum of hydraulica, the Class of mobilia, the Order of actuatoria, the Family of arborcleavers, the Genus of splitto, we have the species splitto rammaximus. Of course, we better know this species by its common name, the heavy-duty log splitter. And although a log splitter can be purchased at the hardware store, it certainly has unique features like the 2-stage pump and kick-off valve.
Splitto rammaximus aside, you can see the title of this article, so you know we’re talking about the animal within the Kingdom of mechanica, the Phylum of hydraulica, the Class of mobilia, the Order of elevatoria, the Family of terraliftae, and the Genus of bucketransitus – that’s right, the infamous bucketransitus linetechii, better known to most of you as the bucket truck.
Classification foolery aside, bucket trucks are road-legal mobile machines designed to lift workers to heights suitable for working on and around utility poles, trees and the final stages of construction. Compared to worksite dedicated machines, such as scissor lifts, a bucket truck can drive anywhere roads are accessible. A bucket truck has unique safety and performance requirements that differ from those of many other mobile machines, so let’s explore them.
However, first, let me clarify that there are more machines used in utilities than just bucket trucks. Digger derricks auger holes for utility poles, cable reel trailers help handle and feed lines into trenches, and vacuum trucks clear water from holes and trenches. The primary difference between these utility machines and a bucket truck is that the former operates remotely, where the primary risk is to the machine rather than personnel.
Safety above all else
Bucket trucks must be designed and manufactured with the safety of the mechanics and operators prioritized above all else. We must consider that lifting a human life many stories into the air carries its own challenges, so all precautions are made to ensure the technician remains in the bucket and the bucket remains in the air.
Believe it or not, some of the most critical components of a safe boom and bucket are the hose assemblies. Hoses used for utilities equipment, especially those bringing technicians within elbow distance to high voltage power lines, are not the garden-variety 2-wire hoses the local fluid power distributor carries.
You’d be correct to think that any old hose could still power the cylinders need to lift, position and tilt, of which there could be half a dozen cylinders or more. And yes, I did say that a bucket can tilt, and that’s not just because the operator wants to lay sideways for a sense of flying like Superman; quite the opposite, actually. Some booms can articulate on angles to reach tight spots, especially in the case of tree care, so a tilting bucket keeps the technician vertical, improving ergonomics and safety. Furthermore, if the ground is uneven, the boom orientation may otherwise prevent vertical operation without tilting.
With so many actuators, you can imagine how many hoses must be run through and around the booms and joints of a bucket truck. For the most part, hydraulic hoses remain within the channels of the booms, but they will peek out as required to connect to the nearest cylinder, and they will be seen at the boom and arm joints, in most cases.
From the outside, the bucket booms appear typical of any hydraulic machine employing such a mechanism, but in this case, one difference is vast — fiberglass construction. And herein lies one of the primary differences that ensure these machines are safe for operators — non-conductive construction. Because these machines place personnel precariously close to high-voltage power lines, any accidental contact between the machine and a 25,000-volt electrical source should be avoided to prevent harm or death.
Fiberglass, of course, is an insulator. Should the basket or boom come into contact with the power line, the high voltage doesn’t turn the entire machine into a plasma lamp. The bucket technician and ground crew don’t have to worry about being electrocuted since the fiberglass boom doesn’t conduct electricity. And that brings us to the most essential quality of a safe hose assembly for utilities equipment: non-conductive construction.
Non-conductive hose is a must
The hoses used for utilities are a special designation of the SAE J517 standard that requires the use of a thermoplastic cover, synthetic fiber and nylon inner tube. A cover material such as polyurethane is rare for hydraulic hoses but perfect for non-conductive hoses because of its high dielectric strength. Additionally, polyurethane is resistant to arc tracking, which is the process by which current can follow a conductive path on the material’s surface caused by contamination, moisture, or degradation.
Needless to say, steel wire reinforcement need not apply to this dangerous job of containing hydraulic pressure in the face of high voltage. Synthetic fibers such as polyester, aramid (think Kevlar) and nylon (polyamide) provide the tensile strength to the inner tube to contain pressure while avoiding conduction at all costs.
The SAE J517 standard offers up two constructions to the non-conductive hose design: 100R7 and 100R8. Similar to the popular 100R1 and 100R2 designations, they offer “1-wire” and “2-wire” options, respectively. Of course, even in the 1-wire 100R7, there are exponentially more fibers woven around the tube. 100R7 just has a single layer of fiber reinforcement, while the 100R8 contains two such layers.
Even still, you can expect a slight reduction in pressure capacity compared to the high-tensile steel wire in traditional hydraulic hoses. It’s a small price to pay to help ensure workers make it home every night to dinner with their families.
A key identifier of the non-conductive hose is the traditional orange cover material. Orange is a standard indicator of high visibility and hazard awareness, making it clear to technicians that the hoses installed on their machines are keeping them safe. Should a hose fail, under no circumstances should a traditional hose be used as a replacement, even in the interim.
Some of the individual hoses measure upwards of thirty feet, which makes for an expensive replacement, so it might be tempting to use a generic hose. And let’s be clear that there are versions of 100R7 or 100R8 that use synthetic thermoplastic, yet may still be conductive or semi-conductive. These hoses could be black or other colors, but you should only trust your life to orange hoses explicitly expressed as non-conductive, which are tested to dielectric standards.
If there is no chance to avoid the temporary use of a standard hydraulic hose, be sure to cover all surrounding power lines with protective line covers to prevent electrical transmission (many municipalities require these, anyway). And then have your temporary hose replaced when it becomes possible.
The importance of design and assembly
How you route your hydraulic hoses through the boom and basket also plays a role in safety. Whenever possible, keep the hoses safely inside the channels and prevent hoses from passing through gaps or openings in the fiberglass. Bulkhead fittings ensure a proper connection inside and outside the boom structure, leaving no place for hoses to rub against edges. Where hoses pass through joints, they should be adequately supported and protected with hose wrap, while channel edges should be equipped with rubber edging.
Any clamping or fixing systems that keep the hoses in place should be made of plastic or other insulating materials. Unfortunately, nobody is making reliable, high-pressure hydraulic fittings from synthetic or plastic materials, so most connectors and adaptors are still made from steel.
The final touch going into safe hose assemblies for utilities equipment actually begins at the hose shop. The most obvious point to make is avoiding any pressure to replace non-conductive hose with off-the-shelf metal reinforced hose, even if it appears to be a similar color.
When cutting and crimping hose assemblies, technicians should use only factory-approved hose ends suitable for their non-conductive hose. Extra attention should be given to crimp specification adherence, as a hose-end failure while an operator is perched 30 ft up may prevent the bucket from lowering if a work line can no longer provide pilot pressure to the counterbalance valve. Careful inspection of hose assemblies should be the final step.
Some fluid power machines require little in the way of safety considerations, but when precious human life can be taken quickly, no expense should be spared to ensure everyone arrives home at the end of the day. A machine failure is one thing, but respect for the individuals in the Kingdom of animalia, Phylum of chordata, Class of mammalia, Order of primates, Family of hominidae, Genus of homo and Species of homo sapiens, is an entirely different animal (pun intended).
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