Tubing vs. hose in mobile machinery

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By Josh Cosford

tubing vs hose mobile hydraulicsIn this world, there are as many preferences as choices. Dark meat vs. white meat. Baths vs. showers. Wagons vs. SUVs. Every one of these choices is personal, and every one is inconsequential. You don’t care that Bob likes white meat and drives a Volvo station wagon. It’s his choice doesn’t matter in the grand scheme of things. Unless he’s racing his wagon, there’s no practical advantage to his preference.

Some choices in fluid power are inconsequential, such as which of the top brands of D03 valves to use, or what color to paint your power unit (machine grey for the win!). Some choices do matter, and there are practical advantages and disadvantages. Picking a piston pump over a vane pump brings vastly different pumping characters to your system since the strengths and weaknesses of each are diametrically opposed. But what about the age-old battle between tube and hose … is there a clear winner, or is the choice inconsequential?

There is a level of comfort to either tube or hose, and it depends on your past usage and preference. If you came from a shop using primarily hose assemblies, you probably enjoy the ease and speed of a hose assembly, which can be cut and crimped in minutes. If you worked at a machinery OEM, I’m sure you’ve cut your teeth making tube sections to hard pipe a cylinder or motor, and you prefer the clean, permanent appearance of tubing.

However, there are some cases when it makes sense to choose one over the other, or where there is no choice but one. Often, you can plumb an entire system with only tube or only hose, and everything will work just fine. Just fine and optimal are two different things, of course, and what is optimal depends on the application.

Hydraulic hose has come a long way and was once only used when it was absolutely necessary. It was necessary to choose hose when you had two moving components of machinery requiring a conduit for hydraulic fluid to travel. If you had a pivoting arm, boom or bucket, you needed a flexible joint to send flow to the cylinders or motors on the other end. Pipe or tube is obviously rigid and would buckle, break or burst if you attempted to treat them like bendy straws. And herein lies the advantage of hoses—they’re flexible.

Progress has occurred, and hydraulic hose is vastly superior to olden days, especially in pressure capacity. Hose assemblies are available in pressure capacities matching any tube or pipe, and their flexibility has advantages beyond just a joint between two moving components, although that remains its primary advantage.

For some machine designers, hydraulic hose is the only option to plumb an entire system. From a design perspective, it offers a couple advantages over tubing. Because it is flexible and ductile, hose can absorb vibration inherent in any hydraulic machine. Machines plumbed with hose rather than tube appear to operate more smoothly and with less vibration.

The same properties of hose that allow it to absorb vibration allow it to absorb spikes in pressure. If you’ve been around machinery plumbed with hose, you’ll noticed how it comes to attention and stiffen when pressure rises or spikes. Although hose can’t eliminate the pressure spike, it can reduce the damaging effects it has on seals, valves and other components, especially though not meant for high pressure, such as return line filters.

Of course, where there is vibration, there is also sound. Pumps, especially, produce vibration harmonics that can be annoying and grinding to sensitive ears, especially over extended periods. Often times, the sound resulting from a particular power unit or system can be unpredicted until it is fired up. Chasing down the exact source of vibrating tube, fitting, plate or mount can be like finding Waldo.

If you think I’m being unfair to tubing, think again. If you work in a plant or shop with old hydraulic machines, take a pause and go out to inspect that machinery. Take note of the age of the hard plumbing on that machine; chances are it’s the same plumbing that existed when the machine was first commissioned, even if it’s older than I am.

Tubing, when done well, is extremely reliable and maintenance free. The only time it is changed is typically when a modification or upgrade is done to the machine. Tubing resists motion or vibration that, although it can transmit it, doesn’t cause it to shake or move the assembly itself. The inherent flexibility of hose allows it to chafe, rub and scratch against other surfaces if it’s not well supported. Many failures occur at those points of contact on a hydraulic hose, but tubes simply resist motion altogether.

Tube assemblies can be more work up front, but look cleaner and have other advantages. They can still be manufactured to be removed and replaced if the attached components require servicing because they’re made with semi-flexible bends at strategic points. You rarely see a straight shot of tube, but rather an L or S shape to provide some flexibility so the assembly can be pulled and replaced between two fixed locations.

Tube assemblies also provide long-lasting, leak-free service, especially with newer O-ring Face fittings used. Although older assemblies with JIC type flare have been in operation for decades, if a tube does leak, it will nearly always be between the seat and face of a JIC connection.

At this point, I haven’t even mentioned the cost of either option. Tubing has natural high-pressure capacity, and dollar-for-dollar is actually less expensive to manufacture and sell than hose. Tubing can be purchased for pennies per foot, but any high-quality hose with reasonable pressure rating is rather expensive, and the cost rises exponentially with size. A 1-in. ID hose rated for 4,000 psi can be hundreds of dollars, but even including labor, a tube can be made for a fraction of the cost.

I should mention, however, tubes are more labour intensive, and require special machinery. You need expensive bending and flaring equipment, and the investment in that can run five digits. Hose crimping equipment can also be expensive, but you can get by with field-attachable hose requiring only two wrenches. Even a hand-pump operated crimper can be had for a few hundred dollars if you wish to go that route.

 

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