By Josh Cosford
Early hydraulic systems were installed on large machines, which meant they were stationary beasts—and as a result, their plumbing systems could be hard-piped. When someone discovered the usefulness of hydraulic actuators weren’t limited to stationary machines, the first hydraulic hoses had to be invented to allow motion between two points of movement, such as a boom.
Although hydraulic pressure could be contained within a cylinder with leather V-packing seals, a simple consideration of the units we use to describe pressure helps in understanding initial difficulties in manufacturing flexible conduits … pounds per square inch. A square inch is about the size of a very large postage stamp. Imagine stacking hundreds of pounds on that little square inch, and you can what engineers were up against; especially considering only natural materials for their solution were available a century ago. We take it for granted now, but imagine you stood barefoot on a tall Lego block and then pile a couple tons on your shoulders; that’s what hose has to withstand from the inside.
Spoiler alert! They worked it out. Early hydraulic hose was made from an inner carcass of natural rubber surrounded by textile or yarn windings for strength. They would add an additional weave of textile on top for abrasion resistance. Hose construction of this sort would remain in production for a surprisingly long time, and I still come across an occasional hose with a cloth outer carcass.
To be fair, early hydraulic systems didn’t run much pressure, and even 1000 psi was once difficult to achieve. As the requirement for higher pressure evolved—and the capabilities to construct components advanced—so, too, did the capabilities of hydraulic hose advance. The addition of steel wire to the outside of the rubber inner core provided a boost to pressure rating.
By “braiding” the steel wire in hydraulic hose, you add resistance to radial expansion, but allow flexibility in bending. To further strengthen a hydraulic hose and increase its pressure capacity, multiple layers of steel braiding was added. Each later was separated by an additional rubber layer to prevent metal-on-metal abrasion, and eventually hydraulic hose was nick-named after the number of steel braid layers, such as “two-wire hydraulic hose.”
As is typical for any industry, various forms of hose construction evolved, and some hoses would apply spiral wound hose of textile or steel, with multiple spirals being wound in opposite directions. Eventually, high-tensile strength steel would make its way into the market as well, further improving the pressure capacity of hydraulic hose.
One of the biggest leaps in hose technology was the integration of synthetic rubbers. Buna N, or Nitrile, is the most popular rubber used in hydraulic machinery, because of its stability over a wide temperature range, and also its compatibility with most hydraulic oils. With modern materials and construction, hydraulic hose is now both strong and reliable, capable of running up to 5000 psi or more for years. Hoses with four layers of wire braids are now common on hydraulic systems, especially those on large diameter running high pressure, such as excavators. Hose materials and construction techniques will continue to improve as pressure rises in hydraulic systems, and I suspect the use of synthetic materials will pave the way to progress.
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