Insane Hydraulics

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A Few Words on Volumetric Expansion of Hydraulic Hoses

So, we were discussing the volumetric expansion of hydraulic hoses the other day (after I accidentally sprayed myself and the best part of the shop with hydraulic oil by disconnecting a long 1.1/2 hose that "didn't seem to have much pressure in it") - and to my surprise, I didn't manage to find concrete data for the hoses that we use (R1AT, R2AT and 4SH). It appears that even with the existence of a whole ISO standard, called "ISO6801:2021 - determination of the volumetric expansion of rubber or plastics hoses under hydrostatic pressure", hose manufacturers don't fancy including the data in their catalogs. Aside from generic phrases like "fast response hose" or "low volumetric expansion hose" there's no concrete data, like ___ cubic centimeters per meter at ___ bar.

Why is it so?

Well, I think that it is so simply because the volumetric expansion does not matter for "routine hydraulic hose lengths", like at all. This still doesn't mean that hose manufacturers are excused from including the hose expansion data in their catalogs. It's definitely a fault, in my opinion, even if they think that overloading catalogs with data may be confusing for their customers.

Be it as it may, the fact remains that volumetric expansion of a hydraulic hose becomes an issue only when its length is big. Like hundreds of meters big. In that case, the necessity to "inflate an elastic vessel" to reach a certain pressure level can significantly increase actuator cycling times and directly affect both the controllability and productivity.

As it turns out - common hydraulic hoses with rubber tubes and steel wire reinforcements aren't the best choice for extra-extra long lines anyway. They are heavy and thick, and the fact that they use wires imposes limitations on the maximum uncut length that can be manufactured (the braid spools can only hold that much wire).

But hose manufacturers are clever, and a long time ago they invented thermoplastic hoses with synthetic fibers for reinforcement, which makes them lighter, and more compact, and allows the production of much longer cuts (thousands of meters for smaller sizes).

Now, I, unfortunately, have never been involved in a project that would require kilometer-plus hose cuts. It seems that sub-sea applications use them all the time, but there's no off-shoring going on in our region, so no matter how much I would like to see an actuator that is a couple of clicks away for the directional valve, this is not happening any time soon, but looking into this issue made me bump into the Parker's Parflex® division, responsible for the development and manufacture of thermoplastic and fluoropolymer hoses and tubes.

As I was looking through their latest catalog, I was happy to discover that Parflex engineers not only tested the expansion of quite a few models of their hoses but also published the results in the catalog (page 42)! Props to that!

I would like to share a couple of takeaways (I will be mostly concentrating on the -8 hose size, for simplicity, but the numbers for bigger sizes matter as well):

The 510C hose - a general hydraulic SAE100 R7, fiber-reinforced hose - has an expansion of 7cc/foot for the -8 size at 2250 psi. I didn't know R7 hoses were that "inflatable", to be honest. This means that a mere 5 meters (15 feet) of such a hose would need an "injection" of half a cup of oil to get to the 2000 psi!

The 520N is the R8 hose, and, not surprisingly, it behaves much better than the R7 from above - only 3.63 ccs at 3500 psi for the -8 size. So, if your forklift needs a new elevation hose, definitely go for the R8 over the R7 (if it fits, naturally).

The 575X hose is a super strong, 5000-psi-rated hose, that uses aramid fibers for reinforcement. Aramid, in case you didn't know, stands for "aromatic polyamide", and is, essentially, a bunch of tiny synthetic filaments braided into a thread that is several times stronger than steel! Ever heard of Kevlar? It's made of similar stuff! These hoses (-8 size again) expand 4.6cc/foot at 5000 psi.

But of course, there are thermoplastic hoses that use steel wires for reinforcement as well (and I actually believe that their expansion rates should be close to the one of our beloved rubber hoses), and there's data for these as well!

For example - the 590TJ - a hose with one/two steel wire braids - expands 2.88cc/foot at 3000 psi (-8 size). The 560TJ and the D6R, which also employ steel wires, have similar expansion figures - 2 to 3 cc/foot at 3000 psi (for the -8).

Quite interesting, isn't it?

Even though this information may not hold that much of a practical value for my day-to-day hydraulic needs, it still underlines the vastness of hydraulic applications and the consecutive need for the development and production of specialized types of products to support them. And I find this to be really cool!