Hydraulic hoses, Stainless steel vs Rubber

>If this looks like yours, the hydraulic line is best made from
> braided stainless steel AN or "aircraft" hose.

I suppose it's time to reiterate a few things about SS braided hoses (having worked with them industrially for more than a decade).

There's nothing inherently wrong with rubber-covered hoses, and depending upon the material type, they can be superior to SS braided hoses--in both wear resistance and burst pressure. As well, contrary to popular opinion, SS hoses have an expansion rate that is inferior to some of the better rubber-coated hoses.

Typically, these hoses use a Teflon or other fluorocarbon liner which has very little mechanical strength and will balloon easily--that's why there is the necessity for the outer stainless steel braid. But, like a Chinese finger puzzle, the braid is flexible, and the line will swell under pressure until the braid locks.

My favorite story about this was from an engineer with whom I worked who was producing his own variant of the Zink Formula Vee race car. He thought the SS braided hose should be better, so he replaced all the hose and line in the brake system with it. The brakes bled out fine, the pedal felt okay, but on his first hard, high-speed braking on the track, the pedal went to the floor and he went sailing off into the catch fence. Afterwards, the pedal felt fine. Tried again, and the same thing happened. Then, he decided to look up the rate of expansion under pressure and calculated the volume change for all the line he'd installed and it exceeded the stroke volume. (!) Went back to the standard VW rubber hoses and steel lines and all was well again.

The lesson then is to keep the distance short if using such hoses for brakes and/or clutch.

The other issue is wear. One of the best materials to resist abrasion is... yup, rubber. One of the worst... yup, SS braiding. The wires are very small diameter, so they're easily worn through. When that happens, the strength of the hose declines rapidly. It's very, very important that such hoses be clipped in rubber-cushioned clips, especially in areas where there's a high vibration level--one of the hoses laying against a metal surface or edge can wear through in no time at all from seemingly low amplitudes of vibration. Where clipping is not possible, the hose should be wrapped in a hard nylon garter coil (these are available for most hose outside diameters) to keep the braiding surface from contacting other surfaces.

Rubber hoses do have their problems--internal cracking can cause fluid to wick out along the reinforcing fabric. As many people on the list know, very old rubber lines can crack sufficiently internally to form a flap that can block the line or cause pressure not to bleed off after the pressure is released.

But, if one wants to be convinced that there are some very good rubber lines available, just drop by your local industrial supply house and ask to look through their Aeroquip industrial catalog and compare specs for various hose types.

As for the cost being lower than one can make up the hose one's self, I wonder about that. These lines are typically -3 or -4 SAE. In bulk and at wholesale, the line and fittings are worth perhaps $8-12. But, if it's not something with which one has a fair amount of experience assembling, I wouldn't recommend making your own lines--but, much of that $46 is in labor, overhead and profit, not parts.

Cheers.

Michael Porter

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