Whose to know. Couldn't hurt,,, in my PoV...

I will look around at solutions offered here. I didn't see any sig. line info in that next to last post.... I have two of these things now and I would like something, anything, to minimize the possibility of blown hydrolic hoses...

About the twitching, rattling and triggers caused by this thread (lol), especially for the main contributors,,, the way I see it EVERYONE was only trying to do good for thousands of us, ya know? And that's pretty HEAVY!

You guys are amazing! Plain and simple

 

Thank you, sir. Much appreciated.

 

Hard to know considering the failure mode appears to be slipping out of the crimp because the outer sheathing has degraded. But, the slipping/shearing failure could run like material fatigue, often characterized by a linear dependence when plotted on a log-log plot (S-N curve).

In this case, reducing the peak pressure (and stress) by about 30% (~1600 psi to ~1100 psi) could increase the lifetime enormously. Often there is a stress below which no additional fatigue occurs (fatigue threshold). This is true even if there was, say, 20 years of cyclic fatigue life taken from it before the fix was performed. Let's hope here.

 

I have sort of a "down in the weeds" question about this stuff, and your post tells me that you are the first guy ever to visit here who has the chops to address it. So ...

The hose failures that occur in our cars ... thinking only about pressure as a variable ... is it just the peak pressure occurring in the system that would effect failure rates, or does the "shape" (rise time, decay time) of the pressure pulses matter too? From research years back I came to believe it is the latter, but never gained enough understanding to know why this would be true.

TIA.

 

I assume you might have already come across this information already but just in case:

http://www.jagrepair.com/HydSystemEvaluationXK8XKR.htm

http://www.jagrepair.com/VoltageReductionSysXK8R.htm

I am guessing that Gus has probably done more investigation and testing/measuring of the hydraulic roof system than even the original Jaguar designers

Doug

 

Unfortunately, I think the answer is actually that it would be difficult to know.

But, here goes something.... In general, in the world, hydraulic hose failures largely come from cut lines or failures at the crimps, where crimped. For pressure related failures alone, the factor of safety (usually 4X or more the expected peak pressure) is high enough that pressure related failures are extremely rare unless the hose is degraded or nicked. This is the reason that rating a 20,000 psi line for our cars does not particularly make sense. The only reason it could be disadvantageous, however, is if there is line motion that bends the likely stiffer hose that causes it to fail in repeated bending. This seems unlikely, so there is probably no particular problem with substantially over-rating the hose if it makes someone happy.

In our particular circumstance, the crimps appear to me to be failing because the hose and degraded material can slip out of the crimp. Now, the crimp is crimping the degraded material under the metal, and since the material is still, even in the degraded state, not marshmallows, it's not particularly easy to spit the hose and degraded sheath out from under the crimp. We know this is so, because otherwise all of the hoses that flaked off to no sheath at all (other than under the crimp) would have failed. We also have the advantage of low cycle count on the hydraulics. A million cycles is a fairly typical assessment. Over a 20 years, this is about 140 cycles a day/every day. I'd hazard a guess that no one has actually done this in an XK8. Once a day is less than 10,000 cycles. Perhaps someone has done that.

In any case, getting around to the actual question, it's complicated. Circumferential (hoop) stress fails the hose in a burst mode. It is vaguely possible to have a very fast peak that goes well over the burst pressure, but does not fail the hose. Since this does not seem possible here, let's ignore this.

For the crimp, the failure mode is something like shearing out a tiny bit of the central hose allowing the pressure to act on the edge of the braided central hose and the degraded sheath material. Just a little motion fails the crimp, where, in the usual circumstances, the crimp would still hold because the material is contiguous (held together) and packed under the crimp. So, yes, in this case, the shape (usually called impulse, or getting very technical, the integral of pressure with time) probably matters too. The rise time may matter too because of the influence of micromotions on this slipping process. Same with the decay time.

Still, it strikes me that the braided central core will hold pressure for the conceivable future. So, in the spirit of exploration having, at this point, only one of my cars partially rehosed, I've explored
1. Recrimping onto the braided central hose. The problem is that the central hose is meant to hold pressure, but not be crimped against. So, this will fail. Strike one.
2. Cutting the hose, bonding a polymer tube to the end of the hose and recrimping onto the newly 'repaired' end. The hoses, in general, may not be long enough for this, but it conceivably could work with a short extender. However, this seems inelegant. Strike two.
3. An overlay of thick polyolefin shrink tubing about 3 mm in thickness onto the crimp and the near hose with the sheath removed, glued down with bondit for nylon and other polyamides should hold by itself in shear and burst and keep the degraded material from moving. Haven't tried this yet on a spare degraded hose where I can cycle a couple of thousand psi for a couple of thousand cycles in 40C heat to check if it works. That test would be much quicker for me than replacing one hydraulic latch hose. Not yet strike three.

In the end, I'll probably end up just replacing the hoses.

So, the above is what happens if an academic doesn't know the answer to a question..

 

CRB,

Hey, thanks for the effort that went into that response. You've elevated our understanding of the problem. If you do pursue your experiment #3, Please let us know what you learn there. Many of us (me included) are still running original hoses.

This is old news for many here, but maybe not for you so ... my final effort to make a dent in this problem, years ago, was to sort out how to replace the pump's internal relief valve springs so as to reduce peak pressure. This method eliminated the hundreds of dollars of expense involved in installing an external relief valve for the same purpose.

Earlier, another forum member (Reverend Sam) and I worked on adding a resistor in series with the pump's power supply. That was found to "soften" pulse shapes and also to lessen peak pressure (but not as much as we first thought.). The accurate documentation on both approaches can be found in my signature line below.


Thanks again for your efforts.

 

+1 to all the contributors above.

Here's a pic of the failure in my 99 MY XK8. Here the hose/crimp failed on the back side of the pump causing a green wee patch at the rear right tyre (plus shorting out the body ground at the battery! Ouch (I remember that wasn't fun).

In hind sight I wonder how common my failure was where the hose that goes in a radius around the front of the pump unit and circles to the rear outlets. Someone had previously changed the hoses to the overhead latch, probably due to the infamous green shower, as I found the overhead map light console had a real bodge job repair.

Virtually all my hood cylinder ram hoses had shed their black sheathing but were fine operating, so I would think the circumference bursting pressures are fine and contained within the metal hydraulic pipe core. So as Dale says, there is the effect where hydraulic hoses move/straighten under pressure, and in my particular failure location at the fixed pump outlet, that caused cyclic shear stresses at the hose/crimp. It's important that the fact that the minor movement here accentuated the fatigue of this particular hose/crimp union point. So here the pressure no longer acted straight through (due to the pressure impulse wave), as Dale says at an angle at (quote) "......edge of the braided central hose and the degraded sheath material...."

In addition to pressure reductions/voltage drops, all would be good ways to lower the risk, but another would be mechanical - some sort of pipe restraint to keep the hose straight onto the pump outlet, so minimize the pressure impulse driven movement at these particular crimps.