Oil Hose Tension

mlwynn · #1 06-22-2012, 03:06 PM

Hi All,

I am trying to figure out oil hose mounting. I mounted my oil cooler on the engine mount to decrease vibration damage. Several folks had talked about cracking of the oil cooler when mounted on the rear baffle per plans. I carefully measured and ordered integral firesleeved oil lines from PHT. They did a very nice job and made them right to spec.

When I installed them, I discovered that the lines are considerably less flexible than the fuel line I used to mock them up. I can wrestle them into position, but it is something of a battle. The issue is that I need some slack to make up for the engine shake. That requires a bend in the line. Bending the size-eight line requires no small amount of force. I am concerned that this sort of pre-load might lead to failure or cracking of the fittings at the oil cooler or where they enter the cooler. The engine fittings are steel and pretty solid. The oil cooler fittings are also steel but the cooler is less robust that the engine block.

Is this a normal issue? Should I think about taking down the oil cooler mount and repositioning with less tension? I am not sure if these lines will "relax" into position after they have been there a while or are going to be a significant hazzard to engine safety over time. Anyone have insights and experience on this?

Regards,

Michael Wynn
RV 8 Finishing
San Ramon, CA

Walt · #2 06-22-2012, 04:36 PM

Hard to tell without looking, but the hose needs to have a gently curve in it at least so it has some flex capability. The back of the engine where the lines attach doesn't really move that much so as long as you have some slack in the lines you should be ok.

TS Flightlines · #3 06-23-2012, 02:54 PM

Michael---you should have some slack in the hose, preferably 1/2 to 1 inch depending on if you hoses have to route around the engine mount. Integral firesleeved hose is great, but yes it is alot stiffer because of the thickness of the extruded firesleeve.
Tom

DanH · #4 06-23-2012, 08:15 PM

Constant preload is not a fatigue factor. Cyclical loading (a load of variable amplitude) will break things. Engine movement is an obvious source of variable loading on the AN fittings screwed into the engine, so when in doubt use steel fittings...the S-N plot has a knee. Load amplitude below the knee will never fatigue the fitting no matter how many load cycles.

Scott Hersha · #5 06-23-2012, 09:37 PM

What kind of hoses are you using. If you use the Teflon hose it will be stiffer because it's a stiffer type hose. They last a long time, but they don't give much. Synthetic rubber is much more flexible, won't last as long, but is much cheaper. You can re-use the hose ends when you make new ones (about every 5 years), making the replacements quite inexpensive.

AlexPeterson · #6 06-24-2012, 07:12 AM

One of the specifications for hose is the minimum bend radius. You can find this for your hose, and then compare to what you actually have in your installation.

I think it is good practice as well to install hoses such that there is no bending moment directly applied to the fittings. I believe this is described pictorially in 43.13, but don't quote me... For example, it should be possible to begin threading the nut on the hose into the fitting using side loading only. I.e., one hand simply pushes on the side of the hose just back from the fitting, the other turns the nut. No "twisting" (bending moment) should need to be applied in order to thread them together. What this means is that the hose only sees a side loading at that point, and no bending loads.

Sorry that is a bit hard to follow... maybe others more "textually" talented can chime in!

Caveman · #7 06-24-2012, 07:52 AM

Take a look at the on line version of AC 43-13 b. There are pictures of what Alex is describing as well as the minumum bend radius.

john.schwaner · #8 06-24-2012, 08:39 AM

Installing Aircraft Hose -- The 10 Rules

RVG8tor · #9 06-24-2012, 09:02 AM

Quote:

Originally Posted by john.schwaner

Installing Aircraft Hose -- The 10 Rules

Interesting #7 and #10 seem to be the same thing. Unless oil is not considered to be flammable. Maybe this is whey Van's does not send oil lines with firesleeve. My oil lines will not be near the exhaust but I was considering having them firesleeved. I guess it can't hurt.

Cheers

mlwynn · #10 06-25-2012, 04:10 PM

Thanks for everyone's input on this. I had some slack built into the hose, but I think not enough. I had another piece of -8 firesleeved hose (a long one that I miscalculated the clocking) which I installed on one side and then put more of a curve on. If I allow an extra two inches in hose length, the loading is much reduced. This allows me to connect without excess pre-load and I think will be much safer.

I used all steel fittings--in fact all my FWF fittings are steel. I was mostly worried about where the fitting connects into the oil cooler. That seems much less robust that the engine case.

Dan, I read but am not sure what you mean about the s-n plot.

I will send a picture when it is all done.

Regards,

Michael Wynn
RV 8 Finishing
San Ramon, cA