Oleo Struts

[xerxes]

Probably a silly question, but what are the pros and cons of air Vs
Nitrogen filling of the oleo struts. Just how much of a problem would be
the humidity be if they are filled with air. Given the environmental
temperatures would probably never get below freezing
Its for a Cherokee 235. Also pressure Mains at 500psi and nose 300psi..
are these figures reasonably accurate.

Thanks in advance

[The Visitor]

No oxygen in the nitrogen, (or moisture). Under great pressues the
partial pressure of oxygen makes it highly reactive to rubber seals.
High pressure tires are also filles with nitrogen. However in light
aircraft I have found at least on shop using air in oleos. The say
nitrogen is too expensive as the struts are so leaky. ???

xerxes wrote:
> Probably a silly question, but what are the pros and cons of air Vs
> Nitrogen filling of the oleo struts. Just how much of a problem would be
> the humidity be if they are filled with air. Given the environmental
> temperatures would probably never get below freezing
> Its for a Cherokee 235. Also pressure Mains at 500psi and nose 300psi..
> are these figures reasonably accurate.
>
> Thanks in advance

[Ray Andraka]

The Visitor wrote:
> No oxygen in the nitrogen, (or moisture). Under great pressues the
> partial pressure of oxygen makes it highly reactive to rubber seals.
> High pressure tires are also filles with nitrogen. However in light
> aircraft I have found at least on shop using air in oleos. The say
> nitrogen is too expensive as the struts are so leaky. ???
>
> xerxes wrote:
>
>> Probably a silly question, but what are the pros and cons of air Vs
>> Nitrogen filling of the oleo struts. Just how much of a problem would
>> be the humidity be if they are filled with air. Given the
>> environmental temperatures would probably never get below freezing
>> Its for a Cherokee 235. Also pressure Mains at 500psi and nose
>> 300psi.. are these figures reasonably accurate.
>>
>> Thanks in advance
>
>

If the struts are leaking, then they probably need a rebuild. make sure
the shop rebuilding them uses the correct seals (Cherokees use a
quad-ring, not an O-ring. However there are many shops that use an O
ring). Also, make sure the struts are not pitted, as that will tear the
seals in short order. It took me several iterations of rebuilds, but I
finally have struts that don't leak. Also, air will leak faster than
nitrogen if you do have a small leak, plus it will break down the seals.

[Mike Spera]

xerxes wrote:
> Probably a silly question, but what are the pros and cons of air Vs
> Nitrogen filling of the oleo struts. Just how much of a problem would be
> the humidity be if they are filled with air. Given the environmental
> temperatures would probably never get below freezing
> Its for a Cherokee 235. Also pressure Mains at 500psi and nose 300psi..
> are these figures reasonably accurate.
>
> Thanks in advance

Some claim that using air will do all sorts of harm (corrode tubes,
screw up seals, leak more, etc.). However, I have never seen any
scientific evidence, only anecdotes.

Another anecdote:
I have used plain compressed air for 13 years in my Cherokee. When I
replaced the 25+ year old seals, there was no evidence of corrosion,
seal breakdown, etc. Just the normal once-in-a-while oil seeping that
old seals do. Upon rebuilding (after a 4 week or so break in) they
stopped leaking/seeping completely. I have had the same air charge in
there for over one year without the need to gas them up further.

I was surprised that the new seals "stick" as much as the old ones. I
figured it was a normal condition of the design. Several mechanics agree.

The pressures you quote are normal "under load" readings. I usually
extend my struts fully before filling. Method? Push up the spar with
your feet while on your back under the wing - use a 3' pipe with plastic
thread protectors to hold up plane via the jack point. Then gas them up
with a little 250lb. 12v auto compressor plugged into my car with engine
running. I put about 220lbs in the mains and 190lbs into the unloaded
nose (rear tie down pulled tight and tied with wheel chocks on the nose
tire).

3' Pipe can be purchased at Ace hardware right from the bin with both
ends threaded and with the plastic protector caps on them. Compressor
can be had at auto parts store. You MAY have to buy more than one brand
to find one that can actually make it to 220lbs (they MUST be plugged
into a running car to get up anywhere near that high) and has an air
chuck that properly hangs onto the schrader valve. I believe the one
that works is the Trak auto brand with the plastic chuck. I have never
had one of the metal chucks work right. You also may have to futz with
exactly how far down to push the filler chuck to get a good seal. Some
units don't work when pushed down hard but are fine when pushed down
"almost" all the way.

Total outlay of cash is around $25 for this setup. Have a high pressure
air or nitrogen source nearby until you get the setup to work right.

Good Luck,
Mike

[Stache]

"Probably a silly question, but what are the pros and cons of air Vs
Nitrogen filling of the oleo struts."

The only problem with servicing a strut with air is the source of the
air. Many shops have an air/water separator hook up to the compressor.
Some shops don't and you tend to get a lot of water maybe lots of
water in the strut. If you live in an area that gets cold the strut
will freeze due to the water. The water also causes pitting on the
strut crome causing it to cut the seals and leak more.

Nitrogen does not cause the seals to rot and crack as fast as air.
Also some maintenance manuals call out to use nitrogen in their struts
and other air. No water in the sturt is good. If the seals are good
and the strut is not pitted it will or should not leak.

Stache