Cessna nose oleo seals

[[email protected]]

In the Cessna 172's nosewheel strut are several O-rings, but the
one that seals against the chrome piston sometimes twists in its groove
and a spiral failure (tear) occurs. All the outer-cavity oil escapes,
then the nitrogen leaves and the strut goes flat. The failure appears
to happen rather suddenly.
These are MS28775-329 rings. The rubber is an ancient type,
probably around for 40 years or more, and while there are much better
O-rings on the market now we can't use them since Cessna does not
approve them and no one, as far as I know, has done an STC on a better
ring. Some folks recommend Granville Strut seal, which I have tried and
haven't noticed any better life. The stuff is supposed to swell and
soften the seal.
The failure I see seems to be due to the ring sticking to the shaft
and getting rolled over. An anti-friction additive might be a better
bet. Does anyone who has worked on these things know if there are any
approved additives like that?

Dan

[nrp]

I have not experienced or heard of that problem, however in ordinary
industrial hydraulics, I've found that smearing STP on a piston rod
will cut the cyclic operating seal friction in half for several minutes
- even if it is only smeared on the low pressure side of things.

Another possible factor is the surface finish on the O-ring groove and
the groove faces. Too smooth and this might happen.

I'm sure it is not approved, but maybe just assembling the strut with
STP as a prelubricant might help. The "ancient" type rubber is
probably neoprene - which is widely used in industrial applications
today. I doubt the O-ring compound is at fault here.

[[email protected]]

If there is a newer technonogy o-ring that would fix the problem, i
would use it. I've never seen an o-ring policeman.
But is the problem with the oring or the shaft making the o-ring twist?
Quite a few people clean and lube their strut tubes on a regular basis.
It helps the seal keep sealing.


nrp wrote:
> I have not experienced or heard of that problem, however in ordinary
> industrial hydraulics, I've found that smearing STP on a piston rod
> will cut the cyclic operating seal friction in half for several minutes
> - even if it is only smeared on the low pressure side of things.
>
> Another possible factor is the surface finish on the O-ring groove and
> the groove faces. Too smooth and this might happen.
>
> I'm sure it is not approved, but maybe just assembling the strut with
> STP as a prelubricant might help. The "ancient" type rubber is
> probably neoprene - which is widely used in industrial applications
> today. I doubt the O-ring compound is at fault here.
>

[RST Engineering]

"Why, I have no idea who put that o-ring in there. It must have been done
before I bought the airplane. (TM)"


Jim



> These are MS28775-329 rings. The rubber is an ancient type,
> probably around for 40 years or more, and while there are much better
> O-rings on the market now we can't use them since Cessna does not
> approve them and no one, as far as I know, has done an STC on a better
> ring.

[Scott Skylane]

Dan,

While I have heard rumors of people succesfully mixing a little STP, or
even dry graphite powder, in with the hydraulic fluid, I can not say
from personal experience that these will work just fine and last a long
time...

However, Cessna does sell a "cold weather" seal that replaces the O-ring
in question, as well as the two S1628 backup rings on either side of
it. It is a much sturdier piece, has a sort of squared-off cross
section, and appears to have some teflon-like properties. I *think* the
part # is "A2748", but I'd be sure to double check that before ordering.
Aircraft Spruce lists the part for $17.50

Happy Flying!
Scott Skylane
N92054