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Thought I would pass this on for general
interest.
Its often been said that using electronic fuel
injection and throttle bodies that should the throttle body fall off, the pilot
would likely not notice until they tried to retard the engine power - I
have reason to believe this postulation correct {:>).
As some of you know I have been flying with a
plastic cast intake manifold (actually the throttle body mount and "Plenum" to
tie the secondary and primary runners together to get the DEI effect) for the
past 4 years (see attached photo).
Due to a oversight on my
part concerning the angle of the throttle body cable to the throttle body,
it had been difficult to get the throttle plate to close completely
leading to a slightly higher idle rpm than desired (2000 instead of 1600-1800
rpm). To remedy that problem, I had changed the angle of the throttle
cable about six months ago and apparently also increase the static pull of
the cable on the throttle body as well. I noticed upon my
last landing, that on landing roll-out that my "idle" rpm
had increased to 2600-2800 rpm. Humm, may have had something to do with a
longer than normal roll-out.
After taking off the cowling I immediately
spotted the problem. The throttle body plastic casting was loose on its
aluminum mounting plate (bottom of casting). Upon closer examination the
problem was evident - some of the metal inserts placed in the plastic for
the mounting bolts had pulled loose - apparently due to the
increased pull of the adjusted throttle cable.
When I had cast this particular plastic
manifold (number 4 in a series), I could not find the desired metal
inserts you place in the plastic to for the metal mounting
bolts. I really wanted to use a particular type (there are a large
number of various inserts) as I never had one of them pull loose with
my previous three castings. My normal source was out of the inserts
and could not locate a new supplier - I tried several suppliers, but
simply could not find any anywhere. So I resorted to my second choice of
metal insert. It turns out that the substitute insert technique of holding
in the plastic (at least against the force in this case) was not up to this
particular task and forces.
Four years ago, after casting the manifold
and implanting the second choice inserts, I later found a source in Germany
for the type of inserts I wanted to use in the first place (had to buy 100 of
the suckers as their minimum order) . The preferred inserts have a
nylon ball at the bottom of the insert (which is barbed and slotted). When
the bolt is inserted, the bottom of the bolt pushes down on the nylon ball cause
it to flatten and expand outward. The expanded nylon ball pushes the metal
barbs outward and into the plastic with considerable force securely anchoring
the insert in the hole in the plastic. The only better method (in my
opinion) is to cast the insert with the liquid plastic - but, that was just
a bit more involved that I could figure out how to easily do.
So one may ask, why didn't I replace them
once I got the preferred inserts - well, the process of pulling an insert out is
highly likely to enlarge (or destroy) the hole in the plastic. Since the
inserts originally installed appeared to be doing the job, I decided
nothing to be gained by trying to retrofit the new inserts. That is until
I discovered the effects of the "adjusted" the throttle cable.
The failed inserts simply had barbed extrusions
slightly larger than the hole which eventually worked loose under the load and
vibration. In the process, they took a small amount of embedded plastic
material with it. Fortunately, they worked out in such a manner
to leave small grooves in the walls of the hole as they pulled out, but did
not enlarge the overall hole diameter. So the hole in the
plastic is relatively undamaged - with just some small
grooves.
The new inserts expand to fill the hole and
have a barb forming the circumference of the bottom of the
insert that dig into the plastic at the bottom of the hole. So you
have to pull a plug of plastic the diameter of the expanded bottom of the insert
out of the casting (which has a high shear strength) in order to get the new
preferred inserts out. That is one reason why I wanted to go with these in
the first place - plus I had used them before and never had one pull
out.
I don't know of anyone else who is using plastic
castings in this manner, but thought I would past my along my experience.
My experience with this particular application indicates that the various barbed
inserts may not providing adequate holding strength under all
conditions.
My preference is for the inserts which are
slotted and have the nylon expansion ball in the bottom. The tighter you
turn in the bolt the more force the compressed ball puts on pushing the bottom
barbs of the insert into the plastic material. This seems to work well for
high shear-strength plastic compounds. You may want to be cautious about
using them in a hole near the edge of the casting as if there is insufficient
material - the force of the barb expanding could crack the plastic toward
the edge. However, I am a novice in casting plastic and metal inserts - so
you would be well advised to check with industrial experts on your choice of
inserts.
As I always say - "If you are going to make a
mistake - advanced the state of the art and make a new one, don't make the same
old mistake I made - we already know how that turned out!!"
Oh, yes, I will continue to fly with the plastic
manifold. Having it off gave me the chance to check for any cracks or
other signs of deterioration from 4 years of under the cowl use and there
was none that I could find. The ability to form complex shapes easily
using foam molds for casting the plastic and its apparent resistance to
conditions under the cowl makes some plastics look interesting for aircraft
use. While the compound I used, has a fairly high temperature resistance,
I have not gotten to the point where I would use plastic for manifold parts
closer to the hot block or exhaust parts. I have visions of melted plastic
{:>)
FWIW
Ed
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