Paul,
I've just gotta ask - why haven't Tracy's
rubber mounts melted?
Ken Powell
You got me.
Paul Lamar
I checked a picture in his book and the rubber
bushings are
no where near his engine. He used the dynafocal
mount.
Paul Lamar
Let's be a bit more specific about Finn's motor
mount problems. The conical
rubber mounts did not melt.
How do you know? Did Finn take it apart at Shady
Bend after I left?
The primary 'failure' (sag of the mount) is due to
the aluminum conical
mount cups not being correctly fastened to the
plate. They were glued with
JB Weld epoxy and the glue joints failed in shear.
I like this stuff for
many purposes but this is not one of them.
I do agree that the 'bolt in shear' design is not
a good idea. The
Conversion Concepts mount abandoned this approach
before any were sold.
Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com
The 3/8 bolt is NOT in shear. It is in almost pure
bending.
Conversion Concepts would have shipped the mount in
question if I had
not raised serious reservations about it in private
email to them.
Paul Lamar
-Mr dan ruggirello wrote:
Can't help piping in about whether it was a shear or
moment failure. After looking at the solid of the
bolt deformation that Paul supplied, looks to me like
it was failure from shear and a moment.
For one, you can't get that bolt to deflect like that
without having both shear and a moment. Just draw a
free body diagram. Assuming it's a ductile material,
looks like it failed because it was outside the
vonMises ellipse. Which contibuted more? You would
probably have to solve the indeterminant structure
using a finite element model to find out.
My two cents worth.
Dan-
My definition of a bolt in shear consist of two or more plates side
by side tending to move in opposite direction with the bolt passing through
them. If the plates are not side by side and separated by a finite
distance I think Roark would call that a beam with both ends supported
with an opposite couple on both ends. I could be wrong. Unfortunately Roark
does not have a formula for it as far as I can find. The parts introducing
the couples into the end of the beam (bolt) also must be analyzed as they could
be weaker than the bolt and fail before the bolt. Be very interesting
to do an FEA on the configuration.
That is not to say there are no shear stresses in the bolt when it is
subjected to this kind of bending. Of course there are shear forces.
It is possible to design a mount like this but not with pencil
and paper. It also leads to a heavier mount and rather high
stress concentrations.
In any event it is not a good way to design a motor mount.
Most pure space frames (motor mounts) have their members intersect
at a load point. The joints in theory could be replaced with pin joints.
In other words rod end joints. No local couples need be introduced
into the members to complicate and foul things up. All members should be
in pure tension or compression.
I am trying to encourage a simple method here that a layman can use
to design an adequate motor mount. I have provided a look up chart on
the web site to insure the tubes do not buckle. This is proving to
be very very difficult and I have been at it on here going on five years.
It would be better if a course in vector mechanics was a prerequisite
for subscribing to this newsletter :-)
As far as I know the bolt did not fail yet. It could be cracked or
the tubes with the welded bolt bushings could be cracked on either
end.
Paul Lamar
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