Paul, I am not sure I agree with the idea of small sections of
tubing welded together. I had a few projects inside the GE turbine
components plant in Wilmington NC where I got to see lots of turbine
parts made. Early on, the interior spool sections of the turbines
were cut from a large solid block of Inconel. This was incredibly
expensive so they experimented with small sections welded together.
None of the welds were sufficient for the stresses involved partly
because the welding itself altered the hardness in the location of
the weld. The solution was to use Inertial welding. Each rotor is
machined and broached to hold the blades. One part is fastened in a
large (100 ton and up) flywheel and the other is fixed to a large
stationary hydraulic ram. These parts are adjusted to within .002
inches in any axis, then the flywheel starts spinning at 10 to 20
RPM. After the spinning starts, the part on the ram is forced into
the mating surface of the flywheel mounted part, causing the entire
approximately 3 inch mating surface to be one large weld with the
heat being spread out over a large area avoiding localized heat
issues.
I have welded lots of aluminum, and I would be very concerned about
the welds and the adjacent metal being able to withstand the forces
placed on them during flight.
Why wouldnt you make a plug from wood or plastic on the lathe for the
inside and the outside and use it to make a sand casting and cast the
mold yourself? Its not difficult to make a mold. You need sand and
some bentonite clay. Look up any local directional boring contractor
in your area for the bentonite. We use the bentonite to condition the
"mud" when we drill to keep it thick and hold the hole open for the
pipe. After you make the mold, melt the aluminum and cast the part.
Put it on a lathe, clean it up, and make your part.
Kevin Alderman
I think you are referring to these parts.
It all depends on the stress. These are very low stress parts.
There is no tension load as there is a 1/2 inch bolt that goes
through it holding all the parts together. Tension load capabilities
on a half inch bolt is about ten tons :).
Paul Lamar
I thought the first round of these emails were referring to making
the case for the PSRU, not the mounting brackets. Sorry if I missed
something.
The loads on those mounts should be mostly in shear, which should be
fine. The PSRU on the other hand sees tension, compression axial and
torsional loads in differing amounts on every part of the unit. I
would be very concerned about welding together aluminum pieces to
make one of these.
Kevin Alderman
The planetary PSRU case sees no torsional loads unlike a spur gear
case. Using needle thrust bearings on the first bulkhead this about
1000 pounds max thrust. The tube containing the ring gear is about
six inches in diameter. If the wall on that part is .250 inch the
area of the material is about pi times 5.75 time .25 or 4.52 square
inches. Therefor the tensile stress is 1000/4.52 or 222 psi. Since
aluminum is good for 40,000 psi 222 psi is near nothing.
The other load applied is if you are in a spin of one radian per
second and the prop weighs 40 pounds there is a 650 ft pound bending
load applied to the over all gear box. See the attached Skyranch
jpg's.
Lets say it is only 4 inches in diameter on average and the wall is
.25 inch. This is a cantilever round tube beam 15 inches long subject
to bending The load on the end of the beam is 650 foot pounds times
12 or 7800 inch pounds divided by 15 inches or 520 pounds up or down
load. The load is always 90 degrees to the direction of the rotation
in calculating gyro loads.
http://www.engineersedge.com/beam_bending/beam_bending9.htm
<http://www.engineersedge.com/beam_bending/beam_bending9.htm>
<
http://www.engineersedge.com/beam_bending/beam_bending9.htm
<http://www.engineersedge.com/beam_bending/beam_bending9.htm>
The moment of inertia of a 4 inch round tube with a .25 wall is
http://www.engineeringtoolbox.com/area-moment-inertia-d_1328.html
<http://www.engineeringtoolbox.com/area-moment-inertia-d_1328.html>
<
http://www.engineeringtoolbox.com/area-moment-inertia-d_1328.html
<http://www.engineeringtoolbox.com/area-moment-inertia-d_1328.html>
/I_y = π (d_o ^4 - d_i ^4 ) / 64/ See the attached jpg.
pi X (256-150)/64 = 5.2
Going back into the beam bending web site and plugging that number in
gives a stress of 219 psi and the deflection is 0.00001 inches. Also
inconsequential.
Over the years we have gone over this several times in reference to
the stiffness of the NC machined adapter plate. In fact Jerry Hey
built a static test rig. See the jpg's.
Check my arithmetic and go over the steps as I did as I may have made
a mistake.
Paul Lamar
-- Its not the adapter plate that concerns me, its the aluminum weld
joint between the various aluminum pipe sections. I have seen too
many aluminum welds crack and fail due to fatigue to bet my rear end
on a welded attachment like this. I see the test rig has what looks
like either a steel pipe or a DIP attached to a flange. If it is DIP,
usually they thread these and do not bother trying to get a weld to
hold, if it is steel they may have welded it to the flange. The plate
looks like the machined plate made from a single piece of aluminum,
again with no welds.
Our aluminum dump beds on our dump trucks would always break at the
welds. Always. The aluminum handrails we installed on marina docks
would break at the welds, the pipe never gave any problems. Even if a
boat or golf cart hit the railing, the welds would break and the pipe
would bend.
My PSRU if I were going to make one from scratch would be turned from
a solid piece or cast, not welded.
KA
I'm not sure here but I would bet that those dump bodies and hand rails
were not heat treated after welding. The welded assembly that Paul is
suggesting is small enough to be heat treated in a decent home oven. I
wouldn't want to fly behind one that wasn't heat treated after welding.
Mark LaPierre
The railing likely was not. However our dump beds were pre heated before
welding and then heated again to normalize the area and cooled in a
controlled area after welding. We had a particular area of the shop that
we could close all of the doors and then we would use welding drapes to
further isolate the weld area to limit possible drafts, etc. Even with
these precautions, they still tended to crack at the welds and in the
area of the welds. These trucks cost almost 100k, and every day in the
shop is a day of lost revenue. If it came in for a repair we did our
best to fix it right and get it back out. The last thing I wanted to see
was that truck back in the shop a week later for the same issue. Over
my 22 years as a contractor I have learned to be cautious of welded
aluminum. Even aluminum boat trailers that are made in a shop have very
few welds. All of the brackets etc. are drilled and bolted eliminating
the inevitable cracked welds and warranty claims. Imagine the results
of a slightly imperfect weld on the PSRU while you are in the air- it
would end badly. To err on the side of caution I would avoid the welds
in this area totally because you can and without much additional cost or
effort. I own NDT equipment that can test my entire Tracy PSRU for
cracks that I cant see because it was cast as a unit and then turned out
of that casting. Welds are much more difficult to test because of the
irregularities of the surface and differing penetration tends to scatter
the ultrasonic beam.
Just my preference- yours may differ :)
Kevin Alderman
It is also possible to design it with thick wall tubing and machine
screw it together.
The long 1/4 inch bolts Tracy's uses are half the necessary machine screws
needed to hold it together.
Paul Lamar
Did you mig weld those truck beds or tig weld them?
Big difference.
Hogging the PSRU case out of a sold chunk of aluminum takes a long
time unless you have a room size lathe. Machine shops charge by the hour.
Casting shops have disappeared in the US due to the EPA. Continental
engines have
all their casting made in Mexico. Sand castings are half the mechanical
properties
of tubing.
Paul Lamar
The Rotary Engine News Letter. Powered by Linux.
ACRE NL web site.
http://www.rotaryeng.net
You Tube
http://tinyurl.com/beqqxas
Copyright 1998-2014 All world wide rights reserved.