1) This is meant to go on the propeller hub. Thus, the rpm should be
3,000 or less. If you are planning to use the engine in an airplane,
you
will have to get a propeller to spin something like 3000 RPM, so it is
the most sensible way to connect the brake dyno. You get to load test
the PSRU while you are at it.
2) The disk is never going to get hotter than boiling water. If it does
get too hot, pause the test and let it cool down. Turn up the water
flow.
3) Trailer disks are made of stainless steel.
4) I think a large diameter chunk of wood is more likely to come apart
than a small diameter stainless disk.
5) A long lever will make the side loads small. Put the lever on the
side that pushes down so it tends to counter act the weight of the
brake
mechanism. It will also hit the floor if something goes wrong. Make the
lever 4 or 5 feet long to get the side load down to a hundred pounds or
so. The PSRU should be able to take a hundred pounds of side load.
If it
won't, you would want to find this out on the ground. You can put on a
second arm to make the side load zero if it is important to you, but
you
will have to put on a second load cell (or scale).
6) You can easily adjust the hydraulic pressure by closing a closing a
"line lock" valve and then operating the handle of a second valve
closer
to the caliper. As you twist the valve stem in and out, you will
displace fluid and move the brake pucks quite nicely. Nice for fine
tuning. Use a porta-power hand pump to set the starting pressure.
7) A trailer brake has all the parts you need and nifty mounting bolt
holes to boot.
8) A 12 inch steel disk with a 5 inch hole spinning 3000 rpm would
experience a maximum stress of about 4500 psi. Obviously no problem.
At 7000 rpm, this maximum stress goes up to about 25000 psi.
Annealed stainless steel has a yield strength of 30,000 psi minimum.
Thus, 7000 RPM is on the edge. If you were to go with a 10 inch disk
brake instead, this would drop the stress down to 20,000 psi.
Probably a
smarter move if you want to go to 7000 rpm. I wouldn't stand next to
it,
regardless.
Bill Dube'
-------------------------------------------
Thanks Bill.
What size trailer are we talking about?
I think you are assuming a simple disk. Most truck brake disk
I have seen recently have vent holes and the outer mass is displaced
from the plane of the disk as in this 3D. If so there will be a
bending stress
tending to deflect the disk backwards into a cone shape. This will
then vector add to the stress caused by the centrifugal forces.
Also the thermal stresses of spraying water on a red hot disk
are not going to help the stress distribution. No doubt it
will cool the disk.
Paul Lamar
Well, I thought the prospect of the exploding brake rotor had quenched
this hot idea. But, Bill "Cool Head" Dube has re-kindled it with some
good ideas. I think it's a good idea to proof test the PSRU at 300hp.
To absorb the 300hp you will need to take away about 12700 BTU per
minute. This is enough heat to vaporize 13.1 lb or about 1.6 gallons of
water every minute. So, to keep the steam production down, you will
need a good bit more flow than that. As Paul has said, cooling the
rotor will be fairly easy. Cooling the caliper and the brake pad will
be more difficult because you can't get the water directly to the pad.
Spraying the caliper assembly with water will interfere somewhat with
the braking action. You will need silicone high temp. brake fluid.
Doug, if you can get a whole air brake system, it may be easier to set
up the controls than fabricating a hydraulic only system. There will be
an air actuated master cylinder that can be connected directly to the
brake caliper. The air actuator on the master cylinder is usually a
diaphragm type that needs fairly low pressure which you can get from
your shop air and a pressure regulator. Put a bleed hole in the air
supply line and the braking force can then be controlled by turning the
air pressure up and down. Air brakes usually work in the inverse, ie.,
higher air pressure causes lower braking force. So, the pneumatic
actuator will probably work against a spring to unload the hydraulic
pressure.
Aubrey
Aubrey and Bill,
Thanks for all your fine thoughts and ideas. For now I am sort of
cooled
off on the brake idea. No offense but it has become apparent that this
concept hasn't been tested but 'should work.' I already have enough of
these
types of projects (of my own making) to work on. Paul is lending me some
props to test with. So now I only have to make some sort of hub for them
which shouldn't be that hard with the shop machines available.
After the turbo compound initial testing is finished I may want to come
back to this or another type of brake loading idea because of its
ability to
vary the rpms. I'll wait until Ken or I has a higher ratio PSRU built and
see if it can reliably handle 300 + hp. Final testing would be with a
pricey C/S prop.
Doug in Japan.
Yesterday, while driving to pick up my daughter at school, I thought of
the one big problem we always experienced with the several small
friction brakes that I have built. The equilibrium between the brake
and the engine is inherently unstable. This is because the the friction
forces usually go DOWN with increasing speed and temperature. It is
almost impossible to establish a load that will give a stable RPM.
A brake that uses a fluid as the friction medium, on the other hand, is
inherently stable because the friction forces usually go UP with
increasing speed and a stable RPM point can be established. So, I
think it is a good decision to abandon the mechanical friction brake
concept.
Last Friday I stopped by my friend Keith's transmission shop to solicit
his opinions on the issue of Ford vs "other" planetary sets. I
noticed his huge pile of removed torque converters. Could a large
torque converter be used as a fluid load cell with water, if a
continuous flow of water was pumped through it? The load could be
varied by adjusting the level of the water. It would be easy to bolt
the torque converter on the engine flywheel or the PSRU prop flange.
Aubrey
Very interesting Aubrey. Thanks for your insight on that. That could
have been a show stopper. Yes I think a torque converter might work.
Usually they have a stall speed around 2000 RPM but maybe they
will work with way less fluid in the 6000 to 7000 RPM range.
Of course I know people that cut them apart and modify them.
Perhaps some of the fins could be removed to up the stall speed
drastically.
Paul Lamar ...No rotor no motor.
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