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.
Here is a link to a typical trailer disk brake. They show a 5 bolt 10",
but you would want the 6 bolt 12 inch.
http://www.tiedown.com/bvented.html
You would need to get a stub axle to go with it. The whole thing
would be less than $200.
The water would flow in the back and flow though the vents. If you cool
the rotor, it should keep the pads cool enough. The pads will transfer
heat to the disk. I would expect it to make plenty of steam. You could
easily monitor the pad temperature and the caliper temperature if you
were worried.
The disk would not get "red hot" if you spray water on it (in it.) It
will make lots of steam and stay a reasonable temperature. If it does
get too hot, you can pause the test to let it cool down.
The centrifugal stresses are truly modest if you stay at 3000 RPM. About
10% of yield. Nothing is going to change shape noticeably.
This is probably the least expensive dyno brake you could possibly
construct. If it doesn't work perfectly, you won't be much out of pocket.
Bill Dube
I down loaded the pdf file and I can't find any trailer weight rating
for this disk brake Bill. What is the max trailer weight? I thought you
were speaking of a 50,000 pound semi truck trailer disk brake.
At 3000 RPM you would still be disapating the same 48 kw and of course
you would still need a gear box.
My gut based on my road racing
experience is 300 HP would go through this disk brake like a hot knife
through butter. The aluminum caliber might melt. I was assuming a
cast iron caliber. Most road racing cars have four 12" disk brakes
and they are forever giving problems as you are bringing a 2000 pound
car down from 200 mph in just a few seconds. Over one G.
Paul Lamar ...No rotor no motor.
Ganged disks (drums?) on a common shaft, all submerged in a big trough
of water with a firehose feeding it.
Matt Prather
Sounds good to me :)
Paul Lamar ...No rotor no motor.
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