nuckollsr(at)cox.net
Guest
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 Posted: Wed Nov 15, 2006 6:39 pm Post subject: Vacuum Pad Limitations - Weight & Torque... |
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At 12:17 PM 11/15/2006 -0500, you wrote:
| Quote: | Does anyone know how much torque the <http://www.BandC.biz>www.BandC.biz SD-20
alternator places on the accessory pad drive? The reason I ask
is because I put one on my TCM IO-550N engine and want to be
sure it is within the limits prescribed by the engine manufacturer.
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A 20A, 14V alternator puts out 280 watts or 280/745 = .38 HP.
The alternator is never running at its design speed for
peak efficiency (about 5,000 rpm) so just for grins, let's
assume 50% (ugh) efficiency. This says mechanical demands on
the engine are 0.76 hp.
The horsepower is 33,000 ft-lbs of work per minute. Since the
baseline torque limit you've cited is in in-lb, we'll assume
a shaft of 1" radius sticking out the back of your engine. Spinning
that shaft at 3,500 rpm give us a surface speed on the shaft
of 3,500 x 1 x 2 x pi = 21,980 inches per minute or 21,980/12 =
1,831 feet/minute. Loading this shaft to 100 in-lb yields
100 x 1,831 = 183,100 ft-lb per minute or available
HP = 183,100/33,000 = 5.5
| Quote: |
My engine is described by the "Type Certificate Data Sheet" (TCDS) E3SO.
(This applies to most of the Continental IO-550 engine models.)
For those who may not be aware, the TCDS is the "official" source
of limitations on any Type Certified device that the FAA has approved.
The FAA is the official repository of the most up-to-date approved TCDS
and also maintains the historical records as well. You can find them at
<http://www.AirWeb.FAA.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf>http://www.AirWeb.FAA.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf
Anyway... the E3SO (Revision  Type Certificate says that
my engine's "accessory drives" (aka: "vacuum pump pads")
can each be loaded to 100 inch pounds of torque on a continuous basis,
and up to 800 inch pounds of torque on a static basis. A little ***
note indicates that one drive can be loaded to 160 in.lbs if the other
drive is limited to 100 in.lbs, (or in my case, not used.)
It also states that the accessory drive gear ratio is 1.5:1
so if I'm cruising at 2333 RPM or higher, then the alternator
should be turning at 3500 RPM or higher and able to put out a full 20 amps.
(according to the info at <http://www.BandC.biz>www.BandC.biz for it's
SD-20 alternator)
The TCDS also shows that the maximum "overhang moment" of each accessory
should not exceed 40 inch pounds. By checking the specs at
<http://www.bandc.biz/SD20REVC.pdf>http://www.bandc.biz/SD20REVC.pdf
it appears that the SD-20 weighs about 5.72 lbs and the CG is 3.15"
from the flange, for an overhang moment of about 18 inch pounds.
So far, so good!
It would be nice to know if the SD-20 has a "shear shaft"
to limit the damage in the event of a locked rotor. If it sheared around
800 inch pounds of static torque, that would be great to protect the engine.
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It could not have been certified without one, and even
then B&C would not have produced one without a shear section
even if it were allowed.
| Quote: |
So... to my great relief (and no big surprise), the SD-20 doesn't appear
to present
any excessive burden to the accessory drive on my IO-550. Thousands of these
things flying around, so I didn't expect that there would be a problem.
############## EXTRA CREDIT ################
The only significant "unknown" is what the "continuous" & "static" torque
of the
alternator is while fully loaded at my max-rpm's ?
I'll challenge the electrically gifted on this list to come up with a
reasonable estimate
of the torque required to produce 20 amps at 3500 to 4050 alternator rpm.
I can follow it through VxA = Watts, Watts -> HP, HP / Efficiency = Input
Shaft HP,
Input Shaft HP / rpm = Force or Torque or something - but I get stuck
about there.
Bottom line, I'd like to know Input Shaft torque on a continuous basis
over the range of operating RPM's. (Perhaps it's highest at lower RPM ?)
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If the power output is constant, then torque would go UP as
RPM goes down. However, this alternator is never operated at
minimum speed for full output so if you're loading the alternator
at max output for 3,500 RPM and then reduce RPM, then power
input (torque) will go down along with the output.
| Quote: |
If it's less than 160 inch pounds, then we're good to hang at least one
SD-20 (on a TCM IO-550)
If it's less than 100 inch pounds, then you could conceivably hang two
SD-20's -
(although it would take tiny fingers & a small miracle to get them both
bolted on & torqued properly! 
########################################
If anybody is considering hanging two of the SD-20's, or has a radically
different engine,
you might want to look up your TCDS to find the latest "approved"
limitations.
You might find out you have a higher or lower gear ratio, torque limit, or
max overhang moment.
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Correct. As we've observed, the SD-20 doesn't begin to load
the pad to limits so the engine is not mechanically
challenged if one COULD install two alternators.
Bob . . .
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