The big problem with a belt drive is they waste precious space inside
the
cowl by
uselessly lowering the engine. This almost precludes using one rad
under
the engine.
It gives more room of course over the engine which is of little use in
most
airplanes. In only a few designs like the Kis Cruiser and the Lancair
320/360 is room above the engine real tight and that only by an inch or
two.
Paul Lamar
There are at least four critical issues with the belt type PSRUs. I'll
tackle them in the
order that I have seen the most problems with the myriad of units out
there.
First and foremost: Many belt type PSRUs ignore the very high (~1000
lbs)
side loads
that the belt (ANY belt) imposes on both of the shafts. Each shaft
needs a
pair of bearings
to pick up these side loads. A key design requirement of any PSRU is
that
it must keep the
loads in the engine the same as the car application, i.e. ONLY torque
coming out
of the crank - side and thrust loads must be very small. This implies a
crankshaft pulley
that has two bearings (one on each side of the pulley) and a
misalignement-capable
coupling. There are various commercial couplings available, such as the
double row
roller chain wrapped around two closely located sprockets, that permit
SMALL
angular
and lateral misalignment.
Second - Rolling element bearings have absolutely finite life and fail
dramatically. They
MUST be properly designed for the load-speed combination and then
replaced at
regular intervals (hours of use, not condition) and inspected at perhaps
1/4th of expected
life intervals. So, if the design manual says B10 life is 3000 hours, I
would want to look
at the rolling elements and race every 750 hours or less. BUT - since
we
all make errors,
I would want to look at the first one ever made every 250 hours.
Sealed
bearings
eliminate the requirement for separate seals and a lot of effort to make
them work,but
preclude inspection of the bearing. I don't like this. An open bearing
with removable
seals can be opened up, degreased, inspected, regreased and resealed.
More work
and design effort, but much safer.
Third - The output shaft and flange should look just like the end of
the
crank on
a similar HP Lyc or Continental. Many look like a 1/2 scale replica.
Since
the
life of these is proportional to the DIAM to the FOURTH POWER !!!!! A
little
too small is WAY too weak. This is critical as the small shaft will
fatigue and break.
Many under sized PSRUs survive because they only fly 10-20 hrs per year
and are
crashed in a couple of years. Not enough time to fail, YET. Also, most
aircraft
conversion make nowhere near the claimed HP, which also saves a bunch of
butts.
Fourth - The belt system must be properly engineered TO THE
MANUFACTURE'S
RECOMMENDED HORSEPOWER AND TORQUE AND SPEED design factors.
This is often ignored. They will work for a while, but are probably
overheating and
ovewrstressing the rubber and fabric. Bad news.
This is not a condemnation or recomendation of any specific belt-type
PSRU.
This is
just how this particular cowardly mech eng would evaluate one if my
soft, pink,
irreplacable butt is going to ride around in the air behind it.
Bill Freeman
Long EZ builder & pilot
BSME, MSME
Hi Bill,
I am in total agreement with everything you stated. The only thing that I
would add, is not to place too much emphasis on the manufacturers catalog
design specs. These guys are ridiculously conservative in their
recomendations for sizing (for liability reasons I guess??). I have been
involved in several projects where we used the catalog to size components
initialy. One that comes to mind was a gear drive that was attached to an
eight HP IC engine. I dutifully went through the design calcs as presented
by the manufacturer, and without thinking too hard about it ordered the part
number that popped out. The gear that I got in the mail was almost as big as
the engine!!!!! I thought I must have made a mistake so I went through
everything four times, had friends go through it etc. They had something
like a 20 factor of safety built into their catalog charts. When I got out
the Machine Design book and used the materials that the gears were made from
and my own factor of safety of 2 I got a much more reasonable size of gear.
I have also sized some chains and gotten similar results. According to the
catalog the chain drive in my Jeep Cherokee transfer case is hopelessly
undersized, it should snap like a pretzel, but it seems to work just fine.
The auto guys size it with their own calcs and run a bunch of tests to get
some empirical data for life etc. Some of the Hi-Vo design stuff I have
gotten lately seems to be a bit more realistic. I guess that the lesson is,
when it comes to fatigue life, you are going to have to do some testing to
really nail it down. This means we are all going to be guinea pigs if we use
a new design. Unless you can afford to do life tests on 20 plus parts (the
minimum required to get statistically valid results) and make your own
bathtub curve for all the parts. Plus the testing required to determine what
the operating conditions for a test are. (something most of us and
incidentally most of the psru manufacturers cannot afford to do)
Conclusion: CAVEAT EMPTOR !!!
Monty
Some good points Monty. Thanks.
I might add that those are life calcs in the belt and chain catalogs.
Not max loads!!!
The auto guys are only transferring 30 HP most of the time. In the case
of a four wheel drive the engage dog clutch is probably on the driven side
so the chain is only used when one is actually in four wheel drive. If one
towed a large load, in four wheel drive up a steep hill, 100% of the the time
the chain would not last long.
The auto engineers are well aware of the
low duty cycle of the automotive environment. Now in fact they are using
computers and sensors to limit power in the event of an out of design duty
cycle demand on power or torque. Even in Europe cars are now governed to
150 mph (much less than max HP) while some are potentially much faster
than that. At one time you could run a European car flat out (max HP)
for as long as the traffic would allow.
We are very fortunate indeed with the rotary as Mazda has not figured
out how to match the durability and life of the rotary to the low automotive
duty cycle and cheapen it up yet. With Ford involved I suspect and fear
that won't be long. BTW I do however think the oil injection quantity is
going in that direction. Mainly for smog reasons.
Paul Lamar
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