P Ponk throwing belts

Betmerick

Betmerick

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Betmerick
Hi all,

I just got my continental 470U modified to a 470-50 P Ponk engine. During break in the alternator belt keeps coming off. Run up okay, takeoff ok, climb ok, once in level flight belt pops off. Time is around 15 minutes. Happens every time!!! Tried 2 separate belts (continental and Cessna) Belts not torn up but not used again. Manufacturer helpful and actually flew to me to change out starter housing abut to no avail. Any ideas? Plane is a 182R
 
have heard of sticky crank weights causing this
'vibrational dampeners'
 
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Manufacturer should have checked the alignment when they were out. The vibration idea is interesting.
 
azblackbird said:
Wouldn't they also spin balance the pulleys for any vibration/harmonic variances? :dunno:
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I would think so, I also think they would check the belt alignment. I wonder if the pulleys are correct for the belt. Seems implausible but when the AC gets up to speed the wind might be knocking the belt off. If it's a v belt usually the top of the belt should be flush or slightly above the top of the pulley. The bottom of the belt shouldn't touch the bottom of the v in the pulley, unless it's not a v belt. Interesting problem.
 
Betmerick said:
Hi all,

I just got my continental 470U modified to a 470-50 P Ponk engine. During break in the alternator belt keeps coming off. Run up okay, takeoff ok, climb ok, once in level flight belt pops off. Time is around 15 minutes. Happens every time!!! Tried 2 separate belts (continental and Cessna) Belts not torn up but not used again. Manufacturer helpful and actually flew to me to change out starter housing abut to no avail. Any ideas? Plane is a 182R
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Take your pick.

http://www.supercub.org/forum/archive/index.php/t-47542.html

https://books.google.ca/books?id=JEXnFqVTqJQC&pg=PA423&lpg=PA423&dq=sky+ranch+vibration&source=bl&ots=dRG8O3d8ZP&sig=COTBO9oD8ml0Jh8L24R0UwSiDho&hl=en&sa=X&ved=0ahUKEwjGk_G-0JjTAhWoK8AKHb4HCh0Q6AEIHDAA#v=onepage&q=sky ranch vibration&f=true
Page 426

http://eci.aero/pdf/EngineTroubleShootingGuide.pdf
 
This is a common problem for 182s with O470 engines. My O-470S engine was doing it for a while and then seemed to stop (knock on wood) on its own. I've read lots of causes for it, but the most likely it is the counterweights causing microscopic acceleration and deceleration of the crank, causing the belt to flail and jump off. For me, it seemed once I got a proper,tension (not too loose, not too tight), the problem seemed to stop.
 
I agree with others, without looking at it, it may be a tension problem.
 
Assuming you have a rear alternator drive with properly aligned pulleys & belts, the internal crankshaft damper system isn't doing its job. What is probably happening is that there is a severe torsional vibration at about 200 Hz (a little below middle C on a piano) in which the rear of the crankshaft is going to and fro, while the propeller is deforming fro and to (read that again). The spinner is moving relatively little when this torsional vibration mode is being excited.

If so, this is imposing a lot of fatigue damage on your propeller and your crankshaft. Maybe there is a different cruise rpm that will not excite this mode of vibration. I suggest you take this up with P Ponk as they should have done a vibration survey on the combination you have, and tweaked the internal damper system to resist this.

One thing - Assuming things are reasonably aligned, it is not a balance problem.
 
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We've had a couple minor issues with belt tension slipping on our Pponk, but that has been over a 1,000 hr period. I really doubt it had anything unique to do with it being a Pponk. As others have mentioned, you need someone to carefully go over the pulley alignments, bushings, spacers, etc to make sure everything is correct.

Our Pponk was built by Steve Knopp's shop and ran as smooth as a turbine the first few hundred hours - incredibly well balanced.
 
nrpetersen said:
Assuming you have a rear alternator drive with properly aligned pulleys & belts, the internal crankshaft damper system isn't doing its job. What is probably happening is that there is a severe torsional vibration at about 200 Hz (a little below middle C on a piano) in which the rear of the crankshaft is going to and fro, while the propeller is deforming fro and to (read that again). The spinner is moving relatively little when this torsional vibration mode is being excited.

If so, this is imposing a lot of fatigue damage on your propeller and your crankshaft. Maybe there is a different cruise rpm that will not excite this mode of vibration. I suggest you take this up with P Ponk as they should have done a vibration survey on the combination you have, and tweaked the internal damper system to resist this.

One thing - Assuming things are reasonably aligned, it is not a balance problem.
Click to expand...

So how many props have failure in service from this and how many cranks have broke? Throwing belts seems to be pretty common so where is the concern from the manufacturer regarding proper troubleshooting and identification of bad damper system? Is the problem real or imaginary?
 
I agree it may be the crankshaft damper system. When I had a Viking I consistently lost belts (IO520) but only at certain cruise rpm's. After several trips to several shops it turned out to be the damper. Once fixed, never had another problem.
 
Alignment aside, throwing belts is only an indication of a damper problem, but from the OP's description it is happening often. Glen R's experience above is a good indicator of a fix, except that getting to the damper system involves a major teardown to remove the crankshaft assembly. Measurement of the damper pin diameters and the IDs of the damper mass sleeves important but it is the slight difference in these diameters that determines the effective tuning frequency (actually vibration order).

I'm experienced in vibration and dampers as a mechanical engineer for hi-rise building sway, but I'm not a propeller expert. I got the explanation from a tech wheel at McCauley maybe 30 years ago. Losing a blade at cruise wouldn't be very fun.
 
Keep bugging Steve until he fixes it.
 
Let'sgoflying! said:
have heard of sticky crank weights causing this
'vibrational dampeners'
Click to expand...

This was the cause on a friend's 310 that had 520s with belt-driven alternators.
 
Jeff Oslick said:
We've had a couple minor issues with belt tension slipping on our Pponk, but that has been over a 1,000 hr period. I really doubt it had anything unique to do with it being a Pponk. As others have mentioned, you need someone to carefully go over the pulley alignments, bushings, spacers, etc to make sure everything is correct. Our Pponk was built by Steve Knopp's shop and ran as smooth as a turbine the first few hundred hours - incredibly well balanced.
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Just like they should be from any reputable engine builder. With that said I doubt they balance or shim the alternator pulley as that is usually an aftermarket bolt on part from a different manufacturer speced for that particular motor.
 
I wonder if a prop balancer would pick up on the vibration? I haven't done it, but I recall that some can do a spectrum analysis and then download it in graph form, so you can see what the frequencies of concern are. Might be a good troubleshooting tool, to narrow down the possibility that the internal dampeners are the problem before opening the engine.
Just a thought.
 
No, a prop (or whatever) balance does not have any way of picking up this vibration as these rigs are sensitive to only linear vibration. For example, a propeller etc balance job only picks up linear vibration in a particular axis (or a pair of linear axes). Instead what's needed is an input from crankshaft rear torsional vibration. There are few transducers that can do this, and they would require a slip ring system for power and for signal output. The insidious thing about torsional vibration in general is that as a pilot you cannot hear it or even feel it.

Someone with a sharp musical ear may be able to pick it out as a subtle engine harshness overtone at certain rpm. I was able to hear a faint 6th harmonic (of crankshaft rpm) ringing in our 172M at about 2100 rpm, which would be about 210 Hz, which is about A below middle C on a piano. There is no rear crankshaft damper system in a Lycoming O-320 but the vibration could be heard. We just didn't cruise there for that reason even though it isn't prohibited in the manual.
 
It seems odd to me that crankshaft vibration would be a contributing factor to the tossing of an alternator belt that is not directly connected to the crank. But stranger things have happened.

Now on the other hand, I could see the recent camshaft gear concerns and MSB Continental released about them being a direct result of crankshaft vibration and/or damper problems.
 
JAWS said:
I wonder if a prop balancer would pick up on the vibration?
Just a thought.
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IT should, Steve's 0-470-50 is always run on the test stand prior to being shipped, it should have shown up then if it was a vib.
 
Betmerick said:
Hi all,

I just got my continental 470U modified to a 470-50 P Ponk engine. During break in the alternator belt keeps coming off. Run up okay, takeoff ok, climb ok, once in level flight belt pops off. Time is around 15 minutes. Happens every time!!! Tried 2 separate belts (continental and Cessna) Belts not torn up but not used again. Manufacturer helpful and actually flew to me to change out starter housing abut to no avail. Any ideas? Plane is a 182R
Click to expand...

If the OP lives in Indiana why is it assumed Pponk built the engine? Did I miss a post?

Is this the same alternator from before the conversion? It isn't the same crank, right? Or can U cranks be used? I can't remember. If my plane had that symptom I'd ask a friend to let me borrow an alternator and swap it to see if the problem was different. I like to start with the simple and work the problem from there. Please report back with the solution once you happen upon it. Good luck. That's a great mod and you deserve to be enjoying it, not fighting with it!
 
You have to understand what is probably happening here. The crankshaft is winding up, then un-winding, then winding up, and then unwinding, etc about 210 times every second. The amplitude of that TORSIONAL vibration at the crank rear is probably on the order of one or two degrees peak to peak. It is violently humming and using the belt to drag the alternator armature with it. If I did my sums correctly, that's about 1000 g's of linear acceleration for a bug riding on a 2 inch radius of the armature (the proof is an exercise for the student.... :)).

And no it would not be caught during an engine test cell run because this torsional vibration relies on the propeller to vibrate in the opposite direction. Only the spinner gets a smooth ride in this game.

The engine assembler should have verified the pin and bushing diameters of the damper masses - and that's assuming the original PPonk modification has taken the damper design under consideration in the first place.
 
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nrpetersen said:
The engine assembler should have verified the pin and bushing diameters of the damper masses - and that's assuming the original PPonk modification has taken the damper design under consideration in the first place.
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And the weight of the individual parts associated with the system. ;)
 
Like a grandfather's clock, the absolute value of the weights is not critical. It is the effective lengths of those little pendulums that is.
 
There are other, less egregious causes for belt problems to rule out before one should assume a counterweight problem, especially in a recently built engine.

I had a good discussion last week with my prop manufacturer about crank vibration and counterweights. Interesting stuff. I'd still look at the alternator pulley and the flexibility of the alternator mount first.
 
Definitely look first for a simpler fix. Alignment and belt tension are important. Balance probably is not.
 
It would be really interesting to mount a high speed video camera or even just a video camera to see the belt coming off in flight. It might help with diagnosis.
 
nrpetersen said:
Like a grandfather's clock, the absolute value of the weights is not critical. It is the effective lengths of those little pendulums that is.
Click to expand...
And like a fine tuned (expensive) grandfather clock, those weights are within grams of each other. ;)
 
azblackbird said:
And like a fine tuned (expensive) grandfather clock, those weights are within grams of each other. ;)
Click to expand...

So are all weights. It's just a question of how many grams... :D
 
Answer: IO550. It's the only option that costs enough to be effective and it eliminates the belt driven alternator.
 
l8evator said:
Answer: IO550. It's the only option that costs enough to be effective and it eliminates the belt driven alternator.
Click to expand...
I just installed an IO-550 in a 180. It has a belt-driven alternator.
 
Do gear driven alternators ever pose a threat to the entire engine, if it fails?
I don't think the belt driven ones do.
 
My B&C gear driven alternator on my Cub has a shear section built into the drive to prevent engine damage from a seized alternator.
 
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