Belt Drives and design

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DanH

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If you can find the right guy, the belt manufacturers do have stiffness information. Don't use the "a" word! I've attached a sample plot for a Dayco RPP Panther. It's been quite a while since I looked at belt drive design, but I vaguely recall the Panther as being middle of the pack in terms of stiffness, some being softer and the Gates Poly Chain (for example) being way stiffer. It's all about the tensile fiber, and some tooth deflection.

Mr. Simpson mentioned (to paraphrase) not knowing where the first fundamental might be with a belt drive. The typical system common to light aircraft seems to ballpark between 25 and 45 hz, with 25 incorporating a soft torsional element, and 45 running without. Obviously shaft lengths, belt length, sprocket support, and so on come into play along with the distribution of inertias.

Jeron's Raven drive was very clever, incorporating a moderately soft element in parallel with a friction damper, both within the upper sprocket. There were no small diameter shafts, which helped keep stiffness up within the rest of the system. It seemed to work very well.

One of my own experiments was a viscous damper, installed in parallel with a Centaflex. It significantly reduced the measured amplitude of vibratory torque (to about 90 ft-lbs) when in resonance at 20 hz (800 RPM) on a 3-cyl Suzuki, steady state part throttle. As a prototype, it had some mechanical weakness and was ultimately removed, but there was real promise. A quick patent search said a major manufacturer already had a patent on a similar damping device parallel to the springs in a truck clutch, so I did not pursue it further.

Different concern, but with an inline 3-cyl sans balance shaft, don't forget about the block wobble. Keep the propshaft short.

Ya'll keep up the good work.
 

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plncraze

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Thanks for the info Dan! I appreciate all the info you have on the this site and VAF. Could you post more pictures of your Suzuki?
 

wsimpso1

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Just calibrate you guys, an even fire four cylinder at 1200 rpm is firing twice per rev, or about 40 Hz. If your resonant frequency is 45 Hz, you will be amplifying big. At 1350 rpm, this thing can resonate without upper bound. You would need to idle at or above 1700 rpm to be out of danger, and 2000 rpm would be better. With typical gear ratios, prop speed is around 800-900 rpm, which still is not bad.

Go to 25 Hz and a four-banger can get to a safe idle around 1200 rpm, threes even lower.

Billski
 

dog

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Thirty years ago, I could have had a complete auto machine shop that was belt driven for free. It has been around forever. It had modern tools too. The land became valuable, the third generation liked his other business, so when dad retired, they took the “ portable “ stuff home and scrapped it. This was electric. In the middle of the room was an armature that was about 8’ in diameter that drove the belts. Looked like an electric waterwheel. They still used the cylinder grinder if someone would pay for it. It made perfectly round cylinders with a surface ten times cleaner than a boring bar and hone.
This discussion is very interesting. I am starting to get some familiarity with the terminology of Torsional Vibration and am reviewing all I have done and seen about belt and shaft drives.
The newer belts seem to have potential for use in aircraft for positioning propellers so as to acomadate design considerations that dont work well with direct drive.
Making the connection with old mill gear and TV is important to me.And it is exciting to think about designing a belt drive for aircraft use and then bieng able to predict that drives vibrational characteristics and adjust them on paper. Before building it and channeling some old time millright foo and get it to run smoooth.
 
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DanH

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Just calibrate you guys, an even fire four cylinder at 1200 rpm is firing twice per rev, or about 40 Hz. If your resonant frequency is 45 Hz, you will be amplifying big. At 1350 rpm, this thing can resonate without upper bound. You would need to idle at or above 1700 rpm to be out of danger, and 2000 rpm would be better. With typical gear ratios, prop speed is around 800-900 rpm, which still is not bad.

Go to 25 Hz and a four-banger can get to a safe idle around 1200 rpm, threes even lower. Billski
Even when designing for a first natural frequency below the operating range, the user still has to pass through it at startup. Put another way, the design should cheerfully withstand the resonant vibratory torque, because it's not really possible to avoid it. Then it can be treated as an RPM range prohibited for continuous operation.

That said, pushing it as far down the RPM range as practical is all good, because it reduces the applied power. When passing through a resonant range, it is important to use as little manifold pressure as possible. I recall one fellow relating how he preferred to shove the throttle forward smartly to get past the big shake range. Not good.

Vintage replica, so I idled the Suzuki 3 at 550, where it sounded like a Model A and turned the prop very slowly. A little tweak of the throttle got it past the 800 resonant peak, after which there was no further limitation.

Oddly enough, I don't seem to have a photo of the whole, assembled drive on the airplane, and back in those days I was still drawing with a pencil on a big white board and butcher paper. Here are two from fabrication.

The old drive (purchased from Reductions Inc in Canada, a real POS) and the new design were both modeled. I had a lot of help from a mentor, Dr. Steven Crow. We teamed up because I wanted a drive and he wanted to prove some very complex software. Anyway, I've attached a list of model inputs for an early iteration of the new drive. The detailed list is inertias. We only inserted three stiffness values in this model.

The JN4C was Grand Champion Lightplane at S&F in 2000, with the viscous damper installed, and a Wheatstone bridge on the propshaft. Nobody ever asked about the tape wrapped around the shaft, or the odd blue flywheel ;)

 

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plncraze

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Thanks For the picture! The ones in the Experimenter were just a tease. LOL
 

plncraze

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Wow! Thank you so much. I just went through everything that you sent with the picture. It is much appreciated.
 

plncraze

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An old issue of Contact! Magazine had a picture of one but there was no follow-up to that.
 

wsimpso1

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Even when designing for a first natural frequency below the operating range, the user still has to pass through it at startup. Put another way, the design should cheerfully withstand the resonant vibratory torque, because it's not really possible to avoid it. Then it can be treated as an RPM range prohibited for continuous operation.
There are some pieces missing here. Imagine a system with a nice linear spring - say a drive shaft between two inertia - say the engine and prop. Imagine also that there is energy being diverted away - this is a low damping case. This system will have a very clearly defined natural frequency. Run within a quarter octave of that natural frequency and some amount of engine output from each firing stroke will be added to the vibration at natural frequency on every firing stroke. It will be amplifying. If the system is not very close to natural frequency, more and more energy will find its way to other vibe modes and amplitude will be capped at some level. Run right on at the natural frequency, and amplitude will grow pretty much without bound until something breaks.

Now if we set up such a system where natural frequency coincides with firing frequency at cranking, we can cause all sorts of havoc when the engine only catches on one or two cylinders and just hangs on at cranking speed. It amplifies, the vibration stroke gets larger and larger, torques get to damaging range, and things can get broken. Same thing happens if the combination of spring rate and inertia puts natural frequency at idle speed or above. Set the engine to that speed and it amplifies, potentially with damaging effects.

So what does the intelligent engineer typically do to get this system to behave? We deliberately set this natural frequency neatly above cranking and below idle, usually at least an octave from each. In this way, idle does not cause amplification, nor does cranking. As for start up, where we have to accelerate through the natural frequency, we want the natural frequency suitably higher than cranking speed and suitably below idle speed so that once the engine is hitting on all cylinders it quickly blows through the speed range where amplification occurs. If it only gets a few firing pulses within a quarter octave of natural frequency, the vibration does not pick up amplitude to the danger level before it is back out of range.

Now if we just can not get the natural frequency below idle, maybe we can put it a quarter octave or more above idle. Hmmm. In cars this is BAD JUJU. You can drive the car at any rpm, and if you match natural frequency and push down on the gas, you can make big torque, make big energy available, and yet speed won't change fast because the tires are hooked to the pavement. Nope in cars, we shove the natural frequency about half way between idle and cranking. Running fans and propellors though, the torque we can react is related to prop speed. At low rpm, we can not make much torque, which can reduce the energy available, slowing down the rate of amplification. Might be OK, but definitely still operating in the danger zone. Better be ready to add a softer element at the engine end when you get to test. Yes, we will likely have to beef up the system to stand the resonance without damage, and when we do increase its strength, well, we tend to increase its stiffness, bringing the natural frequency higher and into a more dangerous zone. This can be a fatal flaw... Usually a soft element upstream will be lighter than the beef up downstream and way less likely to kill the program.

I have never liked the idea of prohibited zones. There will always be times when the operator has to runs there. The one I see regularly is speed/descent restrictions while IFR and being in a line of airplanes on an arrival/approach that drive you to a particular airpseed. "How far into that prohibited band can I run?" While none of us want to precipitate a failed crank or prop, another 100 rpm will keep me on altitude and airspeed for this approach.

One other thing to input here. There is an increasing reliance upon elastomeric bearings in these systems. One of the neat things about giubos is they are both an energy converter and an inherently rising rate spring set. That is, the higher the torque applied through them, the higher the spring rate. The effect is one where the natural frequency is not so precisely defined, and it varies with the swing of the vibration. Try to set at resonance, and as the swing amplifies, the resonance changes while the gadget converts some of the swing to heat... There are upper bounds here too. The heat is made in the rubber element and is hard to remove, so it warms up and can make new failure modes. The heat being made tends to change the spring rate too, further altering the natural frequency and usually reducing the damping coefficient, which then tends to propagate to failure, so you really only want to rely upon this feature in transients, not in steady state. The increasing spring rate can bring resonance with it as you increase torque (and rpm), which can also be a fatal flaw. Best is still to just put natural frequency safely below idle.

I do wonder if DanH's case was not one of lowest frequency being coincident with firing, but of it being coincident with another higher forcing function. Almost all four stroke engines have 2x firing that is about 1/4 of the amplitude of max firing and is usually more diffuse than firing. It would be near idle in many engines, and noticeable, but is less likely to be damaging. There are other orders too - variations between banks, between cylinders, etc. They can be particularly vexing to identify and solve for, but usually they produce annoyance, not broken parts.

Billski
 

rv7charlie

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Hi,has anyone ever built a belt drive for a mazda rotary?
Well, this is odd. I could have sworn I posted to this thread in response to that question, but it isn't here now, nor do I see it in the history of my posts.
Anyway, Ken Welter, in the NW USA, did it over 20 years ago, and actually made them available for sale for a while. the engine was a 13B installed on a Coot amphib. Used nitrous for water takeoffs when heavy.
Link to his old website, from the wayback machine:
rotary coot
 

dog

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Well, this is odd. I could have sworn I posted to this thread in response to that question, but it isn't here now, nor do I see it in the history of my posts.
Anyway, Ken Welter, in the NW USA, did it over 20 years ago, and actually made them available for sale for a while. the engine was a 13B installed on a Coot amphib. Used nitrous for water takeoffs when heavy.
Link to his old website, from the wayback machine:
rotary coot
And you did do that and I have the page bookmarked.Bunch of stuff zapped.
 

rotax618

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I guess the BMW rubber donut in the Rotax and SPG gearboxes successfully moves the resonance frequency lower than idle, maybe if one were incorporated in a belt drive system the belt would survive longer. I believe Raven redrive used a “soft cushion” drive much like that found in the rear wheel hubs of motor scooters an small motorbikes, unlike the Rotax donut it was in the prop hub (output shaft)
 

AdrianS

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I've seen a cush drive destroy itself in about 10 mins on a dyno - the operator was running a small 3 cyl diesel at light throttle near resonance, and the soft elements overheated and crumbled.

Same drive was just fine with a small block v8.
 

TLAR

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I see the college level chit chat pointing to all the challenges that exist, but alas no solutions.
Does anyone know of a reduction drive that doesn’t work?
The rednecks have got it figured out, maybe you college guys will eventually catch up if you keep on “talking about it “
 
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