Q for engineers: How do proven cogbelt or multi-V reductions survive without 'damping' mechanism?

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Aviacs

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Nope. That would be putting all the load on one or two teeth, and it would fail. The cog belt runs on sprockets that have many teeth engaged at all times.

That is a useful observation, perhpas qualified with "depends on ratio and diameters involved"?

IOW system where the pinion is a substantial part of the volume of the drum (have not checked to see where things begin to interfere) but say 3:4 or 7:10. Drum size "largish" - needs to be somewhere over 8" or 9" given the belt diameters available for, say 75 HP. At that point, then, the issue becomes derating (overbuilding) & whether the weight advantage continues to apply?

I'm wondering about thermodynamic issues.
Presumably the drum could be alloy, finned &/or ventilated at some energy cost.

smt
 

Aviacs

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Just draw 2 circles as you're thinking about them, and you'll see what he means. Two different size circles can only touch at one point, if the smaller is inside the other.

Mathematically you are correct.
In the mechanical whirrled it is not quite that trivial.
If it was, for friction purposes it would not matter the diameter and size ratios of smooth drivers; even fairly "rigid" materials.
In fact, it does.

For shredding knobby rubber belts, however, i take your point.
:)

smt
 
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wsimpso1

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Yes, I agree. That said, rather than "distributed mass and distributed spring", perhaps it would be better to stick with the classic presentation, a series of individual inertia values connected by stiffness values. Again, the goal is the promotion of understanding with a non-FEA audience, not absolute accuracy of result.

Modeling a distributed spring-mass as a string of springs and masses is the classic way of doing it and exactly what I had in mind. Trouble is that this scheme is all still pretty involved for a non-professional in that field. I remain highly skeptical that amateurs will do any of this math...

Bill, I've gotten pretty good results modeling one piece wood props as a single accurate inertia determined with bifilar suspension, or at most two inertias, hub and blades.

I don't doubt it. Wooden props tend to be pretty stiff and thus transmit firing vibe all the way out.

When I say "pretty good results" I mean subsequent live run results vs prediction.
I am interested? Prediction of vibe amplitude? Eigen shapes and frequencies?

I understand you have huge experience with automotive drivetrains, but have you modeled, built, and tested a PSRU?
What I have huge experience with is isolating engine vibe from downstream components. When you spec the spring rate range properly, all sorts of sins do not need to be chased later.

As a matter of fact, I have worked on an engine-drive-prop problem for an airplane, solved their problem, and demonstrated that they had nothing else in range vibration-wise.

The folks working on it are a pretty smart bunch, they had a couple race car guys involved, and some of their schemes had run for two minutes before coming apart, throwing pieces for hundreds of feet, their test rig was all scarred up and repaired from having broken shafts and prop blades flailing about, etc. They had racks full of broken pieces from schemes they tried that had failed. I modeled their scheme, proposed solutions, we implemented one, and it worked on both eliminating broken parts and obtaining a smooth idle. Not only did the modeling point the way to the solution, it also allowed us to clear the operating range of other vibe modes, and resulted in over 200 hours of successful test runs. Then we rented a Rotec (I am fluent in their use and have used them at three different companies now) and cleared the operating range with all cylinders and with cylinders not firing... So yes, I know something about the topic of modeling and measuring vibration. I have also seen the results of folks who have tried lesser methods and messed up test stands, dyno cells, and prototype vehicles.

I have talked about this in the past, and I do not expect non-professionals to run analyses of these topics even for educational reasons. There are a couple approaches, they are somewhat non-intuitive and mathematically dense, and requires good programming to get right, even at a simplified level. The other way is to model the system on CAD, transfer it to FEA, and turn loose the Eigen analysis tools. Both require a lot more than a passing interest on the part of the analyst to get anything meaningful out of the process. These are paths that I do not expect we will get from our interested amateurs...

Concepts I have been trying to get out to folks. Doing the analysis? Nah...

Billski
 

wsimpso1

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Question about cog belts, not including the vibration considerations to start:
Could a cogbelt be used inside a "drum" as an internal ring gear, to get a close center distance drive?
If a "reasonable" life hours is attainable, advantages considered over steel gears are 1.) accessibility for the homebuilder 2.) replaceable elements or cheap to replace systems, 3.) lighter weight

smt
If you mean to use one gear with internal teeth and a second gear with external teeth, to obtain a small offset between the two gears, yes, it is done and works well in steel gears. To do it with cogbelts though is problematic a couple ways. If we are not reading you OK, let's get into where your creation is taking you...

The smaller sprocket with external teeth and driven by the engine could probably be COTS, but the larger sprocket would be interesting indeed. COTS sprockets have teeth on the OD. You would need teeth on the ID of the drum, a belt with teeth on both outside and inside of belt, and it would have to fit the drum. The only way cog-belts are inexpensive is when sprockets and belts are current part numbers... Teeth on inside of drum are needed to drive the belt as are double sided cogs on the belt.

Now, how are we going to keep the double sided cog belt on the ID of the drum? With the small sprocket pulling on one tooth at a time, it is also trying to stretch the belt on side and compress it on the other. The compression side will be trying to press the belt against the drum, but the tension side will be trying to pull the belt away from the drum. As long as there more centrifugal force pushing the belt out against the drum than there is forces from tension trying to lift the belt to the inside, it will stay put. When centrifugal is over come by effects of tension, it will be lifting of then be slapped back down as it approaches the small sprocket engagement. Not a good thing for long life.

Then comes the already mentioned issue of making the belt and smaller sprocket live with only one or maybe two teeth engaging at a time. This is gear territory for contact stress. Let's calibrate you here. 180 hp at 2700 rpm is mean torque of 180*5252/2700 = 350 ft-lb = 4202 in-lb. If the smaller sprocker is on the engine side and has a 2" pitch radius, that means the linear force on a gear tooth is 2102 pounds. If your sprocket was 3" long, that is 700 pounds/inch of tooth. With maybe a 0.1" patch carrying the load, that is 7000 psi average. Not impossible yet...

Let's remember that these cog belt drives out there are pretty much assumed to be "stiff" systems for a transmissibility perspective. That means that the belt does not see mean torque, it sees all of the torque variation through the firing cycle. Peak torque in such a system is 4 to 8 times mean torque. Let's say 4 times - stress at teeth is 28000 psi. Sorry, molded rubber won't stand that. The sprockets would also have to be made out of something pretty sturdy too. Your everyday rag and phenolic sprockets won't stand that, nor with 356 aluminum alloy. Maybe 7075-T6 or 4130 steel, but then you are into custom parts.

Once you get into custom sprockets, well, why don't we just talk gears running in oil and get a lot more compact?

Billski
 

wsimpso1

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That is a useful observation, perhpas qualified with "depends on ratio and diameters involved"?

IOW system where the pinion is a substantial part of the volume of the drum (have not checked to see where things begin to interfere) but say 3:4 or 7:10. Drum size "largish" - needs to be somewhere over 8" or 9" given the belt diameters available for, say 75 HP. At that point, then, the issue becomes derating (overbuilding) & whether the weight advantage continues to apply?

I'm wondering about thermodynamic issues.
Presumably the drum could be alloy, finned &/or ventilated at some energy cost.

smt

Gear ratios needed are in the area of 6000/2700 = 2.22 (car engines to conventional prop speeds) up to 9000/2200 = 4.1 (snowmobile/water sled to more modest prop speeds), so the diameters would be substantially different.

Thermodynamics is "the study of the relations between heat, work, temperature, and energy". What are the thermodynamic issues?

To this sort of power transfer, the belt and sprocket sets are usually at least 98% or more efficient, so 1-2% of engine power will be lost. This is heat and mass transfer - not thermodynamics. That most cog belt drives are external tooth sprockets and operating in free air, they usually require no extra cooling. With your internal drive intent, yeah, the drum might need some radial fins cast in, like on the flywheel of lawn mower engines, and air routed to it and away from it. I suspect that long before you get to overheat issues, you will be dealing tooth stresses in the sprockets and cog belt. Maybe you can make the sprocket sizes large enough to allow belt and sprockets to live, and by then the natural cooling will be adequate.

Billski
 

DanH

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I modeled their scheme, proposed solutions, we implemented one, and it worked on both eliminating broken parts and obtaining a smooth idle. Not only did the modeling point the way to the solution, it also allowed us to clear the operating range of other vibe modes, and resulted in over 200 hours of successful test runs. Then we rented a Rotec (I am fluent in their use and have used them at three different companies now) and cleared the operating range with all cylinders and with cylinders not firing...

Have you related details about this project previously? Sure sounds like a great educational opportunity. Folks often grasp more from application stories than dry theory. People love stories.

So yes, I know something about the topic of modeling and measuring vibration.

Yes, obviously. I was referring to working with simple models...perhaps imperfect, but reasonable approximations. Back in post #20 of this very thread you wrote...

In the simplest model, we have one inertia at the engine (crankshaft, cam drive and shaft, pistons/connecting rods, flywheel, and engine side sprocket) and one at the prop (prop, prop shaft, prop side sprocket) and then spring rates of the stuff in between. This two-mass system is directly solvable and can be a pretty accurate description of a properly built belt PSRU.

....
but later seem to insist on complex models using complex tools. If the resident pro convinces the audience only the complex tools will work, we have a self-realizing prophecy...they ain't gonna try.
 

Aviacs

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Bill -
My first airplane, the one i learned to fly in, was a Cessna 175.
So i immersed myself in the lore of geared engines.
Nothing else quite compares to that almost "flying behind an electric motor" sensation . Which was my mechanic's surprised reaction when he told me he had taken it for a spin out of curiosity after an annual one time. Then there's the performance on 175HP with an 84" prop compared even to 180HP 172's.

So^^^^that's the excuse for my geared fixation.
It happened while i was being birthed as a pilot.

Go back further and my first go-cart had an internal ring gear final drive.
I just was mercilessly exposed to all the wrong influences in my formative period. :)

Interest is limited to about 75 HP. 100HP max design consideration.
The musings are philosophical aggravation with all the HP & performance a VW derived engine leaves behind with direct drive, 3,600 rpm, and tiny short props. If the engine could be run at 3500 to 4,000 cruise and prop rpm kept around 2700 - 3000, a usefully longer prop could be used. Maybe it will never be practical to gear one. But i can't help thinking about it.

So, the idea considered "what if" a suitable width belt was bonded (fixed/adhered/riveted/vulcanized &/or?? to be determine once the overall idea was vetted) to the inside of a properly sized drum. Then use as large a metal pinion as fits without interference. Both to approach or attain the 3:4 ratio, and to reduce the individual tooth stress. Belt and cog to be off the shelf & "common" or the idea does not make sense economically. As also posted the thought exploration of "can it be lighter" than a steel gear drive and housing is also part of a viable equation.

A few others have pointed out the errors in my thinking, "cog belts are just not made to work with minimal tooth engagement"

smt
 

rv7charlie

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The musings are philosophical aggravation with all the HP & performance a VW derived engine leaves behind with direct drive, 3,600 rpm, and tiny short props. If the engine could be run at 3500 to 4,000 cruise and prop rpm kept around 2700 - 3000, a usefully longer prop could be used. Maybe it will never be practical to gear one.
It's been done, successfully. Valley Engineering flew one every year at OSH, back in the mid 1990s. You can't buy them anymore, since the patriarch of the family passed away, but some info is is still on the VE website.
If you follow that link, and then click on the 'photos' button, there are quite a few pics of their drive mounted on a VW. Here's a pic of the plane it was installed on:

1656276221084.png
That prop is big. They flew it of the ultralite strip, and were off the ground and outclimbing everything except the helicopters; even the Kolbs, which are really impressive at getting off and up.
They later converted the plane to biplane configuration, apparently because of the long wingspan on a low wing plane.

edit: Also, search the Matronics Kolb List archives for posts from Rick Neilsen. He's flown a redrive-VW on a Kolb MkIII for probably a decade.

Charlie
 
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wsimpso1

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Have you related details about this project previously?
I have talked about it a few times. I do not remember which of my posts went into detail. Maybe I will search it up, fill in ALL of the details, tell the long story, and bore you folks to tears... Maybe not.


I have some 9000 posts, and a fair fraction are about torsional vibration, resonance, etc. Fact is I have solved an airplane propulsion vibration problem. It was an interesting 12 weeks and we were lucky that their problem was posed in such a way that a simple solution would work. Trying to insert a manual tranny clutch hub or dual mass flywheel in the system would have been the next step, would have had its problems, etc, but it was never needed. That the whole project was stillborn after the tooling vendor's warehouse burned was a major disappointment. I really wanted to see it fly. Got to know Ed Gillispie (famous test pilot) on the program. "He has a thousand stories and they are all true" Captain Jim Mynning, telling me to listen when the man talks. Some of the folks I have had the good fortune to know.


This basic topic was covered in my undergraduate program in ME340, and is a required topic for a program in Mechanical Engineering to be accredited and issue degrees. The problem is that it is one of those classes that came at the students like trying to drink from a firehose during their sophomore or junior year, gets some crucial equations memorized to pass the tests, and then gets dismissed by the students as a "where in the $%^& am I EVER going to use this?" Later on somebody like me talks about the topic in an engineering seminar on torque converter design and function, and gets a room full of blank looks and two guys that go "that is what this is about?!"

Billski
 

Aviacs

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It's been done, successfully.

Also, Steve Bennett was working on an inline geared version when he passed away.

If it can be close to inline, it would fit in my Sonerai 2. :)

A big, out in the breeze unit would work well for many apps.
It would kill enough of the streamlining on a Sonerai to make it a moot exercise.
An internal cog/ring gear would allow just enough room to put the engine and the propshaft close enough to where there would be minimal penalty for thrustline, or streamlining.
No sure of the weight for Marcotte, and AFAIK no VW installations?

smt
 

BBerson

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Interest is limited to about 75 HP. 100HP max design consideration.
The musings are philosophical aggravation with all the HP & performance a VW derived engine leaves behind with direct drive, 3,600 rpm, and tiny short props. If the engine could be run at 3500 to 4,000 cruise and prop rpm kept around 2700 - 3000, a usefully longer prop could be used. Maybe it will never be practical to gear one. But i can't help thinking about it.
The Limbach (VW) in motorgliders runs at 2700 -3500 rpm with a 63” prop. The Grob G109 is 1820 pounds gross weight.
 

rv7charlie

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The Limbach (VW) in motorgliders runs at 2700 -3500 rpm with a 63” prop. The Grob G109 is 1820 pounds gross weight.
Ground clearance for the prop?
Ground roll on takeoff?
Climb rate?

No doubt it makes good sense on a motorglider. Minimum prop drag with engine off, gear legs can be shorter, and climb performance isn't typically critical. Works OK on stuff like the Sonex models, too, for some of the same reasons. But for a Cub-like a/c, or a WW1 replica, or other stuff with a lot of drag & low cruise speeds, the inevitable compromises need to be slanted toward more effective low speed mass flow, which the large diameter slower turning prop buys.

One *potential* advantage of a reduction drive isn't talked about much; by running at a somewhat higher rpm, the engine can be operated at the same power level but at a much lower BMEP in the cylinders. Has the potential to allow the engine to last longer, even though it's spinning faster. Another big advantage (if the drive is properly designed) is taking the bending (gyro) loads off the crank. VWs *usually* get away with direct drive at lower power levels, but higher power & heavier props mean broken cranks unless you do something like this. The reduction drive solves that problem, too.
 

BBerson

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Ground clearance for the prop?
Ground roll on takeoff?
Climb rate?
I never noticed the ground clearance, it is a taildragger.
Ground roll solo is 700 feet.
Climb is 600fpm then 450fpm after 2000 feet. (old engine)

I have the typical motorglider three position prop, which is great for increased performance. But just using low pitch would provide Cub speeds and good takeoff.
 

Bille Floyd

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How do those sailplanes , with the doors that open behind
the pilot, and the pylon goes up with a prop one end, and
an engine on the other ; how do they get away with such
a long belt ? I see that a lot ; do they survive many hours ?

Bille
 
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