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Belt Drive theory and basic knowledge ... for a practical example

I have no clue how belt drive systems is calculated, in terms of teeth, diameters, belt length etc ...
So if some kind minds would be willing to explain me in a detailed enough way for this particular project to succeed, without going too deep, that would be awesome.
What don't you understand about belt drive systems? The diameter of one pulley over the other is going to give you the reduction are. I would think belt manufacturers are going to spec ratings for speed and load. Did you have more specific questions?

I would look into ultralight aircraft solutions. I knew a guy years a go that was building a Kitfox if I remember correctly in his shed and it had a small two stroke with a gear reduction drive that is was probably not too far from what you need. I bet there are some reduction drives that are close to what you are wanting.

I know it's not the answer to your question but having RC aircraft experience and having been a UAV pilot I find it hard to imagine there isn't an existing engine that will fit your needs. A DA-170 will swing a 29" 3-blade prop. https://www.desertaircraft.com/collections/da-engines/products/da-170
 
I agree 4500RPM sounds fast but maybe it's OK.

It sure is fast but acceptable with the correct prop. Guys that run the DA-170 engine I referenced spin 30" props in excess of 6,000rpm with the aircraft not moving which is a tip speed over 500mph. RPM increases a lot as the engine uploads in the air at speed too. A lot of things from model airplanes to Cessnas to helicopter rotors have tip speeds that approach transonic speeds.
 
By the way, that setup was made about 20 years ago. Not sure if the outfit is still around. Notice, however, they used a banded belt instead of a cogged belt. It gave them a lot of flexibility with ratios
 
I'm going to leave a drive-by post here, without a ton of thought:

16 HP sounds like a lot of work to transmit through most small belt drives like for robots. Using a motorcycle for an example, the belt drive on an old Kawasaki KZ305 might only carry 15-18 HP and its pulleys are wayyyy too big to fit into the volume you have. The smaller the pulley pair, the higher the tensile load on the belt. So the belt situation will only get worse as you go smaller.

Toothed (timing) belts also lose life span as the bend radius gets smaller and smaller. Your design would be both high load and extreme bending around the drive pulley.

Are you sure this hasn't been done already as a gear drive or something? I thought the model aircraft world had done everything.
The belt drive Harleys push a lot more HP and torque so yes, an appropriately sized belt can work.
 
Do you have a Machinery's Handbook?

I have the 29th edition and belts and pulleys start on page 2484. Synchronous (timing positive) belts, which is what I think what you want, starts on page 2528.

I think I remember something about using an odd number of teeth on either the drive or driven side so that irregularities on the belt or pulleys will wear evenly across the pulleys and belt.
 
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Back a ways ,a guy bought a plug programmed lathe off me to make parts for his production light aircraft ......some months later his father asked me to buy it back,as the money was needed for his funeral...........he'd crashed in the prototype plane .
 
Do you have a Machinery's Handbook?

I have the 29th edition and belts and pulleys start on page 2484. Synchronous (timing positive) belts, which is what I think what you want, starts on page 2528.

I think I remember something about using an odd number of teeth on either the drive or driven side so that irregularities on the belt or pulleys will wear evenly across the pulleys and belt.
The same thing applies to automotive gearing, differentials & transmissions. Called a hunting tooth design. That is why the ratios in diff. gears and transmissions are never an even ratio.
 
Hello guys,

Many thanks for all your inputs.

Gonna read that docs, thanks for the scans of the book.

Well, it's not much about the discussion of the existence of an already made system, there is not such thing. Spinning a 30" prop is not the issue. The issue is I want a 35" pitch ( extremely coarse ) for reasons I do not wish to justify here as it's not the topic. But the torque required will be consequent.

Then there is like everything in aero world a WEIGHT and VOLUME constraint. Volume as cowling space is limited, even more with the need of a reduction system.

Moki are far from being powerful enough to spin that prop, did all calculations already.

Without going further into belt details and spec, first question I still for unanswered yet is ... :

How can I find out how much output torque/HP will be obtained with a given known input ( combination of RPM/HP/TORQUE ), with various reduction ratios ?

Like, if an engine delivers 6.6 HP at 8400 RPM ( 5.6 NM torque ), and I need a 25.5 NM torque output, at whatever speed.

I really need to start from here to narrow it down and eventually find an appropriate engine and reduction system.

Thank you
 
Its far simpler to copy something successful ,than to design your own system ,which may need many revisions to work satisfactorily ......and cost lots of dollars every time you change components .............Any reduction drive will cost up to 10% of your available power ............the higher the reduction ,greater the power loss ..................available torque is multiplied by the reduction,but again ,with a loss of around 10% .
 
Hello guys,

Many thanks for all your inputs.

Gonna read that docs, thanks for the scans of the book.

Well, it's not much about the discussion of the existence of an already made system, there is not such thing. Spinning a 30" prop is not the issue. The issue is I want a 35" pitch ( extremely coarse ) for reasons I do not wish to justify here as it's not the topic. But the torque required will be consequent.

Then there is like everything in aero world a WEIGHT and VOLUME constraint. Volume as cowling space is limited, even more with the need of a reduction system.

Moki are far from being powerful enough to spin that prop, did all calculations already.

Without going further into belt details and spec, first question I still for unanswered yet is ... :

How can I find out how much output torque/HP will be obtained with a given known input ( combination of RPM/HP/TORQUE ), with various reduction ratios ?

Like, if an engine delivers 6.6 HP at 8400 RPM ( 5.6 NM torque ), and I need a 25.5 NM torque output, at whatever speed.

I really need to start from here to narrow it down and eventually find an appropriate engine and reduction system.

Thank you
crankshaft dyno the engine - that will give you real torque and horsepower figures. Add your transmission system and dyno the prop shaft output. One divided by the other gives you the efficiency of the drive. Engineer from those figures and additional tests.
 
Everything needs a baseline assumption to start. If you just use 90% efficiency for a single reduction, like several previous replies stated, you will be close enough. Your project will not fail because the actual efficiency was 85%, or 93%. If you really want to know more precisely you are going to have to build and test.
 
Hello guys,

Many thanks for all your inputs.

Gonna read that docs, thanks for the scans of the book.

Well, it's not much about the discussion of the existence of an already made system, there is not such thing. Spinning a 30" prop is not the issue. The issue is I want a 35" pitch ( extremely coarse ) for reasons I do not wish to justify here as it's not the topic. But the torque required will be consequent.

Then there is like everything in aero world a WEIGHT and VOLUME constraint. Volume as cowling space is limited, even more with the need of a reduction system.

Moki are far from being powerful enough to spin that prop, did all calculations already.

Without going further into belt details and spec, first question I still for unanswered yet is ... :

How can I find out how much output torque/HP will be obtained with a given known input ( combination of RPM/HP/TORQUE ), with various reduction ratios ?

Like, if an engine delivers 6.6 HP at 8400 RPM ( 5.6 NM torque ), and I need a 25.5 NM torque output, at whatever speed.

I really need to start from here to narrow it down and eventually find an appropriate engine and reduction system.

Thank you
On paper it is simple math ,approximately a 4 to 1 reduction @ 2100rpm. On paper The HP will be the same. Actual output will make the paper calc's less because of transmission losses. Remember HP is a work calculation , it is not measured. Torque and rpm are measured and then HP is calculated. So your statement "at whatever speed" doesn't make sense, the speed will be exactly what the reduction ratio is(that doesn't change). But to have that exact out put rpm you would have to increase the input torque to cover the transmission losses which will increase HP required.
You can work the problem two ways ,You only have so much HP available then your out put is predetermined,or you need so much HP then your input has to be variable. Make sense?
So in your example you would have to go to 5 to one or more to cover the losses but your output rpm decreases.




As I said before look into Gates Polychain belts. They will answer most of your questions about HP per pound and efficiency . The power transfer of Polychains is among the highest of all belt drives.
 
Nah, it's a focke-wulf, needs a focke-wulf engine :)

View attachment 437799

If'n I lived in the countryside with a little shop behind the house all paid for, you can bet yer bippy we know what I'd be making ....
Those are beautiful engines. However, I never understood why they went with exposed rocker arms. I think it would have looked even better with dual valve covers on each cylinder like the Saitos. Way easier to maintain too
 
As for the HP problem, find an engine with more power than you think you need, then design your reduction to hit the rpm you need at the rpm range for the engines peak HP output. And as mentioned before, there will be losses. Friction, heat, vibration, all drains on your system. A well designed system will help reduce these losses. Belt manufacturers will have data on expected losses and horsepower ratings for different types and sizes of belts. Martin is another good resource
 








 
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