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

#### Hueyman

##### Plastic
Hello gentlemen !

I have a project in a small RC plane. For room and power reasons, I cannot use an existing solution to turn the propeller that needs to be " scale " looking, and flying, meaning a large diameter ( 73cm, 29" ), high pitch prop. Skipping details, I would need great torque at lower RPM than what is common.

Output would need to be in the ball park of 4500 RPM ( prop ) that would require according to my calculation a torque of 16HP/12 000 Watts/25.5 NM.
On the input, I have a choice of various two strokes and four strokes reciprocating engines. Usually they produce their optimum power between 6000 and 10 000 RPMs.

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.

Here is some 3D I did.

Cheers,
Hueyman

Your pulley size pictures seem weird given the reduction.
Also maybe look at belt limiting surface speed for various types of teeth.
Also when you get up in the 3:1 think about engaged teeth on the driver pulley.
This arc of contact is changed by the spacing of the pulleys.
Stock Drive Products has lots of basic design tools and advice for this.

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Belt size is calculated by how much power can be transferred into the rubber on the small pulley. Check some toothed / timing belt catalogs to determine the size of the belt needed.

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.

<snippitty>

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.

<snip snip>
16hp does sound a lot, though it is a much larger prop than I first read. I unsucessfully tried toothed belts on a 25hp tractor implement and the belts needed were much larger than I thought.

I would not consider anything with a 29” dia. prop to be a “small” RC plane

Ultra Lights with Rotax motors are toothed belt drive ...as are Harley Davidsons .......16 hp would be no problem ,bike drives transmit 100.

Ultra Lights with Rotax motors are toothed belt drive ...as are Harley Davidsons .......16 hp would be no problem ,bike drives transmit 100.
not gunna fly with a harley attached

Look at RC helicopter drives, maybe get some ideas there.

Off hand a two to one ratio will take 10,000 rpm to 5,000. So you may need slightly more then 2/1 ratio of pulley diameters.
FYI. Engine camshaft drives use exactly a 2:1 ratio. That may be within camshaft loads for a big v8 engine?
BilL D

A three blade, 29" diameter prop spinning at 6000 RPM.

Just seat of the pants engineering, but that sounds like an awful fast rotation speed to me. Check my math please, but for the linear speed of the tips of the three prop blades I get 826 km/h. The speed of sound is around 1235 km/h so the tips of your prop will be going about 2/3 the speed of sound.

I have run model airplane engines with 4" props at 10,000 RPM or close to it. But scaling that up to a 29" prop means the speed would need to be reduced to 1400 RPM or so.

Perhaps that speed is correct, but will your prop be able to withstand the forces that will generate, without flying apart? Will you be using a commercial prop that is rated for that speed or one of your own manufacture?

Would not be able to run a two stroke upside-down to get output shaft closer to the height of the prop-shaft and use a straight planetary gearbox instead?

Hi guys,

Some replies in no particular order :

The thread subject is more regarding how belt drive works/are calculated to find the right ones, and especially having the right formulas to play around with, like how much torque/hp would I get output with X input etc... I have no clues on this

Maybe direct drive with toothed gears like on electric helis is simpler, with ratios usually around 10 : 1.

No 6000 RPM is target, I said 4500.

The 3D drawing is more like a sketch of principle, don't take into account sizes, proportions etc, just to show current idea.

Best tip speed should be around 0.9 Mach, to leave room for advancing prop before Mach 1, but once again that's not the subject, the prop will be manufactured accordingly by specialist.

Such gearbox exist but more for electric motor which you can play more flexibly with various Kvs. Also an example of belt drive for similar four stroke engine, no details in it though.

Would you prefer slant gear direct drive for Space and transmitted torque reasons over toothed belt ?

How can I find out how much output torque/HP with a given input, with various gear ratios ?

Is 80/90% a good efficiency factor for typical reduction systems for calculations ?

Planetary gearbox would be the Rolls but extremely expensive, and space is the most limiting factor here, that Focke Wulf 190 cowling is not that big, and glow/nitro/gasser engines do take lot of room.

Cheers,
Hueyman

Perhaps a V twin might be a better option if you're out of space? 16HP twin could be found in lesser motorcycles or generators, and probably for cheaper, with better accessories and spares than a dedicated RC engine?

Anyway, yes 80-90% is a good baseline for reduction efficiency in most cases and the types of reductions you're looking at here.

For 16 HP you're looking at something in the size of 50mm width, but as other have said it depends on radiuses of pulleys and so on. If you want a crash-course in belt transmission design almost every manufacturer/supplier have some kind of pdf on their websites.

FW190 would be a good candidate for an outrunner considering the original had a radial 801.

Hi guys,

Overland, thanks for the catalog, interesting read, gonna investigate.

Ok ViktorS, I'll search that way too.

Regarding math, what is the formula to find out output power, after geared ?

Like If I have 9 Hp or X torque NM at 8000 RPM, what do I get after geared down to 4500 RPM ( 1,8 : 1 ) ?

Thanks

Look for belt manufacturers' guides. They give horsepower and torque ratings, etc. Search string should be the manufacturer name, type of belt and "PDF" at the end.

Down load or get a Gates Polychain belt catalog. They will transmit a lot more power at a smaller belt than the regular timming belts. Their catalog has all the engineering info you will need.

A common size for classic bike primary drives is 35mm .....reliably transmit 40hp.....pulley alignment must be accurate ,or belts will fail.........consequently the wide (up to 70mm) belts used in light planes must be run on rigidly mounted parallel shafts .

I agree 4500RPM sounds fast but maybe it's OK.

You are basically looking at a 2:1 ratio which is not too bad. But your diagram shows the engine at 6000-10,000RPM and the prop 'must spin at 4500RPM'....so let's assume you don't expect the prop to be at 4500RPM when the engine is at 10,000RPM and also at 6,000RPM.

I don't think it's a tough application. My finish mower spins at 540RPM and can bog down my 30+HP tractor engine - all through a 5/8" V-Belt. 16HP is not a lot for a belt to transfer.,,,especially at those higher speeds. Put a plain old V Belt on it and get on with life. The 10,000RPM might be an issue but you'll learn soon enough if it is or isn't.

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