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Increasing gear ratio by replacing pulley?


kalle11
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Hi!

I am looking for  a unit around 30hp with an as big belt reduction as possible and found that the black bull corsair fits the bell in weight, cost and power.  However the reduction ratio of 1:3 is a little less than I would really want. I am in need of a big reduction ratio to turn the a slightly bigger propeller slower.  Realizing  that designing, building installing a whole new belt reduction unit to a para motor unit is no small feat I thought of another possibility:

Can I just replace the engine pully with a smaller one to get a 1:4 reduction ratio?  

 

Obviously it maybe depends on the bearing size of the original pully and other things..hence my question if it is feasable?

Any home-/real engineers or DIY´ers here?:)

If I am correct, the big pulley is 15cm and the smaller one is 5cm which means that I would need a 3.75 cm to get  a 1:4 reduction

Or perhaps there are some  paramotor units with a  high reduction ratios around 30hp of 1:4 or more?

Thanks for taking your time to reading my post

Best Regards

/Kalle

 

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Hi, the smaller the small pulley is, the higher the belt tension needs to be to prevent slipping. Assume the belt contacts the small pulley for 1/3 of its circumference. For 5 cm pulley this contact length is 65 mm and for a 3.75 cm pulley is 37 mm. ie only 56% Thus you would need nearly double the belt tension to prevent slipping.....and the belt would not last very long. It is even worse than this if you keep the pulley centre distance the same as the smaller pulley reduces the angle the belt contacts the pulley even further.

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1 hour ago, AndyB said:

Hi, the smaller the small pulley is, the higher the belt tension needs to be to prevent slipping. Assume the belt contacts the small pulley for 1/3 of its circumference. For 5 cm pulley this contact length is 65 mm and for a 3.75 cm pulley is 37 mm. ie only 56% Thus you would need nearly double the belt tension to prevent slipping.....and the belt would not last very long. It is even worse than this if you keep the pulley centre distance the same as the smaller pulley reduces the angle the belt contacts the pulley even further.

Thank you! So even this small detail about contact area ruins the idea I guess?

  By "not last very long" you mean lasting a really short time like a couple of hours ?

Oh well, is there anywhere online where one could purchase a custom made reduction drive of 1:4 in this case, or do one know a higher reduction ratio of some paramotor unit other than this one? It seems near impossible to find any reduction drive suppliers anywhere.. custom made or not. 

Thanks

Regards

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You may want to research belt selection as well as they can jump past or below your adjustment range. J section poly V or micro V belts. Any supplier of PT components will ask for centre distance, Min Max adjustment, and pulley diameters. You may find that anything standard off the shelf, will require machining. Personaly I would take both to a good machine shop if you can not buy standard and manufacture from scratch with the correct material for the application. Also you need to look at what material is left if you reduce the small pulley diameter. It may not be physically possible to go smaller due to clearences, shaft diameter, nut washer etc, boss projections yada yada.

You may also need to change both pulleys custom made to suit belt centres and ratio if a belt size is non existant for adjustment when changing one or both pulleys.

It's worth mentioning that drive design and ratio, if done correct are measured at the PCD of the belt. In the case of micro v belts, it's a mil or so bigger than the OD of the pulleys. It's at the point you see the chord or PCD of the belt. All other V type drives calculate the PCD of the belt and drive design below the pulley OD.

Andy's bang on, the same applies if you only change the large pulley ( not your issue ) and the only way to compensate is to extend your centres. 

Edited by Kran
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  • 1 year later...

You could fit prop directly to engine , but it would be the size they use on a model airplane , because you could be running 8000+ rpm ,so they fit different size pulleys ,  or gears to slow the prop speed down , then you can run bigger prop , 

this is a very very basic explanation,  

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1 hour ago, cas_whitmore said:

You could fit prop directly to engine , but it would be the size they use on a model airplane , because you could be running 8000+ rpm ,so they fit different size pulleys ,  or gears to slow the prop speed down , then you can run bigger prop , 

this is a very very basic explanation,  

Okay but now suppose if i use honda engine gx270 its rpm is already arround 3000 to 3500. 

You are right that for lowering rpm of prop we need to use bigger pully for propellers.

But i do not modify my engine for higher rpm then can we attach propeller direct to engine shaft??? 

My whatsapp is +92-333-4868797

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