Help: re-gearing transfer of power via pulley + belt / double sheave vs single sheave

Hey Friends. I am working on a new build this winter. Modifying a 48" flail mower to have a fairly kickass offset. But before I begin I want to sort out one issue. The mower was built to be operated by a tractor that is capable of a 800rpm pto speed. The tractor I'll be using it with uses the standard 540. Sooo I need to change the pulley sizes. (Im not keen to replace the gear box)

Here's a pic of the pulley drive: https://drive.google.com/file/d/1N9N...ew?usp=sharing

The mower is 48" and is going to be run by a 21 hp tractor so that gives you an idea of power load - so not huge, but not small either. The system uses a B size belt. Right now the two pulley sizes are 7.75" and 5.5". In order to get the right speed it would seem 9.75" and 4.75" would work. 9.75" is about as big as I can fit under the coverbox without re-making the box.

The problem as I see it is that (based on my google searches) 5.5" sheave is the minimum recommended size pulley recommended for for b size belts. I have experience developing power transmission systems with belting before by trial and error on a from scratch 11hp airblast sprayer built (I intend to share this with you all on the site soon) It was really frustrating having to replace belt after belt and pulley after pulley - It can get expensive. In short I have respect for underbuilt power transmission systems. I want to built it right the first time.

My question is: Do double pulley systems reduce heat build up in the v belt drive system significantly? Would it reduce heat build up enough to run an "undersized pulley" OR is the minimum size pulley rule hard and fast enough that it will still heat up even thought the load is divided between two pulleys? After all 4.75" is not that much smaller than 5.5"

A second question is: Can I solve this with one of those cogged belts like a BX. I hear they can tolerate smaller diameter pulleys somehow. With a cogged pulley could I get away without even upgrading to double sheave pulleys? That would clearly be the cheapest and very much easiest option.

Thanks for your input!

Jed

If you go with a twin groove pulley drive, be sure to specify "matched and tagged" v belts. As for me, I would redesign it for a timing belt drive. Good luck on your build. Flail mowers are neat.

Owen, Thank you for the reply! Sounds like great advice regarding the belts.

I'm really curious about why you you would do a timing belt drive. Sounds interesting! I've never seen it on a farm impliment tbh. And what exactly do you mean by a timing belt drive? Like the same style of belt that is on a timing belt system on an engine?

Thx

Sorry that I didn't post to the forum, but what I said was there are timing belt (positive drives) available today that can replace some chain and vee-belt drives. Even some of the motorcycle brands are using them on their primaty and secondary (high torque) drives, hence replacing the chain drive. Some of these belts are reinforced with Kevlar cords which make them extremely strong. In some cases, you may be able to use a smaller diameter pulley to achieve a more desired ratio. They are a positive drive as long as you stay within the design parameters. They run quiet and smooth as well, and require zero lubrication. The one diadvantage is they are not self cleaning around dirt and mud, like a chain and sproket would be.Thanks for letting me opine. Others may disagree, but I'm talking from 45 years of industrial experience. Good luck on your project.

You might ask at your industrial power transmission dealer such as Motion industries Bearing and Chain & supply or others about going with a Poly V belt drive for automotive use they are commonly called serpentine. Poly V drive systems can come in a wide arrangement of rib counts from 4 ribs up to 20 ribs and "V" grove widths from 2.34mm for the J series to 4.7mm for the L series to 9.4mm grove widths for the M series and up to 800 hp transmission of power
Here is a catalog showing the ratings pulley sizes and center distances available from 1 supplier
http://www.industrialbearings.com.au...1327992533.pdf
The worst thing about a synchronous belt IE cog or toothed drive is there is no possibility for slippage they are a positive drive system just like roller chain if you were to use that as a drive system, I would highly recommend a torque limiting friction drive as well

Double sheave pulleys would be the best option, matched notched belts would be preferable.
However, if you don't mind replacing the belt more often, single pulleys have sufficed on the existing
setup.
Timing belts are not suitable for the application and if they were, you'd be horrified at the expense
of the components!

Remember KISS!