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

[jedidiahwiebe]

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