must have diameter that is a bit larger at the middle of the pulley.
Oiler, we will take that sweeping statement as a small difference in language.
Our 1952 baler (it was not the earliest model that had a similar drive system), a Massey Harris 701, used an endless flat belt. Probably 4 inches wide. For maximum grip, minimum slip, it ran on a flat pulley. A flat pulley needs a lip either side or some other tracking aid. A flat pulley can run with more tension than a crowned pulley without mis-shaping the belt. Modern automotive toothed cam belts run in a guided tooth pulley. Note I say a, as the other members will allow alignment-float for the belt.
Our baler used an engine of about a 20HP at 2000rpm. That belt might screech a little if the engine was stalled out quickly and also a bit when the main machine was put into operation ('wadder and ram mechanism'). Otherwise they were never any trouble. A loose belt would not have coped with that operation.
1958steveflying,
Crowned pulleys go back to the steam era and probably before.
I believe there are better solutions in the 21st century, than relatively loose belts flapping around.
Loose because the belt length was made to approximate measure (and could be repaired many a time before it was of no further use for the user) but the belt clips were a weak link. Loose because of the variations along the belt length and the necessary slippage which always occurred.
However, for a flat belt you will probably find you need the more contact area than the micro V belts which use the V wedges as extra friction. You may need bigger bearings as well, to withstand the extra radial loading (the centre point will invariably be further outboard than a narrower belt). You will almost certainly need a belt-tensioning device.
BTW most tractor belts using flat belts and crowned pulleys operated at around 2600 feet per minute. Probably a figure handed down from steam traction engines.
The questions you might ask yourself are why did V belts virtually make flat belts redundant, and why a lot of drives are now micro-V. Answering these q's will explain why you are spending a lot of time on a less cost efficient drive.
Is your flat belt fastened together with crocodile clips? If so, give it a rest and send it to retirement. If an endless belt, get a suitable pulley and tensioner and it will work.
Regards, RAB