This thread shows the drive mechanism of the SB590:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=22699&page=1

As I understand it the belt drives both the reel, and the clutch pulley for the rear drum. There is no clutch for the belt on the 590 (but there is on the 430).
Here is the SB430 belt drive, showing the separate belt clutch and drum clutch:
[Linked Image]

Here is a partly dismantled SB590 belt drive:
[Linked Image]

The SB590 misses out on the whole belt tensioning lever: it has a belt tension adjuster inside the belt box, but no external way to declutch the belt.

On the 590 there is no belt clutch - when the engine runs, the belt runs and therefore the reel runs. However it has the same drum clutch as the 430, so the rear drum only runs when you squeeze the clutch, and the drum speed depends on how hard you squeeze the clutch. So, there are two ways to control the speed of the mower over the ground:
1. Set the engine speed at a level that provides just sufficient power so it does not bog down and threaten to stall when mowing. Generally, moderate engine speed should be sufficient, and excessive engine speed will make it necessary to slip the drum clutch more than necessary.
2. Expect the drum clutch to be slipping to some extent, all the time. It is designed for this, and will take a long, long time to wear out. Point 1 above is included as a sort of refinement: excessive engine speed wears everything out to no purpose, but even if you run the engine really fast, you just slip the roller clutch a bit more to control the drum speed to what is comfortable for you.

Using a variable slip rate on the drum clutch to manage roller speed is a traditional Scott Bonnar approach on their reel mowers whether they had chain or belt drive. The mowers had large drum clutches to ensure the clutches could be slipped for years without any problem developing.


Last edited by grumpy; 04/10/12 04:30 PM. Reason: Add pictures