OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Removing drive roller from SB 590

Hi grumpy, Pauly80 and Kempe, well at long last it seems we have got an great archive for removing the rear roller and ring gear of the SB 430 and 590 Diplomat.
I think Pauly's old man needs a mention also, as he helped immensely with the pulley removal.
It would be great if we can have some detailed pics of the new replacement steel ring gear....grumpy is right in his assumption, it is purpose built to fix the original prob with the cheap nylon gear...I honestly don't know what Scott Bonnar were thinking when they designed that piece of #*%$ .....oops! I guess I'm not being very "Diplomatic"

I think they were caught out, Deejay. Calculation would tell them that the ring gear didn't have to transmit much torque, and nylon gears are very quiet as well as not needing lubrication. The problem is, sooner or later someone will find a way to put a much higher load on that gear than anyone expected - either trying to climb a kerb, or during a service operation. If you have a weak gear, the normal approach is to ensure the drivetrain can't transmit enough torque to harm it, and I'm not sure they did that. Nevertheless I'm a bit sympathetic to Scott Bonnar, in this situation. However I do think they should have published a manual telling people how to dismantle that mechanism, and made the manual widely available. The combination of not enough information, and a fairly vulnerable gear, was bound to end badly in a lot of cases.

Here's the metal ring gear for archive purposes.
Fitted it today and everything works great and cut the lawns already.
Great to have the old SB back again as it nearly went to landfill and replaced with a cheaper rotary which would have been a shame for a Santa Anna Lawn.

Interesting design. They made the hub, web and gear a single item so it is easy to fit and service. However they chose to fabricate it from sheet, except for the hub. The hub is welded to the web, but the ring gear itself is fabricated from multiple layers of sheet, pop riveted together. They may have pop riveted the sheets after stamping them into circles of the right diameter, then cut the gear teeth, then bolted the fabricated ring gear to the web. However I notice that the gear teeth are triangular, not involute, in shape so I doubt they were cut: more likely they were stamped with the teeth included. This enabled them to use cheap stampings for the gear. The downside is that the stampings will be mild steel, so the gear is fairly sure to be dead soft. Also, the triangular teeth of the ring gear will not mesh properly with the involute teeth of the mating pinion, and because the pinion has so few teeth, it may lose the argument. On the positive side, the ring gear itself will still be a whole lot stronger and tougher than the nylon was. The hub should be fine, since it is now welded to the steel web.

With stamped teeth, this gear should have been quite cheap to make. Someone is probably making a huge profit on them.

Hi, I've just joined this site and have found this particular topic o be of great use as I've inherited a 590 and it has lost drive when engaging the clutch. I'm not mechanically minded or skilled so I've put the machine into a shop to try and get a quote to get it fixed.
Can anyone tell me how much the metal ring gear costs and where it is available from?
I live in country WA

I paid $132 plus delivery from Turf Machinery Services

Gezza, depending on whether you have examined the belt drive and clutches on your mower, you may have a much simpler and less expensive problem than kempe had. Most times, loss of drive is just a belt adjustment or perhaps clutch relining issue.

Thanks. I'll check with the repair shop and advise.

I have been to the shop and inspected the part and it is the same problem. The plastic has cracked in two places and the replacement has now been ordered.Thanks for this forum and the experts available to help.

Thanks for letting us know, Gezza - it gives us another useful case for the archive. I'll close this thread.