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 and Kempe, well spotted grumpy, I looked at the end of the shaft yesterday and thought the same....methinks foul play!
I also agree with you on a good soaking with penetrant, according to the advice I have received, it should slide off revealing the keyway.
Kempe, could you please take some pics of the roller-half when you get the boss off, for archival purposes it will come in handy for future reference....Thanks mate. Plus the fact we would all like to see what's behind the ring gear....he he...

Have removed the boss.
I did as advised on here and soaked it overnight in RP7.
Then I screwed the end nut on (to protect the thread) and held the the axle vertically and hammered down on the concrete.
The weight of the half roller acted like a hammer and contacted the boss squarely and slowly removed it.
Will post pics later tonight of it all disassembled

Well done Kempe, you just invented a poor-man's slide hammer puller. Hope you didn't break any important concrete. I'll look forward to the pictures - as mentioned earlier, we badly need an archive on that SB roller drive.

Thanks Kempe, looking forward to seeing the pics...well done mate!

Pauly80 deserves all the credit as he is the one who posted how to remove the pulleys.
When the ring gear arrives I will lay everything out flat and post a pic of the assembly and where all the washers, circlips and spacers go.
Let me know if you want any other angles photographed, as it wont be getting assembled till next week when postie delivers the new ring gear.

Axle shaft showing the key and pawl holder



Axle shaft with pawls in place (there's another 3 pawls on opposite side to drive the other roller



Outside view of left roller with ring gear removed, those 3 holes look a bit dodgy and are badly drilled


Inside view of left roller


Inside view of right roller


Outside view of right roller


Moderator edit: Sorry folks, these pictures were all from an outside picture service, and we didn't notice that until most of them had timed out. I am localising the surviving ones now, but we've lost a lot of them.

I just grabbed the camera to take some pictures but Kempe's done an excellent job of that already. Now I got some spare time to start removing some rust wohoo!

Mine looks exactly the same. No difference I can tell between the 430 and 590 including the three dodgy drilled holes. I'm guessing they rolled out of the factory like that.

Do you guys think a silicon lubricant would be best where the 3 pawls sit? I'm guessing anything greasy could clag up. It probably doesn't need anything but I would like to make sure everything is at its absolute best before putting together.

Those three dodgy holes look as if they are cored rather than drilled - that is, they were on the pattern used to make the iron casting, so they are cast, not machined. I'd need to see a closeup of the surface finish to be sure of this.

It looks as if the four pop rivets inside each roller-half are holding the ratchet wheel that those pawls engage. That is, the finely machined ratchet wheel is on a separate part which is riveted on to the cast roller-half.

Can we see a more detailed pic of each end of the axle shaft? I can see the left hand end, where the collar slides on over the key, fairly well but can't see detail on the right hand end. I'd also like to see detail of the collar itself, so we can see how the ring gear drives it. I'm guessing there is something like a spline on the inside of the ring gear and the outside of the collar. A key would be sufficient for a metal gear, but not a plastic one, and the design was intended to use a plastic gear. If my guess is right, you may find you get a replacement collar with your steel ring gear, so a simple key can be used, instead of having to broach an internal spline in the steel gear.

I notice both roller halves have pressed in bronze bushes at each end - I take it these are a close fit on the axle shaft. They will need lubrication.

You could use a dry lubricant on both the pawls and the bushes. In the old days we used stuff called Dri Slide, which was a suspension of graphite powder or molybdenum disulphide powder in a very thin volatile hydrocarbon. You dripped the Dri Slide over the joint line, and the liquid carried the powder onto the bearing surfaces, then evaporated leaving the dry powder behind. That sounds like the right technology, but you might try asking a mower dealer's service technician - sometimes they know a special trick, sometimes they don't. If they propose using anything that leaves a film behind, I'd keep looking. You might also try asking a counter-jockey at a Repco outlet - so far I've found them quite knowledgeable, and they have a good range of fairly exotic products.

So, the little pinion on the pulley-clutch shaft meshes with the ring gear, which is splined to the collar, which is keyed to the axle shaft. The axle shaft rotates with the ring gear, and turns the double pawl-block in the center of the axle. The pawls engage the ratchet wheels riveted to the two roller halves. That gives a positive forward drive, but either roller half can over-run the drive in the forward direction only.

Your right Grumpy it is cast iron

Close up of left side axle shaft


Close up of right side axle shaft


Close up of boss with part of ring gear, there is no key way securing it just a tight fit with the criss cross pattern to grip.
One little crack with age and it will start to spin.
The boss is one inch and a quarter high

No evidence of being splined or keyed, looks like a tight fit held by the roughness of the criss cross pattern.
It's the weakness in the system as I see it.
Will look again tomorrow in daylight as it does seem strange

Kempe, that is a knurled pattern on the boss (i.e lathe-turned criss-cross pattern), and I think the nylon gear was molded over the knurling. It isn't nearly as bad as if the gear had just been pressed over the boss, but it isn't all that strong. Obviously you won't be able to re-use that hub with a steel gear. Most likely the steel gear includes the bush, all on one piece, with a keyway in its bore. It should work properly this time.

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.