OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers That Nylon Gear Scott Bonnar 590

Hi

I'm a total newbie and owner of a 590 reel mower. I bought it recently and after playing with the motor have managed to get it running beautifully (to my ears that is). I have been through the forum and read the great post on removing the back roller. Buoyed by the post I removed the back roller and discovered a cracked nylon gear. It was easy enough to remove but the question now is where can i get a replacement. Can anyone help me locate a source for a replacement for the nylon gear?

Hi Queenslandryano, and a warm welcome to the forum.
It's really nice to welcome another Scott Bonnar owner on board, as we all like to learn new things and tinker here.

Unfortunately, I am the bearer of bad tidings....the replacement metal stamped gear that we used to recommend as a replacement, is now out of production...the business owner has now retired and his business is now defunct.
I don't know where you will source one...I only wish I did...
Would be a great opportunity for someone to tool up and re-produce them...as, what the failure does, is render the machine as a very heavy push mower.

Once again, to OutdoorKing.

Thanks for the reply and welcome DeeJay. Bad news indeed. I might try some local fabricators and my local semi-retired metal turner. If all else fails I might replace the back rollers with wheels and use it as a slightly lighter push mower. Will keep the group posted.

The aftermarket gears were a pretty ugly solution to the problem in any case: they consisted of a stack of steel stampings riveted together, and their teeth were triangular, not involute form. This means they damaged the small aluminium pinions they had to mesh with.

Cutting gears requires a milling machine, a dividing head or rotary table, and the specific involute-form gear cutter to match that gear. While most serious machine shops have the milling machine and dividing head, not so many have a full range of involute cutters. Cutting a gear with the wrong cutter gives a result about as bad as those steel gears with the triangular teeth, but at the same high cost as cutting a perfect gear. Accurate gear cutting also requires making some calculations about center distance corrections, and setting the milling machine up very accurately. Doing this without having the original drawing of the part, can be quite time-consuming. Essentially, it is a job for a trade-qualified machinist, not a fitter and turner or other tradesman. Having said that, keen amateurs make gears for antique machinery all the time, sometimes with moderate success - but it will take a number of hours, it may take more than one try, and usually it won't be an accurate, well-cut gear. Incidentally, once the equipment is correctly set up to make one gear, it can make a dozen more quite quickly.

Thanks Grumpy. I have managed a bodge in the interim with a hose clamp which is holding, but definitely short term.

I was even wondering if I could get a 3D printed copy made. I definitely have too much time on my hands!!!!

Mike

If you are that desperate, Mike, you might consider repairing the old gear. Sometimes the nylon cracks radially, but sometimes it comes loose on the steel boss without much radial cracking at all. If that is the case, you may be able to weld a flat web onto the boss, and screw the web to another web on the opposite side of the nylon gear. The plastic would have to be tightly sandwiched between the two flat steel plates, and attached with a multitude of screws on different radiuses. If you are interested, we can discuss the detail. I haven't heard of anyone doing it, but it sounds possible if the plastic web of the gear isn't all cracked up, and none of the cracks extends far radially.

Thanks Grumpy

My bodge with the hose clamp is working. Your idea might be the longer term solution as the nylon hub is pretty not too bad.

Mike

mmm having the same problem, just bought a diplomat 430 i think I am also wondering about how to open the roller to remove/ replace the gear. mine has a large crack. Can it be plastic welded? has anyone tried any of the drive gears out of modern gear driven wheels? They look like a similar size.

I do not have the tools to measure the inner and outer diameter, the mower is not with me, and as yet I have not removed the gear. can some provide a picture to count the teeth and the measurements of the inner and outer diameter. I wouldn't be surprised if a variant was used in other equipment.