# Logan 6560 Back gear



## richz (Apr 4, 2012)

I have a broken tooth on my back gear. I found a gear at work. I put a gear gage in both gears and they both come out to 20 degree 12 D.P. 75 tooth. The gear that I want to use is slightly smaller. Will this work? My plan is to to bore and sleeve the new gear.


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## CharlieW (Apr 5, 2012)

I guess it depends on how you define "work".  It will work as well as any set of gears that are not properly spaced to create the correct mesh of the teeth.  The teeth have complex geometry and when properly spaced the teeth have very little backlash and move smoothly together.  Using a smaller gear gives the same result as having gears that are set too far apart and do not mesh as they were designed to do.  The smaller gear also affects the spindle speeds, slightly, when the back-gear is engaged. 

I can't tell you what will definately happen if you use the smaller gear as I do not have any experience running gears that the mesh is not correctly set. I suspect you may have excessive gear noise or quicker wear of the teeth on the 2 gears.  

I would either find a gear with the correct number of teeth or repair the gear you have. There are several ways, to replace the missing tooth, and it can be done sucessfully by grinding the replacement tooth by hand if you do not have equipment to re-mill the replacement.

Charlie


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## richz (Apr 5, 2012)

Charliew both gears have the same number of teeth.  It is .045 smaller and there is an adjustment for backlash. While I am at it what should the backlash be?


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## CharlieW (Apr 5, 2012)

The simple answer is they should be as close as you can get them with them still meshing smoothly together when you rotate them.  Since the back-gear assembly is made up of 2 gears on each end of the back-gear assembly, I would think that if you adjust that gear to mesh corectly, the gear on the other end of the assembly will be too tight.

The techincal answer is a little more complex in this situation.  A spur gear with 72 teeth and a Pitch of 12 has a PD (pitch diameter) of 6.25 and an OD of 6.1417, whether it is 14.5 or 2 degree pressure angle.

Technically speaking, for example, if you had 2, 75 tooth gears, the distance between them would be their PD added together.  In the example of 2 75 tooth 12 Pitch gears, each gear has a PD of 6.25" and  the distance would be PD 6.25 + PD 6.25 = 12.5" from the center of one gear to the center of the other gear. 

Since both of your gears have the same number of teeth and seem to have the same Pitch of 12, they should be the same diameter.  It is likely that the smaller gear you found is either cut incorrectly or has a different number of teeth such as 74.  Gears are precision and if their specifications are the same, they should have the same diameter but especially the same PD.

Charlie


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## 8ntsane (Apr 6, 2012)

At .045 differance, I wouldnt worry to much about it.
I doubt it would effect the way it meshs to the other gear. I would just do a very carefull set up, and roll it over by hand and look/feel for any binding. You say you have ajustment for back lash, so in that case, you should be good to fit the new gear up. Just remember, after your gear is fit into place, turn the setup by hand. Check for binding while turn it over. The end gear train should have other gears in there, that have no ajustment for BL. See what they have, and try to match it. If you want to see the effects of setting the BL loose, or tight, you could pick up some gear marking compound from a automotive supplyer.It would show tooth contact patch patterns at various depth settings. Its a yellow paste, you just brush on , and roll it back and forth, then look at the pattern.

The tooth profile on the replacement gear, if it is indeed the same would show you how well is meshes together. If you can get it setup to the point where you feel no binding, and a reasonable BL setting, only then turn it under power. I would run at the slowest speed, and work your way up from there. If your lucky, you should be good to go.

Good Luck


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