Alignment, and eccentric bushing, gearbox rebuild
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  1. #1
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    Default Alignment, and eccentric bushing, gearbox rebuild

    A first post with a really hard question, at least so it seems to me.
    Trying to rebuild a cast gearbox. A failed bearing wallowed out the bearing bore-- to egg-shaped. Not having specs. on gear center-to-center, I indicated on the hole assuming very little damage on the "good" side. I bored the housing -through, both sides, for insets. On assembly with new gears a and shafts, one of the bearing pockets has error, of position (center), of about .016"--which resulted in the gears being too tight.
    I am thinking about trying to build a bushing with .016" eccentricity (that is, center of the ID offset .016" to the center of the OD) to rotate about to get a good bearing fit. I consider a good bearing fit as minimal backlash but turn by hand with no tight spots felt. The box is a right angle and speedup with three spur gears after the right angle.
    Anybody done this?
    I am a novice machinist, but lots of years in design and manufacturing.

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    I would personally place a solid plug with a good interference fit then bore it out like you did before but this time in the right place.

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    Quote Originally Posted by Strostkovy View Post
    I would personally place a solid plug with a good interference fit then bore it out like you did before but this time in the right place.
    Thanks for your reply. The bore was really difficult with extreme hard and softer spots. Further, the bearing seat requires an internal O-ring groove-- and, my experience and accuracy is much better on the lathe than the mill. I did a search on this site for "eccentric bushings" and found a thread on the use of such in a Sunnen honing machine, apparently as an extended center adjustment
    method. Yes!!! Always better to do it right the first time. And after a fail, I always can think of better methods for the next time- though my old or low-end equipment and tooling has some limits.

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    Eccentric cams and bushings are pretty common for adjustment, but they need some method to adjust them easily and retain them in the adjusted position.

    The issue with an eccentric bushing in this case is that it doesn't give you a nice linear adjustment, but it rotates around a point and it's very likely nowhere on the circle it can move gives proper fit to multiple gears.

    It would be easiest to measure exactly what the offset it supposed to be, make the bearing bushing on a four jaw chuck with the appropriate offset, and press the bushing solidly into place.

    I think you will have less machining time even if it takes trial and error in making a press fit bushing than you would have in making an adjustable bushing and an adjustment tool and a way to secure it, and still probably have some more trial and error.


    A quick trick that might help you here: to remove inserts like this you can make a weld bead all of the way around the inside of the bushing. Once it cools it will shrink and slip out. Also works for bearing races when the rest of the bearing has been forced out.

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    In case you DO want to figure out center to center distance..
    gear-calculations.jpg

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    Thanks for the helpful information. Today I made an eccentric bushing. I did the ID with the inside O-ring groove in the 3-jaw. Switched to the 4-jaw and machined it with offset. This immediately took the pressure off the gears. However, this action did show me that the other side is also out a slight amount, so I will do that side also, though to a lesser amount--.006" or so.
    My insert, today, was not rotatable. I am using a light press fit with short nickel welds in a machined face groove. This is a lightly loaded shaft -6205 bearings. Further, the inserts are flush with the housing, and they get a cover plate in the bolt up to the machine.
    While I find the eccentric bushing interesting, the big deal is get the bore right the first time and use a concentric bushing. I might repair several of these boxes if I can work out a reasonable method.

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