Bearing problem: Stuck/Shifted bearing

Evan Byrne wrote:
So I was riding my uni yesterday and as I'm taking a break I see some
shavings of powdercoat on my cranks and I see that my right bearing had
shifted out and was almost hitting the cranks, this caused the lips of
my yuni bearing holder to scrape the crank.


Its quite stuck, I can't move it with a screwdriver and a hammer. What
shal I do?



Hey,

I had the same problem on my '07 KH29!
The bearings on almost all tapered interface hubs (square & ISIS) are
designed to be press fitted on the axle. See:
http://en.wikipedia.org/wiki/Interference_fit
As you may read in the link above the pres/interference-fit assembly
relies on the friction between the connected parts (axle and bearing
inner race).
So in production process mother parts (like inner race) are machined to
a slightly smaller size and father parts are done slightly bigger.
In practice, however, because of production tolerances, a bad (or
worst) scenario may happen if one uses a bearing with the inner race
size located on the high side of manufacturing tolerance on a shaft
sized in the low side of the allowed tolerance! This will obviously
lead to poor press-fitting.
Coming back to our situation, its no wonder that the bearing slips if
the friction is not big enough. This tends to happen more on bigger
wheeled-unicycles because the higher frame flex induces higher side
forces on the bearing!
I have all 3 KH unicycles (KH20, KH24 and KH29) all with Moment hub. I
encountered this bearing slip problem on KH20 and KH29.

OK, now what to do?
First, I recommend AGAINST altering the axle or the bearing surface
with any tool!!! This is NOT good!
Additionally I either don't like the solution with spacers between the
bearing and crank arm. This is suitable IMO only for NON-tapered spline
designs (old KH, yellow QU-AX). I hope you immagine why...
Your best bet would be to find some Loctite or similar adhesive
designed for this very problem (e.g. Loctite Bearing Mount 641...).
These are low viscosity liquids that applied on the axle/shaft (before
putting back the bearing) will increase dramatically the adherence of
the press-fit!
Unfortunately they are hard to find and expensive!
I used with success Loctite 243. Although this is a medium strength
thread locker it proved to cope well in the bearing application too!
Note:
get the bearings out, clean (degrease) everything, apply generously
Loctite on both axle and inner race of the bearing, press them back
*quickly* (use the part of the seat post of your KH that you may have
cut out...) and leave overnight!

Hope this helps,
Flaviu


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UAC wrote:
[...]
Your best bet would be to find some Loctite or similar adhesive
designed for this very problem (e.g. Loctite Bearing Mount 641...).
These are low viscosity liquids that applied on the axle/shaft (before
putting back the bearing) will increase dramatically the adherence of
the press-fit!
Unfortunately they are hard to find and expensive!
I used with success Loctite 243. Although this is a medium strength
thread locker it proved to cope well in the bearing application too!
[...]




Loctite does make press-fit locking compounds which are better suited
to this job than a thread-locker. I've used both the Loctite 609
(medium strength, likely to be removable) and the Loctite 648 (maximum
strength, probably only removable by application of near-destructive
250C heat) and have found the Loctite 609 ideal for holding bearings on
square-taper hubs. Loctite 609 is a lower viscosity than 641 and better
suited to something which is already a press fit. Both are used with
the Loctite 7649 primer. My local industrial supply house ordered them
for me; expect to pay around $20 for the locking compound and another
$20 for the primer.

Before assembling with 609, be sure that you have the spacer that sits
between the bearing and hub in place. If you don't, you'll accidentally
hammer the bearing on so far that you may not be able to grab the
bearing with the puller arms to extract it. Be sure you have a good
puller; it is required to remove bearings held on with 609!

I've done roughly 10-15 wheels with the same tube of 609 and have more
than half left. Not a single case of a bearing pulling since I've
started using it!


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I think the problem lies in my flexy frame. I used too thin wall tubing.
My next frame will be much stiffer and thicker.


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Evan Byrne
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If you've already taken the crank off and tried tapping the bearing back
into place with a hammer (which i know is not a good idea...but it
works if you're careful) with no success, you may have to damage the
bearing to get it off and buy some new ones.


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Chexjc wrote:
If you've already taken the crank off and tried tapping the bearing back
into place with a hammer (which i know is not a good idea...but it
works if you're careful) with no success, you may have to damage the
bearing to get it off and buy some new ones.




Oh yeah the hammer and tube method worked fine, its just my frame is so
flexible they keep working thier way out.


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Evan Byrne wrote:
Oh yeah the hammer and tube method worked fine, its just my frame is so
flexible they keep working thier way out.


Oh, sorry I didn't read the whole thread. Perhaps if you added spacers
between the bearing and the crank they wouldn't slide.


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Yes but the crank cant tighten against the spacer, it needs to tighten
onto the hub.


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