I see. I went and found a Chinese catalogue with a search facility so I can
find the numbers and dimensions in future.
Now this may be a naive question but it puzzles me: A 5/8" shaft is 0.875 mm
or 0.034" thicker than 15 mm. This seems quite a lot. Do all shafts have to
be turned down to fit the bearings? The difference seems quite a lot to
reconcile by using just heat and cold for a press fit.
You would find out that a caliper is very handy to have for checking bearing
Many (most) ball bearings are all metric, but some applications use a
combination of metric and inch dimensions, with inch used as the I.D.
Generic (or used domestic brand) calipers are available for about $20, even
for generic digital. Most digital calipers switch between metric and inch
for instant conversions, and also can be reset to zero at any point along
Dial calipers are generally either metric, or inch, although there are some
that indicate both on a single tool.
Shafts and housing openings are machined to sizes which are appropriate for
the desired type of fit for ball bearings, otherwise bearing sizes are
selected to fit existing parts.
I am not sure that I get your point. Are you saying that the tolerances on
bearings are so loose that one has to get a handful and go through them with
a caliper to find which matches your shaft?
Looking through McMaster-Carr web site there seems to be a wealth of both
metric and imperial shafts as well as bearings. In my simple mind one would
use a 5/8" ID bearing for a 5/8" OD shaft. The tolerances quoted are 0.0003"
(converted from metric) for the bearing ID and 0.003" for the OD shaft
I am puzzled why in the case of this particular piece of equipment there are
15 mm ID bearings supporting a 5/8" shaft (measured in the middle with
calipers). To me it implies a degree of machining to achieve a fit, but why?
It's really pretty simple. Ball bearings were originally designed to metric
dimensions because they were not developed in the USA or England and
millions of them were designed into products. The vast majority of them are
still made to metric dimensions although there are ones which are entirely
or partly inch dimensioned. The USA manufacturers commonly machined their
parts to fit the standard (metric) bearings. If you have a 5/8" shaft
running in a X202XX bearing, the shaft has been machined down to fit. There
has to be a shoulder of some kind to establish the axial location of the
shaft and bearing.
To remove the shaft you have to remove one bearing with it. The bearing will
slide out of the housing with a little force from the shaft, though there
may be a snap-ring or other retainer that has to be removed first. It is not
considered good practice to apply force thru the balls but often there is no
other way to remove the bearing. A light tap on the shaft end from a soft
faced hammer will usually do it. The bearing should be re-usable unless it
is corroded in the housing and a lot of force is necessary to get it out.
If one went through them with a caliper, one would get very
frustrated trying to find variations in a given part number. You will
need something with a lot more resolution than a caliper can give you. :-)
And you expect to measure 0.0003" variations with a caliper?
There is a very good reason for this -- which the maker of an
early double-sided 5.25" floppy drive did not understand. He mounted
the spindle in a pair of flanged outer race bearings which slipped into
a cylindrical bore. On one end of the shaft was the cup which drove the
floppy. On the other end was simply a tapped hole. (1/4" shaft, FWIW).
They slipped a pulley over the end of the shaft, and tightened a screw
and washer to hold the drive pulley onto the shaft. They then painted
over the end with Glyptol.
Some months later, I was finding double density floppies very
difficult to read, while single density was still easy. Pulling off the
drive belt, I discovered cogging in the spindle.
The bearings were radial thrust bearings, and the tightening of
the screw had put axial thrust on them, causing them to wear out quite
rapidly. When I got new bearings for the drive, I spent a little time
on the lathe and turned up a spacer to go around the shaft between the
bearings, and to hold them just a tiny bit farther apart than the
cylinder in which the outer races mounted would do. This allowed me to
tighten the screw firmly enough to keep the pulley from slipping without
putting a serious axial load on the bearings.
In the case of your assembly, the 15mm shaft is turned down to
5/16" so the bearings will slide onto the shaft and then stop at a
certain position. The reason for this is that there are some parts
missing from your device which press against the outside end of the
inner race, and support the wheel or buffer -- held in position by the
nuts being tightened against a washer to hold the outer surface of the
wheel or buffer.
Without that step in diameter from 5/8" to 15mm, this tightening
would put a serious axial load on the bearings. Your bearings were made
for radial thrust only. You *could* use angular thrust, but to take the
various loads, they would need to be larger and more expensive. The
full 15mm diameter of the inner portion of the shaft provides a spacer
for the inner races so you can tighten the nuts as much as needed.
I suspect that you will find a step in the bore where the OD of
the bearings mounts, to keep them from moving too far towards the
center. Fairly light pressing should move the far bearing out of its
bore, and the shaft inwards through the inner race on the near side.
You will need to find a setscrew on the pulley to allow you to release
it from the shaft.
Note that you will need to use a lathe to make the spacers
between the outside end of the bearings and the wheels or buffers before
you can use this. I doubt that you will be able to find the parts
needed, as they are made for the task at hand. I have similar things on
two grinders with built-in motors. They have similar steps on the
shafts to keep the ball bearing assemblies from being pressed in too far
and suffering similar fates.
Perhaps someone who has the same model can measure and draw up
what you need to make. But you still need to make a new shaft to get
around the bent end.
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I would imagine it was machined down to allow a shoulder to press the
bearing inner race against. Most applications with a rotating shaft
have a slip fit on the outer race and a press fit on the inner race.
Check out these sites for info on shaft and housing design and shaft
and housing fit. http://www.ntnamerica.com/pdf/2200/shaftdes.pdfhttp://www.ntnamerica.com/pdf/2200/brgfits.pdf
I would not suggest reusing the bearings. They're pretty cheap and
readily available and it's not worth the risk. I'm going from memory
but I think you stated this was a 6202Z? If so, it most likely has two
shields so you would need to order a 6202ZZ. The 6202Z is normally
stamped on the shield because the factory doesn't know if it will be
used in a single or double shield application. If you order with one Z
you will only get a single shield. Good luck.
I didn't mean to suggest that ball bearings were individually hand selected
from batches of bearings to achieve the correct fit.
A more-common all metric dimension bearing would likely be cheaper than a
metric bearing with an inch I.D. in most cases.
If the manufacturer had size and strength constraints a less common bearing
may be more suitable.
When machine manufacturers are designing their parts that the bearings will
mount/mate to, they will typically machine their shaft and housing
dimensions to match a commonly available bearing.
When GLU (guys like us or maintenance/repair folks) are working
with/repairing existing machines, the bearing mating parts are first
measured, then an appropriate, commonly available (with any luck) bearing is
ordered/selected to fit the application.
As you can see by the other responses, the buffing arbor shaft was machined
for several reasons. Positioning the shaft's length relative to the arbor's
housing primarily, and to fit the bearings' I.D.s, and to use common
mounting hardware.. 1/2" nuts.
The shaft remains stronger in the center (providing shoulders for the
bearings), utilizing a commonly available pulley I.D., held in place
laterally, and commonly available mounting hardware for the mounted
If the shaft were just 1/2" diameter, shaft collars or some similar hardware
that wouldn't compromise the shaft strength would be required to lock or
locate the shaft in place (more hardware generally means higher
manufacturing cost), and the bearings would've been more expensive.
The arbor manufacturer had a responsibility to market a fairly safe product,
one that wouldn't fly apart, safe enough for a DIY-type to take home and
There are ball bearings that have integral locking features for securing the
inner races to shafts, but they are more expensive than an ordinary ball
The machining that was performed on the arbor shaft wasn't an expensive
operation, performed on automated machines it was probably completed in
probably less than a minute from raw barstock to a finished, threaded part.
Who knew that such a simple piece of equipment would provide so many lessons
in engineering and associated history and economics? Thanks everybody for
such a detailed explanation. Some of the links provided are useful as
reference. Again, my Google technique has proved deficient as I have been
looking for them or something similar without success for the last two
In the final analysis the whole thing cost me $5 and most of the procedures
suggested seem to involve greater costs than I can justify at present. I
shall just keep looking at it 'cos it's pretty and an idea for good use will
come to me in time :-)
On Thu, 16 Oct 2008 19:19:08 -0700, "Michael Koblic"
Find a piece of allthread and two nuts. Put that in the "ears" on the
left side (belt side), adjust the nuts so it's quite snug. This is
to support the casting and keep it from flexing. The nuts go inside,
Make a plate with a hole in it to support the piece but clear the
bottom end of the shaft. The hole should be slightly larger than the
OD of the shaft.
Press on other end of shaft with hydraulic press. The shaft should
Don't hammer. Hammering can result in peening. Steady force is
If I understand you correctly the casing would be supported as well as the
bottom bearing but the top bearing would be sacrificed? Also I am not quite
sure what the "belt side" is: In this case the belt is looped in the middle
over a pulley, between the two bearings. Or do you mean that these things
are usually driven by a pulley on the left side?
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