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GMKlenklen
11-18-2008, 10:07 AM
Hi all!

I'm working on a project for one of my classes, and I can't seem to find an answer to my question:

Say I've got a bearing with an outer diameter of .75"
What size hole do i need to put in my bearing block such that the bearing will sit snug in there and not rattle around? I probably need to know what tolerances are available from my machine shop (like how many decimal places it can resolve, right?). Is there some sort of equation that can be used to figure out the holes for any size bearing?

Thanks!
and I might as well give the link to pictures of this thing...
It's supposed to traverse a parabolic path using magnus effect and a 5 foot per second crosswind... thusfar we have gotten blown across the cource before making it up to the inflection point... so we need more thrust. wish me luck?

EricH
11-18-2008, 10:24 AM
Bearings need a pretty tight tolerance because they need to be pushed in. 0.75" is as small as you can go with the hole, so you really want it slightly bigger. I don't think there is an equation, at least, I can't remember one.

I would assume that the shop can get to within 0.005" fairly easily in metal. That should be enough--if the hole for the bearing is 0.755" in diameter, I would expect the bearing to press in a bit, but stay pretty well. Smaller is better, though, because it's pretty easy to remove material that is there with, say, a reamer, but pretty hard to add it back. If they go down to 0.001" tolerance, that's even better.

AndyB
11-18-2008, 11:14 AM
Sounds like you're looking for a class III (http://www.engineersedge.com/class_iii.htm) or class IV (http://www.engineersedge.com/class_iv.htm) fit.

GMKlenklen
11-18-2008, 11:15 AM
ok sweet... last time i CADed something, i kinda ended up fudging the values for everything... some stuf worked, others didn't... I suppose i should get the parts and measure them just to be \$@#\$@#\$@#\$@# sure and not make a fool out of myself again!

EDIT: woah, i wonder what it word filtered lol
guess i should use cleaner language :p

Cory
11-18-2008, 02:23 PM
Bearings need a pretty tight tolerance because they need to be pushed in. 0.75" is as small as you can go with the hole, so you really want it slightly bigger. I don't think there is an equation, at least, I can't remember one.

I would assume that the shop can get to within 0.005" fairly easily in metal. That should be enough--if the hole for the bearing is 0.755" in diameter, I would expect the bearing to press in a bit, but stay pretty well. Smaller is better, though, because it's pretty easy to remove material that is there with, say, a reamer, but pretty hard to add it back. If they go down to 0.001" tolerance, that's even better.

That's actually backwards-if you're pressing a bearing into a hole, the hole needs to be smaller than the bearing.

a good press fit for a 0.750" bearing in aluminum would probably be about 0.7495, off the top of my head. There's all kinds of charts available if you google "interference fit chart" or something similar.

=Martin=Taylor=
11-18-2008, 02:33 PM
It depends a little on what kind of material the bearing block is.

Plastic deforms a lot and holes can be made much smaller, while aluminum and steel should be closer to the actual size.

EricH
11-18-2008, 04:42 PM
That's actually backwards-if you're pressing a bearing into a hole, the hole needs to be smaller than the bearing.

a good press fit for a 0.750" bearing in aluminum would probably be about 0.7495, off the top of my head. There's all kinds of charts available if you google "interference fit chart" or something similar.Oops... I forgot about the real version of "press fit". You can press a bearing into the hole size I suggested, but gravity will press it out pretty easily.:o

vivek16
11-18-2008, 05:45 PM
A good rule of thumb for reasonably sized bearings (say .75-2") is a .005" interference. You can get more specific as others have said but it will work.

-Vivek

Cory
11-18-2008, 07:14 PM
A good rule of thumb for reasonably sized bearings (say .75-2") is a .005" interference. You can get more specific as others have said but it will work.

-Vivek

I wouldn't generalize that much. 0.0005" is good enough for a 3/4" bearing. I had to make a part for a large bearing (about 1.5-1.75" or so) and the reccomended max interference was 0.0002"-0.0003". The extra 2 tenths can make a HUGE difference in how freely the bearing can turn.

GMKlenklen
11-19-2008, 03:19 AM
oh god, this is getting complicated...
if i say .7500" hole for a .75" OD bearing, that will work, right? I should be able to put that in there with my bare hands right? i think i'm gonna use hotglue to keep this stuff togeather if i have any problems :p

btw, so u can sorta see what i'm workin on:
i have what seems to be the final render up
the motor will be driving the shaft and the rotor at about 300-700 rpm...

Andy A.
08-19-2009, 05:31 PM
Press fits are a surprisingly complex subject. You are straying into an area where a lot of numbers come into play and assembly technique matters if you want to do things 'right'. If you want to fudge it, I suggest 'skipping' the problem by drilling a .750 hole and using a layer of locktite on the OD of the bearing. As long as you are careful to keep the bearing aligned and the adhesive out of the bearing it works pretty well.

I'm sure some folks here will cringe while reading that, but it does work. Consider it as an option if you don't have access to a shop with a method of cutting a hole of the size you need (jig boring and/or reaming are great ways to do it).

.745 is a good value for pressing this size bearing into aluminum. Different materials and bearing sizes will change that, so don't use it as the cure all. I prefer to stray on the looser side of press fits in FIRST, so I would recommend that you try to keep the tolerance on the hole size positive so the hole will tend to be closer to .750 then not.

As a general practice I don't model press fits in CAD- bearings and their holes are just line to line fits. I'll save press fit instructions as a call out on the drawing and in FIRST I'll also include a table showing the maximum and minimum material conditions for a given tolerance. Opinions on that will very from designer to designer and shop to shop.

Good luck.

-Drew

Daniel_LaFleur
08-20-2009, 11:49 AM