Pages: [1]   Go Down
Author Topic: Pushing axles into bearings  (Read 776 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Jr. Member
**
Karma: 0
Posts: 62
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Hi,

I brought some 3mm diameter axles along with some ball bearings (said to be 3mm inner diameter).  When I try to push the bearing into the axle, I found that they are impossible to fit.  I tried to hammer it in, and broke some of them already.

Afterwards, I measure the dimensions with a caliper, the axle diameter is 3.00mm (damn, it's good smiley-lol) but the bearing inner diameter is only 2.80mm.  

Why's that?  Is the inner diameter of ball bearings deliberately designed to be undersized?  Or, am I just simply brought the wrong parts?  Is there a way to push a 3mm axle into a 2.8mm ID bearing without breaking it?

Thanks.

Gary
« Last Edit: May 25, 2013, 02:01:31 pm by ma_hty » Logged

0
Offline Offline
Tesla Member
***
Karma: 145
Posts: 9671
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Probably the easiest thing to do is chuck the shaft in a drill and use emery paper to see if you can reduce the size some. Also, I've gotten a ~.5' shaft into a bearing (drove shaft out with a hammer) by making sure the shaft was smooth where the bearing fit, then freezing the shaft in the freezer for several hours. Took the shaft out of the freezer and it dropped straight into the bearing. As it warmed up it expanded into the inside of the bearing. Bottom line, you can heat your bearing in a oven and freeze the shaft, then quickly see if you can make them fit.   
Logged

Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   smiley-cool

SW Scotland
Offline Offline
Edison Member
*
Karma: 17
Posts: 1398
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

How did you manage to measure the bore of the bearing.  Even with the proper tools, measuring small internal diameters is difficult to do accurately.  However, on the basis that your measurements are accurate, there is no way a 3mm shaft will fit into a 2.8mm bearing.  If crude work is the order of the day then chucking the shaft in a drill machine and using a file whilst the shaft is rotating should get its diameter reduced.  The correct machine to use is of course a lathe.  The correct procedure is to get bearings of a size that matches the shaft - 3mm bearings fit onto 3mm shafts.  NEVER hammer a bearing, hitting with hammers bursts their balls.
Logged

Offline Offline
Jr. Member
**
Karma: 0
Posts: 62
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

How did you manage to measure the bore of the bearing.  Even with the proper tools, measuring small internal diameters is difficult to do accurately.  ...

I tried to measure the inner diameter at different angle with the caliper inside as many times as I can, and took the maximum reading as the inner diameter.  Though its not extremely accurate, I guess it is still useful to report.
Logged

SW Scotland
Offline Offline
Edison Member
*
Karma: 17
Posts: 1398
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Should have said that any burrs on the shaft end will prevent it entering the bearing.  Also, if the shaft is cut off square it'll be virtually impossible to get it entered.  Always ensure end has a slight taper to provide a lead into the bearing.
Logged

Offline Offline
Newbie
*
Karma: 1
Posts: 14
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Usually one dimension of bearings are designed to be a press fit and the other is slip fit. Every bearing I've dealt with has been press fit on the ID. While hammering is not the best method it can be done with damaging the bearing. You just need to make sure you only hit it on inner race of the bearing. Usually when I am replacing a bearing that is pressed on a shaft I'll take the old bearing and open it up a little with a dremel, then us it press the new bearing on.

I'm sure if you have access to a vice or if the shaft is short enough to fit in it, but you could use it to press the shaft into the bearing. Like Jackrae said put a slight taper on the end of the shaft with a file to make things easier

.2 mm undersize press fit sounds a bit tight. Hopefully your measuring was off otherwise you might want to try a different brand of bearing with better quality control.  I just double checked my machining operations hand book and it says that a good rule of thumb is .001 mm per mm of diameter for a typical press fit.  So ideally the ID of your bearings should be no tighter than 2.97 mm.

Logged

Siena Italia
Offline Offline
Full Member
***
Karma: 2
Posts: 149
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

In a bore of only 3 mm the thickness of the caliper blades will necessarily cause a significant underestimate.  I suspect that the bearing is the right size for a press fit on your shaft.  As others have said, you can use temperature to help, but no matter how you do it pressure should be applied ONLY to the inner race, not to the balls.  I generally use a wrench socket with a loose fit on the shaft and a vice, or for longer shafts, a bar clamp.
Ciao,
Lenny
Logged

Offline Offline
Jr. Member
**
Karma: 0
Posts: 62
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

In a bore of only 3 mm the thickness of the caliper blades will necessarily cause a significant underestimate.  ...

I think its the bearing quality problem.  I brought some more ball bearing afterwards, and this time, from a proper vendor (NSK to be precise).  The inner diameter is measured to be 0.03mm smaller than 3mm (the measurement is not extremely accurate, but it should still show a rough picture).  

I will only buy bearings from proper vendors from now on, crap no more.

Thanks for your help.   smiley
« Last Edit: May 26, 2013, 05:19:16 pm by ma_hty » Logged

Australia Mate!
Offline Offline
Sr. Member
****
Karma: 3
Posts: 284
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Cheap bearings are just that , CHEAP , NSK are my preferred choice, followed by FSK then CBC. NSK you're paying for accuracy, most bearings a pressed onto axles, this allows for even distribution across the surface area.
Logged

SW Scotland
Offline Offline
Edison Member
*
Karma: 17
Posts: 1398
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Crap bearings usually relates to ball quality.  For a bearing bore to be 0.2mm (8 thou) undersize does not suggest poor quality, it suggests an incorrect size.
Are you absolutely certain you actually bought 3mm bearings and not 7/64" bearings because your measured 2.8mm bore is near enough the equivalent of 7/64" (2.778mm)
« Last Edit: May 27, 2013, 05:48:51 am by jackrae » Logged

0
Offline Offline
Tesla Member
***
Karma: 145
Posts: 9671
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
I brought some 3mm diameter axles along with some ball bearings (said to be 3mm inner diameter).

Where did you buy the cheap bearings so we can have a look?
Logged

Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   smiley-cool

Offline Offline
Newbie
*
Karma: 1
Posts: 14
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
I brought some 3mm diameter axles along with some ball bearings (said to be 3mm inner diameter).

Where did you buy the cheap bearing,,s so we can have a look?

A better question might be what bearing numbers are they? Some tools or fan motors have special bearings, I refurbished the motor on my table saw, and when went to replace the bearings which I thought were standard 6206 bearings (40mm od, 17mm id, 12mm) but they didn't fit the shaft since shaft was actually 5/8". I didn't notice the continuation of the  part number (6206XXX) usually specifies shielding, was actually designating a non standard ID.
Logged

Pages: [1]   Go Up
Jump to: