Another Spindle Bearing thread

[jroddds]

So I got a BT30 spindle of the import kind similar to this:
ATC spindle BT30 Spindle CNC Milling Rounter Electric Spindle Motor 220V with Belt for BT30 Spring + Drawbar|motor 220v|220v motor|220v electric motor - AliExpress

The bearings were OK. But I desired to upgrade. So I go some matched angular contact bearings to replace the radial grooves it came with.
There are a set of 3 toward the spindle nose and a pair towards the rear.

I'm mostly asking about the rear bearings. The front bearings run true and everything is great. I didn't have the forethought to realize the rear bearings have no form of preload. The inner races butt against a shoulder on the shaft, but the outer races have no shoulder. There is a rear plate that bolts on, but is nearly 50 thou away. The bearings I used are a match pair of tandem (facing the same direction).

So my question is: do I need to make a shim to add some preload? My father (a retired mechanic, who is a smart guy) seems to think they will be fine to run free. I am wondering if the outer race will float away from the inner race.

Now I'm not so sure I should have left radial groove bearings as the rear pair.

I attached a cross section of the spindle someone else made. Be advised, there are threads and nuts to compress both sets of inner races.
Would welcome any thoughts on the subject.

 

[atex57]

On a setup like this one set of bearings must float to accommodate heat expansion.

Ed.

 

[Peter from Holland]

Keep in mind that a better pair of bearings need better tollerances on the whole thing to get a better spindle
So look up tolerances needed for the bearings you want to use and then see if the spindleshaft and housing meet those tolerances somehow


Peter

 

[jim rozen]

On a setup like this one set of bearings must float to accommodate heat expansion.
Ed.

+1

If you somehow managed to put preload on those two rear radial bearings the spindle is then overdefined and bad things
will happen when the temperature of the thing changes in use.

Often the inners of those two bearings are clamped together on the spindle and the outers float in a tight fit in the bore of
the outer casing. In this design they are probably a tight interference fit on the spindle so they don't move apart in use.

The old b'port M heads were done this way, angular contact pair at the bottom and a single radial at the top.

 

[Tyrone Shoelaces]

For obvious reasons most milling machines spindles are designed with a bearing arrangement that allows for thermal expansion away from the business/cutting tool end.


Regards Tyrone.

 

[dian]

how can a tandem bearing pair function without preload on the outer races? that doesnt make sence to me. what am i missing?

 

[jim rozen]

All of the thrust load is taken by the lower bearings. The upper ones are there to take radial loads only to locate the top
of the spindle inside the housing.

There are three rather than two lowers, and two rather than one uppers, based on the load the spindle experiences, I believe.
This may be done to reduce the overall diameter of the spindle assembly.

 

[jroddds]

how can a tandem bearing pair function without preload on the outer races? that doesnt make sence to me. what am i missing?

This is what got me stuck. Should I flip one of the bearings so they are not tandem, rather back-to-back.
Or replace them with standard radial groove bearings?

In a back-to-back arrangement, the outer races would be forced against each other, but could still float as a set. The downside, is that they were matched as a tandem set. Expensive to replace them.

 

[Bill D]

Maybe a wave spring washer under the endcap to press down on the outer race. I see this used in electric motors.
Bil lD

 

[13engines]

To start I'd say at least one of those front bearings needs to be turned around. The one closest to the spindle face would be my first choice but there are other options. As it is you have fixation in only one direction from the spindle face rearward. I can't believe this is a running spindle with any kind of two way axial rigidity. Good luck running this with a high helix end mill or high positive face mill. All the front bearings face forward. When the tool change release plunger attacks the belleville washers the forces are passing right through the bearing set in the direction they're not setup to handle. (I'm assuming those are angular contact bearings even though they're drawn somewhat incorrectly. Seems like it's showing a thin inner ring whereas the outer ring is usually the thinner one. (If not angular contact then disregard most of this post) The clamping arrangement in the supplied picture is normal in that both inner and outer races are clamped against shoulders. BTW the little yellow things are just a labyrinth seal.

When using a triple set load sharing can be a problem if the mating components aren't of high quality fit and finish wise and the bearings are matched as close as possible. I can see you having as good of luck using a back to back pair with match ground spacers in between filling in for the missing bearing. That space will add a lot to their rigidity and likely leave you with a cooler running spindle. The rear radial ball set is fine as long as the inner rings are clamped. You wouldn't lock in the outer ring as there would then be no room for axial thermal expansion in the spindle.

I'm no expert, but this is the best of what I know. Ha... or think I know. :-)

Dave

 

[jroddds]

To start I'd say at least one of those front bearings needs to be turned around. The one closest to the spindle face would be my first choice but there are other options. As it is you have fixation in only one direction from the spindle face rearward. I can't believe this is a running spindle with any kind of two way axial rigidity. Good luck running this with a high helix end mill or high positive face mill. All the front bearings face forward. When the tool change release plunger attacks the belleville washers the forces are passing right through the bearing set in the direction they're not setup to handle. (I'm assuming those are angular contact bearings even though they're drawn somewhat incorrectly. Seems like it's showing a thin inner ring whereas the outer ring is usually the thinner one. (If not angular contact then disregard most of this post) The clamping arrangement in the supplied picture is normal in that both inner and outer races are clamped against shoulders. BTW the little yellow things are just a labyrinth seal.

When using a triple set load sharing can be a problem if the mating components aren't of high quality fit and finish wise and the bearings are matched as close as possible. I can see you having as good of luck using a back to back pair with match ground spacers in between filling in for the missing bearing. That space will add a lot to their rigidity and likely leave you with a cooler running spindle. The rear radial ball set is fine as long as the inner rings are clamped. You wouldn't lock in the outer ring as there would then be no room for axial thermal expansion in the spindle.

I'm no expert, but this is the best of what I know. Ha... or think I know. :-)

Dave

The mock-up isn't mine, but one pulled off the interwebz. So I can't assure accuracy. The original bearings were ALL deep groove radial. I replaced the front set of 3 with matched AC bearings in a TBT configuration. 2 facing rear, 1 facing front. Thanks for the input, seems like the idea of adding preload to the outer races of the rear bearings is a bad idea.

 

[wesg]

The rears would be back to back, either made as a duplex set, or preloaded with spacers in your case. The outer races float in the housing.

Advantage over radials is you can set the clearance. Disadvantage is ... you HAVE to set the clearance.