Why self aligning bearing?

[pgrbffParticipant @pgrbff]

I have just removed the top wheel from my bandsaw to replace the tyres and thought I should probably replace the bearings whilst I was at it. One is particularly rough. I don't have a bearing puller so I haven't been able to remove them. They are stamped 1205 and possibly IMI. I can only find 1205 as a self-aligning bearing, is that right? I would have thought you could get a standard bearing too in the same size?

[pgrbffParticipant @pgrbff]

[Nigel BennettParticipant @nigelbennett69913]

You probably can, but on my bandsaw the wheels can be adjusted angularly so the blade tracks correctly. I presume they need to be self-aligning so as to obtain this adjustment. If they're rigid you may be unable to get the blade to run well and it might keep coming off.

[Clive FosterParticipant @clivefoster55965]

A self aligning bearing would (theoretically) allow the wheel to twist slightly to equalise the load across the blade. Presumably this would reduce stress and wear on the tyre.

I'm somewhat dubious of the physics of the idea as the wheel will effectively be stabilised only by blade tension variations rather than rigidly by the bearing. Sounds to me like crowned pulleys evil cousin!

Clive

[Martin KyteParticipant @martinkyte99762]

Posted by Nigel Bennett on 28/02/2021 17:15:23:

You probably can, but on my bandsaw the wheels can be adjusted angularly so the blade tracks correctly. I presume they need to be self-aligning so as to obtain this adjustment. If they're rigid you may be unable to get the blade to run well and it might keep coming off.

I would have said that the the opposite was true?

regards Martin

[john halfpennyParticipant @johnhalfpenny52803]

I think you are right Nigel.

[Grindstone CowboyParticipant @grindstonecowboy]

I would also agree with Nigel, the self-aligning bearing would allow the tilting of the wheel necessary for tracking adjustment.

Rob

[Joseph Noci 1Participant @josephnoci1]

Tried to fathom what all has been said here and failed…How would a self aligning bearing do duty in this application? My understanding of such a bearing is that the inner race and outer cage can be on non-aligned axes and are not constrained. If this bearing were used in the bandsaw wheel the wheel would just flop about its axis all the time! Also, such bearing cannot be sealed and so sawdust would enter and destroy it soon anyway..

All the bandsaws I ever used had two normal bearings in the upper wheel, onto a shaft that tilted and so tilted the wheel. Where does the self aligning bearing fit??

The wheel has bearings which fit onto the shaft that tilts – the bearing cannot 'tilt'!

Makes no sense to me…

Joe

[pgrbffParticipant @pgrbff]

I spent some time digging out the congealed grease and although I cant be sure I think it is a standard bearing even though the part number stamped on it would suggest a self aligning. I can only see 8 balls and their diameter would suggest only ring of ball bearings.

[Dave HalfordParticipant @davehalford22513]

I understood a self aligning bearing allowed a shaft to go between two bearings like a layshaft that might not be exactly parallel to each other.

[Grindstone CowboyParticipant @grindstonecowboy]

In light of further comments, I may revise my previous posting I'm wrong again.

I was envisaging a pillow block type of arrangement where one end of the shaft could be moved up and down by a screw mechanism to induce tilt, the movement being between outer race and housing, rather than outer and inner race.

But there would need to be a bearing of some sort where the screw mechanism is, so not as simple as I first thought.

However, you CAN apparently get sealed self-aligning bearings, so something learned anyway

Rob

[Martin KyteParticipant @martinkyte99762]

Bandsaw wheels are crowned to allow the blade to track. Each wheel must be aligned so the crowns run in the same plane. Each wheel must be constrained by it's bearing and usually one of the bearing housings is adjustable for into and out of the common plane and for tilt. If you fit self aligning bearing the wheels will just flop about and the blade will come off. Effectively the bearing must oppose the crowning forces not just allow the wheels to fold inwards.

Probably not as clear an exlanation as some might give but that's it for now.

regards Martin

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, a 1205 is a self aligning double row ball bearing with either a parallel inner race that fits on the journal or a tapered inner race that is normally locked onto the journal with a tapered sleeve. Self aligning bearings are normally used as Dave Halford has said, where the bearing mountings are not guaranteed to be totally inline with each other, i.e. not on a truly flat plain.

Regards Nick.

[pgrbffParticipant @pgrbff]

Posted by Martin Kyte on 28/02/2021 22:13:10:

Bandsaw wheels are crowned to allow the blade to track. Each wheel must be aligned so the crowns run in the same plane. Each wheel must be constrained by it's bearing and usually one of the bearing housings is adjustable for into and out of the common plane and for tilt. If you fit self aligning bearing the wheels will just flop about and the blade will come off. Effectively the bearing must oppose the crowning forces not just allow the wheels to fold inwards.

Probably not as clear an exlanation as some might give but that's it for now.

regards Martin

Bandsaw wheels aren't always crowned. In Italy they are mostly flat. Flat wheels are better with wider blades, to run a narrow blade you need a crown.

[noel shelleyParticipant @noelshelley55608]

There are 2 sorts of self aligning bearing ! Those that have the OD of the outer race curved and a coresponding housing that it moves in to allow alignment of the shaft. I have seen round, square, oval and a plummber block type housings. The other type the ID of the outer race is curved to allow the balls and inner race to align with the shaft..

Noelt

[old martParticipant @oldmart]

As said, self aligning bearings are used in pairs spaced apart, to allow for missalignment which would destroy rigidly mounted bearings. They are not usually used singly.

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