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Thread: Firewood saw bearings
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15th May 2013, 02:39 PM #1Distracted Member
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Firewood saw bearings
So a guy wants me to machine a shaft for a firewood saw. The bearings he's supplied are SA 208-24. They are a self-aligning pillow-block bearing with an eccentric locking collar & grub screw. I've never seen these before. Can someone explain the idea of the collar? Is it meant to compensate for a poor fit? Or as extra insurance against heavy loads/vibration? I can't find out much about these bearings by googling. Just that they come up under tractor supply and they're inexpensive. The descriptions all mention the collar but give no detail about its use.
Thing is, I'm trying to figure out what kind of fit I should make them. Been reading the TAFE books and to be honest they're doing my head in. I've always fudged bearing fits, aiming for something like a push fit or maybe light press - and usually getting it wrong. But I feel I should make an extra effort, since it's a paying job* and it needs to be safe and reliable. This thing will take a 20" blade and run around 2000 RPM.
So if the locking ring means the fit is less critical I will relax a bit more. In any case I would appreciate suggestions for the appropriate fit. Thanks.
* Read: NON-paying since I expect to spend 3 or 4 times as long as I can bring myself to charge for...
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15th May 2013, 03:00 PM #2
Hi Bryan,
I've never used those, but I think the eccentric is just to allow for aligning the bearings so they are in-line with each other. The locking ring with the grub screw stops the shaft moving sideways. I wouldn't think fit on the shaft is all that critical, but I'd make sure the grubscrew locks into a key.
Worked on plenty of sawmills, but mostly on the controls side. Never took much notice of the saw arbor..
Regards
Ray
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15th May 2013, 03:12 PM #3Most Valued Member
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An explanation here Brian Y-bearings with an eccentric locking collar
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15th May 2013, 03:26 PM #4Most Valued Member
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This help?
http://www.google.com.au/url?sa=t&rc...,d.aGc&cad=rja
Nev.
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15th May 2013, 03:28 PM #5Most Valued Member
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Tolerance for normal application should be h7.
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15th May 2013, 05:50 PM #6Distracted Member
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Thank you that's a good link.
I'm sure it would if I could open it. I'm on my 'other' laptop and it has some issues. Thanks anyway.
Ok so as far as I can make out, h7 means anywhere from 12 um interference to 25 um clearance. I guess I will just be happy if I can get it within 10 either way.
Thanks.
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16th May 2013, 10:00 AM #7Most Valued Member
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Bryan
I use them all the time
they are very common
there is just one important thing that you must do
Lock the collar up firmly (not hard) in the same direction as shaft rotation.
This is very important!!! Dont be fooled into thinking its the opposite direction. That means that if two bearings are used and both collars are at end of shaft one does up clockwise the other anticlockwise but both in the same direction as shaft rotation.
they work similar to a cam lock...the eccentricity of the collar and inner race does the locking to stop the shaft rotating in the inner race.
what size are you after ?
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16th May 2013, 10:06 AM #8Most Valued Member
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and a nice slide fit (preferable)...to a very light soft hammer fit...no more
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16th May 2013, 10:22 AM #9Most Valued Member
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the grub screws just locks onto the shaft..no key nothing
the locking ring stops the shaft from rotating in the inner race...the collar is fixed to the shaft via the grub screw
I am still suprised at how often bearing counter staff tell inexperienced clients that they lock up in the opposite direction. How would they know they have never been taught or have read the installation book!! so they say what they believe would work.
During my time with another company I called all the service guys back into the office for a service meeting and tried to explain to them how it worked. After a couple of more warranty failures I realised that I had not gotten my message across to them so I had a perspex collar made and had another meeting..they still argued I was wrong...even after I showed them the installation procedure from CBC... and they still said CRAP....I then showed them how it all worked with the perspex collar...we never had another come loose.
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16th May 2013, 10:34 AM #10Most Valued Member
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16th May 2013, 01:07 PM #11Distracted Member
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Brilliant! Thanks Eskimo. Owe you one. Or two.
PS: I'm not after any - I have the bearings in hand. They are 1.5" ID.
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16th May 2013, 03:42 PM #12Most Valued Member
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oh and incidently... its very common to come across these bearings which have been fitted by others...if you cant unlock the collar by hitting it in the reverse direction to shaft rotation one should assume that they may have been put on incorrectly...ie tightened up in the opposite direction to rotation. They dont all come loose (or havent been given enough time) and stuff up the shaft or this has been my experience anyway...but still be carefull.
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16th May 2013, 03:49 PM #13
Hi Eskimo,
Nice work clarifying that, I've filed it away for future reference.
Regards
Ray