Need Bearing Information

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clevinski

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Hi...

After ongoing issues with my mini-mill, I finally identified the spindle bearings as the culprit. The manual describes the two spindle bearings as identical 6206-RS types, but the actual bearings in the mill are a 6206-RZ upper and a 7206-AC lower. While the upper bearing is sealed, the lower is not. The "AC" designation apparently means "Angular Contact". I called the manufacturer, Big Dog Metal Works in California, and they confirm that the actual bearings in my mill are what is currently being supplied in new product. While finding 6206-RS is very simple, I'm having trouble locating a U.S. source for 7206-AC, though I have certainly not yet exhausted all options.

1. Can anyone describe what the bearing numbers mean?

2. It makes no sense to me that the lower bearing is not sealed. It would then require periodic regreasing, which would require disassembly of the mill to the point where the spindle is pulled. Any thoughts or comments on this?

3. Has anyone replaced the bearings in the Real Bull mini-mill before, and if so, what bearings did you use?

4. Can anyone explain the relevance of the "angular contact" designation on the lower bearing?

Many thanks for any information...
 
The 7206-AC bearing could be a old "U" series, where the internal race is split in the middle. It has a single row of balls and when assembled, can handle thrust from both directions. It could be a single row bearing without a split inner race, too. Back in it's day could be had in a class 3, 5, or 7 precision bearing. I've personally never seen a set or use them. My surface grinder has a similar bearing arrangement, too, according to the parts manual. They do come up on eBay at times, and you just about have to have a old bearing manual to help identify one, and sometimes that's still not enough. The guys that know what they have want $$$ for them. If you find one, expect to pay around $100-$200 for one. Go to a bearing house like Motion Industries, and pay much more.

BTW- Is this 7206 bearing a double row bearing?
 
It's actually the leading 7 that indicates angular contact. The AC indicates that it is a 30 degree contact angle (A) if I am reading it correctly. The Nachi site actually has a lot of info

http://nachi-tool.jp/bearing/

Angular contact bearings will only take load in one direction (at least the single row version, which the 7xxx series is), so make sure you get them back in the right way. Angular contact bearings make much better spindle bearings than radial contact bearings. The will take much higher axial loads, and tend to be built to tighter tolerances. Deep groove bearings (6xxx series) are cheaper, that's about the only reason to use them.
 
DMS said:
It's actually the leading 7 that indicates angular contact. The AC indicates that it is a 30 degree contact angle (A) if I am reading it correctly. The Nachi site actually has a lot of info

http://nachi-tool.jp/bearing/

Angular contact bearings will only take load in one direction (at least the single row version, which the 7xxx series is), so make sure you get them back in the right way. Angular contact bearings make much better spindle bearings than radial contact bearings. The will take much higher axial loads, and tend to be built to tighter tolerances. Deep groove bearings (6xxx series) are cheaper, that's about the only reason to use them.
Click to expand...

DMS 100% agree, BUT he has only ONE bearing not a pair. Thats the reason for suggesting the 6000 series deep groove bearing. It will handle thrust, not as much as a 7000 series. I doubt very seriously he's going to find the U series or a "QJ" series bearing.

Here's a QJ series bearing

http://www.ebay.com/itm/SKF-EXPLORE...=BI_Heavy_Equipment_Parts&hash=item3cd1e622e5
 
Those are some precision grade bearings, hence the price. I'm wondering what grade the OEM bearings are? Am I correct to recall that sometimes the grade is etched at the end of the bearing number?
Sounds like the US distributor was not very helpful, they must not hold spare bearings?

I have had some very good success changing the bearings in my Asian drill press and an old Buffalo 15" to DOUBLE row angular contacts. I just true up the quill (as close as I can get it- 0.0005" max TIR) in the lathe and bore the quill a little deeper to fit the new bearing. Of course, the quill must have room for this and the spindle should have a little extra dimension at the ID mount (not totally necessary). These bearings will accommodate axial loads in both directions and give a good deal of "stability" to the spindle. They are available plain, shielded, or sealed.

I use VX Bearings in California as much as possible, because they are very reasonable on most bearings, https://www.vxb.com/ VX has a vast inventory of current bearings. Their web site is somewhat difficult to navigate, but their phone and on line chat contacts are very, very good. I hope that my answer actually addresses your question in part. I do agree that it would be a good thing to have at least shielded bearings at the bottom location. I use sealed because I don't run my machines everyday.

Not every (Asian) manufacturer is all that astute when it comes to spindle design. My bench model "Dura-Craft" drill press had a conventional radial ball bearing at the bottom of the spindle and a (cheap) ball thrust bearing above that. The thrust bearing was doing nothing but performing as a spacer because until the balls in the lower bearing wore to a large degree those balls were accepting all the operational axial thrust. Since that lowest bearing was just a single row RADIAL bearing this was just a good design formula for bearing failure. The system would run for a while and then the characteristic rattling of a failed bearing would develop. Geoff
 
4gsr said:
The 7206-AC bearing could be a old "U" series, where the internal race is split in the middle. It has a single row of balls and when assembled, can handle thrust from both directions. It could be a single row bearing without a split inner race, too. Back in it's day could be had in a class 3, 5, or 7 precision bearing. I've personally never seen a set or use them. My surface grinder has a similar bearing arrangement, too, according to the parts manual. They do come up on eBay at times, and you just about have to have a old bearing manual to help identify one, and sometimes that's still not enough. The guys that know what they have want $$$ for them. If you find one, expect to pay around $100-$200 for one. Go to a bearing house like Motion Industries, and pay much more.

BTW- Is this 7206 bearing a double row bearing?
Click to expand...

Hi, 4gsr,

Thanks for your comments. This is a current production mini-mill, so the 7206AC should be a current production bearing. It is a single-row bearing. Due to the low price of the complete mill, I would be surprised if the bearing was that expensive, but who knows?

- - - Updated - - -

DMS said:
It's actually the leading 7 that indicates angular contact. The AC indicates that it is a 30 degree contact angle (A) if I am reading it correctly. The Nachi site actually has a lot of info

http://nachi-tool.jp/bearing/

Angular contact bearings will only take load in one direction (at least the single row version, which the 7xxx series is), so make sure you get them back in the right way. Angular contact bearings make much better spindle bearings than radial contact bearings. The will take much higher axial loads, and tend to be built to tighter tolerances. Deep groove bearings (6xxx series) are cheaper, that's about the only reason to use them.
Click to expand...

DMS,

Thanks for the very useful information...
 
4gsr said:
DMS 100% agree, BUT he has only ONE bearing not a pair. Thats the reason for suggesting the 6000 series deep groove bearing. It will handle thrust, not as much as a 7000 series. I doubt very seriously he's going to find the U series or a "QJ" series bearing.

Here's a QJ series bearing

http://www.ebay.com/itm/SKF-EXPLORE...=BI_Heavy_Equipment_Parts&hash=item3cd1e622e5
Click to expand...

Hi...

To clarify, the spindle has two bearings. In the parts list for the mill, these are both listed as 6206RS bearings, which I understand are sealed, deep-groove bearings. The actual mill (mine and current production; I checked with the US distributor) has a 6206RZ sealed, deep-groove, upper spindle bearing and a 7206AC non-sealed, angular contact, lower spindle bearing. So there are a pair of bearings but they are not matched. I'm assuming that the lower bearing is an angular contact to take more thrust load when plunging or drilling, but I know relatively little about bearings. I am very unhappy about the lower bearing not being sealed because it is at the bottom of the spindle and chips and such can actually enter the bearing directly, so I'd like to use a sealed, or at least a shielded, bearing in this position.

As I said, I know relatively little about bearings, so I don't know what the difference is between a "U" or "QJ" series bearing and a 7206AC...
 
110octane said:
Those are some precision grade bearings, hence the price. I'm wondering what grade the OEM bearings are? Am I correct to recall that sometimes the grade is etched at the end of the bearing number?
Sounds like the US distributor was not very helpful, they must not hold spare bearings?

I have had some very good success changing the bearings in my Asian drill press and an old Buffalo 15" to DOUBLE row angular contacts. I just true up the quill (as close as I can get it- 0.0005" max TIR) in the lathe and bore the quill a little deeper to fit the new bearing. Of course, the quill must have room for this and the spindle should have a little extra dimension at the ID mount (not totally necessary). These bearings will accommodate axial loads in both directions and give a good deal of "stability" to the spindle. They are available plain, shielded, or sealed.

I use VX Bearings in California as much as possible, because they are very reasonable on most bearings, https://www.vxb.com/ VX has a vast inventory of current bearings. Their web site is somewhat difficult to navigate, but their phone and on line chat contacts are very, very good. I hope that my answer actually addresses your question in part. I do agree that it would be a good thing to have at least shielded bearings at the bottom location. I use sealed because I don't run my machines everyday.

Not every (Asian) manufacturer is all that astute when it comes to spindle design. My bench model "Dura-Craft" drill press had a conventional radial ball bearing at the bottom of the spindle and a (cheap) ball thrust bearing above that. The thrust bearing was doing nothing but performing as a spacer because until the balls in the lower bearing wore to a large degree those balls were accepting all the operational axial thrust. Since that lowest bearing was just a single row RADIAL bearing this was just a good design formula for bearing failure. The system would run for a while and then the characteristic rattling of a failed bearing would develop. Geoff
Click to expand...

Hi, Geoff,

There is nothing on the lower bearing at all except the designation "7206 AC", so nothing to indicate grade. The US distributor does NOT carry replacement bearings; I checked with them. In fact, they had to disassemble a head to verify which bearing numbers were being used.

Thanks for the info on VX Bearings. It sounds like they would be an excellent source, and I will check them out.

With regard to some Asian designs, I have been really surprised by some actual design errors I have encountered. For example, the tailstock on my Asian mini-lathe has a quick-lock feature, but it has never locked well or released fully. I disassembled it (because I broke the adjustment bolt trying to prevent it from sliding) and discovered that the cams that lift the locking plate for the quick-lock moved about 80°, from the bottom, "no lift" position to almost on the side of the cam, providing almost no lift. The cam should be just past the highest point in the locked position. I assumed the quick-lock arm had just been tightened in the wrong position, but then discovered that the shaft is dimple-drilled in that location to accommodate the set screw. In other words, it can only go in that location. I drilled and tapped a new set screw hole in the handle (easier than disassembling and re-dimpling the shaft) and now it works like it was intended. This seems like such an obvious error that I sometimes wonder why these products work at all. Were it not for the fact that my shop is in my basement with no outdoor access, I would have much rather purchased old, used US machines, but that was not in the cards. Still, I've been pretty happy with the mini-lathe in spite of its shortcomings, and the shortcomings provide me with a series of ready projects...
 
Angular contact bearings, as mentioned above, are designed to take thrust loads in one direction only, with nothing except perhaps a weak retainer holding them together in the other direction, so they're almost always used in opposing pairs and preloaded. In a situation where the load will always be in one direction with only minor loads in the opposite direction, the second bearing of the pair might well be a cheaper radial bearing which can some small thrust load, i.e. just enough to preload the AC bearing. The 6206 radial bearing may also be chosen for its higher radial capacity compared to the AC bearing, if it's supporting belt tension loading.

The basic number (6206, 7206) denotes the bearing type and size, and is the same for all manufacturers. The letters vary from manufacturer to manufacturer, and denote other details, like seals or shields, etc... one manufacturer's 7206-AC might be somebody else's 7206-XY. Best thing is to identify the manufacturer of the original bearing (usually it's stamped on the bearing along with the part number) so you can figure out what the suffix means, then you can cross reference it to one that's available from your local distributor.
 
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