Grizzly G0935 Spindle Bearing Surprise

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jdeng

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howdy forum. new member here but long time reader. I've had the Grizzly G0793 G0935 for about a year now. AFAICT it's a clone of the sieg sx3 and it's been a pretty good machine. It has very low hours on it, maybe 15-20, and I've been very kind to it.

A couple of weeks ago it started to stick at very low RPMs so I figured I'd take a look at the spindle bearings. According to Grizzly's part list (I can't link as a new member) and the disassembly doc for the SX3 that I happened upon (on the arceurotrade site), I was expecting to find a tapered roller bearing at the nose and an open cage bearing + thrust bearing at the splines. Well imagine my surprise when I found an angular contact bearing at the nose paired with a single shielded ball bearing at the spline...with no thrust bearing.

Now this appears to be intentional as the preload spacer used on top of the shielded bearing is thick enough to make up for the missing thrust bearing. But the angular contact bearing is indeed shot. it was still well-packed but there are high and low spots. So, I'm left with a few questions:

1) Why was there no thrust bearing? is the presumption that the angular contact bearing can handle the thrust load well enough?
2) could the lack of thrust bearing have caused the (very) premature failure of the ACB?
3) After replacing the ACB, should I add a thrust bearing anyway (coupled with a thinner preload spacer)?
4) Or should I go back to a tapered roller bearing per the (inaccurate) mfg drawings?

I've ordered all the permutations to reassemble this in whatever manner makes the most sense. Meaning I have in hand
- 1 angular contact bearing
- 1 tapered roller bearing
- 1 thrust bearing
- 1 shielded ball bearing

I suppose I could try either/or and see how it feels but I thought to check here first to see if y'all had any guidance. Appreciate your time and expertise

edit: corrected model number.
 
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You need to double check your model number, G0793 is a drill press.

The deep groove ball bearing can be used as an angular contact bearing, it doesn’t have the same axial capacity as a true angular contact bearing, but it will work provided you stay within it’s limits. I haven’t seen the manual yet, so going by your description. Since the DGBB being on top is only providing radial locating and preload for the ACB that is taking the thrust load, it is probably fine to use the same arrangement. That is assuming the way it is designed it can provide preload. Many Chinese bearings are poor quality, so it is more likely it was just bad bearings, or poor installation rather than the design that caused your problem.

Before installing new bearings, check the spindle and housing bore for roundness, if they are out of round, the bearings can fail quickly.

I can’t answer why you have an ACB in there and the manual shows a TRB, must have been a design change somewhere and the manual hasn’t caught up. An ACB will have a lower thrust capability than a TRB of the same size, but a higher speed capability. You would need to take some measurements to see if a TRB will fit in there, they are wider and there might have been changes to the housing and spindle that make switching between the two more involved.
 
Darnit, I triple checked that post and totally missed the model number error. Thanks, it should be G0935. Will update.
  • I had assumed the ACB was poor quality. Model number is 7007A, McMaster has them for $400 and chinese resellers have them for $40/pair. I ordered a genuine super precision nsk from a reputable ebay seller.
  • Thanks for the tip on roundness. Will chuck up the spindle and quill/housing this week.
  • The TRB should be identical, will double check today when it arrives. If I choose that route, it sounds like you're saying I won't need the thrust bearing. Assuming the engineering on the ACB "upgrade" was intentional, and if the TRB has a higher thrust capacity, then I shouldn't need to add it back in?
 
I looked at the manual and now I have a better idea of how the TRB version works. The ball bearing can act as an ACB for preload, but it looks like they used the thrust bearing to do that and the ball bearing is only for radial loads in the version of the spindle in the manual. My guess is since the TRB can handle higher thrust loads, they used the thrust bearing for preload instead of the DGBB like with your version. Spindle bearings don't need a lot of preload on them, so I'm thinking the DGBB on your spindle will work fine with quality bearings without having to install a thrust bearing.

I didn't have the 32907 TRB in my bearing catalog and did a search on that and it is the same width at the 7007A ACB, so from that end it is fine, but the 7007 has on OD of 62mm and the 32907 55mm, so to get that to fit, you would need to sleeve the quill, and possibly make more internal changes. You can verify that when the bearings arrive, but it looks like you will need to stick with the ACBs.
 
Thanks @Ischgl99

just to make sure I understand, is having an ACB generally a superior setup? I've seen various mods/videos of folks replacing their TRB with ACBs on hobby machines, so I guess I've always assumed they're better...lower friction, less heat, easier to preload. But maybe I'm just conflating the higher cost with performance.
 
AFAIK tapered roller bearings, quality being equal, tend to be more suited to lower speed, higher load applications. No particularly critical reason to avoid them but they do need to be well aligned when installed and need the correct preload applied.

For hobbyist machine purposes (although, given some of the honking great machines some people have in their shops, this may be a generalisation too far!:grin: ) I think angular contact bearings generally are considered just as suitable, if not more. Angular contact bearings won't generally take quite as high a load as tapered roller bearings but can handle higher speeds.

One of the advantages of angular contact bearings is the fact that you can get them in sealed form factors and thus there's no maintenance of grease.

They also tend to be more forgiving where getting the preload correct is concerned.

Finally, they can be a good quick 'upgrade' when the manufacturer has cheaped out (looking at you Chinese mini-lathe manufacturers!) and used a pair of single row deep groove ball bearings as spindle bearings, as they can be easily bought with the same dimensions as the single row deep groove bearings.
 
ACBs are generally used for higher speed applications than TRBs. A roller bearing is more rigid, but generates a lot more heat due to the wider contact patch, so they generally get used in low speed, high load applications as SouthernChap mentioned. You need to preload a bearing for its entire operating range, so if something generates more heat, the preload might be on the low side at lower speeds to make sure it doesn't lock up at higher speeds. The lower preload will result in lower rigidity, so something that can be more consistent across the range, would be the best choice.

For lower speed applications in the 2,000-2,500 rpm range, I don't see it worth upgrading from TRBs to ACBs, but at higher speeds, it can have an advantage. Some of the conversions you see are people running the machines at higher than design speeds, in that case, it could be worth it.

Another advantage of ACBs, there is a wide range of precision classes available off the shelf. It can be difficult finding precision classes in TRBs, and even more difficult if you are looking for one in inch sizes.
 
Thanks! This is super useful intel. FWIW the G0935 has a top speed of 5500RPM and I do use 5k-ish speed pretty often.

It makes intuitive sense that the TRB could handle more radial load. I usually stick to 3/8" and smaller for my tools and I hardly ever do any plunging or drilling. So it sure seems like the ACB is the winner...except the price! The replacement I bought is an abec7.

Will update once I get everything sewn back up.
 
I didn't notice the top speed of your mill when I was checking the manual, at that speed, I would prefer ACBs. As you are finding out, ABEC 7 bearings are very expensive. McMaster-Carr sells good quality, I don't know which brand they supply for the ABEC 7 bearings, but I have gotten FAG and INA bearings from them in the past. Another place to look for bearings is BDI, they are an industrial bearing supplier and the prices are generally decent. www.bdiexpress.com

Something of interest, I went back to check the manual to see if the top speed is rated lower with the TRBs, in the specification section, it lists the spindle bearings as angular contact, but the parts drawing gives a tapered roller part number.
 
great resources. I'll admit to taking a chance on an nsk reputable ebay seller.

Nice catch on the specs. Now that I'm able to post links I'll also share the teardown doc (that I only found after I struggled to find the quill pin behind the DRO). Sieg SX3 Spindle refurb starts on page 11.

I'll note the SX3 does *not* look identical to my G0935 but it's very close. Sieg also makes the SX3.5 that *is* identical but I can't find it for sale anywhere other than arceuro and no luck on documentation.

The SX3 has a top speed of 3000 and the SX3.5 is 5000. Makes sense that upgrading the spindle to handle higher speeds would be a primary change.
 
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