Okamoto Surface Grinder Spindle Repair

Hi...After over 15 years of great service in my small grinding shop, the spindle bearings on my Okamoto 12 x 24" surface grinder needed replacement. I sent the spindle out to a spindle repair facility. They changed the bearings and charged me $2500.00. Times turned a little slow and we used the Okamoto probably 8 hours a week. Eighteen months later the bearings went again. I called the same repair facility and they said send it in and they will take a look at it. They called back and said coolant got into the bearings causing failure. They want to charge $2550.00 to fix it again. I raised concern that the machine had little usage and I didn't think the bearings should go that quickly where my Okamoto was used for over 15 years with no problem. He basically told me that new spindles are not like rebuilt ones. He said that the coolant flow was directly on the spindle causing it to get into the bearings. I told him we use guards that came with the machine and coolant was directed to the part on the chuck. I asked him to send it back and he says it's all apart and I would have to pay a fee to put it back together.

Sorry for the long post. Does this see right to you guys? Should the bearings go after only 18 months light use-age? Was the spindle repaired the first time but not sealed correctly? I would like your input. Thanks in advance. Gary

No way in hell. That guy's trying to evade his responsibility for a hack-job if you ask me. If he sent you a spindle that was supposed to be ready to go, it should be ready to go. A quality rebuild means it should last just as long as the first spindle did.

Was this one of the NH spindle rebuilders? Shame if so. I've used one with fine results, but you can always get a lemon.

At the least, this company should comp you for the alleged "put it back together" charge (especially if they didn't tell you up-front about it).

And I wouldn't mind a PM letting me know who did this...

Was there any change to the coolant used or the way it was used before and after? Are the bearings exactly the same? Same clearance, same precsion, same cage material? I see they are Barden but even a great bearing not seated well can go bad. I'd want to eliminate all possibilities when arguing for them to step up.

Is that a buildup of shmoo (mixed grease and coolant) in the races, or did they fill them with grease that much? Or is this an oil lube setup?

If there's a seal they messed up during assembly, I'd say it's on them. Do you have a parts assembly diagram that would show the parts and stacking for review?

Okamoto uses "Ultra Precision" (P2 or ABEC 9?) 7210 bearings mounted face to face. The Barden bearing shown is a universal mount design with a medium preload. It could be mounted back to back or face to face.

Okamoto uses spacers between the two bearings that make up the pair. It may be that the spacers are selected during assembly to provide the correct preload for each set of bearings. Mounting a randomly selected pair of bearings with the fitted spacers could result in a significantly higher preload than intended. There may also be a problem if the spacers that were designed for face to face mounting were used with the bearings back to back.

The most likely cause for a short life on a bearing assembly is grit contaminated grease. Incorrect mounting as described above would also be high on the list of probable causes.

A close examination of the bearing race would be useful. A failure caused by corrosion or grit will not have the appearance of a fatigue failure from high loads.

We have several 12' x 24' okamoto grinders. When the spindle bearings go bad we just buy an entire new cartridge spindle from okamoto.

We run some of our machines 8 hrs a day and I have some spindles that easily make it 15+ years.

I'd have to agree with eKretz that something fishy here.
Almost like you found a parts changer and not a spindle rebuilder.
But then what to do given the hole you are now in?
Tell them to take a long walk off a short pier, get the disassembled parts back at no cost and go elsewhere for the fix?
Obvious that their rebuild is not so good.
Personally if having done this and the results from you I'd be crying over the fail, the whys and bending backwards to fix it.
The Okos front end is not really great protected for flood grinding but even if just plain water with no coolant additive this is bad.

2 cents from the not knowing but you seem now to be trapped. When that happens I tend to run anywhere.
Bob

I would probably just pay the re-assembly fee and have them send it back, send it somewhere else with a good reputation. I'm not a spindle specialist but it does seem fishy.

Did they replace the seals? Or are seals external to the bearing assembly and part of the responsibility of whoever installs the spindle? Hard to judge without knowing the details of how everything is put together.

Here is a 7010 bearing with seals and a p3 class.
the p3 has a turning accuracy equal to a class 9.
7010CTRV1VSULP3 - NSK Super Precision - Quality Bearings

A few more comments.

If there was a spindle bearing failure due to coolant contamination only the front pair would have been damaged. The spindle repair shop is proposing to replace both the front and back pairs. That would imply that the back pair was damaged from some other cause.

When the spindle was originally rebuilt there should have been a test report of measured axial and radial run out at the spindle nose, a set of vibration measurements, and the temperature rise of the spindle after a four hour run time while operating at 1800 rpm.

The temperature check would have revealed a improper bearing preload.

The preload of a pair of angular contact bearings mounted face to face increases with spindle temperature. The steady state temperature rise will depend on the spindle housing design, operating speed, and the initial preload. Installing a set of medium preload bearings and expecting the pair to maintain that preload as the spindle warms up is unrealistic. The Barden catalog shows a medium preload bearing steady state expected temperature rise of 40 deg F .

The expected operating life of a angular contact bearing is proportional to
(bearing dynamic load rating/actual bearing load)**3

A grinding spindle bearing pair will have 1/8 of it's expected life when the preload is doubled.

A heavy ball bearing preload is about twice that of a medium preload.

I suspect that the spindle rebuilder either did not complete a spindle break in procedure or , if they did, ignored the high steady state operating temperature. A knowledgeable rebuilder would have adjusted the spacers to limit the temperature rise or measured the bearing pair axial stiffness after spindle warm up and adjusted the spacers for a medium preload.

Hi...I contacted Okamoto and they did a search on my machine. Apparently the machine was purchased in 1994. That would be 27 years old and not 15 like I said in my original post. I have another Okamoto that was purchased in the "80's. That spindle works great and has never been replaced. For a spindle to go in under two years is very suspect. The original people that replaced it in the first place says they will discount the spindle repair to $2,000.00 this time around. I'm having them send everything back and go from there. Thanks for all your responses. I'm sending a link of this thread to the people who did the spindle repair. Gary

If I were you, I'd immediately ask for a full step by step outline of the procedures performed as part of the rebuild. Then you'll know if they even know what a proper rebuild entails. There are a lot of intricate steps in a proper rebuild.

Also, +1 on whoever said to get the original inspection report (Robert I think?) I'd want to look at that inspection for the reassembly work and see what kind of preload they had on those bearings. I guess it goes without saying but it's a bad sign if they don't have an inspection report...You can inspect the old parts yourself too and see if there are any signs of bearings losing preload or similar things.

RJT said:

Did they replace the seals? Or are seals external to the bearing assembly and part of the responsibility of whoever installs the spindle? Hard to judge without knowing the details of how everything is put together.

Click to expand...

I have never seen a surface grinder spindle with seals.
The shaft area in front of the bearing should maybe be loaded with grease?
If running wet it is a good idea to let the spindle run a while after the coolant turned off.
Otherwise the coolant dripping off the wheel guard and such can weep into the bearings if the spindle is stopped but water is dripping.

I run a lot of water (10 GPM) and the nose on the Okamoto is certainly a weak spot. Yes, for sure the coolant does get to the back bearing.
We had to make an addition to the front nose piece. This not a real problem on the Harigs, B&S, Reid, or Clausing all running the same flood coolant setup.
It is also helpful to not set the SG level but with a very, very slight down in front.
Bob

Okay...they sent the unassembled spindle back to me recently. I looked at the bearings and when I shake them they make a lot of noise and seem loose. It sounds like a baby's rattle. Also, I can move the inner race back and forth with little pressure. Are high Precision bearings supposed to sound like this?

They're angular contact bearings, so it's normal for them to be somewhat loose with no preload applied. And yes they will slide in one direction a bit, that side has almost no lip on many angular contact bearings - you can sometimes slide the bearing so that it comes apart in that direction with a little force. The rattling is concerning though... Did they assemble the bearings dry? Or maybe they were cleaned and degreased after disassembly?

How did the spindle look where the bearings mount? How do the balls and races look? Do they spin smoothly or are they gritty feeling?

Hi Gsharp10:
They made the claim that the second failure was due to coolant ingress.
Did they show any proof to you that their diagnosis was correct?
Obviously they would need to back up their claim.

I'd like to see photos...obviously it's too late now, but whenever I get a claim like this I always ask them to explain their diagnosis and show me the evidence that supports that claim.
That way everyone stays honest.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

The surface of a unworn bearing race will be a diffuse reflector of light. A bearing that has been exposed to a small amount of grit during operation will have a polished race from the unintended lapping compound. A bearing exposed to a large amount of grit and then run will have a rough surface. The last situation occurs because the balls are reduced in size from the grinding action and the bearing assembly then looses its preload. Without a preload the grit cannot be broken down to a lapping compound.

A overloaded bearing that has failed from fatigue will have a pitted surface on the race.

If the race cannot be examined, the bearing pair with the matched spacers can be checked by reassembling the spindle and placing a known axial load on the spindle nose and then measuring the nose axial deflection. The Barden catalog lists the bearing axial spring constant (stiffness) for the low medium and high preloads. The known load divided by the spring constant will give a unworn bearing deflection which should match the actual bearing deflection.

The test needs to be done with a spindle that has been run for several hours and has reached operating temperature.

The Okamoto parts catalog gives the bearing pair and the two matching spacers a single part number. This is one of the reasons why I suspect that the bearing failed from fatigue after a short operating time. The replacement bearings did not have the correct matched spacers. My understanding is that the spindle was removed from service due to noise or poor finish rather than a complete loss of preload.

Thanks for all your help. I ended up just buying a new cartridge spindle from Okamoto as Cash suggested earlier. I was surprised how inexpensive it was. I'll never waste my money on spindle repair again unless there is no other option, and will definitely NOT send it to the people that tried to fix it previously. If anyone needs spare Okamoto spindle parts..I got some!