Live Centers

[RobinHood]

It all started with the rotary broach project....

Took a “ no name “ live center apart to see how it worked and see if I could adapt it for a rotary broach. It was also pretty crusty when turning the spindle by hand. It needed cleaning and possibly new bearings... Well, I have since refurbished three LCs: the “No Name” DM 313, a Hardinge HDC and a Skoda. The Hardinge was part of another tool purchase - the spindle felt like there was sand in it. The Skoda came with my SM 1340 and I knew it had issues.

All centers are working like new after a thorough cleaning and lube. Each center had different innerts which I thought might be of interest.

Here is a pic of the DM 313. It has a common ball bearing followed by a thrust bearing and a little needle roller bearing on the tail end.



The Hardinge has two precision angular contact ball bearings, ACBB, back to back separated by outer and inner race spacers. This whole assembly pushes against a thin, ground washer which sits on a raised boss in the back of the housing and presumably flexes under thrust loads. Me thinks that that is to compensate for the thermal expansion of the workpiece. Clever.

Here is the stack-up


Here you can see the raised boss at the bottom of the housing


And with the ground flex washer sitting on top of it. The pin locates the washer.


The spindle with both bearings, spacers and bearing retainer before they are inserted in the housing


All together


Yes, I did break the cage of the front ACBB on disassembly. So I made one out of Delrin. You can see the busted one (reddish brown colour) and the new (black colour) Delrin one in the stack-up pic above. The reason for the “sand in the bearings” feel was dried out grease. So I switched the lubrication to lost oil lube. You can see the newly machined radial groove and the oil holes in the outer bearing race spacer ring. I also located a little oil hole with a plug screw in the housing.

Lastly, here is the Skoda. It consists of a needle roller bearing with a split conical outer race that gets pushed into a tapered seat in the front of the housing. Then there is a conventional thrust bearing and at the very tiny tail end a small ball bearing.

Here is the stack-up

The white ring acts as a dust shield. This LC is set up from the factory as a lost oil lube system. Note the little oil port and screw in the housing.

Here is a view of the tapered surface into which the bearing cone mates.


There are no markings on the split cone. My guess is that Skoda made that in house. Note how the slit is at an angle (see prev. pic) so that the rollers can glide across it freely. Again, clever as one can take up any play in the bearing by pushing the cone deeper into the housing, thus compressing it, with the front retaining ring.

I’m sure there are many other LC designs out there. These are the three I came across so far...

 

[Alexander]

Very cool thanks for sharing.

 

[PeterT]

Yes, thanks. I've always wondered what was inside those. Looks like the answer is 'it depends' but generally pretty tight tolerance stuff. I guess that explains the cost.

One of my to-do list projects is a mini spindle for mini tool post grinder. Same deal, combinations of radial or angular contact bearings, spring washers or other temperature compensating tricks. Its one thing to make it from someone's plans, but another to understand how they were designed (I don't fully).

BTW I'm sure you know this, but I've had bearings sound great after a full clean & new lube. Sometimes though (upon destructive inspection under magnification) when the balls have been sitting in a fixed position for extended period, there can be a micro divots in the race on the contact points. Fresh lube or especially grease can mask this. So if you get a different kind of growl at some point, that might be why. Mind you my bearings were RC engines & the methanol raises hell attracting moisture out of the air, they are never really swimming in a bath of lubrication like yours.

 

[RobinHood]

I guess that explains the cost.

That could certainly be said about the Hardinge one: all the components were ID, OD and face ground. The ACBBs are New Departure 20203s. They each had “DXT 07/36” hand engraved on them. Someone had taken the trouble to check them on precision metrology equipment. There is a labyrinth seal between the spindle and the front retaining ring. Also, the big pressed on aluminum “debris deflector’s” OD and the housing have a labyrinth seal.
Compare that to the more economical plastic deflector on the SKODA or the “yarn in a groove” for the DM....

Here is a pic of the cheap DM “sealing system”


Excellent point about micro pitting/divots of bearings that have sat stationary for a long time; I will have to keep an eye out for that.

 

[Janger]

Thanks for sharing - interesting insides.

 

[Tom Kitta]

Does anyone know how to put these live centres from Skoda back together? I have 4 MT4 centers. I got all the bearings.
The thrust bearing goes in first this is followed by the split metal ring. The needle baring goes onto the spindle. It now appears you need to press the spindle into the ring with the needle bearing on the spindle.

But how? It feels this requires some precision equipment as even slightest out of line movement of the spindle gets the needle bearing to loose needles.

I assume if you have it apart you know how to put it back together.

 

[RobinHood]

Hi Tom, can you confirm that your Skodas have the same parts as the one I show above.

If yes, there should be no need for any special jigs/tools or any press to reassemble them. The thrust bearing has two hardenend washers (races) with one ID beeing slightly smaller than the other one (you probably already know this). On mine it was the washer with the writing on it that goes right after the roller bearing. So the sequence is (as you correctly pointed out already): thrust washer with larger ID first. Thrust ball bearing. Thrust washer with smaller ID. Put these three components in the base of the housing. Then put the tapered, split ring in the housing - it only fits with the taper towards the back. Slide the needle bearing on the spindle shaft - it slips on really easily by hand. Now insert the spindle with the needle bearing on it into the housing - again, it should just slip in with some resistance ( the resistance comes from the thrust bearing washer with the smaller ID engaging on the spindle ). The needle bearing should not have to be forced as it is a loose fit on both the spindle and its expanded outer race at this time. Once the spindle is fully seated in the housing, the rear ball bearing can now be inserted from the back. I just used a small socket to gently tap it home. Install the retaining clip. Screw on the end cap. I lubed everything up with light spindle oil at this point. Install the plastic front seal/wiper and the front retaining ring. I tightened it down with a face spanner until all the slop in the spindle was gone. The slop is removed by the split tapered roller bearing outer race compressing slightly onto the rollers. Secure the ring with its tiny set screw. Oil the whole thing some more through the oil hole, if desired, and close the oil port off. Your center should now turn freely and be as good as new.

Hope this helps.

 

[Tom Kitta]

Yes, parts are the same, I am just missing some of the plastic retaining rings.

I didn't realize the split ring is tapered nor there is smaller and bigger part of the thrust bearing. I try it today if I get a chance and see whatever I can put it back together.

Thank you for your help.

 

[RobinHood]

The plastic ring seems to serve as a seal. You can probably make yourself some out of PET or Delrin. Robin Renzetti (robrenz on youtube) just posted a video on how to cut thin rings.

Here is the link:

.

That’s how I would make them.

 

[Bandit]

Has anyone has had a News live centre apart? Wondering if what looks like an end cap, on the tapered tang end, is a cap and threaded???