Question about Engineering Ratio of Cast Iron Bearings

cash · May 6, 2019

I am looking to see if somebody has a chart/info on what the ratio is on the fit for in my case what would be on a cast iron bearing. I know it this info has to be in the Bible somewhere.

To be specific I am trying to confirm how the fit on my carriage bore should be in comparison to the table on a 120" Blanchard we are rebuilding.

The pinion gear and bull gear are terrible worn, so this is why I am doing some further checks. The bores do not appear to have any major wear or scoring.

The table rides in an oil bath and the hub is lubricated via the diagonal grooves to draw oil up the hub/bore. Table rotation is a max of 12 rpm.

The diameters I am working with are 21.375" and it has about 4.500" width of contact area.

We are thinking up to 12' a standard of .0005" per inch is used. After this we are not sure.

DMF_TomB · May 6, 2019

i would be wary or cautious using much over .002". too much pressure can break cast iron.

Tyrone Shoelaces · May 6, 2019

cash said:
I am looking to see if somebody has a chart/info on what the ratio is on the fit for in my case what would be on a cast iron bearing. I know it this info has to be in the Bible somewhere.

To be specific I am trying to confirm how the fit on my carriage bore should be in comparison to the table on a 120" Blanchard we are rebuilding.

The pinion gear and bull gear are terrible worn, so this is why I am doing some further checks. The bores do not appear to have any major wear or scoring.

The diameters I am working with are 21.375" and it has about 4.500" width of contact area.

We are thinking up to 12' a standard of .0005" per inch is used. After this we are not sure.

What's the lubrication system like and how clean can you keep that area ? All things being equal and the oil supply being good I'd say 0.0005" per inch of dia is about right. I don't think I'd be looking to make it any tighter a fit.

Regards Tyrone.

cash · May 6, 2019

Tyrone Shoelaces said:
What's the lubrication system like and how clean can you keep that area ? All things being equal and the oil supply being good I'd say 0.0005" per inch of dia is about right. I don't think I'd be looking to make it any tighter a fit.

Regards Tyrone.

Ah- sorry- I should have added this- the table rides in an oil bath. Then the hub has diagonal grooves which will pull the oil up to lubricate the bore. I will do some editing.

Milland · May 6, 2019

cash said:
Ah- sorry- I should have added this- the table rides in an oil bath. Then the hub has diagonal grooves which will pull the oil up to lubricate the bore. I will do some editing.

Hi Cash,
So is the oil at a static pressure, and literally just a pool of oil in the shallow basin shown in the pics? And the action of the table turning gives a "low key" hydrodynamic bearing for both the planar segments and the central spigot? And the angled slot in the spigot then draws some oil up to lube the full height of the pilot diameter?

At so low a max RPM, it wouldn't surprise me if there's not much "driving" of oil up to the full pilot height, or certainly the flow would be low. I'd be tempted to try to adapt a motor driven oil pump (possibly from a truck engine or similar) through a specific feed line to get oil up along the pilot spigot, and perhaps the drive gear too. The main planar bearing are likely fine if submerged in oil.

Having a pumped oil setup would also allow adding a filter, if there's not one in the system as it stands.

I'm actually leaning towards a smaller gap for pilot to table bore, I'd not want it over ~.003-4" to avoid excessive play and lowering capillary/surface tension dynamics of oil within the gap if it stays as passive lubrication. This is just gut feelings, I'm not a fluids engineer.

cash · May 6, 2019

Milland said:
Hi Cash,
So is the oil at a static pressure, and literally just a pool of oil in the shallow basin shown in the pics? And the action of the table turning gives a "low key" hydrodynamic bearing for both the planar segments and the central spigot? And the angled slot in the spigot then draws some oil up to lube the full height of the pilot diameter?

At so low a max RPM, it wouldn't surprise me if there's not much "driving" of oil up to the full pilot height, or certainly the flow would be low. I'd be tempted to try to adapt a motor driven oil pump (possibly from a truck engine or similar) through a specific feed line to get oil up along the pilot spigot, and perhaps the drive gear too. The main planar bearing are likely fine if submerged in oil.

Having a pumped oil setup would also allow adding a filter, if there's not one in the system as it stands.

I'm actually leaning towards a smaller gap for pilot to table bore, I'd not want it over ~.003-4" to avoid excessive play and lowering capillary/surface tension dynamics of oil within the gap if it stays as passive lubrication. This is just gut feelings, I'm not a fluids engineer.

yes- just static pressure- no pump to pressurize it and not really a way to modify.

FWIW the mattison grinders basically used the same set up for oiling. But for the bore bearing they had a ball bearing.

Milland · May 6, 2019

cash said:
yes- just static pressure- no pump to pressurize it and not really a way to modify.

FWIW the mattison grinders basically used the same set up for oiling. But for the bore bearing they had a ball bearing.

With the single drive gear and irregular reaction loads from part to grinding head, I'd expect the table to bounce and orbit as it rotates with a passively fed oil lubed bearing. So I can see the benefit of a more controlled center of rotation from a ball bearing.

Maybe dress the central spigot, then put a liner in the bore of the table made from a friendly material like a softer bronze or even a polymer? A sheet of porous bronze might act like a giant oilite bearing to wick oil up the full length of the pilot spigot.

Milland · May 6, 2019

DMF_TomB said:
i would be wary or cautious using much over .002". too much pressure can break cast iron.

It's a clearance fit for bearing rotation, not a press fit, Tom. But the point is well made if it was a press.

JS · May 6, 2019

Running 12 rpm , I believe that a .0005 per inch would work.

You could probably run a little tighter.......maybe .005 to .006

What type of oil are you using in the machine?

Ballmill 40” journals are running .0016 per inch clearance at 18rpm, but that’s forced lube 5 GPM per bearing and around 150 tons of machine....

Milland · May 9, 2019

Hi Cash,
Come to any conclusions on the bearing rehab?

gbent · May 9, 2019

Fluid dynamics doesn't care much about the diameter of the journal, it just knows about film thickness and viscosity.

cash · May 11, 2019

We did some inspection yesterday. On mt 120" hub vs table there was .022" clearance. This sure seems like a lot to me.

We then checked out my 100" Blanchard hub and table. Wow, that was a difference, that only had .008" clearance.

Both machines appear this was an OEM fit and no modification/sleeving was done.

I am leaning towards sleeving the 120" table to we have about .006"/.008" Clearance. I have an 84" Blanchard which is running and when we have had it apart in the past is in excellent shape, I am thinking I will pull that table to get a 3rd opinion.

number 2 · May 11, 2019

I have no experience with these machines or that style bearing but .006” or .008” for a 21” bearing seems like a pretty neat fit. Not saying it won’t work though but good fits become tight fits as the diameter increases.

One of my common thoughts on these things is no one will notice if it’s a bit loose, but they will if it’s a bit tight. Especially if galling happens. Cast iron needs a little more clearance I believe in case the metal breaks of in a chunk and jams between the two.

Were the two bearings you compared comparable in size and material?

Do the two machines you compared have the same oil also?

Zahnrad Kopf · May 11, 2019

cash said:
We did some inspection yesterday. On mt 120" hub vs table there was .022" clearance. This sure seems like a lot to me.

We then checked out my 100" Blanchard hub and table. Wow, that was a difference, that only had .008" clearance.

Both machines appear this was an OEM fit and no modification/sleeving was done.

I am leaning towards sleeving the 120" table to we have about .006"/.008" Clearance. I have an 84" Blanchard which is running and when we have had it apart in the past is in excellent shape, I am thinking I will pull that table to get a 3rd opinion.

Full disclosure - I've only ever machined two large hubs/bearings like you refer to. Both were kept to 0.0005"-ish/Inch fit. ( one was 0.0004" and the other was 0.0005" ) Both ran for years before I lost track of them.

I have got to think that the 0.022" is very excessive. That offends every measure of reason. Even 0.008" sounds loose to me. I think that as long as you don't get it excessively hot that 0.006" would be where you want to be.

cash · May 14, 2019

number 2 said:
I have no experience with these machines or that style bearing but .006” or .008” for a 21” bearing seems like a pretty neat fit. Not saying it won’t work though but good fits become tight fits as the diameter increases.

One of my common thoughts on these things is no one will notice if it’s a bit loose, but they will if it’s a bit tight. Especially if galling happens. Cast iron needs a little more clearance I believe in case the metal breaks of in a chunk and jams between the two.

Were the two bearings you compared comparable in size and material?

Do the two machines you compared have the same oil also?

The bearings were comparable in size and material. The machines use the same oil.

Table weights are similar, 21,000 lbs and 15,000 lbs

Dan from Oakland · May 14, 2019

Just from a general engineering standpoint, what prevents metal to metal contact between the table and underlying structure on startup? I imagine the weight of work pieces on a 100" machine are not trivial plus the weight of the table(s) themselves would add up to significant loading on a somewhat "dry" bearing. Seems a pressure fed arrangement on the thrust bearing itself would be beneficial- at least for startup until a fluid film gets established. What am I missing ?

adama · May 16, 2019

I would feel 22 thou is getting to the sloppy side of things, 6-8 thou sounds about right for a reasonably tight running fit.

One thing worth considering, that no one else has mentioned, you need enough slop in there so its not a guiding element on that style grinder, it has to have enough play the table can run on - in the axial bearing ring to be accurate for the grinding, it just wants to stop it going to far, so in some ways, even 40 thou probably would not remotely have the negative effects a lot of people would think it should - would have in a lot of other applications. 12 RPM is also incredibly low speed, would expect most solid metal bearing clearances to be found more for larger boat engine type stuff were your into low hundreds of RPM.

Hence it may pay to just leave it as is for now if its not causing a current issue. Tightening it may well change gear meshes depending on the table drive setup? and that may well in turn cause rapid gear wear if they are all nicely bedded to each other at the current normal sloppy point of rotation.

CarbideBob · May 16, 2019

Do people understand that this the center bearing on a Blanchard chuck?
Any influence onto the 90 degree way axis is bad in capital letters.
Tight or "controlled" here is a not a desirable thing.
Yes the wear is high but keep it loose upon rebuild. You certainly don't want this as the guiding member.
Wobble or wander in this axis means jack shit unless you are grinding controlled OD stepped shoulders on a Blanchard which is rare to say the least.
Tight fit here is simply bad and a source of problems when people go after it. This is not a lathe or a mill and should not be thought of with the same logic.
Bob

Dan from Oakland · May 16, 2019

Agree 100% as long as you can maintain an oil film of some sort to prevent further wear, and that's the elephant in the room in this case.

john.k · May 17, 2019

A certain ammount of sidefloat may be beneficial in limiting grooving of the flat pads.The gear mesh could likely tolerate a 008 -010 variation without ill effects.

Question about Engineering Ratio of Cast Iron Bearings

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