Weldon air spindle

[RCaffin]

Need photos. Really really need photos.

I have built miniature air bearings, and they worked just fine, but they are also partly black magic.
And you will need a very good air filter upstream.

Cheers
Roger

[ozzie34231]

The pictures are of the actual bushing. There are two smiles in it which align with two set screws on top of the housing. Internally there IS an undercut in the center. BUT THERE WAS NO OPENING OR HOLE FOR AIR TO PASS FROM OUTSIDE TO INSIDE. If you read the patent there is no mention of air!
The last picture of the bushing is the bottom of it where I have cut a slot through it as an air passage as it is too hard to drill.
Hopefully, today or tomorrow I'll get to the Hardware store for some air fittings and give it a try. The shaft and bushing are clean now, accomplished with Scotch-brite and WD40.
Ozzie

[ozzie34231]

Couple pics

[ozzie34231]

Here is a link to a patent for an air supported spindle, such as the one in Weldon fixtures later than the patent previously mentioned.
https://patents.google.com/patent/US3030744
Note this patent is dated 1962 !
The author talks of extremely tight tolerances between shaft and bushing, and a slight taper in part of the bushing bore. In the drawing the air inlet is at the top and no mention of a desirable location for it. In the published plan for an air spindle by Duclos he specifies that the air should enter from the bottom to assist in supporting the weight of the spindle. it was with that in mind that I created the hole to be near the bottom.
We'll see what happens.
Ozzie

[RCaffin]

Interesting. That is not a conventional air bearing at all. I will have to take Mr Weldon's word for it that it works.

Cheers
Roger

[ozzie34231]

Well, my rig doesn't fly, uh, doesn't float.
With no air it turns very smoothly, although if I keep playing with it I sometimes hit a sticky spot. Turning on the air makes it tighter rather than floating. Pressure changes don't seem to matter until I get up to about 80lbs where it almost locks. Until I get to the higher pressures I can't detect any air escaping though I'm sure it does.
Measuring the shaft at various places show it is round and consistent along its length. The bushing bore is less than a thousandths larger.
Considering the torture I put it through getting it apart, it is surprising that it fits at all.
I'll put it on a shelf and think about next steps. I might put the shaft on a couple v-blocks and check its straightness and confirm its roundness. if it checks out I'll probably make a new bushing.

[ozzie34231]

Roger,
Do you have suggestions?
Thanks,
Ozzie

[handlewanker]

HYmmmmm......I have to ask myself....why would it get tighter with air pressure applied................second question is.....does the air get to the middle of the bush and can it be seen (heard) to do so with the spindle out of the bush.

BTW......I don't have the 2 screws in the body to hold the bush in like you show in your pic.......it appears the 2 cuts in the bush OD are to locate the screws.........maybe mine has paint hiding the screw heads.

Dwelling on the specs of the patent..........as this device is designed to make the grinding of the helix of a cutter super smooth as opposed to the heavy feel (inertia) of moving a cutter grinder table.......I would expect it to be an air spindle as without air it would be just a shaft in a close fitting bush, so there would be no gain to have it on a cutter grinder.

It "could' be used as a stand alone device on a base plate for grinding cutters if you coupled it up to a bench grinder........just needs a second base slide to do the infeed and depth control.

Very basically, this is all a CUTTERMASTER T&C grinder is, as it has an air spindle and a tilting motorised grinding head.

The tilting of the grinding head enables the grinding of the primary and secondary clearance angles without having to calculate the rise of the stone above cutter centreline of a conventional T&C grinder when applying a cutter to the periphery of the wheel..

I think some pics are really needed and I have just accessed a 1 TB hard drive that I stored a wack of info on......the pics are mostly of a Weldon air spindle on the USA EBAY site some years ago and show various bits of kit that the original offerings came with.

I'll post them on the next post as soon as I can get to the files.....Win 10 is like walking around in a strange city looking for a public toilet without a map.
Ian.

[RCaffin]

I agree with Ian - why does it get tighter when pressurised?

Other thoughts: if it was that easy to make air bearings which work like that, why do most (if not all) modern air bearings use the 'mass of small air pockets' (mosap) approach? The ones we built used the mosap approach and did work. We were measuring the change in torsional rigidity of a single wool fibre as the humidity changed. Kinda low forces.

On the other hand, I think I could make a good argument that this Weldon design should lock up when pressurised. The air flow could push the shaft to the bottom, cancelling the clearance there, and making the clearance at the top bigger. That could be a positive feedback thing: more air pressure, more sideways force.

OK, try something else. Could you try pressurising the thing with 1 psi (or less) instead of 80 psi? ie very LOW pressure. I have no idea whether this will work, but surely the thing did work once?

Cheers
Roger

[handlewanker]

Hi, I just went and read up the patent for this device........gee but the guy waffles on about cutter grinding and a device to grind cutters instead of focussing on the actual principle of how it works with the air cushion effect which is the main object of the patent.......my opinion.

The principle is that any medium that is fluid when exiting through a small gap will exert a unilateral pressure on opposite surfaces.....this is the principle that all plain bearings are subject to......auto engines apply oil at approx 50 PSI to allow the bearings to function......metal to metal contact and wear only occurs when an engine is starting up without oil pressure.

From what I can glean from the lengthy and tedious description, the "gas" is applied under pressure to an annular cavity in the middle of the bearing bore and there is a short taper in the bore to distribute the "gas" so that it is forced evenly around the shaft to exit at both ends.

It is apparent that as the "gas" is elastic, a close fit is needed to keep the shaft closely centralised in the bush, otherwise any side pressure applied to the shaft will allow it to deflect etc.......T&C applications are noted in this requirement.
Ian.

[handlewanker]

I agree with Ian - why does it get tighter when pressurised?

Other thoughts: if it was that easy to make air bearings which work like that, why do most (if not all) modern air bearings use the 'mass of small air pockets' (mosap) approach? The ones we built used the mosap approach and did work. We were measuring the change in torsional rigidity of a single wool fibre as the humidity changed. Kinda low forces.

On the other hand, I think I could make a good argument that this Weldon design should lock up when pressurised. The air flow could push the shaft to the bottom, cancelling the clearance there, and making the clearance at the top bigger. That could be a positive feedback thing: more air pressure, more sideways force.

OK, try something else. Could you try pressurising the thing with 1 psi (or less) instead of 80 psi? ie very LOW pressure. I have no idea whether this will work, but surely the thing did work once?

Cheers
Roger

Hi, an interesting observation Rog............if the shaft is deflected to one side due to side pressure being applied from the application making one side gap bigger I would think that as the air is elastic if, when you get more clearance you will have less force to resist the side move......the air pressure would drop as it exits via the larger gap and the other side would move the shaft over and so maintain equilibrium.

That means there will be more force on the thin clearance side due to less elastic forces pushing against the shaft side.........I'm basing this on going to the ridiculous that if you had a gap on one side of .5mm there would be less force present to push the shaft back and more air would be ejected from the bush end.

I think there is a cut off point where the clearance becomes the vital factor in the equation.........the balance of a small gap and an air pressure is a balance of forces that can be applied.

You might get a working air bearing with a clearance of 5mm all round but the air pressure and supply would be enormous......I think that would be the practical factor that narrowed down to practical dimensions means you need to have minimum clearance to utilise reasonable air pressure and consumption.........and as the air is elastic you don't want a huge clearance either or deflection will occur.

The oil film and incompressibility of oil in a car's big end bearing allows it to resist the huge downward pressure of the piston from combustion.....but only if the clearance is small or the oil will be squeezed out to the top of the bearing and it will hammer itself to death quickly.
Ian.

[RCaffin]

In short, while I do know about what I called mosap air bearings, I just don't know about this sort. But I am left to wonder - if this sort works, why the more complex 'mosap' ones?
If you had a 5 mm air gap, then the arguments would probably be very different.
Dunno.

Cheers
Roger

[handlewanker]

Hi, never heard of the MOSAP air bearing design........or any other type either........just have the Weldon one to play with.

Incidentally, the one on my CUTTERMASTER T&C grinder has the same design as the Weldon.......inside it appears to have 2 bushes with a gap between them.....could also be just an undercut in the bore.

I would think that the forcing of air under pressure through a small gap makes the molecules of the air compress to a more denser composition so less elasticity.......pure guess work here.
Ian.

[RCaffin]

There is quite a lot of info on the web about air bearings, but as far as I can see it is all about what I was calling mosap bearings. Please note: this is NOT an official name for this sort of bearing, not at all. But it does describe them. Lots of commercial offerings.

On the other hand, I have not found anything about straight annular air bearings. Any URLs would be appreciated.

Cheers
Roger

[ozzie34231]

The only obvious answer to why pressure stiffens the shaft is that the small hole in the bottom of the bushing is is creating a lift, forcing the shaft up. So, yes, trying a very small pressure will be interesting.
The Weldon manual talks about pressures around 70 lbs to 120lbs! Something is very different between my setup and a working Weldon.
It is difficult for me to experiment by messing with the bushing because it is so hard. Using a Dremel and a diamond blade it took me a hour to cut the air slot.I don't think I have any industrial carbide drills; maybe I could try a resharpened masonry bit. Evidently I need a much larger air passage, perhaps one that does not point at the spindle, if that's possible.
I think my first step is to do some close examination of the spindle. As I said in my first post, I used 4 tons of pressure and heat from a propane torch to separate the bushing and spindle.
I'll set up on my surface plate with a couple V-blocks to make certain the shaft is straight and round, (although every measurement I make of the diameter is within 2 tenths).
If I can't find a way to put a bigger hole in the bushing, I'll need to make a new one, not the easiest job, boring to sub-thou tolerance.
The air spindle designed by Duclos and built by at least a few, can be seen working very well on Youtube. It uses a spindle to housing diameter difference of 0.002" and air passages of 0.136". It is a bit smaller than the Weldon tool.

Ian,
may I impose on you to inspect your Weldon and give us/me some helpful information.
What is the size of the air passage?
Is it straight through?
What is the location relative to the bushing inner diameter? (near the bottom, centered?)
What is the spindle diameter?
What is the size of the bushing bore?
Appreciate your help,
Ozzie

[ozzie34231]

Today I first tried a minimum air setting; no different from no air, and as I increased it the shaft became stiffer.
I set the spindle, (shaft), on V-blocks, on my surface plate. Rotating it with an indicator right above the block showed a tiny bit of runout, less than .0005" on both ends. In the center the runout increases to .0015". So there is some bend in the piece but I'm not sure how significant that amount is. The shaft is about 16" long and the bearing is 5.5" long. Which means that only a third of the shaft is in the bearing at one time.
I found a carbide drill, 3/16", that I didn't realize I had and set about increasing the hole in the bushing. As I stood at the drill press, a thunderbolt thought hit me. It is time to send me to an old farts home. Why am I thinking I can't deal with the hardened bushing; I have dozens of carbide end mills!
When I recover from the realization of my senility in a day or two, I'll work on re-porting the bushing.
I really do think that the fit between shaft and bushing is too tight. Hopefully Ian can shed some light on the proper clearance.
Ozzie

[RCaffin]

I really do think that the fit between shaft and bushing is too tight.
But modifications are not reversible.

Cheers
Roger

[handlewanker]

Hi, too tight is not an error......the clearance is very small and it makes the consumption of air possible without a huge compressor.........at the same time the design requires a very close fit or the air will not be able to do the supporting bit..........it is not a case of the air blasting at the spindle but more a case of the density of the air becoming virtually a fluid like substance as it gets between the bush and the spindle.

Think in terms of a car's main and big end bearings which are soft white metal or some form of lead alloy......with oil at SAE20 - 50 viscosity and a pressure of around 50 psi the clearance between the bearing shell and the journals is approx .001".......it differs with all engines but is in that region for most.........if the bearings wear and the clearance gets bigger you will soon knock the bearings out.

Briefly, on my Weldon, the air passage is central to the shaft and straight through with 1/4 BSP size hole.

I can't remember if you said there was a section in the middle of the inside of the bush that was undercut.......it needs to be so as this is the air distribution point that directs it to either side of the bush.......without the "cavity' in the middle of the bush I don't think the air will be able to do it's job.........according to the patent description if I remember there is a section that is cut back and a short taper too.......from the middle of the bush the air will then push out evenly around the spindle.

Without the cavity in the bush bore centre the air will just force the spindle against the opposite side which appears to be the reason your spindle gets tight with more air pressure.

BTW.....the patent DOES indicate air or even CO2 gas as the medium for the sliding force.

Strangely, the oil in an engine enters the bearings by a single hole, but oil and air have different properties and the engine journals are rotating.

So, to sum up, you need to have a close fit, and also a cavity in the middle of the bush bore with the air inlet leading to it.....air inlet bore size is not important as long as it's adequate.

lastly, I have no real "expert" knowledge of air bearing design apart from what I have gained over the years of hands on use and observation, but what I have in house works and is practically identical in both cases.

BTW, a hovercraft works on the same principle of an air cushion between two close faces.......there is a cut off point where it will not work if the gap is too big no matter how much air you chuck at it.
Ian.

[handlewanker]

Hi, just did some in depth measurements of the bush and spindle and the layout of the holes in the body and bush bore.

The bush bore ID is 42.95mm at the end and 42.89mm at the centre by the recess in the bore.

This means the bore of the bush is smaller at the ends than the centre by .06mm........this would not be interpreted as a taper.

The recess in the middle of the bush is 44.5mm diam.

The spindle measured 42.86mm for it's total length.

The hole in the side of the body is in line with the BOTTOM of the bush......not the middle as i said earlier.......it's also tapped 7/16 UNC.....it appears to go to a groove under the bush that leads to the hole in the top of the bush.....the top hole is tapped 5/16" UNF.

Looking down the hole in the top of the body and you can see two grooves cut into the OD of the bush ........the 2 grooves are connected to the side hole as air flows out of it if the plug is left out.

I pressured the device with 40 psi and it floated and rotated like it was on ......air....LOL, well, ever so smoothly without a hint of friction.

There does not appear to be any screws holding the bush in the body so I assume it is a press fit.
Ian.

[ozzie34231]

Thank you Ian.
In your 2nd sentence I think you have the numbers reversed?
Anyway from the smaller dimension to the shaft is about 0.0012" in imperial.
I find it interesting that the air is routed to the top of the bushing and injected there.

Are you sure that there is not also one at the bottom, which would seem to make a lot of sense?

I have my shaft/spindle in the lathe, one end in a six jaw and the other end supported by its bore riding on the tapered part of an MT3 tool holder in the tail stock. I have a Tenths indicator against the front and another indicator on top.
At both ends I measure 0.0008" TIR, not sure why. In the center I get nearly 0.003" TIR. So my first impression is that it is too bent to machine straight and still work with the present bushing.
I have not put a tool to it to see if I can cut it. A file skips across it like try to cut diamond. It might be, as Ian suggested, that it is hard chromed, I have no experience with that.

I have read about torch straightening and this might be an opportunity to give it a try. Supposedly the steel has a memory of where it was before being bent, but maybe it was heat that bent mine.
If I can cut it I'll just treat it as a piece of stock, machine it to the best of my ability and then make a bushing to fit.

My thought right now would be to machine a spud that is a close fit to the inside of the shaft, and without removing it from the chuck, use it to hold the work centered and drive it. For the other end I'll need to make a plug with a center dimple. Both ends would then run perfectly centered on the bore.

Any thoughts?
Ozzie