[Zathras]

Guys,

I'm investigating options for a low speed spidle required for a telescope tracking mount. Yes very slow speeds which requires axial loads to be supported. I guestimate around 35kg for the max equipment weight with a decent shaft dia of 50mm 7xxx alu. I'm a little worried about starting friction for such a bearing so it may not be suitable since it's important to have very smooth motion, at least when changing direction. (Just looking at a cheaper option than ABEC7 A/C bearings!)

Problem: I cannot find any design info or a supplier of tapered (phosphor) bronze bearings/bushings.

Only info I found was for lathes that use(d) these bearings in the headstock.

Anyone know of any sources for info, texts, manufacturers???

Thanks

[WhiteTiger]

Static load coefficient of friction for general copper-lead alloy runs about .22, phosphor bronze and brass run .35, oil impregnated sintered bronze .12 to .15 depending on grain size and compact. Polished steel on polished steel is .8, for comparison. (I happened to have my old reference sheet within reach )


Tiger

[WhiteTiger]

Personally, I'd opt for 1 needle roller bearing and 1 needle thrust bearing. Best load distribution and least risk of stick/slip, imo.

Way cheaper than abec7's that will give you the same size reference circle.


Tiger

[NC Cams]

a ROLLING element bearing will give the lowest starting friction.

Smallest package size is/are needle bearings as in drawn cup style from Torrington. Ditto that for axial thrust

Next is tapered roller. These have charm as they have reasonable package size and will absorb axial thrust. Friction can be a bit higher as you approach 0.000 axial end play as the ends of the rollers rub on the flanges in sliding fashion as opposed to rolling. As long as you use light oil lube and refrain from preloading them, they'll probably last a lifetime in the application.

Ball bearings will give very low rolling friction but package size (radially and axially) gobble up space pretty quick.

Unless you're really lucky, you won't get ABEC 7 accuracy unless you spend the bucks. Previously posted bearing links will provide you with the runouts to expect for ABEC 7 ISO P4 versus ABEC 1 (P0) or ABEC 3 (P6) or ABEC 5 (P5).

[Zathras]

WhiteTiger
Thanks for the data. At least I can narrow down bearing composition.

NC Cams
I appreciate your expert advice. Thanks

I thought of using needle bearings but once I had seen some commercially built German equatorial mounts use taper roler bearings (2x each axis) I pretty much settled on it. But there are other successful (generally smaller) designs that use the needle radial/axial combo prob becasue it is more cost effective to build. An obviously important consideration.

After reading a fair bit on the astro forums, there are a few considerations for bearing arrangment, one important being large front thrust area to help absorb vibration. This is where the large contact area of taper roller or needle are ideal.

So I'm left with a few arrangements for the large right ascension axis;

1) 2x taper (smaller rear?)

2) Thrust needle+needle front & rear(smaller thrust bearing?)

also perhaps as suggested;
3) deep groove ball (I think) with loose fit + thrust needle against flange & rear bearing ?

The declination axis requires axial load to be taken in both directions so I'd go for #1

I guess there are endless combos but what do you think??
BTW large front ball type bearings are quite feasable in the design due to the large worm gear drive (~200mm dia) betweeen the flange and the bearing. Why not use an oversized bearing here?
It's gonna look a bit like the pic below.

My other concern is of course the manufacturing. Is it necessarily more labour involved in incorporating one type of bearing over another?

[NC Cams]

Not being familiar with your machine's terms, I"m at a loss to give exact recommendations BUT the following trends should help.

Your lowest friction potential will be an un preloaded, lightly oiled ball bearing to accept radial loads and a roller thrust bearing to absorb axial thrust.

HOWEVER, with some slick design/fit-up work, you can have a deep groove ball bearing absorg axial thrust if you keep it limited. 1% to 2% of the radial capacity can easily be handled....

If you spend a few more $$$, you can use a non-precision grade angular contact (IE: 7203BYG). This will absorb both radial and axial thrust and have minimal friction, all things considered.

Needle bearings are relatively inexpensive BUT a pita to fit as you need to either fit a raceway or grind one on the shaft - for what you're doing, an A/C bearing would be the best compromise.

Tapers are probably going to be cheaper than an A/C BUT, BUT they do have a bit more friction potential than a deep groove or A/C ball.

There is no BEST one to use, merely the best compromise with respect to cost, performance, bearing size, packaging constraints and accuracy....

[Zathras]

NC Cams;

Sorry about the terminology I hit you with. It's not that important anyway.

"Your lowest friction potential will be an un preloaded, lightly oiled ball bearing to accept radial loads and a roller thrust bearing to absorb axial thrust."

This is appears to be what is done on a few mounts.
Obviously one to consider.

"...you can have a deep groove ball bearing absorg axial thrust if you keep it limited. 1% to 2% of the radial capacity can easily be handled...."

I have suspected one design has just this setup. A picture (no info) shows what looks like a very large 8-inch deep groove bearing for the front and rear. This puzzled me at the time but as you say this is feasable if you limit the axial load i.e use an oversized bearing.

Looks like A/Cs might be the way to go if tapers have too much friction.

BTW Noobe question;
Something I wanted to know is can burnishing be used for bearing I.D or O.D seats? Wouldn't this save grinding and honing?

Thanks for your advice, much appreciated.

[RICHARD ZASTROW]

Zathras Burnishing is being used for bearing seats. In fact, Cogsdill (burnishing tool mfg.) calls their method "bearingizing" or "bearizing".

[NC Cams]

If a hole is precision bored, you can burnish instead of grinding with key emphasis on the term "PRECISION".

However, if the boring is sloppy, metal cutting is needed to fix the hole geometry and burnising is "rubbing" not cutting. It is pretty hard to "rub" away high spots or true up the geometry in a sloppily bored hole that isn't round.

[Zathras]

Richard ,

Thanks for putting me on to this little (bearingizing) device from Cogsdill, wow it's fantastic! Everyone should have one in their workshop but it looks like it's worth very moocho mula $$$$$
I'd be looking at roughly around 4" ID bearing seat so it will have to be specially ordered... even more $$$$.
Do you know if the Universal Burnishing Tool (UBT-B) can be used to perform the same job?? http://www.cogsdill.com/pdf%20files/usbur_ubt.pdf


Thanks again NC_Cams

[HuFlungDung]

Having built a telescope mount or two, I would not worry about stiction of the tapered roller bearings. I cannot say that I have noticed any tendency to stick before movement begins. In fact, the movement is so free that it is necessary to make careful provisions to balance the tube assembly perfectly, or the scope will easily drift. In practise, you will have to add a brake to each axis that lacks a worm gear drive and clutch assembly. You will have more difficulty trying to control the breakaway friction of your drive clutches than you will with the bearings.

I used standard Timken tapered roller bearings. If you get using something like a 4" bore bearing, you are looking at a housing of some serious size and weight to hold it all. You might want to consider moderate weight of the mount unless this is a permanent installation.

IMO, 35kg is no significant load. I know that ATM books recommend huge axles and mounts, but I believe, now with a few years of machining experience behind me, that flexure is an overblown issue with small telescopes. The real flexure issues come about in sneakier ways than the "bending of your 2" shafts"

For example, every bearing fit must be tight: that means no "slide together" assemblies, because that is looseness. This means that assembly is permanent, because you'll have to heat most OD fits to get them together, or Loctite them together with a space filling compound, which makes disassembly a very difficult procedure.

Where the T for the declination axis joins the RA (polar) axis must be tight. A slide fit, one that is not too difficult to assemble when holding a rather awkward contraption up at mid chest height, means that you've got .001 clearance between the housing and the polar axle. That is where you will get the sneaky flexure of the mount. It will not announce its presence with a clonk or a click, but it will introduce random errors in positioning of the tube, depending again, on where you are pointing the scope and where the weights are.

Your tube itself needs to be incredibly stiff to not introduce some small measure of flexure. The spindly aluminum saddles that one often sees atop these mounts is a joke for stiffness. A lot of this stuff, you cannot really test until you've built the first mount, and then test its stiffness with a dial indicator in various attitudes. That's when you discover what is what.

[RICHARD ZASTROW]

Zathras Study the "how to" instructions on burnishing. This is a process of moving the high spots of a surface into the low spots. When all is perfect, the amount of high material is equal to the missing material from the low spots and the resultant surface is near perfectly smooth. If the starting surface finish is too fine, the low spots are filled and excess material will flake off.

If I cannot have the correct finish as prescribed by the process, I prefer to leave a small amount of "low streaks" in a correctly sized bore and fill them with bearing seating compound when seating the bearing. These "low streaks" constitute less than 1% of the surface area.

Because of the possibility of "flaking" or other particulates being produced, you will want to assure clean coolant/lubricant.

A few "trial & error" bores in scrap pieces should get the correct size. That should let you use the less expensive burnishing tools rather than Bearingizer.

[Zathras]

HuFlungDung

Thanks for the comments. Excellent, I have an experienced DIY'er.
Re balancing, one would require balancing the scope (anyway) by releasing the worm/gear engagement. The provision you're talking about? I didn't think that too little friction would be such a prob since balancing is already required. Just how sensitive/precise balancing is required I had no idea though. Interesting.

Re bearing bore I meant the housing bore of 4" (actually around 95mm) so the ID is around 60mm. Sorry for the confusion. There is a Hungarian mount (called the Gemini) that uses 55mm S/Steel hollow shafts (I am modelling my mount after) which has a scope capacity of around 30kg. This was just a starting point for my design and this model is for permanent mounting.
What precision did you use for your bearings?

Yes, I was worried about the assembly being "permanent" as you say. But are there not mounts, like the Parallax, that have preload adjustment at the back of the axes? How are the bearing fits designed if they use tapered RBs?

Thus the flange at the RA axis attaches to the dec directly. I'm not certain how I will mount this flange since it is obviously critical (precisely perpendicular). Press fit to shaft then machine? or taper shaft end with screws?

Yes, scope tubes always flex. A (very) smart control system is probably the only way to deal with it. I don't think there is compensation for this even in TPoint s'ware.

I agree about puny sadles, they just don't even look right.

Richard, thanks for you comments. :cheers:

BTW sorry for the off topic. I wish there was ATM in the Hobbies section.

[ViperTX]

I would certainly look at the Thompson Linear Bearings it would most likely be low in cost and easy to use.

Another alternative is using the Misumi bearing blocks with Thompson shafts.

[kkilst]

I have seen a lot of discussion of different types of bearing in this thread,
and wondering does anybody has the information of how the construction
of the bearing arrangements use in the PEAUCELLIER linkage (7-Linkages)
, used in duplicating m/c to get very smooth linear movement.
I would be hapy to share the experience on the wood-duplicating using
this method.