How to - very low run out bearing

[dansawyer]

The application is a telescope tracking drive made from a worm gear. The main gear is aluminum, about 6.1 inches in diameter with 192 teeth. The worm is stainless about 3 1/2 inches long in total. The drive thread portion is 1 1/2 inches in the center. The threads are 1 inch in diameter and have a pitch of about 10 tpi. The worm shaft bearing surfaces are 1/2 inch diamter and about 1 inch long, on each end. The maximum revolutions are about 40 rpm.
I have determined a critical factor in accuracy is the precision of maintaining the worm shaft rotation. My question is: What is the most bearing type for this application?

I had considered a pillow block bearing but realized the set screws would offset the bearing surface.
I have read about 'ceramic' bearings but have found specification.
Would bronze bearing offer sufficiently tighter tolerances?

Thank you in advance, Dan

 

[markba633csi]

The slight vibrations from ball bearings might be an issue- my vote would be bronze bearings carefully fitted to the worm shaft
Are you concerned about backlash? You might need some provision to adjust the worm to gear distance
-Mark

 

[macardoso]

Having a bit of a hard time visualizing the mechanism - a picture would help.

Bearing material, more or less, is independent of the accuracy of the bearing. Typically you would choose a bearing based on application (shape and material), and finally by precision class ($0.02 Chinese junk all the way up to $$$$$$.$$ super precision spindle bearings with runout in the sub micron range).

Here are some general questions: do the shaft bearings need to handle thrust loads? I am thinking yes since it is a worm. How much radial and axial load? What runout (eccentricity) is acceptable for the application?

I'd think a set of angular contact bearings or taper roller bearings would be suited for this application since they create very stiff and accurate bearing arrangements with both radial and thrust support.

A good quality ball bearing should have no discernable vibrations when properly mounted. You could actually get some very high end bearings for pretty cheap at this size.

Bronze or Oilite bearings will always be smaller than ball bearings and good for medium load slow speed applications. They are never produced to the accuracy of precision ball bearings, but that may not matter in this case.

A simple back to back angular contact or taper roller configuration, such as the right most image, will provide excellent results. (bearing contact angle marked on each bearing cross section.




Here is a very nice Angular Contact (AC) bearing with a 12mm bore (most quality bearings will only come in metric sizes) for $10. Two of them with a pair of jam nuts to preload them together against a shaft shoulder will give the stiffest shaft application. VXB unlabeled bearings are made to ABEC3 standards AFAIK giving them a runout of 0.0003" inches or better.

7801AC Angular Contact Bearing 12x21x5

7801AC Angular Contact Ball Bearing 12mm x 21mm x5mm 7801AC 25 degrees contact angle Ball Bearing, the inner diameter is 12mm, outer diameter is 21mm and the width is 5mm, Item: 7801AC Ball bearing Type: Angular Contact Cage: Nylon Dimensions: 12mm x

www.vxb.com

 

[dansawyer]

Thank you for this input. There is only moderate radial force.
The force is consistant and non varying. Given that would a bronze or oilite surfaces offer a high tolerance bearing surface?

 

[macardoso]

Thank you for this input. There is only moderate radial force.
The force is consistant and non varying. Given that would a bronze or oilite surfaces offer a high tolerance bearing surface?

Out of the box, I would not say so. They rely on clearance to generate an oil film. As such, general purpose bearings could have more slop than you'd like. A custom machined bearing could give better fit but that requires some effort on your end. Also these bearings only support radial force requiring additional bearings to be added to support the axial forces. Totally doable, but if it were me, I'd buy a bearing (like the Angular Contact ones mentioned above) which can support both radial and axial forces in one package.

Another benefit is angular contact bearings can be loaded together by means of a jam nut on the shaft, eliminating any clearances in the bearing. You can have truly zero radial and axial play.

 

[Mitch Alsup]

I agree with the annular contact ball bearing. My C11 fork mount uses these to support the fork to mount bearing assembly.

The real trick to getting telescope accuracy is the ability to adjust the 6.1" main gear after you mount it to the shaft. You will want to dial the gear in so that its runout is on the order of 0.001" anywhere the shaft turns (less is better still).

After you dial in the main gear, then you adjust the worm gear to the now-concentric main gear clearances.

 

[dansawyer]

The primary objective is astrophotography, unguided for 1000mm for up to 1 minute. I have identified three sources of error, main gear runout, worm gear runout, and worm gear to main gear contact.
My first attempt had visible worm gear runout. I am not sure of the root cause.
My math indicates that over a few minute window the main gear runout is the least significant term. A .001 offest in concentricity in the main gear produces .25 arcsecond variance in a 50 second window. (assuming a 6.1 inch diameter main gear)
The second term is worm gear runout. The worm - main gears angle of contact is about 14 degrees. A .001 radial runout in the worm produces about a 16 arcsecond move in the OTA.
The third term is changes in point of contact between the worm and the main gear creating acceleration deceleration cycles in the main gear. The original main gear was straight cut, there was no attempt to cut the original gear to match the worm through a worm rotation. The new gear is concave cut, my assumption is this is an attempt to provide smooth rotational response throughout a 360 degree rotation of the worm. It is my anticipation that the upgraded worm gear - gear will provide the required response.

 

[macardoso]

I stand by my previous recommendation with this new info. A good set of back to back AC bearings will give you a stiff worm shaft with runout in the tenths if correctly mounted. This of course assumes you have the space for them. Since loads are low, the worm shaft can be reduced in diameter to allow use of smaller bearings and thus a smaller bearing housing.

I would be concerned about the manufacture of the worm. What you need is the worm pitch diameter to have zero runout (or as close to it as possible). I do not know if you machined this, but wither way this is the thing you need to be worried about. The worm major diameter (OD) should be coaxial with the pitch diameter, but we know how that goes. We also would question how good of a fit the worm bore to the worm shaft is if they are separate pieces.

The accuracy of the worm lead and main gear manufacture. Lead error in the worm would likely show just the same as runout.

If your goal is <0.001 runout in the worm, then a proper high precision ball bearing arrangement will be a must. This is in the order of magnitude of runout in a machine tool spindle.