Spring loaded worms?

[Rusted]

18 minutes ago, Peter Drew said:

You've just reinvented the wheel!  (You did ask).  Fullerscopes, at one point, attached their worm brackets wit rubber bushes surrounding the attachment screws.  This was intended to allow sufficient "give" to compensate for eccentricities.  They also had back stop screws to limit the degree of movement.    🙂 

Not quite what I had in mind, M'lud.
I was looking at "torsion" bushes.
Not squashed tap washers.

You are quite right, of course, Sir.
I was seriously overthinking the problem.
The rubber bushes have no need to be individually restrained.
The worm/motor housing can be restrained externally.

Edited November 19, 2020 by Rusted
Drooling idiocy in the face of overwhelming odds.

[andrew s]

Just one thought if the worm is bottoming in the wheel slots the you will never be able to get both edges (driving and non driving) of the worm in contact with the wheel slots at the same time. So however rigid you make it can't remove the backlash.

I say this as one who fought the Fullerscope Mk IV long and hard.

Regards Andrew 

[markse68]

turn the tips of the worm gear down a bit?

Mark

[Peter Drew]

20 minutes ago, markse68 said:

turn the tips of the worm gear down a bit?

Mark

I used to truncate the tips of the worm threads when I manufactured gear sets for just that reason.  How difficult can it be in this day and age to make gear sets properly?.          🙄

[Rusted]

I'm thinking any wear on the worm tips is probably self repairing. It will bed in before the wheel teeth ever wear out.

This morning I drilled more holes, well spaced apart, in the motor housing bases.
Then I sandwiched tiny O-rings [over the fixing bolts] between the motor housings and their support plates.
I had instant Fullerscopes MkIV adjustability without the tears. The motors ran remarkably quietly too.
These new fangled "wheels" I've just invented are great!

[Michele Scotti]

On 18/11/2020 at 17:07, Rusted said:

Thanks for the continuing feedback.

I've been in the workshop adding angle profile, mutual reinforcement to the RA motor and worm housing.

Still experimental but I'm making progress: Even added a crude, push-off screw adjuster.

I'll be back.

 

 

Hi Rusted, it may seems slightly off topic but is the wormgear supported by ball bearings or conical/taperes ones?

[Rusted]

5 hours ago, Michele Scotti said:

Hi Rusted, it may seems slightly off topic but is the wormgear supported by ball bearings or conical/taperes ones?

Thank you. This is a perfectly valid question. 

The Beacon Hill worms are supported by "normal" deep groove, ball bearings. NOT AS CLAIMED by Beacon Hill.

QUOTE: "The matching stainless steel worms are held in sealed roller bearings in substantial brackets and are fully adjustable to eliminate any developing end float."

Reality: There is no adjustment. The worms are brass. The bearings are standard ball bearings. The substantial brackets are flimsy offcuts of channel profile.

I looked at alternatives like opposed, taper roller bearings, a while back, but there were far bigger problems than just the bearings themselves.
The standard worm bearings were held in place in oversized bores with shellac!

Their housings are simple, short, off-cuts of channel section aluminium with the bearings "retained" by just one, tiny, grub screw.
Over-tightening this tiny screw immediately resulted in total bearing lock-up.

So on every [early] slew the worms , along with both bearings would literally slide straight out of their "housings."
Leaving the mounted telescope [a heavy, 7" f/12 refractor] completely free to turn where it would.
That is until I drilled and fitted small screws through four holes in the channel section upstands.
Fitted with oversized washers these easily retain the bearing's outer races in the channel section housings.

I waited months for wormwheels and worms and paid far more than the eternally unchanging "catalogue" prices.
In the end I accepted a pair which had the wrong bore. I had to accept 60mm bores instead of the ordered 50mm.
I was made to feel sorry for the "poor 90 year-old machinist still working in an unheated tin shed" as the vendor's excuse.

The wormwheels finally turned up, blackened with dust from lying in a filthy, workshop environment for years.
I presume the vendor was unable to source a pair of wheels and worms and I was fobbed off with old, unsaleable stock.

This is a section on the 11" wormwheel after hours of scrubbing the teeth clean with a variety of brushes:

Quote from the Beacon Hill Website: "They are the most accurate worm and wheel sets available to astronomers in this country."

[Peter Drew]

Eeeek!       🙂

[andrew s]

Never having never cut a worm wheel I have always wondered how you get them to correctly mesh as you make the cut. Combination of thread spacing and wheel diameter?

Love to know hint hint @Peter Drew.

Regards Andrew 

Edited November 20, 2020 by andrew s

[Peter Drew]

4 minutes ago, andrew s said:

Never having never cut a worm wheel I have always wondered how you get them to correctly mesh as you make the cut. Combination of thread spacing and wheel diameter?

Love to know hint hint @Peter Drew.

Regards Andrew 

Yes Andrew, that is the principle.  In practice it's not so reliable as material hardness, accuracy of the tap used for hobbing the gear teeth and having a "R" in the month all conspire to create plus or minus a few teeth from those required.  Gears for DEC drives are not so important but RA gears are.  My usual procedure was to lightly engage the tap to produce witness marks on the gear blank, these would show whether the teeth were going to match up during a 360 degree pass.  If a large RA wheel was to be made, specially if in an expensive material, I would gash teeth position with a fly cutter having mounted the blank on a rotary table.  When remounted on the lathe, the tap would accurately follow the marks.     🙂  

[sloz1664]

4 hours ago, andrew s said:

Never having never cut a worm wheel I have always wondered how you get them to correctly mesh as you make the cut. Combination of thread spacing and wheel diameter?

Love to know hint hint @Peter Drew.

Regards Andrew 

All we ever need to know is in here:-

 

 

Steve

[Rusted]

The hessian is a nice touch!

Sack cloth and ashes for failed wormwheel machinists?

[Michele Scotti]

Forgive me if I got it wrong by missing some info already stated in this thread.

You have a run-out issue – can you measure that? From an engineering standpoint it would be crucial to quantify the gap to your ideal situation and see what improvement(s) are closing it up.

A 0.01mm dial gauge is pretty cheap nowadays. You could place in the not-hobbed rim or fabricate a probe tip that gets into the grooves (going to take longer to measure but more accurate). Do you have nay mean to fine tune the cantering of the worm wheel? It’s one of the most usual issue and I guess it might have been exacerbated by the50/60mm bore issue?

Also, I’d suggest to put your phone on a stand pointing at the worm gear and take a video while rock the RA back and forth. If you watch the video and see any movement – then you might better guess where the problem(s) are coming from and address that. With mine I can sense less that 0.01mm movements and have spotted stuff I couldn't see by naked eye.

I always thought that springed systems are inherently a ‘compromise’ and I suppose that beyond a level of error they can’t cope with that anymore.

It would be interesting to know from Peter whether the system he was producing were spring mounted or not.

 

Your worm gear carrier doens't look that flimsy IMHO. More concerend about not tapered bearing which wouldn't be a big installation issue if you want to go for that.

Is that the only botched-up tooth of the wheel?

 

[Peter Drew]

1 hour ago, Michele Scotti said:

Forgive me if I got it wrong by missing some info already stated in this thread.

You have a run-out issue – can you measure that? From an engineering standpoint it would be crucial to quantify the gap to your ideal situation and see what improvement(s) are closing it up.

A 0.01mm dial gauge is pretty cheap nowadays. You could place in the not-hobbed rim or fabricate a probe tip that gets into the grooves (going to take longer to measure but more accurate). Do you have nay mean to fine tune the cantering of the worm wheel? It’s one of the most usual issue and I guess it might have been exacerbated by the50/60mm bore issue?

Also, I’d suggest to put your phone on a stand pointing at the worm gear and take a video while rock the RA back and forth. If you watch the video and see any movement – then you might better guess where the problem(s) are coming from and address that. With mine I can sense less that 0.01mm movements and have spotted stuff I couldn't see by naked eye.

I always thought that springed systems are inherently a ‘compromise’ and I suppose that beyond a level of error they can’t cope with that anymore.

It would be interesting to know from Peter whether the system he was producing were spring mounted or not.

 

Your worm gear carrier doens't look that flimsy IMHO. More concerend about not tapered bearing which wouldn't be a big installation issue if you want to go for that.

Is that the only botched-up tooth of the wheel?

 

There was no spring loaded bracket involved as I took the trouble to ensure that the o/d and bore of the gear blanks were concentric and an accurate size, no real problem if these dimensions were machined in one operation.  There was a back stop plate with adjustment screws to prevent the worm bracket from backing off under load.  Never had any complaints or returns!     🙂

[Michele Scotti]

1 hour ago, Peter Drew said:

There was no spring loaded bracket involved as I took the trouble to ensure that the o/d and bore of the gear blanks were concentric and an accurate size, no real problem if these dimensions were machined in one operation.  There was a back stop plate with adjustment screws to prevent the worm bracket from backing off under load.  Never had any complaints or returns!     🙂

Out of curiosity: straight or helical tap?

[Rusted]

Thank you Michele for some useful ideas for diagnosis.

There is only one bad overlap causing the ugly teeth. i can rotate it out of the way in normal use.

I have several dial gauges. Including a digital Mitutoyo reading to several decimals of a millimetre. Typical hardened ball probe.
Reading the eccentricity of the 11" RA wormwheel was complicated by the very long telescopes.
I'd have to remove them to allow free rotation.  Not sure how to read the individual tooth depths. That would involve repeatable retraction and plunging.

I have cameras, lenses, stands and tripods. 4K 180fps slow motion might be quite useful to study worm housing flexure. It could go viral on YT!
A long rod clamped to the worm housing, to magnify flexure, was far too clumsy and vibrated badly.

[Michele Scotti]

I took the liberty to sketch up how I would do approach this - courtesy my daughters' crayons.

Taking the brave pill I'd split the wheel in an inner flange and a crown - the latter being adjustable. The 2 are connected by a plate (I envisage a 5mm steel plate which needs to be machined at least on one face).

Wrt the investigation with camera check out this post: https://stargazerslounge.com/topic/340154-800mm-telescope-project/?do=findComment&comment=3787165

On a video the resolution could be lower but if you use a mild tele and a camera I'm sure you can crack the 0.01mm. Slow-mo won't help, it reduces resolution and I assume you don;t have high dynamic phenomena giong on.. Here yo'd be more looking for semi-static deflections.

Abou the gauge: if the grooves on the wheel are consistent i.e. same height you might reasonably suppose that the wheel grooves and outer diameter are fairly concentric. Long story to say that you can place the guage more conviniently on the rim.

Lastly on the worm gear bearings - I try to briefly look up some taper bearing but I couldn't find anything with an ID smaller than 15mm. What's the shaft diam?

[Rusted]

Thanks. The hubs of these wormwheels are detachable via three large screws. I marked the hubs and wheels to ensure they were never rotated from their original position.
Then I turned brass liners to adapt the 60mm bores to 50mm without ever removing them from the 3-jaw chuck of my lathe until they were parted off.

This ensured concentricity to the best of my turning skills. However, I missed a trick here. I never checked the eccentricity of the original wheels as supplied.
Had I checked the eccentricity of the wheel rims at the time I could have reduced any error by means of rotation of an eccentric brass adapter.
Or, I could have made all new, brass hubs. While I was at it I drilled and threaded two more clutch screws at 120° apart. I drilled the brass adapter sleeves at the same time.

The single clutch pushes a plastic plug against the axis/shaft as a [safety] slipping clutch. Three of these plug clutches, per wheel, greatly increased torque transmission.
One clutch slipped all of the time. Regardless of pressure applied to the screw. There was also a potential risk of eccentricity if the clutch was pushed very hard.
The stepper motor, AWR drives, do not allow manual telescope movement without "losing the sky." Clutch slip was and remains disastrous.

Now you mention it, I think I remember there being no taper roller bearings in the worm shaft diameter. 12mm from memory.
Larger bearings could easily be accommodated in more substantial worm housings. Or the worm shafts sleeved to a larger size.

The problem then is housing clearance from the wormwheel rims. It would require much wider spacing of the bearings.
Cheeks containing larger thrust bearings would require worm shaft adapters to be turned up.  The supplied worm shafts are very mean in length.
Barely accepting a timing pulley hub. Which, again, required two more radial holes be drilled and tapped for extra holding screws. One grub screw was not enough for security. 

The Beacon Hill worms and wheels, as supplied, soon turned into a major DIY effort. Had I not owned a lathe and drill press I would have been completely stuck!
All of this text is very boring, for most readers, but I wanted to ensure that anyone contemplating building a mounting had enough details to proceed safely.

 

[Michele Scotti]

On 22/11/2020 at 18:23, Rusted said:

Now you mention it, I think I remember there being no taper roller bearings in the worm shaft diameter. 12mm from memory.
Larger bearings could easily be accommodated in more substantial worm housings. Or the worm shafts sleeved to a larger size.

The problem then is housing clearance from the wormwheel rims. It would require much wider spacing of the bearings.
Cheeks containing larger thrust bearings would require worm shaft adapters to be turned up.  The supplied worm shafts are very mean in length.
Barely accepting a timing pulley hub. Which, again, required two more radial holes be drilled and tapped for extra holding screws. One grub screw was not enough for security. 

I think your system has high potential - baby steps: if your shaft is 12mm there could be fairly inexpensive angualar bearings (not as good as taper ones but for these loads they would do).  Youd need to pre-load them but it's easily done with your current set-up - if interested I can sketch-up something for you. 

Don't use adapters - you'll add errors. Here you need to remove them oen by one.

https://www.ebay.co.uk/itm/7001-Single-Row-Angular-Contact-Open-Ball-Bearing-12x28x8mm/143541090346?hash=item216bb7382a:g:4NMAAOSwuvpcR2-C

[Peter Drew]

Although I largely avoided these problems in the first instance, it underlines the importance of concentricity and as all machinists know, this depends on items being machined, if possible, in one operation setting.  Adding subsequent components machined subsequently leads to errors, I've often seen portions of worms turned separately, bored and then grubscrewed on to shafts with subsequent run out.  It's quite difficult to rectify an inaccurate wormwheel as you never know if the gear teeth are concentric to the the bore or the rim unless you try it.  It's a bit like judging the quality of a mirror just by looking at it.

I think I would would make a "top hat" bush that was a good fit on the shaft but well undersize of the gear bore which would have had to be overbored.  The bush would have to have a lip wide enough to accept screws to hold the two firmly together.  This composite unit could be trued up on the lathe to a reasonable degree.  If the gear was now mounted back on the shaft and the pressure of the attachment screws was relaxed enough to allow the gear to be tap adjusted, it could be manually rotated by the worm to determine the high and low portions of the full cycle.  This should initially remove the backlash and finally the mesh by noting the feel of the worm as it is turned.  "Feel" is a very good measure of accuracy. Lastly, the screws would be tightened and the feel checked.  Once satisfactory, the gear could be doweled to prevent further movement.    🙂

[Rusted]

Thank you both for your input.

I have yet to test the eccentricity of my wormwheels with the telescopes unmounted.

It now occurs to me that I am also at the mercy of the accuracy of the [inexpensive] flange bearings which support the 50mm axes.
I really ought to check the shafts in rotation, at the wormwheel position, with a dial gauge before assuming the wormwheels, alone, are at fault.
Again, this requires removal of the telescopes to allow free rotation.
I could load some weights onto the top of the PA shaft to simulate the telescopes without their considerable length causing issues.

Assuming the teeth appear uniform I think I should be able to ignore variations in tooth depth.
Would anyone like to suggest an acceptable degree of eccentricity at the wormwheel rims?

Thanks again.
 

 

[Michele Scotti]

On 24/11/2020 at 18:49, Rusted said:

Thank you both for your input.

I have yet to test the eccentricity of my wormwheels with the telescopes unmounted.

It now occurs to me that I am also at the mercy of the accuracy of the [inexpensive] flange bearings which support the 50mm axes.
I really ought to check the shafts in rotation, at the wormwheel position, with a dial gauge before assuming the wormwheels, alone, are at fault.
Again, this requires removal of the telescopes to allow free rotation.
I could load some weights onto the top of the PA shaft to simulate the telescopes without their considerable length causing issues.

Assuming the teeth appear uniform I think I should be able to ignore variations in tooth depth.
Would anyone like to suggest an acceptable degree of eccentricity at the wormwheel rims?

Thanks again.

Brief and rough trigo. --> arctg(um/R)= ca. 1.5arcsec i.e. for every micron (um) of 'detachment' of the worm from the wheel's teeth means 1.5arcsec of backlash and that is the same you'd see at the telescope. 

A more 0.1mm  of 'sloppiness' would give you back 2.5arcmin.

Quick and tad cryptic - let me know if it makes sense though.

 

Wrt to RA flange bearing I would not be concerned - gravity loads them always in the same position and take away any bearing clearance (but not roundness isssue although not likely to be an issue in this application.

Can you test with the scope on? What's the reason to remove it?

[Rusted]

5 hours ago, Michele Scotti said:

Brief and rough trigo. --> arctg(um/R)= ca. 1.5arcsec i.e. for every micron (um) of 'detachment' of the worm from the wheel's teeth means 1.5arcsec of backlash and that is the same you'd see at the telescope. 

A more 0.1mm  of 'sloppiness' would give you back 2.5arcmin. Quick and tad cryptic - let me know if it makes sense though.

Wrt to RA flange bearing I would not be concerned - gravity loads them always in the same position and take away any bearing clearance (but not roundness issue although not likely to be an issue in this application.

Can you test with the scope on? What's the reason to remove it?

Thanks. The telescopes are long and heavy refractors.
My  home made 150 f/10 solar H-alpha telescope remains.
Plus the little 90 f/11, after I removed the 180 f/12.
At one time I had all three mounted simultaneously.
The drives were unable to cope reliably.

Physically the mounting could hold an immense weight.
The drives have always been its handicap.
It was a choice between backlash and flexure. Or repeated stalling on slews.
Sometimes it would behave well but it was always unpredictable. 
I was always careful about maintaining balance in all planes.

Trying to measure the wormwheels in rotation means the telescopes would need constant adjustment.
To keep them safely above level. Or simply to avoid collisions with the huge [timber] pier.

It would be tempting to release the clutches [or even the worms] and rotate just the wormwheels for measurement.
Though I don't think this would be a realistic [real world] test for eccentricity.

[Michele Scotti]

10 hours ago, Rusted said:

It would be tempting to release the clutches [or even the worms] and rotate just the wormwheels for measurement.
Though I don't think this would be a realistic [real world] test for eccentricity.

I agree although the clutch plays a role - I suggest to detach the wormgear assy and attach a camera and mark the wheel. You can then focus on freed axis and make it safely rotate around the pier - then you can take notes of the measurements conviniently fom the video.

Something like this set-up: https://youtu.be/OGJyjb-h-G8

Btw I recall you started a topic to explain the build - can you attach the link pls?

[Rusted]

Thanks. 

I released the worm housing and measured the 11" RA wormwheel eccentricity today.
Maximum variation was only 0.02mm on the rim beside the teeth.
I left the telescopes in place and measured in two x 180 degree arcs.
Without any end loading on the flange bearings I was able to flex the PA by a similar amount.
There doesn't seem to be much to worry about given these numbers.

The most obvious problem is that the RA worm housing flexing when I manually rock the telescopes in RA.
I used 70mm square tubing with a 5mm wall thickness [from memory.]
I was able to see the flexure without needing any aids.

 

Edited November 27, 2020 by Rusted
Typo