Spring loaded worms?

[Rusted]

Hi,

How do you eliminate backlash in your drives when routinely using several meters of equivalent focal length?

I have enough eccentricity in my 11" and 8" worms and wheels to cause tight and loose spots with rigid worm housings.
But I want the exact opposite. Stiff against unwanted motion but perfectly free to turn.

I have tried pivoting the worm housings with various tension springs.
You'd think it was the perfect answer, but it isn't. Not so far.

With the moment of my long refractors there is still far too much backlash.
Add too much spring tension and it just stalls the big, 210 stepper motors.
Push-off screws would surely just short circuit the springs?

With all the experience out there I badly need a fast track fix.
Other than burning well over £10k on a commercial mounting. 😱

Thanks for your time.

 

[Davey-T]

Probably a case of getting the right length / tension spring, maybe doubled up one inside the other.

Dave

[Alien 13]

A similar issue exists with the lower end skywatcher mounts like the EQ3/5 and an effective solution is to unbalance the scope so that the worm drives are under constant load..

Alan

[Peter Drew]

I would try retaining the pivoting worm bracket and spring pressure but introduce an adjustable stop to limit the amount that the amount that the bracket can back away from the wormwheel at the tightest spot.  Hopefully the difference in the backlash inducing eccentricity will not be excessive.  (Though I won't be surprised if you haven't already tried this.)   🙂

[Rusted]

36 minutes ago, Davey-T said:

Probably a case of getting the right length / tension spring, maybe doubled up one inside the other.

Dave

Thanks Dave. But wouldn't doubling the spring be the equivalent of one stronger spring?
I have tried a variety of springs but despite the 287:1 advantage and the huge radius the motors don't like strong springs.

[Rusted]

34 minutes ago, Alien 13 said:

A similar issue exists with the lower end skywatcher mounts like the EQ3/5 and an effective solution is to unbalance the scope so that the worm drives are under constant load..

Alan

Thanks Alan. I've tried the constantly climbing eastern bias to no great effect.
When you are trying to capture solar close-ups with over 3m of focal length you need full and instant control in all four directions.
The slightest backlash and you have lag and then overshoot. The length and moment of the refractors seriously exacerbates the problem.
Any mounting can carry a huge weight quite effortlessly but they just can't control it. 

 

[Rusted]

30 minutes ago, Peter Drew said:

I would try retaining the pivoting worm bracket and spring pressure but introduce an adjustable stop to limit the amount that the amount that the bracket can back away from the wormwheel at the tightest spot.  Hopefully the difference in the backlash inducing eccentricity will not be excessive.  (Though I won't be surprised if you haven't already tried this.)   🙂

Thanks Peter. No, I haven't tried using screw stops. I keep adjusting what I have but it is never the same twice and never for very long.
I'm using white grease as a worm lubricant. I have no idea if this is low friction or whether there is something better out there.
Lower friction would allow greater spring pressure. When the worm is deep into the wheel the friction surface area must rise accordingly.
Worms, in theory, aren't supposed to be in full contact with their wheel. So an adjustable stop would make good sense.
I tried lapping the worms against the wheels, when new, but all it did was reduce the graunching sound.

[Davey-T]

Two weaker springs would have more squishing travel than one strong one.

Dave

 

[andrew s]

Time to give Edward R Byers Co a visit http://www.edbyersco.com/in_stock.html

Regards Andrew 

Edited November 17, 2020 by andrew s

[Peter Drew]

1 hour ago, Rusted said:

Thanks Peter. No, I haven't tried using screw stops. I keep adjusting what I have but it is never the same twice and never for very long.
I'm using white grease as a worm lubricant. I have no idea if this is low friction or whether there is something better out there.
Lower friction would allow greater spring pressure. When the worm is deep into the wheel the friction surface area must rise accordingly.
Worms, in theory, aren't supposed to be in full contact with their wheel. So an adjustable stop would make good sense.
I tried lapping the worms against the wheels, when new, but all it did was reduce the graunching sound.

I'm not a fan of white grease and similar, too thin for my liking when used in high load bearing situations.  I prefer a heavier sticky grease that clings to the bearing surfaces, rather like the Chinese "gloop" that many complain about.  I always used Rocol "Kilopoise" for worm/wormwheel lubrication and a 50/50 mix of Kilopoise and Castrol LM grease on other applications where load bearing was important but needed to still turn in very cold weather.

I never had backlash issues due to eccentricity of the worm or wormwheel as I made sure they were concentric when I made them.  It's not "rocket science".        🙂

[Horwig]

I'll take you back to a thread of mine some time ago with the same problem, to which you contributed, (although I only work at half your FL):

 

Page 2 is the important one , Oddsocks here pointed me in the direction of spring loaded plunger screws, they have worked a treat, and are used in conjunction with end-stop screws, adjusted to be just snug on the tightest part of the worm.

These were the parts that SB use in their mounts:

https://www.mcmaster.com/8476A38/

 

I got equivalents here in the UK from these people:

https://hyquip.co.uk/spring-loaded-devices/6315-carr-lane-spring-plungers.html

 

the ones I used in the end were CLM-8-SSPS-2

 

Hope that this might be of some help to you

 

Huw

 

[Rusted]

8 hours ago, andrew s said:

Time to give Edward R Byers Co a visit http://www.edbyersco.com/in_stock.html

Regards Andrew 

Thanks Andrew. I wish!
BUT, I'd need welding goggles if I had that much 'bling' in a solar observatory.
The telescope making gods are against direct purchase from the  USA.
Freight + import taxes, customs clearance and 25% VAT could easily make the US retail price look [almost] cheap.

[Rusted]

7 hours ago, Peter Drew said:

I'm not a fan of white grease and similar, too thin for my liking when used in high load bearing situations.  I prefer a heavier sticky grease that clings to the bearing surfaces, rather like the Chinese "gloop" that many complain about.  I always used Rocol "Kilopoise" for worm/wormwheel lubrication and a 50/50 mix of Kilopoise and Castrol LM grease on other applications where load bearing was important but needed to still turn in very cold weather.

I never had backlash issues due to eccentricity of the worm or wormwheel as I made sure they were concentric when I made them.  It's not "rocket science".        🙂

Thanks again Peter.

How many Chinese mountings would I need to buy to get enough "gloop" for a pair of wormwheels?
It might still be cheaper than one supplier who wanted 825 Euros for a tin of 'Kilopoise!
I could start a market for scavenged "gloop" from belt drive modifiers and upgraders.
I'll have see what is available over here. A lorry repair workshop who might have something useful.
Just aim for the stickiest stuff they have?   I'll take a teaspoon and a jam jar and throw myself on their mercy.

I'm afraid eccentricity is strictly in the eye of the beholder if you buy a commercial pair of the "finest available" wormwheels in the UK.

 

 

[Rusted]

7 hours ago, Horwig said:

I'll take you back to a thread of mine some time ago with the same problem, to which you contributed, (although I only work at half your FL):

 

Page 2 is the important one , Oddsocks here pointed me in the direction of spring loaded plunger screws, they have worked a treat, and are used in conjunction with end-stop screws, adjusted to be just snug on the tightest part of the worm.

These were the parts that SB use in their mounts:

https://www.mcmaster.com/8476A38/

 

I got equivalents here in the UK from these people:

https://hyquip.co.uk/spring-loaded-devices/6315-carr-lane-spring-plungers.html

 

the ones I used in the end were CLM-8-SSPS-2

 

Hope that this might be of some help to you

 

Huw

 

Thanks Huw. It must be an age thing but I'd completely forgotten about your thread. Some really good ideas in there.

Looking again at your mount and mods I realise that my worm mountings are seriously "undernourished."

I also realise that my guiding technique is using top gear [of 3] on the AWR paddle just to overcome the backlash.
I'd have to wait for [literally] minutes for the lower guiding speeds to have any visible effect.

It's time to get serious about worm containment, hinging and support!

Thanks for all the feedback chaps.
 

Edited November 18, 2020 by Rusted
Clumsy fingers!

[Peter Drew]

A tin of Kilopoise would last you 3 lifetimes of constant DIY telescope building so is probably quite good value!   The last time I bought some about 30 years ago Rocol sold it in small squeezy toothpaste type tubes for a few quid each,  I still have a scraping left.  (NB, don't confuse the tubes with your toothpaste!)          🙂

[andrew s]

1 hour ago, Peter Drew said:

A tin of Kilopoise would last you 3 lifetimes of constant DIY telescope building so is probably quite good value!   The last time I bought some about 30 years ago Rocol sold it in small squeezy toothpaste type tubes for a few quid each,  I still have a scraping left.  (NB, don't confuse the tubes with your toothpaste!)          🙂

I had Kilopoise on a Fullerscope's Mk IV mount it was so viscous it took a month to balance it and only then in the summer. Regards  Andrew 

[andrew s]

4 hours ago, Rusted said:

It's time to get serious about worm containment, hinging and support!

Thanks for all the feedback chaps.
 

Good luck with the modifications.

I had some 12" Byers gears on a Parallax mount. They were just so good until the observatory burnt down. I don't think their bling caused it.

Regards Andrew 

[Peter Drew]

39 minutes ago, andrew s said:

I had Kilopoise on a Fullerscope's Mk IV mount it was so viscous it took a month to balance it and only then in the summer. Regards  Andrew 

That's why I mixed it with 50% Castrol LM grease.  The load bearing was retained but things didn't seize up in Winter.      🙂

[Horwig]

5 hours ago, Rusted said:

Looking again at your mount and mods I realise that my worm mountings are seriously "undernourished."

I'm still working on this mount, don't think it'l ever get finished!

Worm 'containment' has changed quite a bit since the thread above. Angular contact bearings, and flexors instead of pivots on the worm bracket just for starters.

I had good guidance from Tony Owens amongst others, (he suggested the flexors,) his choice of grease for worms was engine assembly lubricant, since it is thin but high performance.

 

Huw

[Rusted]

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.

 

[andrew s]

I am no expert but your wheel "slots" look very shallow compared to those on my Paramount and previous Paralax mount. I do wonder just how many are engaging at any one time. I am sure @Peter Drew will know more than I.

Regards Andrew 

[Peter Drew]

There is a limit as to how deep you can go with the teeth on a fine pitch gearwheel.  Despite the flimsy appearance they are very strong, any drive that could strip these would already have serious other issues.  Think how much pressure you can bring to bear on hose clips which have similar shallow depth.   🙂

[Rusted]

The Beacon Hill wheels have 287 "teeth". That is most definitely on the coarse side of things so tooth breakage is highly unlikely. The thickness of the "teeth" is considerable in comparison with higher tooth counts. I am seeing brightness on the crests of the DEC worm "thread." Suggesting that it is bottoming in its wheel.

Ironically, the DEC worm and wheel set were vastly better quality than the RA. The latter was very rough and the worm distinctly eccentric. The roughness of the wheel machining and cyclic changes in "graunching" with worm rotation were readily audible during slews. Hence the attempt at lapping the worm with the wheel. Based on a couple of hours work it would have taken many hundreds of hours to achieve much change using metal polish. Most of the wear would have gone into the brass worm. The wheel having far more circumference and active surface area than the worm.

The next stage will be adding external restraint for the entire motor assembly. As the worm acts on a tangent, to the wormwheel, any longitudinal movement must be strongly resisted. If there is any movement it will be exhibited directly as backlash. The most obvious ploy would be adding angle profile to the large, 10mm thick motor support plate.

These angle profiles will be provided with adjustable stops. Ideally these stops should be placed close to the ends of the worm. Where maximum longitudinal force is applied. I have collected several [scrap] aluminium box and angle sections which have been looking for a useful purpose.

The largest box section is 100mmx200mmx6mm thickness or 4"x8"x 1/4"! So I can produce any L-profile I am likely to need. I may even be able to contain the entire motor/worm assemblies with a compete box section for greatest stiffness.

[Rusted]

I have a new idea!

The linear movement required to maintain mesh between a worm and its wheel is really rather small.
So why not use rubber bushes expanded into metal tubes as "hinges?"
Just like a car engine mounting or skateboard truck.
Massive restraint against lateral movement but with just enough compliance to achieve their intended purpose.
No need for bonding the rubber to the metal like the heavy duty, commercial vehicle devices.
Using cones, or cupped washers, on both sides of the rubber bushes will stiffen them while providing easy adjustability.
External, screwed stops might still prove useful. Now tell me I have just reinvented the wheel.

[Peter Drew]

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.    🙂