Making a friction drive mount (help)

[matija]

Hi.

Im making a homemade german EQ mount that will use SiTech servos. I already have the design for a worm drive system but to get rid of errors and backlash I want to transform it into a friction drive mount. I don’t have much information on how to make such a thing. What material do you use for the disk? How large should It be? And most importantly how to actually spin the disk(friction roller and so on, should it be touching the disk on the edge or on the face side?…)
If you have any sketches, ideas, photos, information, please do tell. Ill attach images of the mount desigh.

Thanks!

[vlaiv]

Hi.

Friction drive consists of large disk and very small diameter "pin".

Top view looks like this:

It is a bit like spur gears - but without teeth:

Ratio of diameters of large and small roller is reduction.

You need to have enough reduction to be able to achieve sidereal rate.

I've done calculations for stepper motors and if one wants better resolution than one step per arc second (and usually for high end mounts you want at least 10 times higher) - you need around 200:1 reduction (if stepper is 200 steps per revolution or 1.8 degrees and one uses micro stepping of 32 or more).

I don't know much about servos, but I believe they also have resolution? Right? They have internal encoder that is making sure servo is at correct position?

Either look up for that to be enough compared to tracking precision - or see what is lowest RPM that motor can achieve and see to make it some fraction of sidereal (like 0.1 of sidereal).

In any case, for higher end mount, I believe that you need at least 300:1 reduction. That would mean 300mm disk and 1mm pin.

Pin needs to be pressed against large disk somehow. Maybe spring loaded or something like that.

I believe there is open source version of friction drive, let me see if I can find that for you, maybe you'll get some ideas there.

It looks like I was wrong and that OGEM is not in fact open source:

https://www.cloudynights.com/topic/612391-open-source-gem-from-jtw/

However, take a look at E.fric as well for ideas, although I'm not sure if you'll be able to see the friction drive itself in the images.

http://www.geminitelescope.com/efric-friction-drive-mount-german-equatorial/

 

[matija]

1 hour ago, vlaiv said:

Hi.

Friction drive consists of large disk and very small diameter "pin".

Top view looks like this:

It is a bit like spur gears - but without teeth:

Ratio of diameters of large and small roller is reduction.

You need to have enough reduction to be able to achieve sidereal rate.

I've done calculations for stepper motors and if one wants better resolution than one step per arc second (and usually for high end mounts you want at least 10 times higher) - you need around 200:1 reduction (if stepper is 200 steps per revolution or 1.8 degrees and one uses micro stepping of 32 or more).

I don't know much about servos, but I believe they also have resolution? Right? They have internal encoder that is making sure servo is at correct position?

Either look up for that to be enough compared to tracking precision - or see what is lowest RPM that motor can achieve and see to make it some fraction of sidereal (like 0.1 of sidereal).

In any case, for higher end mount, I believe that you need at least 300:1 reduction. That would mean 300mm disk and 1mm pin.

Pin needs to be pressed against large disk somehow. Maybe spring loaded or something like that.

I believe there is open source version of friction drive, let me see if I can find that for you, maybe you'll get some ideas there.

It looks like I was wrong and that OGEM is not in fact open source:

https://www.cloudynights.com/topic/612391-open-source-gem-from-jtw/

However, take a look at E.fric as well for ideas, although I'm not sure if you'll be able to see the friction drive itself in the images.

http://www.geminitelescope.com/efric-friction-drive-mount-german-equatorial/

 

Thanks for oyur response.

Is a 1mm roller wide enough? It seems a little thin.

And yes. Servos have encoders to tell them its posotion. I spoke with the owner of Sidereal Technology and he said that his servos should work just fine. He did warn me about slipping though.

What do you know about slipping on friction drives?

Thanks!

[Mandy D]

@vlaiv Looking at this from an engineering perspective, so I may have some astro points wrong here, would it not be preferable to use multi-stage reduction? Two stages with 3 mm pinion and 150 mm wheel would give 2500:1 reduction. The 3 mm pinion would certainly be preferable to 1 mm. Combine this with micro-stepping and you would achieve you a resolution of 0.081 arc-second, if my arithmetic is correct. I'm not sure about spring loading it, but a rubber tyre on the wheel and a steel pin should provide an adequate friction drive with minimal backlash if the rubber is thin enough.

[Peter Drew]

I agree with Mandy D.  I think you would be better served by a larger diameter drive "pin" and a multi-stage reduction.  The friction drive on my 8.5" refractor mount has a 11" wheel driven by a 2" driver.  The driver is driven by a worm and worm wheel, itself driven by a geared motor.     🙂

[vlaiv]

1 hour ago, matija said:

What do you know about slipping on friction drives?

Friction drives are prone to slipping if they are not balanced well enough.

1 hour ago, Mandy D said:

@vlaiv Looking at this from an engineering perspective, so I may have some astro points wrong here, would it not be preferable to use multi-stage reduction? Two stages with 3 mm pinion and 150 mm wheel would give 2500:1 reduction. The 3 mm pinion would certainly be preferable to 1 mm. Combine this with micro-stepping and you would achieve you a resolution of 0.081 arc-second, if my arithmetic is correct. I'm not sure about spring loading it, but a rubber tyre on the wheel and a steel pin should provide an adequate friction drive with minimal backlash if the rubber is thin enough.

I agree with you, but as far as I know - all friction drives out there are in fact single stage. I also think that there is no rubber involved either.

It is precision machined steel against steel with enough tension.

18 minutes ago, Peter Drew said:

The driver is driven by a worm and worm wheel, itself driven by a geared motor.     🙂

Worm is intentionally avoided with these designs because of backlash.

Pure friction drive has no backlash and it guides exceptionally well because of this - it is very responsive.

This is primarily design intended for imaging and as such even suffers from "random" periodic error (misnomer, I know - can't be periodic if it is random) - but tracking error is small enough and smooth so it is easily guided out. This design is intended to be guided for imaging.

If I were to design friction drive, I'd probably go with two stage design.

CNC machined cycloidal drive for motor reduction and then 50:1 - 100:1 friction drive.

[Rusted]

Just a note: A drive pin on the face of any practical disk size would suffer from slippage.
The variations in drive radius [on the disk] would fight each other across the length of the drive pin or width of a drive wheel.
There would be simultaneous, multiple drive ratios depending entirely on the contact radius on the large disk.

I built an equatorial platform half a century ago. Using a sharp edged rubber gramophone drive wheel and a long plastic cone for drive speed adjustment.
The plastic cone drove a long, threaded rod and nut, via reduction spur gears, for altering the speed of the equatorial drive.
 

[tomato]

Here is a schematic of the original Mesu 200 drive reduction, courtesy of @steppenwolf, 3000:1:

 

Some precision engineering needed.

[vlaiv]

1 hour ago, tomato said:

Here is a schematic of the original Mesu 200 drive reduction, courtesy of @steppenwolf, 3000:1:

So it is two stage after all!

Even so, very thin driving pins can't be avoided - these look like 2mm ones.

[tomato]

Yes, and interesting that the first stage employs a dual disk, presumably less chance of slippage and mechanically a more stable drivetrain.

[Tomatobro]

I was trying to figure out why the frame drawings had such tight dimensions?

[gorann]

I assume one reason we know so little about the internal workings of a friction driven mount like the Mesu 200 is that they virtually never brake down or need service so no one except Lucas Mesu may ever have taken one apart.

[tomato]

7 hours ago, Tomatobro said:

I was trying to figure out why the frame drawings had such tight dimensions?

Is that just a result of the CAD package doing the math?

[matija]

@tomato does the motor axis pin need to be tentioned with another roller like a bearing with a screw & spring ot can I just tention the pin itself?

[tomato]

I don’t know how the pin is tensioned, but I do know it is critical to the operation of the mount. A Mesu owner attempted to take his apart to have a look which resulted in the mount having to go back to Lucas to be set up again so I’m afraid it’s not something I will be investigating. 
Whatever the design is it is pretty robust. Mine is still fine after nearly 10 years of use, including some unintended abuse along the way, such as trying to run the mount with the locks on.

[matija]

@tomato Is 2mm diameter for the pin really enough? The mount will drive a 15kg scope max.And how thick do you think the disks have to be? 10mm? 5mm? Im seriously considering that design.

Thanks for your response, it was very helpful indeed.

[tomato]

The Mesu drive housings are about 25-30mm thick(I’ll measure them properly and post the figure on this thread) and that includes the cover. Given the first stage drive has dual disks I would think they would be about 5-10mm thick at most. I’m sure the data will exist to give recommended contact areas to transmit a given torque load.

[gorann]

Why not try to build a harmonic drive mount? Here is a guy doing it:

https://hackaday.io/project/967-diy-harmonic-drive-equatorial-mount

[Astrobob1]

same too i start to build a  fork friction  drive  mount same as Gemini Intregra , the load capacity is  about 200 kg but is more important calculate  the baricenter and the unbalance weight i have assumed a max unbalance weight about 10 kg.

I use two disk one 350 mm diameter and  10 mm diameters thickness 8 mm  material Aisi 304.

[ollypenrice]

I have used two Mesu friction drive mounts and consider them perfect. Whatever my budget, I would never choose another make, and I host two of the most obvious rivals in my robotic sheds.

The Mesu has a two stage reduction and I think that the metallurgy is very important.

If Mesus slip, I haven't noticed. My number one mount lost not one single sub to guiding error in over ten years of commercial use. I know that this is hard to believe but there it is.

Olly

[whipdry]

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Edited January 4 by whipdry
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[whipdry]

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Edited January 4 by whipdry
re-think