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Homemade friction drive ideas? #ascom #question

Matija Sircelj
 

Hi.

Im making a homemade german EQ mount that will use SiTech servos. I already have a 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 2 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!
Don W
 

Hi Matija,

Friction drives must have similar tics/rev to worm/gear drives, which means for SiTech you need around (or more) 10 million tics/revolution for good tracking (for imaging).

SiTech servos are (were, since they are no longer available) 10:1 gear ratio times 2000 encoder tics/rev.  Those numbers with 360:1 worm gear ratio = 7.2 million (not quite enough).

OK, now consider a friction drive with a disk driven by a servo driven pin, with the pin rolling on the edge of a disk.  If the disk is 360 mm diameter, to get 360:1 means the pin has to be 1 mm diameter.  No way is that going to work!

So consider a pin about 8 mm diameter, that would give the friction drive 360/8 = 45:1 ratio, needing 8:1 gearing between the servo (10:1) just to get to 7.2 million tics/rev.

Now the real problem with friction drives: selection of materials for the disk and pin drive.  The most successful friction drive mounts are the Mesu mounts.  The earliest ones had a problem:  after extended operation (a lot of use) the friction surfaces deteriorated leaving a powder of metal debris on the surface that made the drive rough and noisey.  Mr Mesu changed the type of metals used to eliminate the problem.  I don't know what metals he uses today.

Stainless steels are known to gall easily.  Steels rust. Aluminum is generally too soft.  Titanium is expensive and difficult to work with.  So I don't know.

Don W
SIMON
 

For size of wheel the answer has to be a large as possible,  it's possible mesu includes this in his specs so have a route around his site,  but I'd say it was about 10" diameter, (I can't measure as it is enclosed on a stainless steel case)

Material:Mesu uses stainless steel



Matija Sircelj
 

Hi Don.

I found a design which I plan to use. The thinest pin will be 8mm in diameter. The final reduction ratio is 1:1875 making my tics per rev = 37.500.000  Ill attach an image in this message. I do plant to make my disks out of well polished stainless steel. If they ever need replacement I will make more but that will probably take years. I will also make a tensioning mechanism.

In this design I use a 1:15 reducer. I did however find a 1:50 harmonic drive for just over 150usd. Here is the link to that: https://www.ebay.com/itm/224931782181?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=ZKPtnJVoQwy&sssrc=2349624&ssuid=a4iwdr0otkw&var=&widget_ver=artemis&media=COPY

Thanks for your responses
Matija Sircelj
 

Hi.

I plan to make a two stage system just like in your mesu mount. My main disk can only be 8” due to physical limitations. If I can get my hands on a decent 1:100 harmonic drive to attach to my motor, my final gear ratio will be 1:12500. That would get me (20000tics/rev * 12500) 250.000.000 tics per rotation of the main disk.

I will indeed use stainless steel for my disks. I will polish all of them very well and maybe harden the thinest ones. Here is a sketch of what it will look like:

Thanks!

Matija
Don W
 

Hi Matija,

Be careful with designing your tics/rev.  SiTech can work well with 10 million to 30 million tics per rev.  But 250 million tics won't work at all - WAY TOO MANY!

Don W
Matija Sircelj
 

Hi, Don.
Thanks for informing me. I redesigned my friction system so that there is only a harmonic drive(1:50), roller pin, and the main disk(1:25), That gives me a ratio of 1:1250 or 25.000.000 tics/rev. That should work. I do worry about slippage though.
Again thanks for the information.
 
Matija
Don W
 

Hi Matija,

Yes 25 million tics should work fine.  I have been using 29 million tics on my SiTech since 2008 with about 1.67 degrees per scond slew speed.  Not fast, but I spend most of my time tracking, not on GoTo's.

Friction drives always have a concern for slippage.  Just balance the scope and cameras very well.  Then the main condition for slippage will be acceleration and deceleration.  Fortunately SiTech has a control in ServoConfig to reduce accereation if you do get slippage.  Not all mount controllers have this capability.

Don W
Matija Sircelj
 

Hi Don.

Great to hear. I wonder, will the harmonic drive compromise the elegant performance of the friction drive system of the mount? I hear they tend to have quite some periodic error. 

I plan to make my main disk 15mm thick and the roller 10mm in diameter. They’ll be machined out of stainless steel ad then properly polished. I will design a tension mechanism to keep the motor pin/roller from having any bending momentum. I wonder how much pressure should I be expecting to put on the roller? 500N? 1000N?

I am thrilled to be using your motors and controller for my build. I love and appreciate the slow acceleration and deceleration option. That will help a lot. 

Do you by any chance know when will the motors be available in your online store?

Thank you for your response!

Matija
Don W
 

Hi Matija,

There are many types of stainless steels, ssum are gummy to machine and others are great to machine (cut with lathe or mill).  Generally I believe using the same alloy stainless for mating parts is very likely to gall (tear the surface, ruining the assembly).  Polishing doesn't prevent galling.  You need to contact a metallurgist to recommend which alloys may work.

Don W
Roland
 

Don,

You’re quite right. The best combination is probably some form of steel (certain types of stainless and other steels) and B14 bronze. 

In a worm and crown setup the crown is normally bronze and the worm is steel. B14 is actually quite self lubricating and is easy to machine precisely.

Many mounts (eg AP) use aluminium crowns which can be anodised to increase surface hardness but I’m not convinced about the long term wear. 

But certainly using the same metal for mating parts may be problematic. 

Roland 

On 19 Nov 2022, at 03:56, Don W <westergren@...> wrote:

Hi Matija,

There are many types of stainless steels, ssum are gummy to machine and others are great to machine (cut with lathe or mill).  Generally I believe using the same alloy stainless for mating parts is very likely to gall (tear the surface, ruining the assembly).  Polishing doesn't prevent galling.  You need to contact a metallurgist to recommend which alloys may work.

Don W

Mark Volle
 

I’m not a metallurgist but I did try a friction drive with stainless on stainless and it wasn’t good - 

Surprisingly it was also a problem with an anodized disk - the anodized surface is almost too hard and too abrasive but an anodized pinion was able to drive almost any other metal - challenge being that if it slips, it grinds into the other material very quickly but the apparently rougher and harder surface of the anodized 6061 drive pin grabbed really well

I’m somewhat spoiled as I managed to get several worm and gear sets and stopped trying the friction drive - 

I did end up using servo motors with harmonic drives to turn the worms - which has seemed to work well - 

Mark


On Nov 18, 2022, at 23:20, Roland <mrcas62@...> wrote:

Don,

You’re quite right. The best combination is probably some form of steel (certain types of stainless and other steels) and B14 bronze. 

In a worm and crown setup the crown is normally bronze and the worm is steel. B14 is actually quite self lubricating and is easy to machine precisely.

Many mounts (eg AP) use aluminium crowns which can be anodised to increase surface hardness but I’m not convinced about the long term wear. 

But certainly using the same metal for mating parts may be problematic. 

Roland 

On 19 Nov 2022, at 03:56, Don W <westergren@...> wrote:

Hi Matija,

There are many types of stainless steels, ssum are gummy to machine and others are great to machine (cut with lathe or mill).  Generally I believe using the same alloy stainless for mating parts is very likely to gall (tear the surface, ruining the assembly).  Polishing doesn't prevent galling.  You need to contact a metallurgist to recommend which alloys may work.

Don W

Matija Sircelj
 

Hi Mark.

I am now searching for the very best materials to use in my system. For the pinion, you recommend aluminium 6061(anodised). Do I, when machining (before anodising) the pinion have to polish it really well, like I was told to do with steel? 

Also I really cant figure out what to use for the main disk. Maybe bronze like it was recommended earlier? One of the many stainless steels? I would imagine, that the 6061 pinion will be hard enough to not have any surface denting with all the pressure being put on it.

Do you think that I will experience slippage with the main disk being 15mm thick and the pinion being 10mm in diameter?

Thank you for your reply.

Matija
Matija Sircelj
 

Thanks!

We have an experienced metallurgist at my high school. I hope he can help the find the disk material. One user recommended I use 6061 anodised aluminium for the pinion. What do you think?

Thanks again,

Matija
Mark Volle
 

Matija,

I found that the anodized surface was very “grippy” against stainless and plain steel but an anodized part isn’t necessarily the most perfectly uniform or round as the anodizing “grows” the surface - which if grown thick enough could then be ground to a precision finish and diameter afterwards but it is essentially a very hard surface on a relatively soft material.

Thicker disks increase the amount of surface contact between the parts which can lower the tension needed or increase the load for the same tension but ultimately the tension between the two parts will be a big part of how suitable a specific material is - 

Ground and polished tool steel dowel pins are readily available and might make good pinions - and are available in very hard materials and precisely ground - they would have very little flex

Some of this really depends on your performance targets - how perfect do you want the result to be? “”Perfect” is extremely challenging while “really good” is much more forgiving -  

Mark


On Nov 18, 2022, at 23:53, Matija Sircelj <matija.sircelj@...> wrote:

Hi Mark.

I am now searching for the very best materials to use in my system. For the pinion, you recommend aluminium 6061(anodised). Do I, when machining (before anodising) the pinion have to polish it really well, like I was told to do with steel? 

Also I really cant figure out what to use for the main disk. Maybe bronze like it was recommended earlier? One of the many stainless steels? I would imagine, that the 6061 pinion will be hard enough to not have any surface denting with all the pressure being put on it.

Do you think that I will experience slippage with the main disk being 15mm thick and the pinion being 10mm in diameter?

Thank you for your reply.

Matija
Roland
 

Mark,

I pretty much agree with your thoughts. An additional problem with anodising is that if, for some reason, you get a hard particle on the wheel, you run the risk of breaking through the anodising and then you only have the soft metal underneath. At most, anodising is maybe a few hundred microns thick, as far as I can recall. 

Matija,

One other option is to make sure that one part wears more quickly than the other and can be replaced on a regular basis. So you need to make it so that it is easily accessible. 

You need to find a compromise between the coefficient of friction, the force applied and the precision you require. Bear in mind that anodising (and chroming) increase surface hardness but reduce the coefficient of friction. 

I am by no means an expert but certainly the choice of materials is fundamental. Of course, providing you can construct a system which will allow it fairly easily, you can also experiment with different materials. 

If I had to make a call on it, I would go for bronze against steel. If you harden the steel, the bronze will wear out over time and you’ll need to replace it from time to time. You can have both precisely ground so that the dimensions are uniform. You could also turn and then lap but I think the ground option is better. 

But I’m sure there are other choices possible. 

Hope this helps. 

Roland. 

On 19 Nov 2022, at 10:07, Mark Volle <mark.d.volle@...> wrote:

Matija,

I found that the anodized surface was very “grippy” against stainless and plain steel but an anodized part isn’t necessarily the most perfectly uniform or round as the anodizing “grows” the surface - which if grown thick enough could then be ground to a precision finish and diameter afterwards but it is essentially a very hard surface on a relatively soft material.

Thicker disks increase the amount of surface contact between the parts which can lower the tension needed or increase the load for the same tension but ultimately the tension between the two parts will be a big part of how suitable a specific material is - 

Ground and polished tool steel dowel pins are readily available and might make good pinions - and are available in very hard materials and precisely ground - they would have very little flex

Some of this really depends on your performance targets - how perfect do you want the result to be? “”Perfect” is extremely challenging while “really good” is much more forgiving -  

Mark


On Nov 18, 2022, at 23:53, Matija Sircelj <matija.sircelj@...> wrote:

Hi Mark.

I am now searching for the very best materials to use in my system. For the pinion, you recommend aluminium 6061(anodised). Do I, when machining (before anodising) the pinion have to polish it really well, like I was told to do with steel? 

Also I really cant figure out what to use for the main disk. Maybe bronze like it was recommended earlier? One of the many stainless steels? I would imagine, that the 6061 pinion will be hard enough to not have any surface denting with all the pressure being put on it.

Do you think that I will experience slippage with the main disk being 15mm thick and the pinion being 10mm in diameter?

Thank you for your reply.

Matija
Matija Sircelj
 

Ronald,

I see, and would you recommend making the pinion or the disk out of bronze? I imagine it would be better if the disk was bronze. 

I still cant even remotely figure out how much force will have to be pushing on the pinion for the mount to drive nicely. My mount head will weigh about 40-60kg(without  CW) and my telescope will be 25kg MAX. Most likely closer to 15kg. So lets say I make a D=10mm pinion out of steel and a D=250mm disk from bronze, about how much force are we talking here? 500N? 1000N? I will attach a photo of the tension system.

Thanks!

Matija
Matija Sircelj
 

Hi Mark,

Yes, I will most likely make the pinion out of stainless steel and the disk from bronze. With the pin being 10mm in diameter and the disk being 15mm thick, I imagine there would be almost no slippage?  If there would still be some leftover, I may also add some rotary encoders with a 30mm shaft hole. I found some well priced 5000ppr trough-hole encoders on Aliexpress.

I don’t expect my mount to be perfect, but I do want to achieve 7-10 minute exposures at 16kg payload. I also don’t want any backlash. Luckily, the friction and harmonic drives have none. Only periodical error I can easily guide out at gear ratio 1:1250.  Thats what Im expecting to see.

Thank you for the reply.

Matija
Roland
 

Most of the periodic error will probably come from the pinion if it is slightly out of round. That’s why I’m suggesting you precision grind and harden it. Of course, you may be able to go with off-the-shelf pinions. 10mm would seem ok to start. 

However, of what will you make the pressure wheel?

This aside, over time a bronze disk will wear if the pinion is hardened. To avoid making a large bronze disk, you could think about using an annulus which fits precisely on a disk which you could make of another material. That way you only need to change the outer annulus. You could even experiment with different materials fairly easily. Just make the annulus out if a different material and slip it onto the main disk and fix it in position. 

In terms of the pressure, I have no idea.  Much will depend on how well balanced you are and what friction you need to overcome. 

R. 

On 19 Nov 2022, at 13:05, Matija Sircelj <matija.sircelj@...> wrote:



Ronald,

I see, and would you recommend making the pinion or the disk out of bronze? I imagine it would be better if the disk was bronze. 

I still cant even remotely figure out how much force will have to be pushing on the pinion for the mount to drive nicely. My mount head will weigh about 40-60kg(without  CW) and my telescope will be 25kg MAX. Most likely closer to 15kg. So lets say I make a D=10mm pinion out of steel and a D=250mm disk from bronze, about how much force are we talking here? 500N? 1000N? I will attach a photo of the tension system.

Thanks!

Matija
1962405A-7490-4051-BAB8-6F9604D59B12.jpeg
Mark Volle
 

Matija,

I would strongly recommend supporting the pinion with bearings on both sides of the disk - even with precision worm drives, having bearings on both sides of the pressure point is a huge improvement.

Using a spring loaded pressure roller rather than or even in conjunction with a tensioned pinion mount should be really useful.

The “spring” can also be a metal arm with “flex” in it that is tensioned down by an adjusting screw - the spring travel will be very small but the forces may need to be considerable so something that we would normally think of as more rigid can actually make a nicely controlled spring.


Mark


On Nov 19, 2022, at 04:05, Matija Sircelj <matija.sircelj@...> wrote:

Ronald,

I see, and would you recommend making the pinion or the disk out of bronze? I imagine it would be better if the disk was bronze. 

I still cant even remotely figure out how much force will have to be pushing on the pinion for the mount to drive nicely. My mount head will weigh about 40-60kg(without  CW) and my telescope will be 25kg MAX. Most likely closer to 15kg. So lets say I make a D=10mm pinion out of steel and a D=250mm disk from bronze, about how much force are we talking here? 500N? 1000N? I will attach a photo of the tension system.

Thanks!

Matija
<1962405A-7490-4051-BAB8-6F9604D59B12.jpeg>
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