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Friction for Large/Small Altitude Dob Bearings

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#1 highertheflyer

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Posted 07 July 2014 - 07:15 PM

With so many home made dobs in the past, I've used altitude bearings riding on various Teflon/laminate surfaces.
Mostly the altitude bearing diameters have been quite large in relation to the optical tubes and somewhat more than 150 percent of the diameters of the primaries.
The latest has been a 12 inch diameter altitude bearing surface wiping on a pair of 1 inch Teflon pads, each side.
These pads squeeze high on the sides of the bearing surface, perhaps at 150 degrees or so.
I was noting the ease at which the altitude axis was moving, even with gentle wind forces upon the telescope.
So the subject came up.
Will a proportionately larger diameter DOB bearing produce less or more stictations/frictions, given the same relative positions of the Teflon and laminate riding surface?
Again, given the same Teflon area surfaces, wiping upon a small diameter altitude bearing/versus wiping upon a large diameter altitude bearing, produce more or less stictations/frictions?

Help me with this thought.

Jim

#2 Starman1

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Posted 08 July 2014 - 12:27 AM

Larger pads=more friction
Pads farther apart = more friction
Pads closer together = less friction
Larger trunnion=more friction
Smaller trunnion = less friction

2 kinds of friction:
Start-up friction (stiction)
Kinetic friction (moving friction)
When Stiction is a lot higher than Kinetic friction the scope jerks when it starts moving.
When both frictions are too low, the scope is ultra-sensitive to balance and needs a brake to hold it in place when changing eyepieces, and needs to be rebalanced every 10-15 degrees of altitude.

Different materials respond differently. Ebony Star Formica on teflon has a higher stiction than kinetic friction. FRP on teflon has a higher kinetic friction, more closely matching the start-up friction. Ebony Star on PFA has a higher stiction and kinetic friction than PTFE (Teflon) and is a little easier to control if moving up and down to center the object. That higher force, though, may not feel right.

It's hard to satisfy everyone in terms of movement. I dislike the start-up jerk of Ebony Star on teflon, but disliked the higher start-up friction of PFA.
I like the kinetic friction, but want to only lower stiction. I tried lubricants on the formica and teflon, but they lowered BOTH the stiction and the kinetic friction, making the scope too sensitive to balance. Raising kinetic friction a lot and stiction a little makes them the same but then the scope is disagreeable to move, so PFA was out.

If anyone has an idea of how to lower start-up friction WITHOUT lowering kinetic friction, I'm all ears. Everything that lowers one of those frictions seems to lower both.

#3 schang

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Posted 08 July 2014 - 07:41 AM

2 kinds of friction:
Start-up friction (stiction)
Kinetic friction (moving friction)

If anyone has an idea of how to lower start-up friction WITHOUT lowering kinetic friction, I'm all ears. Everything that lowers one of those frictions seems to lower both.

Don: If I recall correctly, the startup (static) friction depends on the weight of the OTA only, other things being equal. If the scope has high static friction that causes stiction using low friction materials like Teflon, then the only way to decrease it is to reduce the weight of OTA (for example, go for truss tube, light weight eps, and finder). Once the scope is in motion, the kinetic friction depends on the frictional coefficient of the materials only. So it is possible to optimize the smoothness of the scope operation.

#4 schang

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Posted 08 July 2014 - 08:24 AM

There may be a way to help if you can not change anything wrt the weight of OTA...

Since we do not view low lying objects in the sky, so anything lower than, say, 30 degrees is not worth to observe due to atmosphere effect. So we can rearrange the Teflon pads in such a way that they are not symmetric on the bearing. For example, put the front pads in a lower position than the back pads on the bearing. The angle thus created can induce sliding force that may help to reduce the static friction, depending the angle... I have not tried this, because I do not have such jerking issue. You may want to experiment and tell us if it works.

#5 Starman1

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Posted 08 July 2014 - 09:30 AM


2 kinds of friction:
Start-up friction (stiction)
Kinetic friction (moving friction)

If anyone has an idea of how to lower start-up friction WITHOUT lowering kinetic friction, I'm all ears. Everything that lowers one of those frictions seems to lower both.

Don: If I recall correctly, the startup (static) friction depends on the weight of the OTA only, other things being equal. If the scope has high static friction that causes stiction using low friction materials like Teflon, then the only way to decrease it is to reduce the weight of OTA (for example, go for truss tube, light weight eps, and finder). Once the scope is in motion, the kinetic friction depends on the frictional coefficient of the materials only. So it is possible to optimize the smoothness of the scope operation.

Well PFA raised the start up friction AND the kinetic friction until they seemed very close together. It just took a lot more effort to move than I liked. FRP on teflon works great on the azimuth bearing but results in too much kinetic friction in altitude, IMO.
Sizing the pads can keep pressure to about 15 psi on a scope of any weight, and this seems just about optimum for kinetic friction--very smooth, very little effort, yet stays where it's put.
I'd just like a little less startup friction.
Beveling the teflon pads to create rounded edges helped.
There are probably better materials than formica-on-teflon to use for bearings, and I'm always on the look-out for the scope with the optimum feel so I can ask what materials are used.
I've owned two 12.5" truss dobs, and one had very little stiction and also very little kinetic friction--it needed to be balanced for every eyepiece in every position. My second requires twice as much force to move, yet the scopes are of nearly identical weight. However, the second scope has 19.5" trunnions, while the first had 12" trunnions, and smaller trunnions, by virtue of how the vectors of force are applied, move easier.

#6 Jon Isaacs

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Posted 08 July 2014 - 09:43 AM


2 kinds of friction:
Start-up friction (stiction)
Kinetic friction (moving friction)

If anyone has an idea of how to lower start-up friction WITHOUT lowering kinetic friction, I'm all ears. Everything that lowers one of those frictions seems to lower both.

Don: If I recall correctly, the startup (static) friction depends on the weight of the OTA only, other things being equal. If the scope has high static friction that causes stiction using low friction materials like Teflon, then the only way to decrease it is to reduce the weight of OTA (for example, go for truss tube, light weight eps, and finder). Once the scope is in motion, the kinetic friction depends on the frictional coefficient of the materials only. So it is possible to optimize the smoothness of the scope operation.


Shien:

If the coefficient of friction is constant, then the resultant frictional force only depends on the applied normal force which in this case is the weight of the telescope.

However, the situation here is more complicated than a simple vertical force and the resulting friction. This is moment or torque, the larger lever arm of the larger trunion bearing diameter increases the torque required to rotate the bearing. The observer has a lever or moment arm as well, the distance from the center of rotation of the altitude bearing to wherever he/she is pushing. So, larger bearings do increase the force required to move the scope.

The angular position of the bearing pads affects the force between bearing pad and the bearing because as the pads are moved further apart, while the weight load is constant in the vertical direction is constant, the force normal to bearing surfaces is equal to:

Fn = W/(cos A) weight divided by the cosine of the angle.

Essentially the it's like a wedge, the steeper the angle, the more the bearing pushes outward and the greater normal force to the bearing surface and the greater the frictional force.

As far as larger pads resulting in greater friction, that would have to be a material non-linearity issue because from a physics standpoint, the assumption is friction is linearly proportional to the applied load, a larger area spreads the load over a larger area, reducing the bearing pressures but the net forces remain unchanged.

So, larger trunions = greater moment, more force required to move the scope.

Bearing spread out so the angle of the applied force in increased = more force required to move the scope.

Kinetic friction versus Startup friction: This is property of the particular bearing system..

It is my thinking/experience that a relatively stiff, "heavy" action is desireable. With a light action, the kinetic energy and momentum stored in the scope become a factor so the once the observer stops pushing, that energy/momentum must be dissipated and the scope overshoots, the result is a jerky motion. The commercial Dobs with the roller bearing azimuth axis are an excellent example of the need for friction for proper tracking motion, I find them unusable, a light pull and the scope is moving and overshoot in inevitable.

I setup my Dobs so they are firm so that the motion is slow and smooth without overshoot. I also suspect that the stiction or the difference between static and kinetic friction has a constant component and so a heavier action reduces the effect. In any event, a firmer action, results in less jitter and overshoot because it is more highly damped, things happen slower.

I think it's pretty clear looking at the premium Dobsonian that they are designed with large trunions that do increase the forces required to move the scope. I often hear of buttery smooth action but smooth and steady is where it's at for me. The action of my 25 inch F/5 Obsession is quite amazing, you can just move the object about the field of view as desired at magnifications in 600x + range. Part of this of course is the long lever arm that a scope with a 10 foot focal length provides, but a good part of it is the appropriate level of friction those large trunions provide.

The Correction Tension spring system that I first saw with the GSO Dobs manufactured for the first Orion XT-series Dobs increases the force required to rotate the scope by effectively increasing the weight of the scope, this has the same basic effect as increasing the diameter of the trunnion bearings.

Just some stuff to think about...

Jon
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#7 Pinbout

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Posted 08 July 2014 - 10:03 AM

Sizing the pads can keep pressure to about 15 psi on a scope of any weight, and this seems just about optimum for kinetic friction--very smooth, very little effort, yet stays where it's put.



on my 8" stellafane dob I had to cut my 1" sq az pads a lot smaller to get that buttery but stiff feel since the scope is pretty light trying to keep to the 15psi. :grin:

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#8 schang

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Posted 08 July 2014 - 10:37 AM


Don: If I recall correctly, the startup (static) friction depends on the weight of the OTA only, other things being equal. If the scope has high static friction that causes stiction using low friction materials like Teflon, then the only way to decrease it is to reduce the weight of OTA (for example, go for truss tube, light weight eps, and finder). Once the scope is in motion, the kinetic friction depends on the frictional coefficient of the materials only. So it is possible to optimize the smoothness of the scope operation.

Well PFA raised the start up friction AND the kinetic friction until they seemed very close together. It just took a lot more effort to move than I liked. FRP on teflon works great on the azimuth bearing but results in too much kinetic friction in altitude, IMO.
Sizing the pads can keep pressure to about 15 psi on a scope of any weight, and this seems just about optimum for kinetic friction--very smooth, very little effort, yet stays where it's put.
I'd just like a little less startup friction.
Beveling the teflon pads to create rounded edges helped.
There are probably better materials than formica-on-teflon to use for bearings, and I'm always on the look-out for the scope with the optimum feel so I can ask what materials are used.
I've owned two 12.5" truss dobs, and one had very little stiction and also very little kinetic friction--it needed to be balanced for every eyepiece in every position. My second requires twice as much force to move, yet the scopes are of nearly identical weight. However, the second scope has 19.5" trunnions, while the first had 12" trunnions, and smaller trunnions, by virtue of how the vectors of force are applied, move easier.

Don/Jon: It appears that the larger trunnion dob is the one you have issue with, am I correct? As I stated, other things being equal, the OTA weight affects the frictional force alone. Since you are not likely to change the 19.5" diameter trunnion, nor you can reduce the weight of a truss OTA, so that limit the options you have. You can experiment with different Formica surface (smoother one) to reduce the friction. As Jon indicated, we do need certain amount of resistance to operate the scope without much jerking motions, so that needs to be optimized. I wonder if you can experiment the front pads position in your 19.5" diameter dob by move them lower than the back pads and see if this helps? I was thinking may be the sliding angle has some impact on this, and it is quite easy to find out.

#9 Starman1

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Posted 08 July 2014 - 11:02 AM

In the October 2003 issue of Sky & Telescope, Martin Lewis discusses different bearing materials. It was there I first saw mention of PFA and I acquired a strip of it to replace the teflon bearings on my scope.
As suggested in the article, the start-up friction AND the kinetic friction were higher than teflon, but closer to the same, meaning the scope started moving but then did not suddenly move faster with the same force applied.
I just didn't like the higher force necessary to get the scope moving, and I didn't like the higher force to keep it moving.
So I went back to teflon.
And I'm still looking.

#10 highertheflyer

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Posted 08 July 2014 - 12:15 PM

Excellent briefings on means to blend frictional and kinetic frictions together!
Thank you all.
To understand just why this OTA is easily blown up and down in the wind, may I offer these thoughts.
The OTA consist of a lightweight 10" concrete forming tube, 65 inches in length and pivoting upon a 12 inch bumpy Formica bearing.
The 12 inch diameter bumpy bearing surface supports a total of 19 lbs of OTA weight.
And resting on a total of 4 square inches of Teflon surface, space 150 degrees apart, spread high on the Formica bearing.
I've tried substituting a plastic sold at home stores used beneath the legs of furniture, and a slider pad of sorts.
But that seems to increase the frictions, both at rest and in motion very dramatically.
So my easiest remedies, based on the thought that this 12 inch bumpy Formica, resting upon 4 square inches of teflon, riding high on the cradle, with just 19 pounds of total weight, will not be giving me the 15 lbs/square inch of "grab"?
A pulling force of a spring might be the solution, but esthetically not a pretty sight tho!
And substituting different plastic makeups, will increase the stictations and kinetic energies required, might be the next step?
Thanks again everyone.

Jim

#11 highertheflyer

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Posted 08 July 2014 - 12:22 PM

In the October 2003 issue of Sky & Telescope, Martin Lewis discusses different bearing materials. It was there I first saw mention of PFA and I acquired a strip of it to replace the teflon bearings on my scope.
As suggested in the article, the start-up friction AND the kinetic friction were higher than teflon, but closer to the same, meaning the scope started moving but then did not suddenly move faster with the same force applied.
I just didn't like the higher force necessary to get the scope moving, and I didn't like the higher force to keep it moving.
So I went back to teflon.
And I'm still looking.


Hi Starman.
Just wondering where you can purchase the PFA as I'm unfamiliar with it.
Jim

#12 Pinbout

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Posted 08 July 2014 - 12:29 PM

might be the next step?



pics!

#13 Jon Isaacs

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Posted 08 July 2014 - 02:46 PM

Since you are not likely to change the 19.5" diameter trunnion, nor you can reduce the weight of a truss OTA, so that limit the options you have.



The location of the bearing pads also affects the force required to move the scope. By placing them closer together, the force is reduced, by placing them further apart, the force is increased.

Jon

#14 Jeff Morgan

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Posted 08 July 2014 - 02:51 PM

It is my thinking/experience that a relatively stiff, "heavy" action is desireable. With a light action, the kinetic energy and momentum stored in the scope become a factor so the once the observer stops pushing, that energy/momentum must be dissipated and the scope overshoots, the result is a jerky motion. The commercial Dobs with the roller bearing azimuth axis are an excellent example of the need for friction for proper tracking motion, I find them unusable, a light pull and the scope is moving and overshoot in inevitable.

I setup my Dobs so they are firm so that the motion is slow and smooth without overshoot. I also suspect that the stiction or the difference between static and kinetic friction has a constant component and so a heavier action reduces the effect. In any event, a firmer action, results in less jitter and overshoot because it is more highly damped, things happen slower.

I think it's pretty clear looking at the premium Dobsonian that they are designed with large trunions that do increase the forces required to move the scope. I often hear of buttery smooth action but smooth and steady is where it's at for me. The action of my 25 inch F/5 Obsession is quite amazing, you can just move the object about the field of view as desired at magnifications in 600x + range. Part of this of course is the long lever arm that a scope with a 10 foot focal length provides, but a good part of it is the appropriate level of friction those large trunions provide.


Of course we are venturing into the area of personal preferences, but I would agree with Jon on this. If you have access to a Equatorial mount, release the clamps and see how you like it. It sure is "buttery smooth". And if you observe where there is no breeze and never change eyepieces, it might just work for you.

But for the rest of us, some resistance is desirable. If you go with smaller bearings, you likely will need some type of add-on tensioning strategy such as clamps, springs, or sliding weights. Large bearings are easier.

Forget about 15 psi and 1.x times mirror size. They are no more relevant to the problem than yesterdays baseball scores.

The frictional formulas are nice and will suggest something close. (I have used them too). But this is one area where where experimentation and hands-on experience pay off. Remember, you are dealing with a preference more than a metric that requires six decimal accuracy to work well. There is lots of room for error on the high side.

You might prefer other plastics too. After the Sky & Tel article came out I thought the Holy Grail had been discovered and ordered it that day. I was less than impressed and sold it off to another CN member for minimal cost. You can search the ATM forum for "PFA" and I don't think you will find many people raving about it. Teflon still rules.

A few years ago I did finally discover the Holy Grail for smooth Dobsonian motion - it's called ServoCAT. Not cheap, but it does have a few other little benefits too.

#15 Pinbout

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Posted 08 July 2014 - 03:46 PM

A few years ago I did finally discover the Holy Grail for smooth Dobsonian motion - it's called ServoCAT. Not cheap, but it does have a few other little benefits too.



I want one. :bawling:

#16 Starman1

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Posted 08 July 2014 - 04:46 PM

In the October 2003 issue of Sky & Telescope, Martin Lewis discusses different bearing materials. It was there I first saw mention of PFA and I acquired a strip of it to replace the teflon bearings on my scope.
As suggested in the article, the start-up friction AND the kinetic friction were higher than teflon, but closer to the same, meaning the scope started moving but then did not suddenly move faster with the same force applied.
I just didn't like the higher force necessary to get the scope moving, and I didn't like the higher force to keep it moving.
So I went back to teflon.
And I'm still looking.


Hi Starman.
Just wondering where you can purchase the PFA as I'm unfamiliar with it.
Jim

Boedeker Plastics.
and others.

#17 Jeff Morgan

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Posted 08 July 2014 - 11:27 PM


A few years ago I did finally discover the Holy Grail for smooth Dobsonian motion - it's called ServoCAT. Not cheap, but it does have a few other little benefits too.



I want one. :bawling:


Money makes the world go 'round :grin: Much easier to think of it in terms of value delivered.

For most of us Dob owners, a drive system is somewhere between maybe 5th to 99th on our priority list. It just seems so counter to the Dobsonian mythos. But so are truss tubes, high performance mirror cells, 2 speed focusers, etc, etc, etc. In short, every "Dob" innovation since 1980.

With the benefit of 20/20 hindsight if I were to lose it all in a disaster tomorrow and had to start from scratch my priority list would be:

1) Premium mirror (home made or commercial);
2) Mirror cell and structure;
3) Drive system (ServoCAT or EQ platform, depending);
4) SkySafari/SkyFi;
5) Quality observing chair or ladder; and
6) All the other toys of convenience.

Solid optics, structure, and mount. The guys before the Dobsonian revolution knew this implicitly.

#18 Chriske

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Posted 09 July 2014 - 04:26 AM

Hi guys,

Did not read the complete thread, so maybe someone mentioned this before..
About that alt-friction, why not replace one teflon pad -on each side- by a roller bearing..? By relocating the teflon pads friction on the altitude axis can very accurately and easily be tuned.
As a matter of fact, on my next project I will altogether remove all teflon pads and replace them all with roller bearings. To control it's motion I will add a small teflon pad to have some friction.

#19 Jon Isaacs

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Posted 09 July 2014 - 05:35 AM

Hi guys,

Did not read the complete thread, so maybe someone mentioned this before..
About that alt-friction, why not replace one teflon pad -on each side- by a roller bearing..? By relocating the teflon pads friction on the altitude axis can very accurately and easily be tuned.
As a matter of fact, on my next project I will altogether remove all teflon pads and replace them all with roller bearings. To control it's motion I will add a small teflon pad to have some friction.


If you read the thread you would have seen it stated that friction is your friend. If there is not enough friction, the motion will be jerky because the momentum of the scope becomes significant.

If you replace one bearing, you cut the friction in half, that has the same effect as cutting the diameter of the trunnion is half. Premium Dobs use those large trunnions/altitude bearings just so they have sufficient friction.

If you consider that those large bearings carrying the entire weight of the OTA are required to get sufficient friction, a single Teflon pad with most of the load carried by roller bearings would not be sufficient.

That's how it looks to me..

Jon

#20 Starman1

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Posted 09 July 2014 - 06:10 PM

Jon is right. Reducing friction isn't ideal because then balance becomes too sensitive and the scope won't stay where you leave it when moving it by hand. My scope is nearly ideal in that regard.
It's the start-up friction that is too high. If I want to move the scope only a couple minutes of arc at high power, by the time enough pressure is applied to get it to move, it zooms past where I want it to stop.
If I want to move it 15' to move an object to the other side of the field to watch it drift, then it passes the jerky start and smoothly moves to wherever I want it to stop.
My desire is to lower the force required to get it to move without lowering the friction to keep it moving.
The extra stiction makes the scope less sensitive to imbalance, and makes changing eyepieces easy. But it makes use a little difficult.
In contrast, the azimuth is nearly perfect: it takes almost the same force to get it to move that it takes to keep it in motion. The difference? FRP instead of Ebony Star formica.
Hmm.

#21 Chriske

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Posted 10 July 2014 - 01:56 AM

It's all a point of view.
The first thing I tell my pupils during course is : When you start building your mount, friction is your enemy and you have to learn how to reduce it. Point is, the location of your teflon pads depends on the weight of your scope.
Once your scope becomes to large teflon pads will not suffice and you need to find other ways to deal with that heavy load.
As a matter of fact I'm busy building 200+ lbs scope(bino). I do not even think of using teflon pads. Yes I will use pads on the very rim to act as a 'break' for both alt/az. It's a much easier approach to deal with friction.
Needles to say, for small/lighter telescopes teflon pads are the best option.

#22 Chriske

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Posted 10 July 2014 - 02:35 AM

Read some more...(not all)

About kinetic friction versus startup friction.
Strangely enough never had to deal with that ever during our courses. The only thing we compensate for is eyepiece changes.
All course telescopes use teflon pads + formica. The formica we're using is not the flat one. It's a rather uneven surface, so it has no full contact with the teflon pads. We also add some teflon spray.
For the az bearing we always place a 1" to 2" ring shaped teflon pad at the very center. At the very rim we place 3 very small teflon pads. All the weight of the scopes and it's rocker is supported by the teflon ring at the very center. The outer teflon pads are only there to 'balance' the scope / keeping it from 'tilting'.
Some tuning needs to be done because the center teflon ring has to be fraction of a millimeter thicker compared to the rim pads. Depending on the weight and the diameter of the az disk, the inner teflon disk needs to be lifted using only a few layers of cellophane.
Works like a charme.

#23 schang

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Posted 10 July 2014 - 06:52 AM

(Part)

All course telescopes use teflon pads + formica. The formica we're using is not the flat one. It's a rather uneven surface, so it has no full contact with the teflon pads.

I was suggesting a while back about using a "smoother" Formica for trial purposes...My thinking was that under the heavy weight of the OTA, the Teflon pads would "yield" or "deform" to those high spots on the Formica surface, thus increasing the total contact surface area between the Teflon and the Formica, and therefore the static frictional force.

Using different materials or sliding mechanism for those heavy OTA weights with braking control look like a better solution than the Teflon pads, which I'd try if I had one of those monste OTA.. :cool:

#24 davidpitre

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Posted 10 July 2014 - 07:14 AM

Chris.
Do you have any photos of what you are talking about? I'm having a hard time following it.

#25 Pinbout

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Posted 10 July 2014 - 08:01 AM

Yes I will use pads on the very rim to act as a 'break' for both alt/az.



are you talking about an adjustable friction clutch?






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