Formica bearing surface for dobs

[markse68]

I’m in the process of drawing up a new dob base and looking at ideal bearings for it. There’s a lot of stuff out there on CN etc about using textured Formica laminate with ptfe sliding bearing pads and I should really research some more but it seems the old favourite textured pattern Formica is no longer available anyway but another “stardust” is. But it’s hard to obtain other than in full sheets and rather pricey!

What is it that makes these textured formicas so ideal when used with ptfe low friction bearing pads? And what is the ideal texture? It’s hard to determine what the texture actually is from photos. My guess is that ptfe on smooth Formica is just too slippy and  that the texture has 2 functions- it embeds slightly into the soft ptfe when the scope is stationary giving a little bite and increasing the stiction slightly, whilst at the same time reducing surface contact area when the scope starts moving possibly reducing friction and making it more slippy? Ideally I think we want something that’s effortless to move when we want it to but that will hold position enough to withstand breezes and slight out of balance when eg changing eps.

If anyone has the old preferred texture Formica “ebony star” could they describe the texture or perhaps try to photograph it so it shows? Ditto the newer “stardust” pattern?

i found this stuff on eBay- it’s not genuine Formica but has a sort of orange peel texture- and it comes in orange! It’s a kind of smooth bumpy texture by the look of it. Might it work? Are the preferred formicas a sharper texture? 

 

[Chriske]

Two possible alternatives.

If you own a printer, print a bearing in PLA and use it with Teflon pads. Works even better then formica, done it a few times already the last few years. Most printers do have a 200mm print-platform, that should be enough.

Second , a small shim of shine metal screwed onto a wooden disk. In the rocker a sheet of felt instead of the usual Formica, works surprisingly well also. After a few months you need to replace that strip of felt.  And if you do own a printer, here again you could use that printed bearing instead of that wooden disk/metal strip.

Using one or the other, there is no backlash at all.
I've used all my life formica/teflon. My last telescope(bino) I made a few months back, I did use the combination PLA/felt. That is the best bearing I've ever used on my dobs.

[marcopolo]

Quote

What is it that makes these textured formicas so ideal when used with ptfe low friction bearing pads? 

It is because the texture allows the slip face on the teflon to work without and suction caused by micro-vaacuums or pocket of air. A bit like a sucker holding low pressure on a piece of glass. Smooth on Smooth can cause "suction"

[Chriske]

these textured formicas have fewer contact points, resulting in a very smooth 'ride'. The 'rougher' the formica, the smoother it will go.
Think of ball bearings. Ball bearings have just point-contacts, That's the reason why they work so smooth.
+ of course, formica is a very 'slippery' material.

[johninderby]

This gives an idea of the sort of texture needed. This is the az bearing on a Bresseer dob.

Edited May 23, 2020 by johninderby

[markse68]

12 minutes ago, Chriske said:

these textured formicas have fewer contact points, resulting in a very smooth 'ride'. The 'rougher' the formica, the smoother it will go.
Think of ball bearings. Ball bearings have just point-contacts, That's the reason why they work so smooth.
+ of course, formica is a very 'slippery' material.

Ball bearings work by rolling though. PTFE is very soft- under pressure I’d have thought it’d deform and the little bumps would dig into its surface. Friction is sometimes a bit counterintuitive- smaller peaks mean higher pressure 🤷‍♂️It’s a conundrum. I’ve ordered some orange coloured orange peel textured stuff anyway

[markse68]

7 minutes ago, johninderby said:

This goves an idea of the sort of texture needed. This is the az bearing on a Bresseer dob.

Ooh that’s very rough looking. I guess one advantage is any abrasive dust particles can fall into the grooves avoiding being dragged around and scoring the surface maybe 🤷‍♂️

[markse68]

Or is there a weird property of ptfe that lowered its friction with increased pressure- like a skate on ice?

[johninderby]

The textured surface is always used on the underside of the dob base and the teflon on the top side of the base plate so the textured surface is above the teflon.

Edited May 23, 2020 by johninderby

[markse68]

Just now, johninderby said:

The textured surface is always used on the underside of the dob base and the teflon on the top side of the base plate.

Always? Makes sense from a dust accumulation perspective 👍 It’s not on my current dob base but that’s a diy job. 

[johninderby]

Have thought about buying a big sheet of Stardust and  selling it on in smaller dob sized bits.🤔

https://shop.morlanduk.com/formica-f1782-stardust-3050x1300x0-7mm-laminate-quarry-single-sided

[johninderby]

1 minute ago, markse68 said:

Always? Makes sense from a dust accumulation perspective 👍 It’s not on my current dob base but that’s a diy job. 

Should be done that way but of course diy doesn’t have to. 

 

[markse68]

The orange fake Formica stuff I just ordered was £18 for 600x600mm sheet- seemed reasonable. Will wait and see if it’s ok. Is the pic you posted the Formica stardust? Or similar?

[johninderby]

No it’s the stock Breeser textured ring which is actually metal. but does remind me of ebonystar.

Edited May 23, 2020 by johninderby

[markse68]

So it does appear that the coefficient of friction for ptfe running on a hard surface does decrease with increasing contact pressure! I didn’t know that. So the same weight dob sliding on ptfe pucks on a smooth flat surface will have greater frictional force than one running on the smooth bumps of textured surface like your steel or Formica where the decreased contact area increases contact pressure. V Interesting

https://wisconsindot.gov/documents2/research/WisDOT-WHRP-project-0092-08-13-final-report.pdf

Edited May 23, 2020 by markse68

[John]

With my mount, the precise location of the PTFE pads (3 on the azimuth, 2 on each altitude) was apparently quite important to the smoothness of the movement.

The pads bear against black laminate of some sort which covers bottom of the rocker box and the edges of the alt bearings. Works very well

I have heard of old vinyl LP records being used as azimuth bearings !

 

[markse68]

22 minutes ago, John said:

I have heard of old vinyl LP records being used as azimuth bearings !

Not sure they’d make good az bearings- I’d be worried about the wow and flutter 🤪

im going to break with conventional wisdom and use 4 ptfe bearing pads- triangles are overrated 😉 

[ollypenrice]

I'd have thought that the key attribute is not the absolute level of friction but the similarity between the static and moving friction. When they are the same the mount doesn't stick then over-respond to pressure. Perhaps this is the main advantage of the textured surface?

Olly

[markse68]

18 minutes ago, ollypenrice said:

I'd have thought that the key attribute is not the absolute level of friction but the similarity between the static and moving friction. When they are the same the mount doesn't stick then over-respond to pressure. Perhaps this is the main advantage of the textured surface?

Olly

Could be or is it the opposite? You don’t really want your scope floating on air as it’ll never stay where you want it and would be a nightmare changing eps etc unless you had separate brakes. When I fitted a lazy Suzan bearing to the old base I have it was most impractical until I added extra friction. My proposal is that the textured surface has 2 benefits- it reduces kinetic friction but it adds a bit of static “friction” which improves it usability.  It’s not real friction- that remains low but the little bumps slightly embed in the soft ptfe giving an easily overcome mechanical keying that’s just enough for practical purposes. That’s my theory anyway 😉

Edited May 23, 2020 by markse68

[ollypenrice]

1 hour ago, markse68 said:

Could be or is it the opposite? You don’t really want your scope floating on air as it’ll never stay where you want it and would be a nightmare changing eps etc unless you had separate brakes. When I fitted a lazy Suzan bearing to the old base I have it was most impractical until I added extra friction. My proposal is that the textured surface has 2 benefits- it reduces kinetic friction but it adds a bit of static “friction” which improves it usability.  It’s not real friction- that remains low but the little bumps slightly embed in the soft ptfe giving an easily overcome mechanical keying that’s just enough for practical purposes. That’s my theory anyway 😉

It's from David Kriege's book that I first came across the idea that static and kinetic friction should be equivalent and it struck me as a good insight. When I once ran out of Turtle Wax for my 20 inch Dob I used a different brand of polish, chemically similar but available in France. It was an absolute disaster. The mount would stick-stick-stick then luuuurch past the target. I no longer have the big Dob but Turtle Wax is still the best vehicle polish I know of. 😁

Olly

[markse68]

13 minutes ago, ollypenrice said:

It's from David Kriege's book that I first came across the idea that static and kinetic friction should be equivalent and it struck me as a good insight. When I once ran out of Turtle Wax for my 20 inch Dob I used a different brand of polish, chemically similar but available in France. It was an absolute disaster. The mount would stick-stick-stick then luuuurch past the target. I no longer have the big Dob but Turtle Wax is still the best vehicle polish I know of. 😁

Olly

Logically it makes sense as that’s the worst thing with my mount at the moment- the stick-slip that makes fine adjustments such a pain. I’m just not getting how the bumpy surface would either reduce stiction or increase kinetic friction to balance them out 🤷‍♂️ But I’ve never used a good ptfe bearing mount either so I’m completely guessing about all of this 😂

But I think a bit of controlled stiction would be beneficial- fluid heads for cameras are designed to have a uniform drag with zero stiction for smooth camera moves but you can’t walk away from them without applying the brakes- and they all have brakes!

Edited May 23, 2020 by markse68

[jefrs]

There is the word 'stiction' that aptly describes the problem, yes it is a real word for a real problem - "the friction which tends to prevent stationary surfaces from being set in motion"

[johninderby]

reading this thread has reminded me that I need to order a tin of Turtle Wax. 🙂

[ollypenrice]

36 minutes ago, jefrs said:

There is the word 'stiction' that aptly describes the problem, yes it is a real word for a real problem - "the friction which tends to prevent stationary surfaces from being set in motion"

Absolutely true. It's also highly applicable in the case of motorcycle front suspension where the telescopic fork, under braking, has its inherent 'stiction' greatly increased.

Olly

[Chriske]

The best way to control the AZ friction to perfection is to add 3 extra teflon pads in the very centre of the rocker, between rocker and groundboard.
These 'central pads' are there to support the complete weight of the scope. The three pads at the outer rim do no more then balancing the rocker on the groundboard.
To start with, these central teflon pads need to be just a little bit thinner then the outer teflon pads. No more then a fraction of a millimetre or so.
Next I test whether the scope does run smooth. And I mean very smooth and no backlash at all.
If the AZ runs to heavy I take the rocker and scope away from the groundboard and add a very thin layer of plastic under the central pads and test again. I do not(never) add something under the outer pads of course. If the scope still runs to heavy I'll again add a thin layer of plastic under all 3 central pads and repeat if necessary .

I keep on doing this until the scope really floats above the outer teflon pads.
Doing this, for larger scope it might be necessary to repeat this the next day, when the RA 'bearing' is 'set'. After that the scope will stay on floating for the rest of it's day.

Needles to say for that you need a piece of formica around the central pin of the rocker as well.