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Barograph 'stiction'

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modeng200012/05/2022 11:58:57
307 forum posts
1 photos

My barograph mechanism has a fair amount of 'stiction' in the pen movement and so I wonder if replacing the simple sleve bearings for point bearings as used for escapement wheels would be worth doing.
I realise that the friction between pen and paper is one source of stiction but feel that the simple holes through the two posts is probably another.

Brian Wood12/05/2022 12:08:24
2566 forum posts
39 photos

You may well be right

Brian

modeng200012/05/2022 12:31:03
307 forum posts
1 photos

Brian, it would be fairly easy to make the change and to revert if no improvement.

So I think if it might help I should try. Just wasn't sure it would help.

John

Brian Wood12/05/2022 13:51:48
2566 forum posts
39 photos

John,

I don't think you have anything to lose by trying your ideas out.

It might be instructive to look at other instruments of this type to see how they have dealt with suspension matters; the forces acting on the device that is trying to trace the changes in barometric pressure will not be high and plain sleeve bearings will demand some of that as you suspect.

You might be pleasantly surprised at the result

Brian

John Haine12/05/2022 13:51:58
4673 forum posts
273 photos

I think you mean "balance wheels" (sorry, inner pedant at work!). It might help just to dismantle, clean and oil the bearings.

modeng200012/05/2022 14:18:13
307 forum posts
1 photos

Joohn,

Yes of course I mean balance wheels, sorry. I have tried to upload a photo but alas failed miserably.

The linkage is very basic, just pins through holes. So I think where a 2.5mm dia brass rod passes through two clearence holes in a pair of uprght posts is possibly where some of the sticking is taking place.

(I did manage to place a picture on the Home Model Engine Machinist site)

John

gerry madden12/05/2022 14:32:41
252 forum posts
132 photos

One of the interesting characteristics of PTFE is that its static friction is the same as its dynamic. This helps avoid 'stiction' issues. Not sure if you would be able to work this in to your application... it might make things too big and therefore might increase friction to above what you already have, so perhaps the balance wheel pivot is better.

Gerry

Dave Halford12/05/2022 14:46:46
2050 forum posts
23 photos

The chart recorders at BT used a simple balanced pen similar in construction to a Lenco record player arm suspension. A vee shaped cutout balanced on knife edge. No pen arm sleeve bearings at all.

David Jupp12/05/2022 14:47:21
835 forum posts
17 photos

I seem to recall from University materials classes that for PTFE friction increases when direction of movement changes, so it isn't as good as you'd expect in oscillating systems. If movement is always in same direction friction remains low - almost as if there is a 'nap' on the surface that has to brushed into place by initial movement.

Swarf, Mostly!12/05/2022 17:02:50
668 forum posts
73 photos

Long, long ago, someone told me that instruments for motor car dashboards used crude pivots for the sake of ruggedness and longevity but relied on the vibration when the engine was running to vercome the friction.

If you can bear it, strap a buzzer to your barograph!

Best regards,

Swarf, Mostly!

PS: OK, OK, I know that opening sentence is too long, sorry.

gerry madden12/05/2022 17:32:23
252 forum posts
132 photos

David, I didn't know that about oscillating movements. Interesting. I had planned to use in a clock escapement, perhaps I wont now .

There is another material with similar CoF, called Vespel. Do you have any experience of this ?

Apologies if this is a thread hijack.

Gerry

modeng200012/05/2022 17:33:18
307 forum posts
1 photos

"If you can bear it, strap a buzzer to your barograph! "

This is something I had considered but it would have to be clockwork!!

David Jupp12/05/2022 17:48:42
835 forum posts
17 photos

Gerry - I've heard of Vespel, but have zero experience of it.

I also seem to recall that PTFE gives lower friction if there is at least some moisture (humidity) around - strange given that it was allegedly developed for the space programme...

Andy Stopford12/05/2022 19:35:48
158 forum posts
18 photos
Posted by David Jupp on 12/05/2022 17:48:42:

I also seem to recall that PTFE gives lower friction if there is at least some moisture (humidity) around - strange given that it was allegedly developed for the space programme...

The space programme thing is a myth - PTFE was discovered (more or less by accident) in the thirties. Its first notable use was in the Manhattan Project to protect the plumbing of the Oak Ridge Gaseous Diffusion Plant from the corrosive effects of Uranium Hexafluoride.

Interesting about the poorer performance for oscillating systems. I wonder if its the same for other slippery plastics like nylon and acetal.

bernard towers12/05/2022 21:06:49
618 forum posts
109 photos

And the countries last Barograph and Barometer maker is closing for good next month after something like 175years!!!

Robert Atkinson 212/05/2022 21:57:58
avatar
1208 forum posts
20 photos

Swarf's comment on a buzzer is spot on. Many aircraft altimeters (basically aneroid barometers) have internal buzzers to overcome stiction. Altimeters have very good bearings but stiction is still an issue.

Robert G8RPI.

modeng200013/05/2022 07:15:15
307 forum posts
1 photos

So I suppose Bernard, these will now be made abroad like a lot of other things these days. Which company is about to stop trading?

Robert, so this is why folk tap the barometer as they pass by watching it settle to the current reading.

A barometer really indicates the pressure when requested by giiving it a tap however the barograph normally sits on its own most of the time so stiction becomes a problem. I have started the modification so hopefully I'll find an improvement. An electronic device is the answer but finding one with a printed chart output is another matter and it would need an electric supply not just a spring.

Thanks for the suggestions to my question.

John

Michael Gilligan13/05/2022 07:31:49
avatar
20182 forum posts
1053 photos
Posted by modeng2000 on 13/05/2022 07:15:15:

So I suppose Bernard, these will now be made abroad like a lot of other things these days. Which company is about to stop trading?

.

… which reminds me to ask you, John: What barograph are you using ?

MichaelG.

.

P.S. __ a recent eMail from Jeffrey Formby Antiques mentioned this book, which might be of interest:

https://www.formby-clocks.co.uk/cgi-bin/sh000001.pl?WD=collins&PN=c-e-p5.htm&utm_source=BenchmarkEmail&utm_campaign=May_03_2022_Email&utm_medium=email#aN5082

http://www.barosbooks.co.uk/b.html

 

Edited By Michael Gilligan on 13/05/2022 07:50:15

modeng200013/05/2022 09:10:51
307 forum posts
1 photos

Michael, it is a dome barograph by Russel Scientific. Quite compact and I believe this has compromised the design by being enclosed in a 4"dia by 6" tall glass dome.

The Philip Collins book looks really interesting, thanks for the link.

John

modeng200013/05/2022 10:16:47
307 forum posts
1 photos

There is another possible reason for the pen sticking, as this is a small barograph the chart length is shorter than for a normal barograph therefor the distance the pen moves across the chart is also shorter so giving less chance to unstick as compared to a full size barograph..

John

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