Increasing Friction??

Hi Guys,

This is a piece off a Gustav Becker wall clock. This piece is part of the escapement mechanism. However this clock has a friction washer that pushes the tri pointed brass onto a 9mm Dia piece of brass. The pressure of the spring is there to maintain position but allow adjustment if necessary.

My question is as the pressure from the steel friction spring puts pressure on the 2 brass surfaces, the smaller being 9mm Dia is there a way to optimise friction between the 2 brass surfaces whist not actually soldering them together or changing the spring.

Obviously or should I say I think flat smooth brass must have a lower friction rating that rough but making the surfaces rough would reduce contact area.

Optimising friction means the clock is less likely to go out of beat.

I would be grateful for any suggestions on possible solutions.

regards

Chris

The way I get around this is to use a curved slot and a locking screw....not really 'orology', and not really engineering either but it works and you can be reasonably sure it will stay as set.

Bob, thanks for reply can you elaborate a bit so I can understand where the curve slot and screw goes for your remedy/

Chris

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That’s strange :

I always thought that optimised spring pressure allowed the beat to self-adjust.

I shall watch and learn.

MichaelG.

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Edit. __ How uncanny !

I’ve just found a very similar enquiry dating back to 2014, on NAWCC

https://mb.nawcc.org/threads/gustav-becker-verge-crutch-assembly.113877/

Edited By Michael Gilligan on 14/01/2021 18:23:01

That is the beat adjustment. The pin at the other end of the arbor is apparently lined up with a window before starting, and the escapement should then be in beat.

There is a thread on the NAWCC bulletin board that discusses this movement.

Gustav Becker Verge Crutch Assembly | NAWCC Forums

It shouldn't need an lot of friction, the force it's transmitting is only the drive from the escapement to keep the pendulum ticking. If it is slipping out of beat when running, try cleaning and roughening the two mating surfaces (slightly) to increase the stiction between them - and don't oil them!!

Edit - Michael got there while I was typing!!

Edited By Peter Cook 6 on 14/01/2021 18:26:26

This mechanism on this clock  is not intended to be self adjusting.

Chris

Edited By Chris TickTock on 14/01/2021 18:26:15

If the spring isn't providing enough friction, how about resetting or replacing the spring instead of trying to bodge it?

Thanks for link I think I will try a small washer behind the spring first.

Regards

Chris

I'm not qualified on the clock aspect of this but a few general remarks may help.

Although there's a relationship between surface area and friction, I don't think there's much reason to worry about a bit of roughening in this case.

When a smooth bar runs on rollers across a flat surface, friction is almost zero provided nothing bends. But when the bar hits an end-stop, it can't move even though the contact area is low. So on one hand low surface area means low friction, on the other it doesn't. Designers exploit this all the time.

Two rough surfaces can engage with low surface area and still have high friction:

But! How a rough surface behaves over time is likely to change. In the above example pressure at the contact points is high, so the material tends to deform or cut, i.e. the two blocks polish each other and friction drops. However, once polished flat another effect may kick in: If both materials are soft, the whole surface may tear, causing friction to rise again.

One way of controlling friction is to make the contacts out of different materials, one of which is harder than the other. Then the hard material tends to dig into the soft one, keeping friction high until the soft side wears out. Disc brakes are a sophisticated implementation; the disc is polished hardened steel, while the brake blocks are soft and replaceable. Not any old soft, pads are made of a hard material embedded in a softer matrix, both which are heat resistant. The combination balances friction and wear so vehicles stop efficiently without having to fit new brake blocks every 50 miles.

In a clock, the soft side could be paint, or a fibre washer but materials like that don't last. Instead the existing arrangement features a steel spring pressing on a brass plate. I think this is good design, but what could possibly go wrong?

One possibility is that the brass has work or age hardened over the years, and the spring now skates over it. Another is that the spring started with a sharp edge, and it's worn flat. A third is the spring has lost tension and brakes more weakly than when new. Dirt may be acting as a lubricant: kitchen grease and nicotine are both slippery. In an old clock, all these factors might work together to reduce the mechanism's grip.

Practically then, the answer could be sharpen the spring bearing edge if it's rounded, or to gently rough up the brass especially if it looks burnished, or just a good clean. Theory - if a practical horologist disagrees, listen to him!

Also possible the problem is the other way round, where a sharp strong spring has embedded itself deep into soft brass and jambed. Doesn't fit the symptoms though.

I can see why people like working on mechanical clocks - very interesting.

Dave

I can't see what retains the spring but since you can't do much about the co-efficient of friction between the two brass parts then the only other factor you can alter is the force holding them together. As you say you don't want to change the spring then can it be moved closer to the brass part so it applies more force.

If there is no way to move the spring what about annealing the spring, bending it more and then rehardening so once back in the same place it is applying more force.

Another option would be to introduce a material that has a higher co-efficient of friction in the form of a thin washer between the two brass parts.

As-machined grey cast iron more like it on most cars. Some motorbikes have a low grade stainless steel for appearance sake. But its never hardened or polished for obvious reasons, ie coefficient of friction.

When disc brakes first appeared on street motorcycles, manufacturers (including Triumph) used chrome-plated discs. Mediocre in the dry, but boy, were they a lot of fun in the wet! Slippery as snot. Then the chrome wore off unevenly and you had on-off-on braking every revolution. Ah, wonderful days!

Which is an effect that any ten year old cyclist could have warned them about. I wouldn't want a bicycle without disc brakes now, although they are very fussy due to their small size.

Ive also put a small washer under the spring "legs" to increase the force if they are slipping. Useful if your trying to eliminate a problem.

Peter

It may be helpful to put a washer of what is known as 'red fibre' - used for fibre washers on petrol fittings and in electrical parts as an insulator - between the brass surfaces. Search hard and you might even find the same stuff but black, if you prefer.

I am guessing that the crutch is set by rotating it to what seems a good position, listening to the ratios between tick and tock, and making final adjustment again by sliding. And once set, until small brother or big dog nudges the whole thing again, it will carry on ticking and tocking steadily with both parts of the device moving in unison.

Cheers, Tim

Edited By Tim Stevens on 15/01/2021 18:16:30

Thanks Guys,

I have made a spring to replace the original one. I annealed a piece of old mainspring first made the spring bent it then hardened it. Unfortunately the spring broke when fitting die to the metal being too brittle when clamped with dental pliers to get the pin in. I think after hardening I will temper next time and see if it improves the situation. After all a spring has to have give in it or it is not much of a spring but has to be hard enough to retain shape.

Chris

Edited By Chris TickTock on 15/01/2021 18:19:32

Spot on Tim. Personally I do not like this set up as clever as it might be it is prone to lose beat. The clock is a trio gong and while if left alone the position will hold it is not as reliable as other methods such as fixed position or grub screw. Others might disagree but that is my take. If I was a botcher I would solder the thing up as it is my clock but it goes against the grain. fibre washer noted, thanks.

Chris

Update.

I thought about this over night. If I think if a small part of a clock is of questionable design and merit should I 'improve' it. Well you will get different opinions that goes without saying. But if its a minor variation from original design I think it is acceptable. The person doing the repair along with the owner must decide here it was much easier just me.

Anyway i have done it.

regards

Chris

Edited By Chris TickTock on 16/01/2021 15:38:37

Good solution Chris. I've done the same in the past - just have to be careful that everything is aligned correctly otherwise things can move as you tighten the screw.

Russell