Alternative to a ‘fly’ for braking a clock striking train

[lfoggyParticipant @lfoggy]

A few years ago I built an 18th century style English bracket clock based on plans published by John Tyler. The clock strikes the hour through conventional rack striking. Although the clock has been running well, I was always aware that the striking train was quite noisy when operating. This is a feature of all clocks of this design as well as long case clocks. The whirring noise comes from the fly which spins rapidly and serves to brake the striking train when it is unlocked. Most of the noise seems to come from the engagement of the warning wheel with the pinion on the fly arbour. How could this be reduced? First thing I tried was making the warning wheel out of nylon. This reduced the noise considerably, but it became evident that this would not be a durable solution. I thought of dispensing with the gear/pinion arrangement completely and driving the fly arbour with a belt, maybe using an O ring. This would probably have also been quieter but again vulnerable to failure. I do want my clock to have the same durability as an original 18th century example. At around this time, I made a desktop gadget consisting of a thick copper tube and magnet. The magnet dropped down the tube is dramatically retarded by the Lenz effect. This led me to wonder if eddy current braking could be used to control the striking train. A quick test on the clock depthing tool with an aluminium disc mounted on an arbour and some small neodymium magnets was encouraging.

With no magnets the disc spins freely.

With the magnets close to, but not touching, the disc, there is obvious retardation.

I therefore removed the fly from the clock completely and replaced the warning wheel with a 1mm thick 45mm o.d. aluminium disc. Four small neodymium magnets were mounted in a pure iron yoke and arranged with a 0.25mm air gap between the magnets and disc. This braked the striking train very effectively. Too effectively in fact, as the striking frequency was reduced to a ponderous one strike every three seconds. To reduce the braking effect, I reduced the thickness of the disc to 0.5mm and added some slots to interrupt eddy current formation.

I now have a clock that strikes at one second intervals with no audible additional sounds at all – success. Added benefit is that the system has much less inertia than the fly arrangement meaning reduced strain on the parts when the striking train is suddenly arrested by the gathering pallet hitting the pin on the rack. Durability is ensured as this arrangement has fewer moving parts than the traditional design.

The magnets will eventually lose their magnetism, but this happens over centuries -easy to replace as well. The only other problem I can think of is iron dust sticking to the magnets, maybe from the pinions wearing over time. This design may even be more reliable than the traditional fly. Only time will tell.

Few things in horology or model engineering are new, and I can’t imagine that this is a new idea, but I’ve not seen reference to it anywhere hence this post which I hope is of interest to other clock constructors. I will be incorporating this into future clock projects.

[BazyleParticipant @bazyle]

A nice bit of lateral thinking so thanks for publishing it first? on here. The Foggy Retarder might become a standard fitting though not so many mechanical clocks being made now. Personally I rather like the noise of the warning with its anticipation of a burst of greater activity and wonder if the sound of the bell without the background whirring would sound ‘right’ now.

[Michael GilliganParticipant @michaelgilligan61133]

Very clever … and nicely executed

Well done !!

MichaelG.

[renardiere7Participant @renardiere7]

Superb, well presented too!

[gerry maddenParticipant @gerrymadden53711]

A great idea and thanks for reporting your work.

I had to ‘attend’ to an old Sigma precision gauge a few years ago and was surprised to discover that this machine contained a strong magnet with a foil disc running in a slot in the magnet. The slot had become clogged with steel particles which was impeding the disc. As it didn’t look easy to clean, I considered an easier plan might be just to remove the offending unit. A quick trial showed the machine was totally unusable without it. Slightest movement of the probe and the needle was out of control, bouncing of the stops and taking a week to settle. A great lesson in the effectiveness of induction damping.

The thing is, the casing of my gauge housing wasn’t exactly full of holes, but despite this, the magnet seemed to have somehow attracted and captured every bit of steel passing by. So do take care over where you put your clock and how its protected from the elements. Perhaps you should consider placing a few ‘decoy’ magnets inside the cover near any holes in the cover or internal wearing parts. Of course, don’t put them near the escapement or you will get a degree of induction damping in that too with obvious effects on timekeeping!

 

[gerry madden]

On gerry madden Said:

Perhaps you should consider placing a few ‘decoy’ magnets inside the cover near any holes in the cover or internal wearing parts. Of course, don’t put them near the escapement or you will get a degree of induction damping in that too with obvious effects on timekeeping!

 

Decoy magnets are a great idea, I will add a few. There will inevitably be some iron dust from the pinions over the years.

All steel parts in the clock apart from the pinions are 304 stainless and my escapement consists of an aluminium body carrying tungsten carbide pallets so no chance of interference there…..

[Dave SParticipant @daves59043]

Seiko make a watch escapement that uses eddy current breaking controlled by electro magnets.

There is also a tuning fork/magnet eddy current brake escapement – I bet Michael can find the patents.

Dave

[Nigel Graham 2Participant @nigelgraham2]

Very ingenious solution!

The same principle is used in the analogue speedometer / tachometer.

The drive spins a bar-magnet inside a thin-walled aluminium cup, inducing in that eddy-currents proportional to the speed of rotation. Their own field’s reaction to the bar-magnet pulls the cup round against the controlling torque of a hair-spring.

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