Basic Clock Design

[James AlfordParticipant @jamesalford67616]

Russell.

Thank you. That makes perfect sense now. I was thinking along the lines of the clock that I have already which uses a chain which simply loops over a toothed wheel, rather than being wrapped around it, as below.

Regards,

James.

[AjohnwParticipant @ajohnw51620]

It might be worth mentioning that in Russel's arrangement the winding barrels are usually screw cut to guide the wire carrying the weight. LH thread in a design I have seen.

It's also possible to add a sort of "dummy" barrel to keep the weight well away from the pendulum if the gearings doesn't do that automatically. It might be put high up and to the left for instance.

I'm trying to get my head around the escapement and weight on the end of the pendulum. It's too cold in my alternative work area currently to start chopping up al for a 3d printer frame.

John

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[James AlfordParticipant @jamesalford67616]

John,

Thank you. A screw-cut barrel? Well, that will be challenge 73: how to cut a thread without the screw cutting gears for the lathe. I have a small. low geared-high torque 12v motor which I may rig up to the lead screw. Matched carefully, I imagine this could be pressed into use to cut a one-off thread.

Regards,

James.

[AjohnwParticipant @ajohnw51620]

You could do it more easily than you might think with a suitable hand chaser James. It's needs a bit of practice but the easiest way to start is to use the chaser straight on to lightly mark the pitch lines. Those can be used to judge how quickly to move the chaser along by hand. They tend to self correct once they have cut into the work a little way but if the feed isn't fairly even they can produce a drunken thread.

The only catch really is that the width of a chaser with several teeth on it can mean that some waste material is needed at the end of the work for the chaser to run into. A design I have uses 14tpi for 1/16 dia stranded phos bronze wire.

It just needs a hand turning rest but I have been known to use a heavier boring bar for that. Best just buy a chaser and try it on some scrap. There are usually a few on ebay. The right speed is best found by trying it. Too slow and it's more difficult to feed evenly and if too fast it's hard to keep up.

Once upon a time threads were often cut this way.

John

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[julian atkinsParticipant @julianatkins58923]

Hi James,

Lots of older clocks do have a 'thread' on the barrel, but I personally cant see any engineering point to this, let alone an horological point. If there is a reason that I am not aware of, I am sure the experts will post a reply!

Cheers,

Julian

[speelwerkParticipant @speelwerk]

A small pin at an angle to guide the wire after the first turn is enough to make it wind correctly on the barrel. Niko.

[James AlfordParticipant @jamesalford67616]

Thank you for the ideas. I had forgotten about chasers, but have, somewhere, a selection of adjustable half-nut chasers that I inherited from my grandad. They range from quite small to very large.

James.

[AjohnwParticipant @ajohnw51620]

There is usually plenty on ebay

**LINK**

Another type that people have made themselves when tracy tools used to sell sets of single die head chasers rather than the usual set of 3 is this type

**LINK**

I was in there once when they were in Dartmouth and he wanted to know why people bought the die head chasers. I've often kicked myself for not buying a set off him. Actually I'm pretty sure that he knew why they were selling well and was suggesting I bought a couple of sets.

If your thread chasers are the flat adjustable type it should be pretty easy to make a holder.

Before splashing out cash though it would be best to try it on a cheap one but it really is fairly easy. Once the tool is tending to pull itself along it's easy to keep a constant feed. Trying to use them on internal bores is much much harder.

I should add that a clock restorer pointed out that brass could be chased using screw thread gauges, stoned up on one side to get a sharp edge and held in a simple holder. The usual fold out type of guage. Never tried that myself but it's another option. He was old enough to have had to cut threads this way as part of his apprenticeship.

One thing to watch for is that the usual hand chasers are often handed so the profile slopes one way for lh and another for rh. They vary. The thread gauges are so thin that shouldn't matter. Machine chasers, the square ones without a tang can also be used by hand, in a holder of some sort.

John

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[Russell EberhardtParticipant @russelleberhardt48058]

It is usual to "screw cut" the barrel but not with the usual thread form. It should be half round to comfortably accommodate the line. I used the screw cutting facilities on my lathe with a half round form tool but the traditional method used by my ancestors would have been to cut it by hand on the lathe using a single point hand graver. That takes some skill to do neatly though!

As has been pointed out you can get away with a plain barrel but should be careful to angle the approach of the line correctly. If it gets wound with big gaps it will eventually bunch up at the end of the barrel and cause a variation in the driving force as a result of the variation in diameter.

Russell

[AjohnwParticipant @ajohnw51620]

I would have thought any thread form would be better than none Russel.

I have wound many close wound coils by using a sort of guide so that the wire runs neatly right up against the previous turn. Similar problem. To do it reliably the guide needs to be a significant distance from the coil that depends on the width of the coil. I would have thought this would be difficult to achieve reliably in a clock. For a coil say 2" long I would reckon on needing to put the guide at least a foot away. A jig I once made was more like 2ft long. Having found the problem I didn't want it to be too short.

On the design I mentioned the dummy barrel is also screw cut so that guides as well. It may be a handy arrangement to remember because as the re wind time is increased the diameter of the gear on the main barrel tends to go up which puts the wire closer and closer to the pendulum or the barrel size needs to go up as well. I would have thought that was a better option but designs I have seen tend to just increase the number of teeth on the gear. Odd really as a bigger barrel should give more driving torque for the same weight and even more drop of course.

John

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Edited By Ajohnw on 25/04/2016 09:47:26

[Martin KyteParticipant @martinkyte99762]

Could I just point out that a two fall line to a pulley on the weight has the advantage of stopping the weight from rotating (twisting) or swinging in the same plane as the pendulum, as well as doubling the running time.

regards Martin

[AjohnwParticipant @ajohnw51620]

Interesting point Martin. It explains why that form is often used.

As there are some clock people about I have an obscure question. Started by the dead beat escapement spread sheet I linked to earlier. It allows the impulse angle given to the pendulum to be set but suggests that the swing of the pendulum will be more. From looking at them the escapement doesn't limit the pendulum swing, it just provides the impulse to keep it going.

So if the weight on the end of the pendulum is changed and the impulse is kept the same the actual pendulum swing will change as well. Maybe too light a weight would cause the escapement to hit it's wheel and damage it or prevent it from working correctly ????? Or in other words just how is the weight set and are there other implications?

John

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[Martin KyteParticipant @martinkyte99762]

Not an obscure question at all.

The function of the escapement is to provide enough energy to the oscillator (pendulum) to make up for losses due to friction caused by air resistance losses in the suspension etc. The impulse occurs at intermittently at the same point of the swing and is triggered by the pendulum. The magnitude of the impulse is governed by the driving weight the gearing of the clock and the duration of the impulse. The magnitude of the swing will be dependent on the impulse and the losses and independent of the weight of the bob. A heavier bob will result in more stored energy in relation to the energy losses. This is the Q factor. As the losses are variable it is desirable to make the losses as small as possible and the stored energy as large as possible thus causing the variability of the losses to have a smaller effect on the stability of the clock.

regards Martin

[AjohnwParticipant @ajohnw51620]

Thanks Martin. It's interesting how it can all be made to fit together. I had been wondering about this aspect as I would guess that the resultant swing of the pendulum does have some bearing on how efficient the escapement is. Hence mentioning it in the spreadsheet.

It's also interesting to note how the person who posted the thread in another forum went about getting to grips with this area. Escapement and pendulum first to get some idea of the drive requirements and rewind interval.

The wall clock we have in the hall really aught to go. It's there because I only see it going in and out of the house.

I've seen good reports on the use of carbon fibre for the pendulum rod but haven't really seen any conclusive information on temperature effects other than it's low. Some comments about moisture absorption but wooden rods can be made to work pretty well without much complication. This should help further as it will concentrating the weight nearer the end of the pendulum.

John

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[Russell EberhardtParticipant @russelleberhardt48058]

Hmm. . . I think Martin is correct in that the pendulum swing will stay the same with varying weight but only assuming the losses remain the same. Ie., if a lead bob is exchanged for an aluminium one the same size the swing will remain the same. However if the bob weight is reduced by reducing its size the air resistance will fall and so the losses will fall and the swing must be greater to compensate.

All that is rather academic though. The pendulum swing Is not critical. It is usual to install a couple of pins in the backplate to limit the swing and prevent damage to the escapement.

Russell

[Martin KyteParticipant @martinkyte99762]

The efficiency of the escapement is really just the amount of energy delivered to the pendulum less the energy dissipated in friction. For a Graham Deadbeat it's a maximum of 50% with 45 degree contact angles on the pallets. Efficiency is not really that important. You can always within reason provide more power. The primary aim is to ensure constancy of period. The ideal pendulum clock would have a completely detached pendulum swinging in a vacuum with no friction. As soon as you take a measurement from it you change affect it's swing even if you just use light. Adding an escapement where the pendulum has to unlock the impulse really interferes with it.

It would be handy to have a chart of limitations of pendulum clocks so you could see the limiting factors in order of importance.I've never come across one. Many of the refinements (some of which have been mentioned in this thread) would make no real difference to the going of the clock unless the bigger issues have been sorted out first. I would suggest that unless the pendulum is in a case (isolated from draughts) and is rigidly suspended with good temperature compensation all of the other refinements are a waste of effort for a weight driven clock.

Carbon fibre is good for a pendulum rod (actually has a negative coefficient of expansion), invar (try Ian Cobb for supplies) is easy so long as you support the pendulum from the middle and as you mentioned wood is better than iron. You need to ensure it's well varnished to avoid moisture variation.

Rewind interval is easy. The centre wheel goes round once per hour, you know the gearing and barrel diameter and then can work out how much drop you can have.

regards Martin

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Ajohnw on 25/04/2016 13:25:29:

I've seen good reports on the use of carbon fibre for the pendulum rod but haven't really seen any conclusive information on temperature effects other than it's low.

I have use unidirectional carbon fibre re-enforced epoxy tube for the pendulum rod of my regulator. It is specified at -0.1 to -0.3 ppm/C expansion co-eff. I have compensated it by careful positioning of the pendulum bob suspension and a short length of steel in adition to the carbon fibre rod. I haven't been running it long enough to get any temperature data yet. I'll have to wait for the hot summers we get in the South of France. I did notice some initial creep at first even though the stress in the pendulum rod is only about 0.5% of its UTS. That settled down in the first week or so.

Russell.

[Martin KyteParticipant @martinkyte99762]

Nice regulator Russel.

Martin

[John HaineParticipant @johnhaine32865]

Yes, that is a very nice clock.

An article in a recent HSN mentions that carbon fibre is a little hygroscopic, and I remember hearing this from someone else too a while back. I'm not sure how significant this is. I wonder if it is possible to seal the surface or if there might be better resins? Have you seen anything Russell, or any affects?

On the question of pendulum amplitude, yes it is determined by the loss, not the mass as long as the losses are mainly proportional to bob velocity. Aluminium and lead bobs of the same size and shape should give the same swing everything else being equal, but the lead one will take longer to settle down. The desirability and relevance or otherwise of high "Q" divides horologists!

[AjohnwParticipant @ajohnw51620]

Thanks Russell. That fits in with what the 2 design book says – better than invar. Does the negative sign indicate it shrinks as it warms up ?

John

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[Russell EberhardtParticipant @russelleberhardt48058]

Yes, the negative sign does indeed indicate that it shrinks with increasing temperature.

As far as hygroscopic effects go I believe that there is a minute amount of swelling with water absorption however, with longitudal unidirectional fibres I would expect any swelling to affect the width and not the length of the rod. From my sailing days I remember that we used to paint glass/polyester hulls with an epoxy barrier to prevent osmosis so epoxy is certainly better than polyester resin. Personally I'm not worried about moisture as we have very low humidity in this part of the world .

Russell.

[AjohnwParticipant @ajohnw51620]

I suspect that the hygroscopic concerns would be weight changes rather than size. Looking around I managed to find this – aircraft research datat

It fits in with other comments – slow and takes some time to reach equilibrium. That source also mentioned that in this temperature range the effects are more rapid as the epoxy goes rubbery. At normal temperature it takes too long to form a useful test.

John

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[John HaineParticipant @johnhaine32865]

Yes, if the weight increases the cg of the pendulum will move upwards and it will slightly speed up. But 80c and 90% RHUL is quite extreme!

[Russell EberhardtParticipant @russelleberhardt48058]

Interesting John. A quick calculation shows that if my 30g pendulum rod gains 1.5% weight the clock would gain about 1 second a day. However as the other John said 80 C and 90% RH is pretty extreme. Our house averages 20 C in winter and 30 C in summer with RH between 35 and 45% so I doubt that I have anything to worry about.

However, when I get the clock installed in its case ( the case just needs staining, waxing and glazing now) I will make a point of recording going rate against interior temperature and humidity. It will take a full year though to get any meaningful results though.

Does the paper you found give any information on the fill factor of the composite? Obviously the greater the proportion of fibre the less the effect will be.

Russell.

[AjohnwParticipant @ajohnw51620]

No I couldn't find anything like that Russell. From memory some time ago the fill factor for high strength can't be that high but I don't remember the numbers.

I did some sums on what I might do and it works out at 0.1gm. Two things would reduce that, temperature but I get the impression that it just takes longer to reach the equilibrium point and more importantly it wont gain that much water because the relative humidity will be lower. I can't see epoxy behaving the same was as a desiccant does.

I'm wondering about a "mini regulator" based around a 1 sec period pendulum but suspect the space for the drop weight will be insufficient for a reasonable time between winding. On the other hand I can't see why that can't be geared up.

John

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