I have some ratchet wheels from previous versions with 50 ratchet teeth and 10 tooth pinion.  Smallest is 125mm OD which is a bit big.

Here's a photo of the current state of the clock.  To get an idea of scale the chain wheel is 125mm OD.

The 125mm ratchet wheel might work with a gear on the 8t sprocket with 50 teeth or more.  Need to do some calculating do see if this would be viable.

1. 8t sprocket want to rotate at 400s/rev.
2. Motor shaft rotates at 200/256s/rev = 400/512s/rev
3. This makes total reduction required 512:1

This is purely a power of two so ratios of 5:1 are out it seems.  No problem, just need to design and print new ratchet wheel and gears.  I think this can be achieved with a spur gear on the 8t sprocket and matching ratchet wheel pinion.  8:1 gear ratio and 64 tooth ratchet wheel.  This assumes the pawl works directly off the motor shaft but a 2:1 reduction gear would make the sprocket to ratchet wheel ratio a more manageable 4:1

The teeth on the 64 tooth ratchet wheel seem rather small with 120mm OD as fits nicely in the clock case so I'll have a rethink.  Could use 32 teeth and 4:1 motor to crank wheel.

CAD assembly model.

Components of the striking mechanism hidden.

Back plate added to show space available.

I think I might put the auto-winding and pendulum pushing stepper motor below the clock face (hood) behind the body door.  I don't think all the auto-winding mechanism needs to be in full view and I think it would make things a lot easier.

On 30/10/2018 at 15:55, Gina said:

Model of clock "works" in case.  The dial isn't really white, it's black but the CAD software adds white lines round everything.

Another plan...

Using the 4096Hz square wave from the clock module wanted a step-down ratio of 512:1.  The ratchet wheel has 64 teeth so that leaves 8:1 ratio.  If I changed to 1024Hz the remaining step-down would only be 2:1.  This would mean the pendulum drive gears would need to be a 25:16 step-up ratio.  Not sure this would be a good idea, though admittedly the load is very light.  There isn't a setting of 2048Hz though this could be simulated in the Arduino sketch.

With a 1024Hz square wave from the RTC, the reduction required is 128:1.

The trouble with ratchet wheels is complication - needs ratchet wheel, fixed pawl and driven pawl plus a crank arrangement.  Advantage is it gives a high reduction ratio bur there are other ways of getting a high reduction ratio viz. worm gear and epicyclic gearing.  I don't think a worm gear lends itself to printed plastic. 

The epicyclic gear system as used in my Giant Wall Clock might be an alternative to ratchet wheel.   A 64:1 ratio with 63t and 64 t gears would be rather too large with decent sized teeth but 32:1 should be doable with 31t & 32t gears and mod 3 size teeth.  Dividing 128 by 32 gives 4, an easy gear ratio.

It would be nice if I could use the 25t gear on the motor that drives the pendulum to drive the auto-winding as well but a 4:1 ratio would mean a 100t gear to match which would be far too large.  A 50t gear would give only 2:1 and need a 64t gear on the epicyclic drive.  The upshot of this is that a tooth size of mod 3 is too big for an epicyclic gear system in the space available.  It may be possible to juggle modulus with gear ratios to make this possible.  Some of the gears in the clock already are mod 2.5 so this should be feasible for the auto-winding system.

With a 25t gear on the motor it should be possible to use a 50:1 reduction in the epicyclic gearing.

1. Overall ratio required = 128:1
2. If the epicyclic gear ratio were 50 the epicyclic drive gear could be 64t say.
3. Motor gear ratio would be 64:25.
4. Total ratio would be 50x64/25 = 128 [tick]

Now to see how big the epicyclic drive gear is with mod 2.5.

The above makes the biggest gear 160mm OD.  This would make the drive gear more than mod 2.5.  So now trying epicyclic ratio of 40.

1. Overall ratio required = 128:1
2. If the epicyclic gear ratio were 40 the epicyclic drive gear could be 80t say.
3. Motor gear ratio would be 80:25.
4. Total ratio would be 40x80/25 = 128 [tick]

Later...  No joy with that.  The 80t gear would be 200mm OD and that's too big.

Latest design :- 

1. Motor gear with 25t driving the epicyclic system gear if 64t mod 2.5.  Gear is 163mm OD.
2. Epicyclic gears will be 49t fixed and 50t driving the 6t auto-winding sprocket with 8t pinion mounted on the 64t gear.

Here is a new model of the clock assembly.  The modulus of the driving gears (green and blue) has been increased to 2.75 but I think 3 would be better.  The epicyclic gears are mod 2.5.  I'm showing the latest gears as solid until I am sure it's the final design, then I'll add spokes.

Changed to mod 3 for the motor drive to the epicyclic gears and it fits nicely.

New assembly model.

After a problem with the bed heater, I now have my 3D printer working again and printing the 64t gear.  OK so it's not blue. 😄

That print turned out to be perfect for the job and will be the final print.  I've been printing more of the gears with good results.  I now feel like a break from printing and looking at the design of the strike mechanism.  The main parts are done but there remains an auto-winding system.  With the main clock auto-winding gears taking up more than half the free space, the AW system for the strike needs something smaller.  There is also the problem that the drive varies by 12 to 1 over the 12 hours unlike the timepiece which is constant.

This model of the clock assembly shows the strike mechanism as far as I've got with the design plus the AW for the timepiece.  The large wheel on the left with blue cam turns once per ring of the gong/bell so a cam on the axle of this will operate a clapper lever.  I was planing to drive the large wheel with step down gearing from a sprocket carrying chain and weight similarly to the timepiece but I wondering whether some lateral thinking can come up with something simpler.

One possibility I have in mind is to use cord rather than chain unwinding from a drum (a common system in longcase clocks) with some mechanism for re-winding the drum between striking periods.  Usually this would be manual but I want to design an automatic system.  (I don't want to wind up the strike anymore than I want to wind up the main timepiece.) 

The way manual winding works is to have a ratchet system between the drum and the strike mechanism.  The motorised winding could drive the drum with another ratchet.  This ratchet would only engage when winding (otherwise it would stop the drum driving the strike mechanism).  I'm thinking the motor winding would take place either every half hour or when the weight had dropped a certain amount.

I'll continuing posting my ideas as the come into my head 😀

Every half hour seems logical.  Three thoughts :-

1. Maybe I could use a similar arrangement to the strike mechanism. 
2. A wheel that turns less than a revolution when the clock strikes 12 could be would back up until it's back to "square one". 

No.2 seems simpler.  It would want gearing of something over 12:1 as the striking shaft rotates 12 times for 12 o'clock.

OTOH for every 12 hours :-

• A knot in the cord that engages a lever when the weight is at the top to stop the winding.  The winding could be started by a cam on the hour shaft.

A calculation for winding every 12 hours :-

1. Weight would descent by 1+2+3+4+5+6+7+8+9+10+11+12 = 78 times the amount for one strike (revolution of the big wheel).
2. Distance from bottom of hood/clockface to floor = 1.3m, less height of weight and pulley of 200mm give 1m clear so 2m travel of the cord.
3. Amount of cord for one strike = 2000/78 = 25.64mm
4. If this were on the same shaft as the strike wheel the drum diameter would be 25.64/π = 8.16mm - which is far too small.
5. With a 4:1 gearing from the strike shaft this would become 32.7mm.  5:1 gives 40.8mm - better.
6. 78 factorises into 2x3x13 so at 6:1 the drum would turn 13 times to wind up the strike system giving a 50mm diameter drum. Seems OK.

I will have a lever pushing the pendulum every 2 seconds and I think it could be used to wind up the strike mechanism.  This could take half an hour or more giving up to 30x30 = 900 strokes.

More calculations :-

1. A ratchet wheel with 64 teeth and 160mm OD will fit nicely in the space
2. This will need 13 revolutions so a total of 13x64 = 832 - comfortably below the 900.
3. Total time to wind is therefore 832x2 = 1664s = 27.73m

This gives minimal force needed to wind the strike mechanism yet taking a reasonable time.  The winding sequence can be started at 12:15 and will end just before 12:43.  Seems like a plan - just need to design the parts now.