Graham Escapement design

[kit]

Art,
I've been playing with the Graham escapement variables in Gearotic and have found what I think is an error in the design.

As the lock angle is increased the angle of the impulse faces changes until, from about 2.75 degrees up, there is no impulse action at all.

I've found a video on YouTube which is from an interactive model of the escapement. This version includes an adjustment of 'impulse angle'.
https://www.youtube.com/watch?v=saFkfpoJm4g


Kit

[ArtF]

Kit:

Not so much an error as an adjustment. Gearotic sets its impulse pallet to the most efficient angle for power transfer,
but it has to do that within the confines of the other adjustments.  The pallet adjustment is a vectored angle of force,
vectored between the tooth pushing on it, and the shaft position of the pallet, if you start a simulation, and set the speeds slow, watch the push the tooth gives to the pallet as each tooth slides up the angled pallet lock for that side,
in the simulation its easy to see the push happen, one can easily imagine the energy being pushed on that arm, 
but that  push has to happen at a vectored angle to the shaft in order to transfer the maximum power to the
pallet arm and then to the pendulum.

  Ive looked and it seems proper to me, its angles are those provided by the British horological societies as those
most efficient. It does change depending on lock angle, but that's part of the specification.

  If you've built one to test, what you should look at is.. does the tooth push the pallet to tilt as the tooth on either side pushes it as they turn.. If so, the question then becomes why does the pendulum not get that energy..

  Sorry if I'm missing something, I never discount the possibility of an error, but the pallet angles on GM aren't
adjustable because they always adopt the most efficient transfer angle allowed by the other settings.. I felt
that by allowing too many settings, I wasn't doing anyone a favor as its then possible to design a pallet
that is less than as efficient as it can be. ( None are 100% efficient..). As a result, the angles are computed
to the max power transfer. I will take a look to see if a more negative drop can be added rather than a 0 limit..
you never know, may help in some designs..

Art

[kit]

Art,

I'm not at my home computer so cannot send you a screen-grab but with the following set-up, at the extremes of the range I know, the thrust faces of the pallet are definitely sloping the wrong way to take any power from the teeth. The adjustments are set to ensure clearance between the teeth and the pallet so should be a realistic arrangement:

diameter 120mm, teeth 30, back angle 9, forward angle 2, tip relief 1.5, toothspan 7.5, drop angle 1.25, lock angle 4,

For lock angles below 3 degrees the simulation shows the ratchet tooth sliding slowly up the end face of the pallet before rapidly snapping onto the other leg of the pallet as expected. For 3 degrees and above the pallet face releases the teeth immediately and the ratchet jumps almost instantly to the next tooth.

I've just tried a switch to inches in case this is a millimetres-only effect but (after changing the shaft diameters from 5 to 0.25) the same problem applies.

A very informative move is to zoom in to a close up view of the palette ends and ramp the value of lock angle between it's limits to see how the shape changes.

Kit

[ArtF]

Kit:

  Oh, I agree completely, those settings with a lock of 4 degrees are impossible, and wouldn't work, but the lock angle adjustment allows 0 - 4 degrees no matter the settings of the other values. The same settings with a 0 lock seem ideal.
  The only purpose of the lock adjustment is to make sure the tooth drops onto the lock face when it ticks , so at zero I
see it does just that, so the settings are fine as they allow for the lock adjustment, but they DO go too far in allowance,
you definitely always have to ensure what your designing works in reality, when I give adjustable distances, I don't verify they will work in all mixtures.. The rest of the setting look pretty good for a stable motion..

Art

[kit]

Aha! There's a lot more to learn about this clock-making lark than you realise at first.

I wanted to increase the lock angle as my current design has the pallet often catching the ends of the ratchet teeth, which knocks the pendulum off it's pivot, and the whole wheel train can free-run. Perhaps I need to wait until I've extended the Y-axis of my CNC router to 850mm and can cut the holes for the ratchet and pallet shafts on a new plate there rather than rely on a ruler and pillar drill to set the spacing.

I see that the higher lock angles work fine with fewer teeth. This also gives a longer duration push to the pendulum and makes other dimensions a bit less critical for the same diameter. I might try a 10 tooth ratchet with a 3:1 reduction to the second hand. This would also remove the friction of the relatively long shaft-within-a-shaft-within-a-shaft from the ratchet itself.

Plenty to think about!

Regards

Kit

[ArtF]

Yes, more than most think..

  I allow wide tolerances , you just never know whats needed.. :)

Art