Basic Clock Design

Harrison, and Clock B, used circular cheeks, not cycloidal. This is quite explicit. Anyway real pendulums, which are compound so they have their mass distributed, are not made isochronous by cycloidal cheeks. It seems that Harrison knew this.

I just doubt if Harrison's chops were cycloids Michael. On the other hand they might be. Huygen's aims was to correct the swing of a pendulum and it seems very little actual use was made of the idea despite lots of work. Harrison#s protractors would be well aware of Huygen's work as well.

The best ever pendulum is often reckoned to be down to this bloke. Change of subject but again not in any way a basic clock.

**LINK**

However note the need for a microscope to set the swing and other factors. That one it seems could detect changes in gravity due to the position of the sun and moon.

John

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Indeed ... 'twas my error

Cycloid only works for a 'simple' pendulum.

MichaelG.

John, that link doesn't seem to work for me. Was it the Loomis and Brown article on the Shortt clock tests?

I am currently drawing up plans for a Brocot escapement for this clock, rather than my original design. I have read that the diameter of the pallets should the same as the gap between the tips of the escape wheel teeth, less the drop. What would be the consequences of using smaller sized pallets than this? The main thing that I can see is that it may run out of "operating" surface if the pendulum were to swing too far, so the cut-out part of the pallet tries to engage, rather than the curved face.

Can anyone please advise?

Thank you.

James.

It looks like the data base is off line John. Perhaps best to try again in a few days. Odd it's one I have made a lot of use of at times and I hope that mentioning it on the web hasn't caused them to restrict usage. When it's back on line you could try searching Astronomical Clock. The paper was written by F M Fedchenko. It is mostly of academic interest though.

You would be increasing the recoil James if you mess about with the other face so best stick to the design. The other curved face just provides clearance for the pallets and could be replaced by the other form that has a straight angled line instead. The curved form provides stronger teeth. It's possible to cut that with a curved form tool first and then a suitable slitting saw. I'm stuck for a suitable linux cad package that will draw one easily so will have to do it on paper.

So far I have found that a 60T one will be about 95mm dia and a 120T one about 190mm dia. Irksome as I would prefer 120T.

John

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Edited By Ajohnw on 08/05/2016 10:05:58

This is the first of 13 video's "describing" the making of a replica Harrison's RAS regulator I mentioned earlier.

**LINK**

Personally I have always wondered if his grass hoper escapements needed power applying with a lot of care to prevent them from running away. He also made wooden ones for probably rather high end domestic use. There is also a youtube video showing some shots of a  replica some one made. It's called Harrison's Wooden Clocks.

John

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Edited By Ajohnw on 08/05/2016 12:17:43

Edited By Ajohnw on 08/05/2016 12:18:26

John,

Thank you for your reply. It took me ages to work out why it would increase recoil. To my logic, it seemed that a smaller pallet would reduce the recoil, after studying some videos and thinking about it, I think that I can see the reason.

I shall carry on drawing.

Have you tried Onshape CAD? It is an online system and sketches or drawings can be exported in dxf and dwg formats, amongst others.

Regards,

James.

Draftsight is a comprehensive 2D package produce by the Solidworks people. It is available for Ubuntu and its derivatives. I find the current version works fine on Linux Mint.

Russell.

I'll see if I can find some images that give the main aspects of the design James. The size of the pallet given that the active face is set at the correct angle shouldn't make any difference to recoil. Altering the angle will.

I've just had 1/2 hr + on my knees, huddled up praying to my PC with a vacuum cleaner. HP make wonderful workstations that even have a blow through heatsink on the cpu - pity they don't assume that some one will need to clean it one day. I had to undo 4 screws with a 15mm long phillips bit as no room for anything else. I've even got sore fingers now. As the cpu was running rather hot it became extremely slow.

I'll have a look at the cad package suggestions. I had looked at LibreCAD which is a spin off from Qcad. It will do it but it's cumbersome to use purely with a mouse so I have made some suggestions as to how they might improve it. I might load Qcad but it isn't clear as to what remains after the 15mins of pro sample disappears.

John

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The adsabs link is working fine this morning, John

MichaelG.

As things tend to interrelate there doesn't seem to be much "do this" and all will be ok design information on Brocot escapements. It seems that there may be a list of ideal designs about some where but I can't find it. Best I could pull together is this.

What it really doesn't cover is the rake on the active face - the flat side of the tooth as other factors such as the number of teeth spanned and the length of the escapement arms alter it.

The best way to view that seems to be looking at what happens at the start of lock when the pendulum carries on swinging, Left hand pallet in the view on the top left. The rake at that point needs to be tangential to the curve of the pallet swing. As it's a curved path it can't be precisely replaced with a straight line so there is a need to play with escapement arm length and the number of teeth spanned to flatten the curve as much as possible.

It can work as the video clearly shows. The drop, 0.1 pitch clearance can be altered by rotating the pallets slightly and as De Carle points out the impulse angle can be reduced by moving the pallet up a touch but not by so much that it can run unlocked.

Well this is how I see if, If anyone knows better or can think of other aspects now would be the time to mention it.

When I ran through the design verbally I suspect I had the tooth pitch incorrect. The graphic design starts with twice the number of teeth sectors. One thought that struck me was what would happen if the flat raked side of the tooth was given a suitable rad but as most things will have been thought about long ago I suspect there is a catch.

John

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John,

Thank you for this information which I shall need to study carefully, I am working on my design of the escapement slowly and shall update this thread once I have something completed.

Regards,

James.

AJohnW

Belated response to your question. The grasshopper does need to be 'powered' at all times. If you remove the drive both arms detach from the scape wheel and the train runs uncontrollably. Grasshopper escapements really need maintaining power mechanisms.

regards Martin

Going back to Brocot the image I posted it shows how it's initially designed, The 3 views, left running up from the bottom. The one with angles is so that actually dimensions can be calculated. The top of the 3 shows the initial construction and the next one the pallets added. The idea is to pick an escaped wheel and draw it out 10 times full sized as shown and then try various pallet positions and escapement arm lengths. If an arc is struck off from the pivot to the edge of the pallet it can be compared with the rake applied to the active face of the tooth. The idea is to minimise the difference between the arc and the rake. Any difference represents recoil so the amount can be measured off the drawing. At the same time the impulse angle can also be measured.

The tangent angle used to set the rake on the teeth need more explanation. They are offset from the centre by 1/4 of the diameter of the pallet either way as this shows.

This seems to give the best compromise angle for some pallet arm length and tooth span. An arc can then be struck off the edge of the pallet and compared with the rake on the tooth and the gaps/recoil directly measured and converted to an angular rotation of the escapement wheel if some one is feeling manic.

All good fun - what !!! An escapement that is easier to make but more fun to design. A few trials against a desired escapement wheel and pallet diameter should soon sort out which way to go.

I suspect that in my case I will need to make the body of the escapement out of aluminium to keep the weight down as it may need long arms due to my need for a high tooth count with a 1/2 sec pendulum.

John

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John, do you mean the pallet frame or the 'scape wheel? If the former, as it's attached to the crutch and through that to the pendulum, a bit of extra weight probably doesn't matter!

The pallet frame John. Not sure about the escape wheel. I'd be tempted to try a high tensile copper aluminium alloy as per the Burgess clock for everything but really wonder if that is sensible as aluminium oxide is rather abrasive and the corrosion resistance of those types isn't as good as others.

I wonder about the mass of the pallet arms because of a comment in the old modern clocks book I linked to. In order to achieve narrow impulse angles the arms tend to need to be long which increases mass and inertia. I'm inclined to agree though that the bits been driven will have far more.

Really it's hard to be sure about any of it until it's drawn out. My feeling about that is it's better to get a clear understanding of the principles before doing that and sorting out the images has helped. I offered Rod what I thought was a very sensible price for his no longer wanted drawing board. He didn't agree. Since then I bought a type of desktop board Reeve's produce off Amazon and a used drafting head. The head had to come from the USA and can be clamped onto anything. Much lower cost and I feel that hand drafting can sort this out much more quickly than picking up and using a cad package. Then a bit of maths to work out actual sizes and angles. Must admit though I am basically a designer and did spend many years on a drawing board with either a pencil or ink.

The amazingly complicated Astro clock I mentioned also has a series of video's on youtube. He prototyped everything before cutting brass. Wood and sometimes perspex for the plates. Looked at that way there is plenty of scope for late variations before all of the brass is cut. Any gears cut could be used with various arrangements so shouldn't be wasted. Same for holders for ball races etc.

As I haven't got a clue about what I am doing I envisage getting there in stages. Escapement and pendulum first. Then how it's driven as there are several options. While it may be possible to spend a lot of time trying to design these options it's likely to be quicker to find out as needed when the choice is made using cheap materials.

John

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Edited By Ajohnw on 10/05/2016 14:18:44

John,

Again, thank you for this information. I shall post my sketches, once i have something that seems to be about right.

Regards,

James.

John

Just a thought on your Brocot design. The more teeth your pallets span the longer the pallet frame arms will be. Thus the radius of the arc they describe will be greater and the recoil less. Not sure how important that is really though.

Russell.

The grasshopper doesn't have a problem with an Ali alloy wheel since the pallets don't rub, they hop out of engagement. But I don't know if there'd be a problem anyway with any abrasive action of the oxide layer.