Jump to content

30sec_exposures_2021.jpg.48851b1871a4bf9500ebd53c3e790d81.jpg

 

 

  • entries
    5
  • comments
    401
  • views
    2,966

Longcase Pendulum Clock


Gina

8,787 views

 Share

Traditional longcase (grandfather) clock but using 3D printed gears etc.  Also transparent acrylic clockface and mechanism front and back plates to show all the works.  The case is made of wood and pretty much traditional shape.  In addition to the usual hour and minute hands and dial this clock will have a moon globe above the main clock face similar to my moon dial clock.  I may add a small seconds dial if this proves viable.  There will also be an auto-winding mechanism driven from a stepper motor.   I'm hoping to add a striking mechanism once I have the main clock working.

 Share

182 Comments


Recommended Comments



This photo shows the probable gear arrangement.  The upper white, green and yellow gears are the new ones, printed in PLA.  The others are the original gears and sprockets (the latter in pink and blue).

117568959_GearTrain01.thumb.png.550ca07d0233c7a1314b67b2d98ca7f7.png

Edited by Gina
Link to comment

Proper drive chain gear added (red).  (The curvature is not the clock but due to wide angle lens.)

699809044_GearTrain02.thumb.png.781f365b4df961a8eaa35f2ccf0976a4.png

Edited by Gina
Link to comment

Calculations for the auto-winding system :-

  1. As shown the chain sprocket/gear to minutes wheel is 4:1 so the drive sprocket (pink & black) will rotate once every 4 hours.
  2. Drive sprocket has 18 teeth and auto-winding sprocket 8 teeth giving a ratio of 9:4 so auto-wind sprocket will turn once in 4x4/9 = 16/9 hours.
  3. Number of seconds for one revolution of auto-wind sprocket  = 3600x16/9 = 400x16 = 6400.
  4. If this has a shaft with a ratchet with 32 teeth, each tooth would need a push every 200 seconds.
  5. A stepper motor would need 200 steps per revolution so that makes 200 steps in 200s or 1 step every second.

A good result.  Instead of a 45t gear on the auto-wind sprocket there will be a 32t ratchet wheel, pushed forward one tooth with each revolution of the stepper motor.

Edited by Gina
Link to comment

We can calculate the time the clock would run for in the event of a power cut stopping auto-winding :-

  1. Circumference of main drive sprocket is about 350mm.
  2. Allowable drop of the weight is about 1.5m and with pulley corresponds to 3m of chain over the main drive sprocket.
  3. Number of turns of drive sprocket = 1500/350 = 4.3 approx.  equating to 4x4.3 = about 17 hours.

So a power cut lasting up to several hours would not be a problem.  The auto-regulation system should have set the timing pretty accurately so the clock should keep time for the duration.

Link to comment

Another photo of the gears roughly laid out on the acrylic sheet.  The ratchet wheel actually has 32 teeth not 45 - that was what was there before.  The auto-winding stepper motor will fit in the lower left-hand corner and have a ratchet pawl attached.  Another pawl will go just above the drive pawl to stop the ratchet wheel going backwards as the drive pawl returns.  I haven't shown the anchor escapement in the top in this view.  The escape wheel shaft will have the seconds hand on the end and a seconds dial will be attached to the 12 o'clock mark of the main dial.  To reduce cutter, this clock will not have numbers beside the dial.  The space top left will have the auto-regulator drive and top right the moon dial drive gears.  Down the LHS will be the striking mechanism with the chain drive for it at the bottom.  I don't think there will be much spare space!

687975704_GearTrain03.thumb.png.c1301faae481b08b7ab0ff6a70ab5b6b.png

Edited by Gina
Link to comment

The yellow minutes wheel (often called the "great wheel" in clock parlance) will need connecting to the minutes arbour/shaft with a slipping clutch so that the time can be set by turning the minute hand.  Another gear attached to the minutes shaft will drive the intermediate wheel which in turn will drive the hour wheel.  The hour shaft will be a sleeve on the minutes shaft with the hour hand attached.  Also on the hour shaft (or maybe part of the hour wheel) will be a cam which will determine how many strikes to perform.  Another cam on the minutes shaft, gives the striking mechanism "warning" and then actual time the strikes begin.

Link to comment

Cam was too big so reduced the step size from 8mm to 4mm.  Might change it again as 4mm steps seem a bit small. 

1568091144_GearTrain04.thumb.png.008c68834ac50aa8d88e1cb8a0952af5.png

Link to comment

Been looking again at the gear train from minutes wheel to escape wheel.  My first attempt was dividing the required ratio of 60:1 for minutes to seconds into the obvious 10:1 and 6:1 but the ratios can be 8:1 and 7.5:1 with gear ratios of  80:10 and 75:10 with an 8:1 ratio being easier to produce than 10:1.  The intermediate gear can have 10t and 75t and still be mod 2.25 to match the other gears so I would only need to redesign the one gear - result!!  I think this looks noticeably better.

793552456_Screenshotfrom2018-09-2709-59-21a.png.30a1658c9c89949e0788aaf093eb2695.png1833942293_Screenshotfrom2018-09-2710-53-59.png.17cef8733be68d8cff48871c7d5a06cd.png

Link to comment

As a little digression, I shall look at the calculation of the drive system for the strike mechanism allowing for the possibility of no winding for 12 hours as a starting point.  I shall deal with auto-winding later.

The amount of motion over the twelve hour period may be calculated from :-

  1. Total number of strikes for 12 hours will be 1+2+3+4+...+10+11+12 = 78.
  2. Using the same chain and weight system as the main clock drive gives 3m as the range of chain motion over the drive sprocket.
  3. I have an 8 tooth sprocket with diameter approximately 50mm giving a circumference of 50xπ = 157mm
  4. Number of revolutions of drive sprocket before weight hits the floor = 3000/157 = 19
  5. Each strike can be allocated 19/78 = 0.24 approx. or just under a quarter of a revolution of the drive sprocket.
  6. To allow some tolerance we will allocate 5 strikes per revolution of the drive sprocket, giving a 5:1 gear ratio between sprocket and main strike shaft.

To prevent the strike mechanism from running too fast I shall use the standard fly-fan governor with a further gear train from strike shaft to fly-fan shaft.  Some experimentation will be required to find the gear ratio required as a calculation would involve the amount of air resistance and the torque driving the fly-fan (which is beyond the capability of my poor old brain!).  A larger fan will run more slowly for a given torque.  I can use the fly-fan I used in the Mk1 clock for the perpetual calendar mechanism.

Edited by Gina
Link to comment

It isn't possible to finish the main clock mechanism without looking at the strike mechanism and the strike needs a cam on both minute and hour shafts.  Another cam is needed on the hour shaft (tube) to drive the moon globe dial.  This means the hour tube needs a gear (to drive it) and two cams.  The minute shaft will have a coil spring (to press the great wheel against the clutch plate), great wheel, clutch plate and cam that starts the strike mechanism plus gear to drive the hours.  The clutch plate can be combined with either the strike start cam or the gear for the hours drive.  With the present design, there isn't room for both behind the front acrylic plate.  I shall now look at the strike mechanism to see which arrangement would be best.

Link to comment

Here is a diagram showing how the striking is controlled to give the correct number of strikes according to the hour.  The 10 o'clock position is shown with the striking just starting.  I haven't shown the parts that control the starting and stopping.  The large snail cam on the left is on the hour shaft/tube and the small snail on the right is on the strike main shaft and driven by the striking gear train.  The bell is struck with each revolution of the small snail and the mechanism is stopped when the ratchet reaches its highest position.  Both cams rotate clockwise.  A latch pawl (not shown) holds the rack as the small snail clear it.  This pawl is released just before the hour and drops the rack, raising the probe until it reached the large snail cam (as shown).

174682179_Screenshotfrom2018-09-3013-59-08.png.c75993216f4373e12ab34988fe0c25bd.png

Link to comment

Since the large snail cam (on the hour shaft) is in front of the front clock plate, it would seem to make sense to have striking start cam (on minutes shaft) in front of the clock plate.  This means the hour drive gear can be behind the front clock plate.  It all depends on how the striking mechanism works out.

Link to comment

This new diagram shows one method of stopping the triking action.  After the 10th strike the small snail cam turns until it hits the end part of the rack.

1912096934_Screenshotfrom2018-09-3015-20-30.png.d71f73096469f26e660a961e1879d2c1.png

Link to comment

Hold on a minute - this isn't right!!  That's not 10 o'clock - it's 3 o'clock!  Thought that rack position looked a bit odd...  OK... Reworking it...

Edited by Gina
Link to comment

Here we go...

First diagram shows shows the rack plus its lever and probe with the small snail cam in resting and starting positions.  Resting and stopping positions being the same.  The second shows the stopped position of the rack part as well.

1207299391_Screenshotfrom2018-09-3019-44-25.thumb.png.b1af338fcd9d9766c97353d1ce02b2a8.png1204777913_Screenshotfrom2018-09-3019-43-15.thumb.png.98b2571f26cb693de7ac6c868bf96fbf.png

Link to comment

I've found another couple of problems with this design.  The curve on the large snail cam between the 12 level and the 1 level fits a design where the probe enters from the top with the pivot on the right (or from the bottom if the pivot is on the left).  Plus the whole thing is too big to fit withing the clock frame.  These two problems are not simple to rectify.  One solution would have been to reverse the rack lever and put the pivot on the end and the probe part way along but this would mean having the striking mechanism on the left hand side of the clock but my design so far has the clock gear chain that side.  It isn't possible to do a mirror image of the striking mechanism as the large snail cam has to rotate clockwise (unless I used a pair of gears to reverse it - which is daft).

Two possibilities occur to me - swap the whole clock round with the main gear train on the right and strike on the left (which is actually how most clocks are arranged I have since found out) or redesign the striking mechanism so that the probe part and large snail cam actually work together.  My way of thinking is that I work from left to right as with writing and to me the main clock gear train comes before the striking mechanism.  That is why I designed the clock that way round.  Clocks usually dont't show the works so it doesn't matter esthetically but mine will show off all the workings as a main design principle.

Edited by Gina
Link to comment

Realised I was thinking in two dimensions but we have three dimensions to play with and the mechanism doesn't have to lie in one plane.  I have in mind a new design that will work with the striking mechanism mainly to the right as I had originally intended.  The lever with the rack on the end will have a peg which will drop onto the large snail cam, so the lever/probe clears the cam by being in front or behind it.

This diagram shows the idea with the peg represented by small circles on the lines that represent the positions of the rack lever.  The large snail cam no longer needs a curved face between 12 o'clock and 1 o'clock.  The line on the right represents the edge of the frame.

917626550_Screenshotfrom2018-09-3023-17-27.thumb.png.192d67e852125ae6b9cbe545bf4d92e3.png

Edited by Gina
Link to comment

Constructing model for rack lever and small snail cam.  This is looking a possibility.  Although the small snail cam looks close to the frame the centre is 30mm away so a 50mm OD gear would fit in easily.

1549362499_Screenshotfrom2018-10-0100-16-44.thumb.png.9393fbdcd93c1d0b39d1fe213618ab69.png

Edited by Gina
Link to comment

In working out the design of the rack lever I found I could have the lever in the same plane as the large snail cam which would make things easier.  Bit rough ATM but I can tidy it up easily enough.

1856381478_Screenshotfrom2018-10-0111-59-16.png.6df9a3eb93853ae2ff700ed97027ce1b.png

Link to comment
Gina

Posted (edited)

First diagram shows the small snail cam that drives the rack upwards and pawl that stops it from dropping back when the cam disengages from the rack.  Second diagram shows the rack in the 1 o'clock position.

160596736_Screenshotfrom2018-10-0114-04-32.png.6bc82c2a97dd018a2a8fe9e929227303.png2025953168_Screenshotfrom2018-10-0112-31-23.png.9a81e81ec6b3584046c6608491b5ed09.png

Edited by Gina
Link to comment

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.