Graham Escapement with Pendulum Adjustments

Builder Joe has a question about Graham escapements:

I have another question regarding my pendulum slowly stopping.  In your FAQ (on our website):

"Now when you get to the escape wheel, put the wheel in with the pallet arbor.  When the pallets are in proper position, gently restrain the pallets with your finger on the arbor.  Now turn the escape wheel.  It should first push one of the pallets out of the way, and this will cause the other pallet to come into contact with another escape wheel tooth.  Keep turning the escape wheel and the pallets should gently rock back and forth."

Should the pendulum be on it?  I have tried without the pendulum, but I can’t get the pallets to keep rocking.

Thanks again, Joe

Clayton answers:

The entry in the FAQ's describes a manual test of a Graham escapement.  It is a check designed to let you determine if the pallets and escape teeth interact correctly all the way around the escape wheel.  

However, what is described there is only a manual test.  Once the pendulum and the drive weights are added and the clock is put into motion, we introduce inertia that will cause the pallets and escape wheel to interact differently than described in that manual test.  In a working clock the pendulum's inertia will move the pallets further than will happen during the manual test described in the FAQ's.  The drive weights will cause the escape wheel to move faster than it does during the manual test.

Slowly moving the escape wheel by hand, and restricting the pallet arbor, should give your Graham pallets a nice rocking movement as the escape teeth move gently from one pallet face to the next pallet face.  Manually restricting the pallet arbor as described in the FAQ's, the pallet arms should not be involved.  

But when the weights and pendulum are added and the pallet arbor free to move, as in a fully functioning clock, more movement is introduced into the mechanism and the pallets are not now stopped by the their faces, but instead are stopped by the side of the pallet's arm.

The FAQ check is a manual check of the escapement.  It is something you can do to test function while the clock is being assembled and laying on its back on your workbench.

However, the actual movement of a fully functional escapement with drive weights and pendulum will be different.

Here is how a fully functioning Graham escapement works...

The pallets are first stopped by the side of the pallet arm.  The escape wheel is held completely still by the side of the pallet arm until the pendulum has completed its motion and reversed its direction.  Then the escape tooth is released from the pallet's arm and the tooth slides across the pallet's face - giving impulse to the pendulum, and simultaneously causing the next pallet arm to enter between the teeth, which puts the opposite pallet arm in the way of the escape tooth on the other side of the escape wheel.

With an understanding of the step-by-step motion of a Graham escapement explained, in the video below we can actually see it all happening.

If your pendulum is slowly coming to a stop, there are a few issues that need to be addressed.

First, do you have enough drive weight to power your clock?  Every clock is different and will have its own minimal drive weight requirement.  Check out my Blog post on Drive Weight for a way to determine just how much drive weight your specific clock actually needs. (link to my blog is at the bottom of the main page)

But what if you find that your clock needs excessive drive weight to power it?

If your clock is in need of excessive drive weight, the second issue to be addressed is the depthing of each wheel set in the clock's train.  This Depthing procedure is described in detail in the Troubleshooting section of my Blog 

The third issue to be addressed is a test specifically for the action of the pendulum/pallets and crutch.  For this test you will want to remove the escape wheel from the system as we will only be testing the pendulum/pallets/crutch assembly, and all of their associated arbors, and spacers.  

With the escape wheel removed from the system, but with all of the parts of the pendulum, pallets and crutch still working together, pull the pendulum's bob about three or four inches to one side and let it go.

The pendulum/pallet/crutch assembly should continue to rock for at least 60 seconds before stopping.  And 90 seconds is even better.

If your pendulum does not continue to oscillate for at least 60 seconds, we need to find out why.  We need to find what is restricting the movement of the system.

One of the most common reasons is that the groove that the knife edge of the pendulum pivot sits in has been carved too deeply.  That groove should be about 1/32" (.75mm) deep.  A too-deep groove will add excessive friction to the knife edge of the pendulum and restrict its freedom of motion. (this does not apply to the clock designs using bearing packs)

Another area that should be checked is the interaction of the crutch pin and crutch.  

If the crutch pin fits too tightly into the slot in the crutch, the binding friction will stop the clock.  

If the crutch pin fits too sloppily into the slot in the crutch, the impulse from the pallets will not be transferred to the pendulum, and the clock will stop.

The crutch pin should fit in the crutch slot and be free, but not sloppy.  As a measuring tool, you should just be able to slip the corner of a sheet of paper between the crutch pin and the side of the crutch slot.

Pendulum Physics

 Clayton,

In your written instructions for the Mantis clock you say to slow the time adjust the Bob up and to speed the clock turn the Bob down. Isn’t this backwards?

I thought to slow the clock you make the pendulum longer and shorter to speed it up. 

Thanks, Cal 

Cal, your query got me thinking that I should create a blog post explaining about the various types of pendulums and hopefully make their action simpler to understand.

There are basically three pendulum arrangements used in modern clockmaking.  These are shown in the Pendulums jpg below.  It is important to notice first that "0" is the pivot point.

Fig 31 shows a standard straight hanging "simple" pendulum with the pivot at the top.  Most of my clock designs use this type of pendulum.  It's fairly simple to understand.  If we raise the bob, it speeds the clock.  If we lower the bob, it slows the clock.  The longer the pendulum shaft, and the lower the bob, the slower this type of pendulum will oscillate.  We can also slow the oscillation of the clock by increasing the mass of the bob. With this style of pendulum, a very short pendulum shaft, or very light bob, will oscillate much faster.

Fig 32 shows a compound balance pendulum.  This is the style of pendulum used on the Mantis, Celebration, Balance, Organic and SwingTime and others.  Compound balance pendulums have the most variation, and are the most difficult to understand...but are pretty cool, because of their versatility.  For example, we could create a compound balance pendulum that is very short, say only 15 inches (38cm) tall, to take the place of a standard "simple" pendulum (Fig 31) that would need to be 42 inches (107cm) long.  Of course, we can make our compound balance pendulum shorter than 15", but the shorter we make them, the touchier, and more difficult to adjust it would become.  I saw one metal clock with a compound balance pendulum that was only three inches (7.5cm).  However, that's a bit unrealistic for our wooden mechanisms.

Fig 33 shows the pendulum arrangement used on the Toucan and Arts&Crafts clocks.  There is a stationary bob at the bottom of the pendulum shaft, and an adjustable bob between the stationary bob and the pivot.  Although it is a variant of the compound pendulum,  this type pendulum works much like the pendulum in Fig 31.  Raise the bob (or lighten it) to speed the beat, and lower the bob (or add more bob weight) to slow the clock's beat.

This morning I was sitting on the couch reading a book and it hit me how to answer your question and explain a compound balance pendulum's action.  It all has to to with what is called "Restoring Force".  The force exerted on the pendulum by gravity.

With a heavier bob on the bottom of the pendulum shaft, there is more restoring force on the bottom.  That's simple to imagine.  (we will not be concerned here with oscillations - only the effects that gravity has on the restoring force of the pendulum)

Think about a pendulum that pivots in the center, like the pendulum example that is shown in Fig 32.  But in our example here, instead of the pendulum being vertical, think of it horizontally.  (if you want a visual, simply turn the Pendulums jpg 90 degrees and look at Fig 32)

With the pendulum shaft being held horizontally, add a stationary, immovable weight at the very end of one side of the shaft, and then add a heavier, adjustable weight to the other side.  Don't let go of it yet.  Keep holding it horizontally.  If you move that heavier bob inwardly toward the pivot until it balances with the lighter, stationary bob at the other end, the pendulum has no restoring force.  You can let it go and the pendulum stays at rest horizontally.

With that same horizontal pendulum arrangement in mind, move the heavier bob down the shaft away from the pivot just a fraction of the distance so that the pendulum is no longer perfectly balanced.  Let it go and the pendulum will begin to move.  The heavier side moves down toward the floor, and the lighter side moves up toward the ceiling.  You can imagine that if the imbalance is very slight that the movement of the heavier side toward vertical is quite slow.

In a second experiment, holding the pendulum at balance horizontally, move the heavier bob all the way out to the extreme end of the pendulum shaft, and let the pendulum go.  The pendulum will now move quite fast to the vertical position (and overshoot vertical, thus the pendulum will oscillate a while.  But we are not concerned with oscillations here.  We are only concerned with the speed at which restoration takes place.)

In the first example with the heavier, adjustable bob essentially raised up (in) toward the pivot, the pendulum has little restoring force and moves slowly to a vertical position.  (thus, when lower adjustable bob is raised it causes slower restoration)

In the second example with the bob lowered toward the bottom end of the shaft, the pendulum quickly moves under the influence of gravity to the vertical position.  (thus, when we lower the adjustable bob further from the pivot it causes faster restoration).

Since Mantis has a compound balance pendulum, as shown in Fig. 32, by raising the bob up toward the pivot the motion of the pendulum is slowed.

We can also slow the Mantis pendulum another way; by lightening the lower bob.  A lighter lower adjustable bob has less restoring force.

SwingTime and Organic have compound balance pendulums also, however unlike the adjustable lower bob of the Mantis, their lower bob mass is not adjustable and stays constant.  On these clocks the upper bob is the bob used to adjust the beat.  Fortunately, the same "Restoring Force" physics apply whether you are adjusting the upper bob or the lower bob.  On a pendulum with a constant weight below the pivot, if we raise the bob that is above the pivot we slow the beat of the clock...alternatively, we could simply make the upper bob heavier.  In this case of an upper adjustable bob, both a heavier bob or raising the upper bob have the same effect on the restoring force of the pendulum.

One more, just for fun.  I created a clock that is yet another variation of Fig 33.  This clock is ALL pendulum.  The only part that is not pendulum is the wall mount.  Everything else is contained in or on the pendulum.  I call this clock "Minimalist."

Since the clockworks is integrated into the pendulum we have a compound balance pendulum that is similar to the Toucan or Arts&Crafts, except that in this case the center bob (the clockworks) is stationary, and the lower bob is the adjustment for the beat.

Because of the clock being integrated into the pendulum, the mass of the pendulum has increased and thus we need lower the bob with a longer pendulum shaft than would normally be the case.  Most of my "Simple" seconds pendulums are around 42" (107cm) from pivot to center of the bob.  The Minimalist pendulum requires about 63" (160cm) to give a seconds beat.  My purpose for this clock was to stretch out and thin down the overall appearance of the mechanism.  Integrating the clockworks into the pendulum allowed me to do just that.

Thin, slim and streamlined, Minimalist is a lovely clockworks with a unique pendulum.

Daisy Dial Train Puzzle

Dear Clayton,  I have a small issue with the Daisy Dial Train on my Mantis. The hour hand consistently stops at about 4:00. It happens pretty much every time that the hour hand attempts to pass roundabout 4:00. 

Since this is consistent, I figure that there is probably a known solution, and maybe this is common. 

The only item that comes so mind for me is the balance of the hour hand. If the hour hand isn’t well balanced, and the short end of the hour hand is heavier, this might explain the consistent stopping at 4:00. Thoughts? 

Just wondering about this. Since it’s so consistent, maybe you have a quick solution? 

Clayton's response:  Aloha.  I'm happy to help.  As you know, the Daisy Dial Train is such a wonderful and unique part of many of my designs - Radiance, Mantis, Balance, SwingTime, Tempo - all use the Daisy Dial Train.   It is such an excellent dial train for very quickly setting the hands to the correct time. 

I think your guess is a good one about the Hour Hand's counterweight end being just a bit heavy, and that should be checked out.  

Checking a hand's balance is easily tested by removing the hand and running a rod through the center hole of the hand and giving it a spin.  When the hand stops spinning, the heavy part of the hand will be pointing at the floor.  Then to balance, remove some wood from the back of the heavy end, or add some weight to the counterweight end.  

However...my first thought was not of the Hour Hand counterweight.  It was instead of the Daisy itself and its interaction with the pins coming out the back of the Tri. 

A most common cause of the Daisy Dial Train not working properly at first is that the pins get stuck on the Daisy's bump as they are trying to pass over.  Once the reason is found and fixed, the Daisy can continue to give decades of flawless motion. 

This sticking of the pin on a bump can be caused by a too-large, or unsmooth bump.  Or it could be that the drilled dowel, that is the Cam of the Minute Hand, does not have enough lift to get the Tri's pin smoothly over the bump.  Here is a screenshot from the bottom of the Minute Hand assembly page that explains... 

 Alternatively, an Hour Hand consistently getting stuck at one point on the dial could be something as simple as the Daisy bump needing a bit of lubricant. 

Since it is such a simple thing to investigate, that is where I'd begin looking. 

I'd first see what pin/bump is getting caught upon (there may be two of the three pins each caught on a bump), then I'd get out my color-coordinated Crayon and add a little paraffin to the offending bump and pin.  It wouldn't hurt to put a bit of paraffin on each of the bumps and each of the pins.  And then back off the Allen screw and leather plug and give the Minute Hand a spin like I show in the attached video.

 

Please keep me posted.  Let me know what you discover and what you did to fix this puzzle. 

Aloha.  Clayton

Excellent Mantis Build Video by Brian Gray!

Brian Gray made this wonderful video of his build of the Mantis clock from cutting to assembly to fully working clock.  He sent us this link, plus some very nice compliments.  Thank you, Brian!

Clayton writes:

Wow!  This is an amazing video, Brian.  You've done an excellent job constructing your Mantis wooden clock build, but you've also done a great job showing how you did it.  You have an amazing workshop with so many excellent tools.  I was drooling!  Your shop makes mine look pretty meager.  Beautifully done, Brian.  You have shown the way these projects are approached by a true craftsman.  I'll certainly be referring this video to other Mantis builders.  Well Done!  Aloha.  Clayton

 Brian answers:

Thank you Clayton! It's been such a pleasure to put together a few of your clock designs, with more coming! I've been a machinist and woodworker for more than a few decades now. But for some reason, clocks have put a huge spark into my inspiration like no other projects in the past. Thank you very for taking the effort to make your plans available. I'm sure that creating an accurate set of plans is much more work than the clock itself, so thank you!

Perfect Involute Tooth Forms?

Interesting question from a builder:

I'm new to wood clocks, but have some mechanical engineering experience. I see that the tooth profiles on the pinions are nothing like those on the wheels they drive (and neither look like involute teeth). 

Why the difference in tooth profiles? It seems to me that the two ought to be the same. To add to the question, it seems to me the quasi-trapezoidal teeth on the driven wheels would be easier to cut accurately, suggesting the pinion teeth should be similar. What do you say?

Regards,

John

Aloha John, it is nice to hear from you.

Your engineering experience has taught you right.  It has taught you about high speed gearing at its finest.  

Thing is...clocks don't move fast so that level of perfection is not required.

As a matter of fact, in my own experiments, it's difficult to notice any difference at all with various tooth forms.

I discuss this at length in my book, Practical Guide.

Take a look at the various tooth forms that I use in my designs.  You'll see straight sided, involute, "random", rounded, and none of them seem to perform much better than any other.

You can make high speed, "perfectly" engineered teeth if you like, but do some experiments on your own and I think you'll convince yourself that pretty much any tooth form works well at low speeds.

The old tymers used to cut triangular tooth forms.  Worked fine for them.  Hacked them out with adz and axes.

Take a look at the teeth on this old clock on the cover of the National Association of Watch and Clock Collectors (NAWCC) journal.  Here is a perfectly fine running clock with possibly the poorest choice of tooth form available...triangles.  Ha!  

And yet................it has been working for centuries.

So, my advice is to make whatever tooth form strikes your fancy and compare it against the "perfect" involute tooth forms and see how much difference it makes in the running of your clock.

Enjoy!  Aloha.  Clayton