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.