2024-11-09

Last modified: 2024-11-09 16:17:39

• Stepper motor clock
• H4 escapement

Stepper motor clock

My clock has stopped working properly. It is intermittently stopping and starting, even when not under any apparent load.

https://www.youtube.com/watch?v=7fOnR9B2Yh0

So it worked for about a year.

Actually it seems to have freed itself up now and it's going properly again. Maybe a tight spot inside the motor? Do we expect that to get better or worse over time?

H4 escapement

https://incoherency.co.uk/blog/stories/principles-harrison-timekeeper.html

https://watchesbysjx.com/2019/09/john-harrison-marine-chronometer-h4-diamond-pallets.html

https://frodsham.com/commissions/h4/

Verge escapement, but:

• with hairspring, obviously
• no undercut on the escape wheel teeth
• impulse faces are 180 deg. apart, directly facing each other
• pallet backs have a curved frictional-rest surface
• the frictional-rest surface is cycloidal, and designed so that the escape wheel pushes the balance towards the extremity
• acting surfaces very close to axis of balance, to increase the "dominion" of the balance over the escape wheel
• large moment of inertia on balance wheel, high angular velocity
• doesn't need to be self-starting

There's no undercut on the escape wheel teeth because otherwise the recoil would be greater as the pallet "catches" underneath the tooth.

If we don't want an undercut, do we want the opposite? Do we want the teeth more like symmetrical V shapes? Just go with straight sides for now.

The curved frictional-rest surface is what allows the balance to have higher amplitudes, and to become kind of detached from the escapement like in a deatbeat escapement.

Putting the acting surfaces of the pallets close to the axis of the balance reduces the leverage so that the escape wheel is applying less torque to the balance, and the balance mostly spins back and forth by its own properties.

A large moment of inertia and high angular velocity of the balance wheel means there is more energy in the balance, which also makes it less effected by the escapement.

You don't need it to be self-starting, because you don't want the escape wheel to add enough energy in a single impulse to set the balance running. You'd rather it only add small amounts of energy with each impulse and you set it running manually.

The watchesbysjx article has this diagram:

But Harrison's fig. 8 has this:

They all show pallets that are 180 deg. apart.

Unresolved questions:

• Why are the impulse faces 180 deg. apart?
• Why are the curved backs cycloidal?

I think in my clock I could basically directly copy the H4 escapement, but put anti-friction wheels on the escape wheel, and a gravity remontoire to drive the escape wheel.