Bench Note: Cannon Pinion and Back-hacking

I was looking over a couple of Seiko 7002 movements that I recently brought back to life and noticed one of them was able to back-hack and thought it would make a good subject for a bench note.

What is back-hacking?

Back-hacking is when a non-hacking movement, our 7002 in this case, can be forced to a complete stop by applying slight pressure on the crown as if trying to set the time backwards a small amount. I have read on forums where enthusiasts sometimes admit to liking the ability to back-hack but it should be seen as a fault requiring correction.

That sounds more like a feature than a bug so why remedy it?

The watch needs a way for the time to be set manually with the crown without putting excessive strain on the gears in the motion work. This is accomplished on the 7002 through an indenting system where the cannon pinion, which supports the minute hand, snaps over a center post extending from the center wheel forming a clutch. This post contains an indentation which should perfectly correspond to a crimp in the tube of the cannon pinion. The crimp on the cannon pinion presses onto the shoulders of the indentation creating a small amount of friction which allows the center[minute] wheel to turn the minute hand. If the friction is too low then the center wheel will drive the center post but the cannon pinion may slip causing the watch to loose count of minutes. If the friction in this fitting becomes great then it will grip onto the center post and in turn transfer force into the motion work in whichever direction the time is adjusted. If set backwards this force will oppose what is provided by the mainspring causing strain and potential damage to a number of components. They key here is to have just enough pressure and lubrication to allow the cannon pinion to be driven by the center post while slipping only when opposing force is introduced.

Center wheel and center post on the left; Cannon pinion on the right

Parts loosely fit together to give an idea of assembly

Here is an example movement that demonstrates proper adjustment and lubrication. Note that the second hand continues sweeping in the proper direction while I set the time backwards. Once I flip the movement over you can see that the balance amplitude stays healthy and the fourth wheel continues turning in the proper direction while the time is set backwards again.

Here is an example movement with a cannon pinion which is either improperly lubricated or too tight. While the time is set backwards the second hand stops and even can be made to run backwards. Flipping the movement over reveals the balance comes to a stop and if you look very carefully you can actually see the fourth wheel reverse direction momentarily as the balance comes back into action.

The problem 7002 in the video above was stripped down again and better attention was paid to the lubrication on the center post which the cannon pinion attaches.  This was all that was needed to resolve the back-hacking in this particular case.  If the issue were to persist then the crimp on the side of the cannon pinion would have been pushed out slightly to relax its grip on the center post.  The video below is the same movement after fixing the lubrication.

 

Thanks for watching!

-Watch Dog

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