An Unusual Striking Mechanism
Here are a few images of an interesting striking mechanism, rarely used, although I cannot think why.
I am going to try my best to describe how the mechanism works with the aid of labeled images. I have done my best to label the components with apropriate names.
1. The 'Rack tail'
3. The Lifter and action spring
5. Locking Pallet
2. Hour Wheel and Snail
4. Cannon wheel and half-hour 'Snail'
6. Positions of wheels between the plates
The images to the left are of the components which make up the front plate of the mechanism, behind the dial.
Between the plates is similar to rack striking clocks, shown by arrows in image 6, I have not counted the teeth and leaves, but will explain the necessary ratios where needed to clarify the working of this clock.
The mechanism for striking a single blow at the half hour is a simple one, The 'Rack tail' (1) is arrested by the cannon wheel (4) at the appropriate distance to allow only one blow to be struck (7), aside from this, it works in the same manner as when striking the hour.
The 'rack tail' (1) is attached to the extended arbour of the second wheel of the train. The spring on the rack tail ensures that this is a loose friction fit and that no slippage is allowed as the train runs, allowing it to be 'gathered' up as the second wheel rotates. The pin at the top right hand of image 1 extends to both sides of the 'tail', one half being arrested by the cannon 'snail' (7) to strike half past and one by the hour snail (8).
7. Rack tail arrested, and ready to strike half past the hour.
8. Rack tail arrested on hour snail.
There is no warning action on this type of striking mechanism to write about, however you are able to watch the fly 'tick' as the locking pallet is pushed aside by the lifter, which moves a small amount with each unlocking of the escapement.
As the time approaches the hour, the lifter is raised by pins on the underside of the cannon wheel as usual. Its action spring begins to rest (9), and then push against the pin on the 'rack tail' which is unable to move because the locking pallet is in place. The top of the lifter then pushes the locking pallet aside (10), allowing the force of the action spring to overcome the friction fit of the rack tail to second wheel arbour, and the rack tail falls on to the snail.
When the lifter is released it falls away from the locking pallet and striking begins.
The locking pallet wheel makes one turn for every division on the hammer pin wheel, so that it is in posision to lock after each strike of the hammer (as with the gathering pallet on rack striking). This allows the second wheel and 'rack tail' to control the number of strikes sounded.
9. Lifting begins as we approach the hour.
10. Locking Pallet is released ready to strike