Hammer mechanism
The rack mechanism is a type of striking mechanism in a wheel clock , which is mainly found in antique table and wall clocks. The peculiarity of the rack mechanism is that, unlike the older lock disc mechanism, it always performs the correct number of strikes even when the trigger is repeated before the next trigger by the movement and even after the pointer has been adjusted. This feature allows the repetition ( repetition ) of the shock by hand and a change of the clock without a resynchronization for percussion is required.
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Components
- Hourly scale : coarse worm-shaped disc with graduated radii, which precisely limits the fall path of the rake depending on the time. It is often attached directly to the hour tube and thus connected to the hour hand. Striking mechanisms with a quarter-hour strike often have a separate scale with four radii that sits on the shaft of the minute wheel. For striking mechanisms that can strike every single minute, for example, the scale for the minute strike is shaped differently and sits on the shaft of the minute wheel.
- Release lever
- Drop lever or computing lever : locks the rake as a pawl
- Rake : Lever with a toothed arch or a toothed side, whereby each tooth corresponds exactly to a hammer blow, "Bim-Bam" or "Cuckoo".
- drive wheel
- Additional wheel with drive (typically only for watches with 8 or 14 days running time)
- Lifting pin wheel with drive with lifting pin
- Creator wheel with engine and creator and pen with incidence cam. Creator is basically a gear with only one tooth or pin, turns 360 degrees during a hammer blow and lifts the rake by one tooth.
- Guide wheel with drive and guide pin
- Windscreen : an aerodynamic brake that stabilizes the speed and thus the impact frequency
function
The fact that more than one blow can occur after triggering is made possible with the rake hammer mechanism with the rake and the relay . The relay disk sits on the hour wheel and is therefore driven by the movement of the wheel clock. This ensures that the mutual assignment of the displayed and posted time cannot be lost.
As with the lock washer striking mechanism, the first blow takes place after the actual process begins. The second (up to twelfth) blow can only occur when the rake has fallen and the release lever d + a of the first run-up wheel is blocked in the release position. The second highest (to lowest) segment of the scale corresponds to a corresponding arithmetic position. With the Creator , the rake is lifted back one step (tooth) with each rotation. In its uppermost position it releases the lever d + a again. The appropriate number of strokes has been made, the striking mechanism is at rest again.
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For an additional half-hour stroke, for example, the release lever with the second pin on the minute wheel is only moved so far that the first run-up wheel is free, but the rake does not yet fall. The season is also shaped for the extension to quarter beats: between successively deeper cuts for the additional hour beats at 2 a.m. to 12 p.m., the season is not cut (first quarter, 1 beat), followed by a simply deep (second Quarter, 2 strokes) and a double deep (third quarter, 3 strokes) incision.
Even with rack bar mills, it can happen that the beat does not match the displayed hour. There are several reasons for this:
- The hammer mechanism had previously run out and the rake remained in a low position (for example 12 o'clock) and then, after winding the rake hammer mechanism, it also strikes 12 o'clock, for example, as soon as the blow was triggered for the first time after winding. This can happen even at half an hour. The next time a beat is triggered, the hammer mechanism should sound the correct number of beats again without any corrective action.
- Sometimes the hour hand sits so loosely on the hour tube that the hour hand was inadvertently turned in relation to the hour tube and the hour scale when setting the clock. If this is the case, the hour hand can be turned back to the correct position with little force.
- If the minute hand of the watch is set against its normal direction of movement, a wrong beat can sound when the half or full hour is reached.
- If stroke errors only occur when manually triggering the stroke repetition before or after the full hour or at certain times of the day, the hour tube is not optimally positioned in relation to the rake.
Rack bar mechanisms are mainly used in stationary clocks. For this, the mechanism is designed in such a way that gravity is used for the falling of the rake and drop lever. The release lever and hammer lever shaft often have a return spring, even on stationary watches. For movable clocks (e.g. travel clocks, pocket watches), rakes and drop levers are additionally provided with springs, which, with their spring force, ensure the appropriate movement of the rake and drop lever in every position of the clock or guarantee a certain dynamic of the movement process.
The picture shows a clockwork with a 4/4 Westminster beat (hours and 4 quarters of an hour). In the left area the rake of the hammer mechanism can be seen, which strikes the full hours. The striking mechanism for the quarter-hour strike is on the right-hand side. This is controlled by a small lock washer with four notches. In addition to the notches for the respective termination of the quarter-hour strike, this disc also has a raised portion (cam) in the area of the quarter-hour strike of the full hour. This triggers the beat of the full hour as soon as the preceding 4 quarter-hour beats (= full hour) have occurred. Directly below the lock washer, the lever of the lock stop runs from the left edge of the picture to just next to the pointer mechanism. The lever of the strike stop is in the upper position and thus prevents the release lever of the quarter-hour strike from falling and thus prevents the clockwork from striking.
With Comtoise clocks , the rake is sometimes also straight (see illustration)
In Morbier watches , a release lever with two tips triggers the hour strike again a few minutes after the hour (automatic repetition).
Repeater mechanism
The rack mechanism can also be designed with a repeater, which is not possible with a lock washer mechanism due to the design. In the case of percussion mechanisms with repetition, the last blow performed is repeated either automatically or on request. More elaborate versions also strike the past minutes (so-called minute repeater).
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