You see these clocks everywhere. They hang on walls of kitchens and offices, quietly ticking away, keeping count of the time left in the day. One of these clocks recently failed so I decided to take a closer look at the inner mechanisms of a clock.
If you think about it, a clock is not a simple task, especially with how it is presented: three rotating pointers on a single shaft, all turning at different speeds. If you directly drive one pointer, say with a motor, how would you put another axis of rotation through the same vector? The solution, as it turns out, is to just wrap shafts around each other.
Here you can see the central shaft, the second hand, inside two shafts, of which there is a step. This helps to locate the hands, which are press-fitted onto each plastic shaft. The hands themselves are made of thinly stamped aluminum. Interestingly enough, while burr is a defect in the stamping process in most cases, for the hands, this actually appears to be an intentional design feature because it allows a stronger fit of the hand onto the shaft. In nicer clocks with nicer hands, the shaft is actually designed to be thicker.
Opening up the clock reveals this mechanism. The transparent gear sits on top of the white gear below. The other white gear on the side actually drives both gears, and this makes sense because the rate of rotation of the hour hand is directly proportional to the rate of rotation of the minute hand, so we would expect to find some coupling mechanism.
The next picture shows the inclusion of the other half of this mechanism.
Here we see yet other coupling gear on the top right, where it connects the gear with the silver shaft to the translucent gear. This entire mechanism is driven by a rather ingenious circuit.
On the bottom of the metal piece is a circuit board with an epoxied IC and metal contacts for the battery. It is not very exciting except for seeing how thin the wires on the orange inductor are. The real magic is how the gear is driven. I determined by hand that the black ring on the bottom of the gear is a magnet, with poles on the left and right side.
How this works is when the inductor is powered, it generates a magnetic field in the silver metal, this causes the gear to rotate because of magnetic attraction. I'm not entirely sure why the rotation is in one direction, but I think it has to do with the induced fields not being equally strong, so the magnet is inclined to rotate in one direction. On the next 'tick', the current in the inductor changes direction, and the magnet rotates again. This is essentially a poor-mans BLDC.
Now I think this is the part that is the reason why this mechanism can run on years off one AA battery. The magnet actually levitates off the support. This means that the friction encountered is very low, and since no brushes are required, it will not wear out. This is aided by two locating mechanisms, the black shaft to hold the bearing up, and the thin silver shaft from the gear that fits into the black support shaft. The magnet is attracted to the metal around it, so it never fully sits on the balck shaft, allowing it to 'levitate'. With the elimination of friction here, the current required to run this mechanism is a lot less.
The case uses a snap-fit mechanism and plenty of locators to hold the many gears in place. One interesting observation I made is that sometimes there would be locators around part of the gears as well, which is perhaps to help in assembly. The parts appear to be injection-molded ABS. It is held to the clock face with by snap fits as well.
No significant electronics to speak of. There is an epoxy blob on the small PCB hidden under the inductor with what looks like a 32.687kHz crystal soldered on.