What is a clock?
  • One thing that all clocks have in common is firstly a power source, secondly a constant and thirdly a counting system.

    Lets start with the humble sundial, the power source is the Sun, the constant is the rotation of the earth and the counting system is the shadow thrown by the stick (the proper name for the stick is actually gnomon)

    Or an Atomic clock, the power source is electricity, the constant is a caesium oscillator and the counting system is a digital display. As an aside, the official definition of a second is, and I quote "The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom", well you can not argue with that, no understanding it either.

    But as far as most wooden clocks are concerned the power source is the drop weight, the constant is the pendulum and the counting system are the hands.

    If you were just to take the escapement wheel, the anchor and the pendulum, and add a drive weight you would have a very important part of a clock, except, of course, the counting system. Not very practical as it would mean you would have to count the escapement wheel revolutions 24 hrs a day to know what time it is. And of course the drive weight would have to be lifted every half hour or so.

    As most escapement wheels turn at a rate of 60 seconds, it is just a question of developing a system that will turn 60 times slower than the escapement wheel, in mechanical clocks this is realised by the use of gears, stick on a hand and you have the minute counter, but as a reduction by 60 times is quite large, nearly all clocks use an intermediate arbor, to keep the cogs at a manageable size, if this was done in one step and the pinion had 8 teeth then the cog would have 480 teeth, that is a wall filling cog.

    Now all that is missing is the hour hand, this of course means a further reduction of 1 to 12, it would actually be possible to do this reduction with just a pinion and cog, eg. 7 tooth pinion and 84 tooth cog, but then the hour hand would turn counter clockwise, which is why it is done with 2 cogs and pinions.

    Apparently it is the case that right handers are more comfortable with clockwise rotations rather than counter clockwise, although I have read that the first clocks mimicked the sundials rotation, but this only applies to the northern hemisphere.

    So if the drop weight was added to the minute arbor we would have a fully functioning clock, so far so good, however there is a snag ( isn't there always). If the winding barrel had a diameter of 2,5 cm that would be a circumferences of 7,853 cm, so for every hour the weight would drop 7,853 cm, a normal drop length is approx. 120 cm, so 120cm / 7,853 cm = 15,2 hours until the weight hits the deck, so the clock would have to be rewound nearly twice a day, feasible, but once again not very practical.

    This is where the drive arbor comes in and adds a further reduction, depending on the cog/pinion used the run time can be considerably increased.

    So this also explains why friction is the biggest enemy of any clock builder, as the reduction also works in the opposite direction, so by the time a drive force of, lets say, 2kg has made it all the way to the escapement wheel all that is left is only a few grams, and that is the pain and joy of clock building.

    Dave