Category Archives: Clocks

Update On Longcase Clock

Completed cad designs of primary longcase movement this morning. Still need to add the driving weight and winding gear but the main gear train is now fitted together in the cad model. I found when I resized the gears from the original design that I needed to include an intermediate carrying plate in order to prevent the 2W gear from clashing with the arbor of the GW.

A clock is really just a series of gear boxes. Firstly comes the gear box for the second hand. This is more of just a direct drive. The pendulum interrupts a 30 tooth escape wheel. Because the pendulum swings in two directions from centre the escape wheel must be half the time increment So 30 teeth on the escape wheel for a pendulum just under a meter equals 60 seconds or 6 degrees for each step of the second hand.

Next comes the Cannon Pinion. This is not fixed to the main arbor but is able to rotate freely on it. You could in fact if you wanted have each of the hands on the clock separated in to an individual dial but of course we want to stack them up. This is achieved by making the arbor of each gear box in the series hollow so that its final driven gear may rest above the driven gear before it. Seconds. Minutes. Hours. This results in the arbor for the seconds being the first to emerge from the clock face. The minutes being next and the hour hands the next.

Where this now gets complicated is that the clock mechanism must be driven by something. It is a common misconception that the clock is driven by the movement of the pendulum. This is incorrect. A clock of this design is driven by a driving weight. What the pendulum does is to interrupt the progress of the escape wheel by the anchor that rests above it. This rotates back and forth in a carefully calculated arc that always interrupts and then releases the escape gear at each step.

Where this gets tricky is that you need a secondary gear stage to take some of the energy of the slowly lowering drive weight and to put that back in to the primary arbor to which the escape wheel is attached. The result of this is a highly effective regen system that keeps the pendulum going for much longer than it would if it was just allowed to free swing itself with no return.

Here are some pictures that break down the mechanism in the movement. The drive weight and its winding gear sit behind what is called the GW or great wheel. This sits on the lower centre arbor ( axle ) and is usually behind it.

At the very back of the mechanism behind the entire movement sits the pendulum. This will be driven by a small rod that pokes through a hole in the anchor. As the pendulum moves from side to side it moves a small rod in an almost transverse line relative to the anchor and extends through it. This provides the timing movement for the anchor.

I have not included the winding gear yet but this mechanism is a one way ratchet. In one direction it is caught up with the driving weight as it falls and pulls around the arbor of the great wheel. In the other direction it can be turned freely and returns the weight back to its initial drop height.

This mechanism is for a 30 hour clock which means it must be wound once a day. At the pinnacle of clock design a long case clock could have as many as 30 weights and run for an entire year without winding.