Intro: Accurate Pendulum Wall Clock
There is something restful about a clockwork pendulum time piece ticking in the background. Clocks with Westminster chimes add even more interest when they announce each quarter of the hour. Then there is the weekly ritual of winding up the mechanism every Sunday morning and admiring the clock smith's handiwork.
So far so good but what is frustrating is that the clock does not keep time. The manual glibly tells you to screw up the pendulum nut one turn to make the clock gain an extra minute a day. However after any adjustment, anything seems to happen. The clock can gain or lose or even do nothing, even worse, the clock can be running quite happily for days or weeks and then suddenly it will start gaining or losing time.
This Instructable looks at fixing the time keeping problems on a clockwork pendulum clock so that it will keep the best time it is capable of.
Step 1: How Bad Is the Timekeeping
Before tinkering with the clock, the first job isto see how accurate the clock really is. After winding up the clock on the Sunday morning and setting the time, the offset from real time was recorded twice a day over the week. The chart shows the clock loses time at the start of the week and then runs faster during the latter part of the week. It looks like the timing is changing as the clock spring winds down. A fully wound spring will make the pendulum swing more and slow the clock down.
Step 2: Tools
Very few tools are required to adjust the timing of a pendulum clock. A spirit level is used to ensure the clock is level. Any sliding surfaces should be lubricated with clock oil, ordinary oil can corrode the metal or damage the protective lacquer. Finally, a key is required to wind up the mechanism.
Step 3: Dismantle the Pendulum
The pendulum is usually hooked into the clock mechanism so that it can be easily removed for transport. Stop the clock by holding the pendulum in the central position and unhook it from the mechanism. Sometimes the escapement will race away without the pendulum, just hold the hanger in the central position to stop any further activity.
The pendulum is dismantled by unscrewing the adjusting nut so that the bob can be slide of the rod. This particular pendulum has a decorative Gridiron that does absolutely nothing be look pretty. The original purpose of the Gridiron was to provide temperature compensation but this was obsoleted by low expansion materials.
Step 4: Inspection
For good timekeeping, the pendulum assembly must be mechanically stable, with nothing that could move in the long term. Any sliding joints should be smooth so that there is no built in tension that could be released giving a change in pendulum length.
On this pendulum there is a metal tab on the bob to hold the bob in place on the shaft. In this example the metal tab was actually digging into the pendulum rod making any adjustments jerky. Also, some of the stress could be released much later causing a jump in time keeping.
Careful bending of the tab upwards gave a smooth sliding action but still held the bob firmly on the shaft. A smear of clock oil gave even better sliding action.
As mention earlier, the Gridiron is only for decoration. The Gridiron should not interfere with the sliding action of the bob and so there should be a gap between the Gidiron and bob.
Step 5: Final Assembly
The adjusting nut end of the bob should also be checked for smooth operation and oiled if necessary. There is a half turn of backlash in the nut before the bob moves. When changing the adjustment from fast to slow, the nut needs turning first to remove the backlash before any adjustment takes effect.
Once we are happy with the adjustment system working smoothly with no grabbing or snatching, the pendulum can be hooked back into the clock.
Step 6: Checking the Beat
Before we adjust the timekeeping, we need to make sure the escapement is powering the pendulum evenly on each stroke to give the familiar tick-tock. First of all level the clock and note the position of the pendulum. Now tilt the clock to the right until the escapement clicks and note the position. Repeat again but tilt the clock to the left until the escapement clicks. The two distances should be the same from the rest position of the pendulum. The clock may be mounted slightly skewed to give equal swings around the rest position.
Rather than tilting the clock to check the beat, the pendulum may be moved from the centre position to hear when the mechanism clicks on both sides. Again, the clock may be skewed slightly to balance the distances.
Step 7: Final Adjustment
The nut on the pendulum needs adjusting to give reasonable timekeeping over several hours, this could take several attempts to find the right position for the adjustment nut. Remember to make the last adjustment so that the bob in resting on the nut to stop any slipping over time.
Finally, the timekeeping was checked again to see if there was any improvement. The timekeeping is much better now, within about a minute per week. Also, it is better to set the time a few days after winding when the clock has settled down into its most accurate period.