What is this clock thing for? #4 – the Geneva stop

You will not see Geneva stops on many clocks but if you do here is an explanation of how they work and how to set up the stops.

EN Welch Whittier parlour clock circa 1895, time and strike movement with Geneva stops

Most American clock movements do not use the middle portion of a long & powerful spring but use most, if not all of the mainspring. Geneva stops address this power issue. They are found on better quality antique movements made by American manufacturers. They are set so that when winding the mainspring you can only wind so far and then no further. As we know, clocks can’t be “over-wound” and this lets you know where the definite stopping point is.

On weight drive clocks Geneva stops are used to prevent undue force coming on the pendulum by jamming the weight against the seat board.

Stop works are used as a compromise by utilizing only the middle portion of a long and powerful spring, which varies too much in the amount of its power in the fully wound and completely released positions to get a good rate on the clock if all the force of the spring were utilized in driving the movement.

Removing the stops will do no harm to the movement but it will not release power as originally intended

You will notice a long “finger” (long point of upper star) on one of the Geneva stop gears and a wider gap (shallow groove of lower star) on the other. You set these up so that when the proper number of turns are completed the long finger meshes with the wide gap, thus preventing any further turning.

Strike side stop in lock when long point of upper star is engaged in shallow groove of lower star
You will need a letdown set to release the springs

Setting up the Geneva stop

Wind up the spring and then let it down slowly until the stop work is locked, counting the number of turns and writing it down. Then, hold the spring with your let down key and remove the stop from the plate; then count the number of turns until the spring is down and also write that down. Alternatively, wind the mainspring until it winds no further, release the mainspring with your letdown key one complete turn and set the stop. It may take more than one try to set it up correctly.

Don’t worry if they have been removed

On many antique clocks these have been removed because the repair person did not know what to do about properly setting them up.

Geneva stops removed

Removing the stops will do no harm to the movement but it will not release power as originally intended.