Ordering a replacement platform

Ordering a replacement platform by post is not hard if you have actual size illustrations and speed information. There are two points to check, namely actual physical suitability in terms of size, depth etc., and the speed of operation. Even if the original platform is missing altogether, those with mathematical ability will be able to calculate what is required.

To check the size, remove the original and compare this with the scale drawings to check for physical differences. The overall size is not important unless it has to fit between the recesses of a pair of carriage clock plates or will foul the dial etc. The escape wheel must be long enough to reach the wheel that drives it. Check with the specifications that the guard surrounding the escape wheel can be rotated if necessary. This often has to be done when the wheel that drives the escape wheel is slightly to one side. This is mostly found on French drum clocks where the platform is mounted on the back plate.

To check the speed, we need to do some calculations about the mechanism. It is clearly almost impossible to tell much by timing the original because if it went properly, it would not be being replaced. It is for this reason you cannot send a platform to a material dealer and simply ask for a replacement. You cannot just look at a hairspring and know how many times per second it will oscillate. But we can work it out from the clock mechanism, because we know that the minute hand shaft must rotate once per hour and the hairspring's function is to ensure that it does. All modern escapements are manufactured to oscillate at 18,000 vibrations per hour, so there is little point in working out the speed of the original. We really need to know if a standard 18,000 vibrations unit will be correct and if not, what value of escape wheel tooth count could be substituted to obtain the desired timekeeping. The calculations are based on some basic fundamentals.
All new units have a frequency of 18,000 per hour and have an escape wheel with 15 teeth. As there are two vibrations of the escape wheel for each tooth, it follows that the escape wheel must rotate 600 times per hour. That is: (18,000 divided by (15 x 2)). The centre wheel (to which the minute hand is attached) clearly rotates at a rate of once per hour, so the gearing in between must be 600:1.

To calculate the number of leaves that the escape wheel pinion must have to give this ratio of 600:1, count the number of teeth on the centre wheel and divide this by the number of leaves on the pinion it drives. Do the same with the next (4th) wheel and multiply the two results together. Now divide 600 by this figure and divide the answer into the number of teeth on the contrate wheel (the one that drives the platform). The result is the number of leaves (or nearest) that the escape wheel pinion should have. If this is not a whole number, use the nearest available. The difference can normally be taken up with the adjusting lever. If the answer is below 6, then the original platform probably had a frequency of 15,000 or less and a modern unit is not compatible. The gearing ratio of the wheels in the train would need to be changed for a modern unit to be of use.

Repairing Your Own Clocks by Mervyn Passmore