Gearless Motion Work 

By Michael Adler

The step down reduction of twelve to one between minute and hour hands is known as the motion work.   This is usually accomplished by using a layshaft from the minute hand shaft with 4:1 and then 3:1 gear pairs, returning to the hour hand which lies coaxially and loose on the minute shaft.   The result also is that the minute hand and the hour hand both rotate clockwise. 

An astonishingly simple motion work was invented by the American clockmaker called Aaron Dodd Crane.   It employs a daisy wheel, but no layshaft and no gears.   This is one of those mechanisms that defy understanding unless they are built and are studied in operation.     It is similar to the harmonic motion drive, which is to be found in many reduction gears from electric motors.

It operates on the principle, like the harmonic drive, of a wave generator.  The wave itself is created by a small crankshaft or shaft offset from the minute arbor.  A daisy wheel with eleven petals is free to move on the shaft, but is prevented from rotating.  The result is that as the crankshaft rotates, it causes the daisy wheel to generate a wave.    The leaves of the daisy wheel mesh with pins on a second wheel to which the hour hand is fixed.  The principle is like that of epicyclic gearing with teeth of unorthodox shape.  The pin wheel is in fact a type of lantern pinion engaging with an internal gear wheel.   Hmm.

Lets start again.  We have our daisy wheel moving on a crankshaft but prevented from rotating.  As the crankshaft rotates, the daisy wheel will generate a wave at its periphery.  A pin wheel in mesh with the leaves of the daisy wheel will be caused to advance one twelfth of a circle after the crankshaft has rotated through 360 degrees.  This gives a twelve to one reduction between the two.  The hour hand is of course attached to the pin  wheel.

The daisy wheel, which consists of eleven 1" triangular plates can be seen in the photograph.  The crankshaft out of line with the main shaft can also be seen.  Now observe the pin wheel which is a structure to which four long threaded pins are attached.  The hour hand is fixed to it.

Finally - how is the daisy wheel prevented from rotating?  Observe the vertical stalk or rod.  This slides between two pins (not shown) on the front clock plate.

No wiser?  Then like me you will have to build it to understand it.  Then have a go at describing it yourself.

There is a full description in the International Meccanoman number 41 January 2004. 

A daisy wheel is constructed from a circle of eleven 1" triangular plates (part 77)  which are bolted to each other.  A bush wheel is fixed to the plates by two fish plates and acts as a hub.  A 5" rod acts as a vertical stalk and is fixed by a rod and strip connector to one of the free holes of the bush wheel on the same side as the boss, by a 3/4" bolt, but separated from the bush wheel by a collar and three washers. 

A cage is built from two 3 1/2" perforated strips which are fixed at 90 degrees to each other on the back of a bush wheel.   Four long threaded pins are fixed in four threaded bosses (part 64) and these are then bolted to the end of each of the strips on the same side as the boss of the bush wheel.  The hour hand, a 3 1/2" narrow strip, is lock nutted to one of the 3 1/2" strips using a 3/4" bolt, but stood off from the strip by 1/2".

A crankshaft is built using two pairs of double arm cranks (part 62b), the pairs being bolted back to back to each other by their elongated holes to provide a displacement of 4mm.  

The minute hand is a 5 1/2" narrow strip bolted to a crank (part 62).

Assembly:  The input shaft (at the rear) is 1 1/2" rod and is fixed in one of the bosses of the double arm crank.  A second 1 1/2" rod (the crank-shaft) is fixed in the other boss.   Place a standard washer on the shaft and then slide the daisy wheel on as well, boss to the rear.   The daisy wheel is free to rotate on the crank-shaft.  Add a collar as spacer, and then fix the second pair of double arm cranks to the rod and fix the 1 1/2"output shaft to it.  Make sure that input and output shafts are in line with each other.

Now slide the cage onto the output shaft with the boss of the bush wheel to the rear, and engage the threaded pins with the daisy wheel.   It is free to rotate on the output shaft.  Finally, fix minute hand to the output shaft.

Action.  The input shaft is rotated clockwise.  This causes the crankshaft to rotate and move eccentrically.   As the daisy wheel is prevented from rotating by the engaged stalk (between two threaded pins on the clock frame), it simply oscillates, and does so inside the cage, which it causes to rotate clockwise.    Put another way, the eccentrically placed daisy wheel creates a wave, which causes the cage to move around it, advancing the daisy wheel by one twelfth of a turn for every revolution of the crankshaft.    

The minute hand rotates with the input shaft, but the hour hand fixed to the cage rotates with an effective reduction of 12:1.  The action is smooth and without binding.  

References

  1. My Own Right Time - Philip Woodward - Oxford University Press -1995
  2. John Wilding - British Horological Journal - July 2003