MANTEL CLOCK with BRIGGS' ESCAPEMENT
designed, photographed and described by  John Stark

This model was one of two clocks that I exhibited at the April 2003 NZFMM Convention in Hawera. My interest in clocks as subjects for Meccano modelling stems from the fact that I prefer making models that have lots of moving parts -especially gears. Clocks are ideal subjects for Meccano model building; well-designed and made they can run unattended for long periods and keep good time too.

There are three critical (and interrelated) factors to consider when designing a mechanical clock:-
(1) The number of teeth on the escape wheel,
(2) the length of the pendulum, and
(3) the reduction ratio between the escape wheel shaft and that of the minute hand (which must rotate once per hour).

Pat Briggs wrote an article entitled "Basic Clockwork" that can be obtained from Michael Adler's website http://users.actcom.co.il/meccano/  Pat provided four formulae to simplify the calculations required.

1) R = 30 * B / E
2) L = 144,000 / B²
3) L = (3600 / E * R)² * 10
4) N = L * M / 22.5

Where:
R = reduction ratio between the escape wheel rod and that of the minute hand.
B = beats of the pendulum per minute.
L = theoretical length of the pendulum in inches.
E = number of teeth on the escape wheel.
N = number of turns of the rating nut to adjust for one minute per day.
M = minutes lost or gained per day.

The trap for inexperienced clock builders in using these equations is that the theoretical length of the pendulum (L) is always considerably less than the actual length required because allowance must be made for the suspension spring support at the top, the finite size of the bob, and for the screwed rod extension at the bottom for the rating nut. In fact, the practical length of the pendulum is likely to be closer to L + 3 or 4 inches.

Designing a clock using these formulae can be an iterative process, but often the first step is to choose the escape wheel. It also helps to know that the theoretical length of a so-called "one-second pendulum" is about 40" and it beats 60 times per minute (L = 40", B = 60). A half-second pendulum (B = 120) is one quarter the length (L = 10"), and a quarter-second pendulum would be only 2.5" long (B = 240/minute).

I wanted to design a clock for the NZFMM Easter 2003 15" cube competition. That immediately constrained the practical length of the pendulum to substantially less than 15". I also wanted to use a 1 _" sprocket (28 teeth) as the escape wheel. I determined that a convenient height for the clock was around 11_" leaving room for a pendulum approximately 9" long. That meant that the theoretical length (L) of the pendulum was likely to be 5 or 6 inches. So now that we know E (=28) and have a best guess at L (say 5) we can begin our calculations.

By rearrangement of equation (2) we can determine that B = 169.7 for a 5" pendulum. Now we can use equation (1) to determine R = 181.82. However, that's not a very convenient ratio to achieve with Meccano gearing, where multiples of, 2, 3, 4, and 5:1 are easiest. So, let's choose R = 180 and achieve this using a gear train of 4:1, 3:1, 3:1, and 5:1. We still have our 1_" sprocket escape wheel (E = 28) so can determine the theoretical length of the pendulum from equation (3). In this case L = 5.1", which translates into a practical length around 9". This will fit nicely in the available space.

The resulting clock is a mantel clock using an escapement originally designed by Pat Briggs (see Meccano Magazine July 1968 p390-392) and a sun-planet electric rewind system similar to that described by John Wilding (see Constructor Quarterly 33, September 1996, p4-11). Despite the electric motor, the clock is weight-driven with the motor and 4 AA cells in battery holders (plus a few extra 2 _" strips) providing the weight required. A mercury switch, which is wired in series with the motor and 6V battery pack, trips the rewind mechanism every few minutes.

Hopefully, the photographs (combined with your imagination) will reveal sufficient detail to enable construction of this clock. The following notes may assist.

1. Four 2" Pivot Rods provide low friction shafts for the anchor, escape wheel shaft and the next two shafts down the gear train. The escape wheel shaft carries a 28t Sprocket and a 15t Pinion. This Pinion meshes with a 60t Gear on another Pivot Rod that carries a 19t Pinion. Below this shaft another Pivot Rod carries a 57t Gear and a 19t Pinion. A standard 2_" axle in midline of the clock carries a 57t Gear and another 19t Pinion, which meshes with the 95t Gear on the minute hand shaft (3_" Axle).

2. The 95t Gear is loose on the minute-hand shaft but turns with it via a metal Spring-Clip friction clutch. This enables the clock hands to be set.

3. Escapements using standard Meccano Sprocket Wheels can be quite tricky to set up. The "anchor" escapement comprises a 1_" Corner Bracket attached to a Double-Arm Crank through its elongated hole. The boss of the Crank coincides with the vertical slotted hole in the centre of the Corner Bracket. Two _" Angle Brackets form the pallets.

4. The mercury switch must be wired in series with the motor and 6V battery pack. The end of the switch that the wires emerge from must face outwards from the clock.

5. I slid a 23c over the mercury switch (be careful not to break the glass envelope!) and mounted it between _" Bolts projecting from adjacent holes on a 6-hole Insulating Bush Wheel. The boss of the Bush Wheel is fixed to a short Threaded Pin, so that it can be rotated to adjust the arc of the rewind. _" Bolts in diametrically opposed holes in the Bush Wheel act as terminals for the mercury switch wires. One of these has a motor lead, and the other one of the battery leads attached. Make sure that the motor runs the correct way when the mercury switch is activated. Reverse the battery connections if not. The arc of the rewind will reduce as the batteries run down.

6. The ideal motor for the sun-planet rewind is internally geared. It must be powerful enough to pull its own weight (plus batteries and any other weight required) up around the 95t Gear and not fall backwards when the mercury switch turns the power off. I used a motor obtained by mail order from Bull Electrical, but they are not always available from that source these days. If a different motor is used, a pawl can be arranged to bear on the pinion on the motor output shaft to prevent roll back (Michael Adler, personal comment.).

7. The rewind should be adjusted (by rotating the insulated bush wheel) so that the motor works through an arc equidistant above (after the rewind has occurred) and below (when the rewind starts) the level of the minute hand shaft. The driving weight (comprising the motor etc.) is greatest when the motor shaft and the minute hand shaft are on the same level.

8. The pendulum comprises a 5" Axle joined to a 3_" Screwed Rod using P/N 64a. The Screwed Rod is passed through the threaded bore of an 8-hole Bush Wheel. A few Wheel Discs are added for extra weight. A hex-Nut below the Bush Wheel locks it in position when the clock has been adjusted to keep accurate time. The pendulum suspension spring is a 1" length of spring steel.

9. Once the clock is level, ensure that the tick is even by adjusting the position of the Angle Brackets on the anchor, the height of the 1_" Corner Bracket above the escape shaft (adjusted by loosening the Bolt (with locknut) in the slotted arm of the Double Arm Crank), and the relationship between the anchor and the crutch.

10. The effective length of the pendulum can be altered by moving the bob up and down to adjust the time-keeping. Set the clock to the correct time and let it run for a while. Record how many minutes (or hours!) fast or slow it is and use equation (4) to determine how many turns to raise (if slow) or lower (if fast) the pendulum bob.

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