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THE CLOCK MOVEMENT

Before stripping your first clock you should know the preferred name of some of the components common to most mechanical clocks, and some of the definitions, so that the instructions given are better understood. Every activity has its own 'language', and communication is easier if that language is understood (or at least enough to get by and build on).

Even the word 'clock' has different meanings for different people. For some it is the case, dial, hands and all the things that can be seen from the outside; to others it is what can't be seen, that is, the 'workings'. In horology, once the 'workings' have been separated from the case, dial and hands it is known as 'the movement'.

Fig 6 A striking clock movement.

The movement comprises a number of components, each of which will come under a certain heading. For example, every movement will have some sort of framework to hold everything together. There will be a driving force to keep the clock going, usually a weight or spring, and a series of wheels of the right ratio to make the hands go around at the right rate and for the right duration. There will be some means for winding a spring or weight without having to turn all the wheels backwards, some means of adjusting the hands independently and, very importantly, some means of releasing the power from the mainspring or weight under control. These mechanisms will now be looked at more closely.

CLOCK TRAINS

To start with then, our movement has two plates: a bottom plate sometimes called a pillar plate or a dial plate, and a top plate sometimes called the back plate. Between the plates there is a series of wheels called a train which usually, though not always, gives a step up in gear ratio from where the drive originates. In an alarm clock there will be two trains, a time train and an alarm train. In a striking clock there will also be two trains, a time train and a striking train. Similarly a chiming clock will have three trains, a time, strike and chime train.

The time train in a simple thirty-hour clock (the name given to a clock that is wound daily) usually consists of a great wheel, which is the first wheel in the clock; a centre wheel, which is the second wheel in the clock; a third wheel and a fourth wheel. The latter may have a long pivot to carry a seconds hand.

The Escapement

Next is the escapement, which allows the power from the mainspring or weight to escape slowly under control. In a pendulum clock there are two main parts to the escapement, an escape wheel and a pallet. In a clock with a balance there are three main parts, an escape wheel, a pallet and a balance.

Sometimes there is another wheel between the great wheel and the centre wheel called the intermediate wheel; this converts the clock to an eight-day clock. When an intermediate wheel is used, the wheel driven by the centre wheel is still often called the third wheel, even though it is actually the fourth wheel in the train. The only time that this may be a problem is when ordering such a new wheel from a material house. Describing its position in the train should overcome any ambiguity.

The great wheel always has some arrangement to hold the mainspring in a wound state; usually this incorporates a ratchet wheel, click and click spring. The centre wheel will have a simple clutch arrangement of some kind to allow the hands to be turned independently of the rest of the train.

The prime function of the escape wheel is to give impulse to a pendulum or balance to keep it swinging. How escapements work will be looked at later.

A balance will vibrate according to the natural laws of springs, and a pendulum will vibrate according to the law of gravity. It should be appreciated that all time-measuring devices involve an 'event' which is repeated in the same space of time over and over again and is counted up. This applies to the earth as a clock, a water clock, a sundial, a pendulum clock, a clock working with a balance or even a quartz clock.

Fig 7 The clutch arrangement on a centre wheel to allow handsetting.

Motion Work

On the outside of the front plate will be found another series of wheels, this time called 'motion work'. Motion work makes use of a cannon pinion, a minute wheel and pinion, and an hour wheel. The motion work gives the 12:1 reduction to drive the hour wheel. A 24-hour dial will have a 24:1 reduction.

Earlier, when talking about the time train, reference was made to a wheel. The word 'wheel' is often used to mean the whole wheel which is usually made of two separate parts fastened together, one part a wheel, the other a pinion.

In horology a wheel is a driver: it is made of brass and has teeth. A pinion is driven, and is made of steel and has leaves. The teeth of the wheel drive the leaves of the pinion. There are exceptions to this, for example the cannon pinion in the motion work; however, it is true to say that generally, pinions make bad drivers.

Endshake and Sideshake

A wheel must have up and down movement between the plates, and this movement is called endshake. There must also be clearance between the sides of the pivots and the plates, and the movement of the pivot due to this clearance is called sideshake. Endshake must always be a minimum of 3/100mm in any wheel or staff; in a long-case clock, it may be ½mm or more, though it need not be as much as this.

Safe sideshake is indicated by a 10° lean in all directions when a wheel is placed in a plate and allowed to lean naturally.

Fig 8 A typical wheel and pinion in a plate.

SIMPLE TRAIN CALCULATIONS

For most repair work, the only calculations that need to be carried out are those for sorting out invoices, mark-ups and profit margins. However, occasionally you may need to calculate the number of vibrations per hour or per minute or even per second that a clock has to make in order to keep perfect time, or you might have to calculate the correct ratio of teeth to leaves of a missing wheel in a clock so that you can arrive at the most likely combination for a new wheel and pinion to be cut.

Calculating Vibrations

When calculating the number of vibrations a pendulum or a balance needs to make per hour, per minute or per second for the clock to keep perfect time, first we need to know the number of teeth used in an hour on one of the time-train wheels. The calculation then becomes the product of the drivers multiplied by two, divided by the product of the driven to give the number of vibrations per hour. If the number of vibrations per minute or second is required, we simply divide by sixty once or twice, to get minutes or seconds.

As most clocks - though not all - have a centre wheel, we take that as a starting point because we know that every tooth will be used once in an hour. (The centre wheel rotates once in an hour, therefore every tooth will be used just once.) Our formula for calculating the number of vibrations per hour looks like this:

The times two in the top line is because the escape wheel advances half a tooth at a time, and the centre wheel pinion doesn't enter the calculation because it comes before the centre wheel, as does the great wheel.

As a practical example, a train count reveals the following numbers:

I recommend that the formula you are going to use is always written down first.

If vibration per minute is wanted, just put '× 60' in the bottom line, or divide the answer by 60. For vibration per second, just put '× 60 × 60' in the bottom line or divide your vib/hour calculation by 3,600 (60 × 60). Should you need the time of one vibration, simply divide your vibrations per second into the number 'one'. In other words, the time of vibration is the reciprocal of the number of vibrations per second.

Very occasionally a clock comes in for repair with a missing wheel, and a calculation has to be made to determine the likely number of teeth in the missing wheel and the likely number of leaves in the pinion. Provided you know the number of vibrations per second, minute or hour that the pendulum or balance makes for the clock to keep time, then it is relatively straightforward. For example, if the missing wheel and pinion was from a long-case clock with a second pendulum, you would know that the pendulum vibrates sixty times a minute. If you were unsure, you could even count the number of vibrations in a minute with a stopwatch. It would bring you so close to a convenient ratio that the correct number of teeth and leaves would be determined.

The following is the train count of a long-case clock with a missing third wheel. Calculate the likely number of teeth in the wheel and leaves in the...