The Clock Jobber's Handybook

By Paul N. Hasluck

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The

CLOCK JOBBER'S HANDYBOOK.

PENDULUMS THE CONTROLLERS.

CHAPTER III

ESCAPEMENTS COMMONLY USED.

A piece of thin sheet metal is the best material to draw such a diagram upon; sheet zinc is convenient First drill a hole to represent the centre of the escape-wheel, and enlarge this to allow the axis of the escape-wheel to go through, and fit when the wheel lies in contact with the plate. Draw the various lines, by means of a scriber, so as to get the position of a pallet-centre, and then gauge the position of the actual pivot holes in the clock. Of course the hole must be drilled in the metal plate to correspond with the pivot hole in the clock. The hole is enlarged to fit the pallet axis, and the escape-wheel and pallets may be tried on the plate. Having due regard to any peculiarities of the especial escapement being examined, proceed to draw the various lines as indicated in the accompanying diagram, and any error in the form of the pallets will be shown by comparison.

It may possibly appear somewhat erratic to suggest that such a method may correct errors in escapements produced by professedly skilled clockmakers. If workmen really worked on correct principles, no doubt the suggestion would be erratic. In practice, however, many clocks are made by men who work alone, without any knowledge of theoretical principles, and who idolatrously worship the "rule-of-thumb." These workmen, by practice, attain considerable skill, and are able to produce good-looking work at a low price. They seldom have any opportunity of seeing the practical result of their labour, and hence have no knowledge of any defects that may exist.

The shape of the pallets may be made to suit the fancy, so long as the faces against which the escape-wheel teeth impinge are kept to the form indicated. In the drawing, the tooth on the right is shown just free of the pallet. The arrow indicates the direction that the wheel travels, the tooth on the left impinging on the pallet forces it upwards, the tooth sliding along its face till it reaches the end, and the pallet on the left will then be in the position to receive the tooth shown inside the circle. Practically the pendulum continues to swing some distance after the tooth has escaped, and the non-acting sides of the pallets must be so formed that they are quite clear of the backs of the teeth. These parts of the pallets are shown drawn from the centre B, and will therefore be correct.

The diagram, Fig. 9, is lettered in precisely the same manner, the proportions being, however, different. The diagram of the escape-wheel is drawn, and the radial lines D C are drawn through the points of two teeth, as shown. The lines E and N are drawn and their intersection at B gives centre of the pallets. The faces of the pallets are determined in much the same manner as previously described; the point 5 being five- sevenths of a semi- diameter from the point of the tooth. These illustrations are merely intended to show the extended application of the principles that have been explained.

The accompanying drawing, Fig. 9, will show the method of designing a pair of pallets to suit a particular escape-wheel. In the first place determine the distance apart of the centre of the wheel A, and the centre of the pallets B. If you are only replacing worn-out pallets, the holes in the plate will guide you. If you are making a new escapement entirely, the following method is useful: Draw radial lines from the centre of the escape-wheel A, through the points of the teeth embraced by the pallets. These lines are marked A C and A D in the drawing. At the point where the circumference of the wheel bisects these lines, erect perpendiculars shown by E B and F B; where they bisect B is the centre of the pallets. From this centre draw a circle through the points of the teeth embraced by the pallets. This circle is shown dotted in the illustration. A continuation of the line E B to H cuts this circle in half. Divide the half into seven equal parts marked 1234567, and from the point 5 draw a line through the point of the tooth. (See diagram). This gives the impulse face of the pallet a. A line drawn through the points of the two teeth of the escape-wheel, shown by K L, gives the face of the pallet b. The circle drawn inside of the one through the teeth is precisely midway between the points of the teeth. This marks the length of the pallet b. The amount of the face necessarily in contact with the escape-wheel between each escape is contained within the angle N B F, and amounts usually to about five degrees. A like amount is set off on the opposite side, M B E. This explanation will make the diagram clear.

Figure 9.

The American clock pallets shown at Fig. 10, are shaped by this method, though, perhaps at first sight, they hardly look so.

Fig. 10.—Pallets of American Clock made of Bent Steel.

Dead-beat escapements are an improvement on the recoil. Regulators and the better class of household clocks have dead-beat escapements. George Graham invented this form of escapement about the beginning of the eighteenth century. The term dead-beat is in contradistinction to recoil. The faces of the pallets in a dead-beat escapement are concentric with the centre of oscillation, so that during the supplementary swing of the pendulum the train remains perfectly stationary. The impulse is given to the pendulum through another face of the pallet which is inclined to the axis of oscillation, the same as a recoil escapement pallet.

Dead-beat clocks, having a seconds hand, and watches also, remain perfectly dead during the greater portion of time. The seconds hand jumps from one division to the next, and remains. With recoil escapements, the seconds hand will be observed to jump from one division to the next, but instead of remaining dead it goes backwards till the pendulum, or balance, has completed its supplementary vibration, then the hand goes forward gradually till the tooth escapes, then it jumps, and then the retrograde motion is repeated.

Reid says of the dead-beat escapement: " On an additional motive force being put to it, we find that the arc of vibration is considerably increased, and in consequence of this the clock goes very slow. There are two causes which produce this : the one is, the greater pressure by the escape-wheel teeth on the circular part of the pallets during the time of rest; the other is, the increase of the arc of vibration. With regard to the recoil, it was observed that an additional force would make the clock go fast, and with a dead-beat the same cause produces the opposite effect." These facts were pointed out in the earlier part of this chapter.

When the same cause produces precisely opposite effects on the two forms of escapement, the means of adjustment are obvious. It is necessary to modify the two forms, and this is now done successfully. Pallets should be so formed that they have but very little recoil, and then a variation in the motive force or in the arc of vibration of the pendulum will produce hardly any appreciable variation in the time-keeping.

Reid says that clockmakers in general have an idea that in an escapement the pallets ought to take in seven, nine, or eleven teeth, thinking that an even number could not answer. This is by no means essential. The distance from the centre of the pallets to the centre of the escape-wheel also is not determined by any rule. The nearer the centres the less will be the number of teeth that are required to be taken in by the pallets. When the arms of the pallets are long, the influence of the motive power on the pendulum will be greater than when they are short. The depth that the pallets engage in the wheel teeth will determine the angular motion of the pendulum necessary for the teeth to escape.

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Hasluck, Paul N. The Clock Jobber’s Handybook. London: Crosby Lockwood and Son, 1889.

This and the following pages are excerpts from the book.