Experimental Mechanics / A Course of Lectures Delivered at the Royal College of Science for Ireland

by Robert S. Ball

THE ESCAPEMENT.

672. Practical skill as well as some theoretical investigation has been expended upon that part of a clock which is called the escapement, the excellence of which is essential to the correct performance of a timepiece. The pendulum must have its motion sustained by receiving an impulse at every vibration: at the same time it is desirable that the vibration should be hampered as little as possible by mechanical connection. The isochronism on which the time-keeping depends is in strictness only a characteristic of oscillations performed with a total freedom from constraint of every description; hence we must endeavour to approximate the clock pendulum as nearly as possible to one which is swinging quite freely. To effect this, and at the same time to maintain the arc of vibration tolerably constant, is the property of a good escapement.

Fig. 99.

673. A common form of escapement is shown in [Fig. 99]. The arrangement is no doubt different from that actually found in a clock; but I have constructed the machine in this way in order to show clearly the action of the different parts. g is called the escapement-wheel: it is surrounded by thirty teeth, and turns round once when the pendulum has performed sixty vibrations,—that is, once a minute. i represents the escapement; it vibrates about an axis and carries a fork at k which projects behind, and the rod of the pendulum hangs between its prongs. The pendulum is itself suspended from a point o. At n, h are a pair of polished surfaces called the pallets: these fulfil a very important function.

674. The escapement-wheel is constantly urged to turn round by the action of the weight and train of wheels, of which we shall speak presently; but the action of the pallets regulates the rate at which the wheel can revolve. When a tooth of the wheel falls upon the pallet n, the latter is gently pressed away: this pressure is transmitted by the fork to the pendulum; as n moves away from the wheel, the other pallet h approaches the wheel; and by the time n has receded so far that the tooth slips from it, h has advanced sufficiently far to catch the tooth which immediately drops upon h. In fact, the moment the tooth is free from n, the wheel begins to revolve in consequence of the driving weight; but it is quickly stopped by another tooth falling on h: and the noise of this collision is the well known tick of the clock. The pendulum is still swinging to the left when the tooth falls on h. The pressure of the tooth then tends to push h outwards, but the inertia of the pendulum in forcing h inwards is at first sufficient to overcome the outward pressure arising from the wheel; the consequence is that, after the tooth has dropped, the escapement-wheel moves back a little, or “recoils,” as it is called. If you look at any ordinary clock, which has a second-hand, you will notice that after each second is completed the hand recoils before starting for the next second. The reason of this is, that the second-hand is turned directly by the escapement-wheel, and that the inertia of the pendulum causes the escapement-wheel to recoil. But the constant pressure of the tooth soon overcomes the inertia of the pendulum, and h is gradually pushed out until the tooth is able to “escape”; the moment it does so the wheel begins to turn round, but is quickly brought up by another tooth falling on n, which has moved sufficiently inwards.

The process we have just described then recurs over again. Each tooth escapes at each pallet, and the escapements take place once a second; hence the escapement-wheel with thirty teeth will turn round once in a minute.

675. When the tooth is pushing n, the pendulum is being urged to the left; the instant this tooth escapes, another tooth falls on h, and the pendulum, ere it has accomplished its swing to the left, has a force exerted upon it to bring it to the right. When this force and gravity combined have stopped the pendulum, and caused it to move to the right, the tooth soon escapes at h, and another tooth falls on n, then retarding the pendulum. Hence, except during the very minute portion of time that the wheel turns after one escapement, and before the next tick, the pendulum is never free; it is urged forwards when its velocity is great, but before it comes to the end of its vibration it is urged backwards; this escapement does not therefore possess the characteristics which we pointed out ([Art. 672]) as necessary for a really good instrument. But for ordinary purposes of time-keeping, the recoil escapement works sufficiently well, as the force which acts upon the pendulum is in reality extremely small. For the refined applications of the astronomical clock, the performance of a recoil escapement is inadequate.

The obvious defect in the recoil is that the pendulum is retarded during a portion of its vibration; the impulse forward is of course necessary, but the retarding force is useless and injurious.

676. The “dead-beat” escapement was devised by the celebrated clockmaker Graham, in order to avoid this difficulty. If you observe the second-hand of a clock, controlled by this escapement, you will understand why it is called the dead beat: there is no recoil; the second-hand moves quickly over each second, and remains there fixed until it starts for the next second.

The wheel and escapement by which this effect is produced is shown in [Fig. 100]. a and b are the pallets, by the action of the teeth on which the motion is given to the crutch, which turns about the centre o; from the axis through this centre the fork descends, so that as the crutch is made to vibrate to and fro by the wheel, the fork is also made to vibrate, and thus sustain the motion of the pendulum. The essential feature in which the dead-beat escapement differs from the recoil escapement is that when the tooth escapes from the pallet a, the wheel turns: but the tooth which in the recoil escapement would have fallen on the other pallet, now falls on a surface d, and not on the pallet b. d is part of a circle with its centre at o, the centre of motion; consequently, the tooth remains almost entirely inert so long as it remains on the circular arc d.

677. There is thus no recoil, and the pendulum is allowed to reach the extremity of its swing to the right unretarded; but when the pendulum is returning, the crutch moves until the tooth passes from the circular arc d on to the pallet b: instantly the tooth slides down the pallet, giving the crutch an impulse, and escaping when the point has traversed b. The next tooth that comes into action falls upon the circular arc c, of which the centre is also at o; this tooth likewise remains at rest until the pendulum has finished its swing, and has commenced its return; then the tooth slides down a, and the process recommences as before.

Fig. 100.

678. The operations are so timed that the pendulum receives its impulse (which takes place when a tooth slides down a pallet) precisely when the oscillation is at the point of greatest velocity; the pendulum is then unacted upon till it reaches a similar position in the next vibration. This impulse at the middle of the swing does not affect the time of vibration.

679. There is still a small frictional force acting to retard the pendulum. This arises from the pressure of the teeth upon the circular arcs, for there is a certain amount of friction, no matter how carefully the surfaces may be polished. It is not however found practically to be a source of appreciable irregularity.

In a clock furnished with a dead-beat escapement and a mercurial pendulum, we have a superb time-keeper.