Fig. 93.—Graham’s Dead Beat.
Fig. 94.—Gravity Escapement (Mudge).
We will now describe the more elaborate forms of escapement, and we will take first the gravity escapement, as it is called. The principle of its action consists in there being a small impulse given to the pendulum at each oscillation, by means of two small rods hanging, one on each side of it, and tending by their own weight to force the pendulum into a vertical position; these rods are alternately pushed outwards by the escapement before the pendulum in its swing arrives at them, and then they are allowed to press against it on its return towards the vertical, so that the pendulum has a constant force acting on it at each oscillation, unconnected with the clock movement. This is carried out in the escapement invented by Mudge, as shown in Fig. [94]. The pendulum rod is supposed to be hanging just in front of the rods hanging from the pivots Y1 Y2, and on swinging it presses against the pins at the lower ends of the rods and so lifts the pallets S1 S2 out of the teeth of the escape wheel. In the position shown the pendulum is moving to the right, having been gently urged from the left by the weight of the pallet rod Y2 S2, and the pallet S1 has been lifted outwards by the tooth acting on its inclined surface. On the pendulum rod reaching the pin the rod is moved outwards and the end of the pallet S1 pushed out of the tooth when the wheel moves on, at the same time pushing outwards the pallet S2. As the pendulum returns towards the left again the rod Y1 follows it, giving it a gentle impulse by its own weight until it returns nearly to the vertical or to the corresponding position in which Y2 S2 is shown. On the pendulum swinging on and releasing T2 the pallet S1 is again pushed outwards by the inclined plane to the position shown in the diagram. In this escapement there was danger of the pallets being thrown too violently outwards so that the teeth were not caught by the flat surfaces at the ends, and Mr. Bloxam improved on it by letting the pallets be thrown outwards by a small wheel on the axis of the escape wheel so that the action was less rapid; he accomplished this in the following manner. On the end of the axis of the last wheel are a number of arms A A, Fig. [95], say nine, about 1½ inches long, which are prevented from revolving by studs L1 L2 on the inside of each hanging rod P1 P2; then, as each rod is pushed outwards by the pendulum, an arm escapes from a stud and the clock goes on one second. Each rod is pushed outwards by the clock almost sufficiently far for the arm to escape, but not quite, so that the pendulum just releases the arm at the end of its swing in the same manner as in Mudge’s escapement, Fig. [94]; but instead of the teeth of the escape wheel pushing the rods outwards there is the small wheel T1 T2, having the same number of teeth as there are arms on the axis and close to them, and the projecting pieces H1 H2 at right angles to each swinging rod rest against the teeth of this wheel, one resting against the teeth at the top and the other at the bottom, so that they catch against the teeth after the manner of a ratchet, and the rods are pushed outwards by this wheel as it revolves. The arms and ratchet wheel are so set that, during the motion of an arm A, to a stud, a tooth of the ratchet wheel is pushing outwards the rod carrying that stud. The action is as follows:—The pendulum having just swung up to a rod and released the arm pressing against its stud, the arms and ratchet wheel revolve, and the tooth of the ratchet wheel, which had been pressing outwards the swinging rod, passes on free of the projecting piece, which can now move backwards to the next tooth; so the rod, being no longer supported, presses against the pendulum rod on its return oscillation. The arms and ratchet wheel revolve until an arm on the opposite side comes in contact with the stud on the other rod, and in revolving the ratchet wheel throws outwards this rod just so far that the arm is not released. The pendulum is assisted by the weight of the first rod to the vertical position, when the projecting piece of the rod comes in contact with the next tooth of the ratchet wheel where it rests until the oscillation is completed, and the second arm is released. It is then forced outwards, and the next arm on that side presses against the stud, when a repetition of the foregoing takes place. In this way the clock is kept going without any direct action of the clock train on the pendulum.
Fig. 95.—Gravity Escapement (Bloxam).
Another very beautiful escapement is that devised by the Astronomer-Royal and carried out in the clock erected in 1871 at Greenwich.[[9]] In this case the pendulum is free except during a portion of every alternate second, when it releases the escapement and receives an impulse, so that there is a tick only at every other second.
Fig. 96.—Greenwich Clock Escapement.