train of less velocity will do, and that sometimes amounts to a savin<? of one wheel in the train, and a good deal of friction; and the blow on both pallets being downwards, instead of one up and the other down, the action is more steady; all which things are of more consequence in the heavy and rough work of a turret clock than in an astronomical one. The details of the construction are given in the Rudimentary Treatise. It has been found expedient to make the dead faces not quite dead, but with a very slight recoil, which rather tends to check the variations of arc, and also the general disposition to lose time if the arc is increased; when so made the escapement is generally called "half-dead."
Fig. 6.—Pin-Wheel Escapement.
Passing by the various other modifications of the dead escapement which have been suggested and tried with little or no success, we proceed to describe one of an entirely different form, which was patented in 1851 by Mr C. Macdowall, though it appeared afterwards that one very similar had been tried before, but failed from the proportions being badly arranged. It is represented in fig. 7. The scape-wheel is only a small disc with a single pin in it, made of ruby, parallel and very near to the arbor. The disc turns half round at every beat of the pendulum, and the pin gives the impulse on the vertical faces of the pallets, and the dead friction takes place on the horizontal faces. Its advantages are that the greatest part of the impulse is given directly across the line of centres, and consequently with very little friction; and therefore also, the friction on the dead faces is less than usual, and scarcely any oil is required; moreover, it is very easy to make. But there must be two more wheels in the train, consuming a good deal of the force of the clock-weight by their friction, which rather more than makes up for the friction saved in the escapement. It was applied successfully to watches, but the expense of the additional wheels prevented their adoption. In order to make the angle of escape not more than 1º, the distance of the pin from the centre of the disc must not be more than 160th of the distance of centres of the disc and pallets.
With the view of getting rid of one of these extra wheels in the train, and that part of the impulse which is least effective and most oblique, Mr Denison shortly afterwards invented the three-legged dead escapement; which, though afterwards superseded by his three-legged gravity escapement, is still worth notice on account of the exceedingly small force which it requires, thereby giving a practical proof of the large proportion of the force which is wasted in friction in all the other impulse escapements.
Fig. 7. Macdowall's Escapement.
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In fig. 8, the three long teeth of the scape-wheel are only used for locking on the dead pallets D and E, which are set on the front of the pallet plate; A and B are impulse pallets, being hard bits of steel or jewels set in the pallet plate, and they are acted upon by the three sharp-edged pins which are set in the scape-wheel and point backwards. As soon as the pendulum moves a little further to the left than is here shown, the long tooth will slip past the dead pallet or stop D, and the pin at B will run after and catch the corner of that impulse pallet and drive it until the wheel has turned through 60º, and then it will escape; and by that time the uppermost tooth will arrive at the stop E, and will slide along it as in the common dead escapement, but with a pressure as much less than that which gives the impulse as the points of the teeth are farther from the centre of the wheel than the impulse pins are. But the impulse is here given with so little friction, that even where the points of the teeth were made identical with the pins, the clock-weight required to keep the same pendulum with the same train (a common turret-clock movement), swinging to 2º, was only one-fifth of what had been required with the pin-wheel escapement; and the scape- wheel which kept the 6 cwt. pendulum of the Westminster clock going for half-a-year, until superseded by the gravity escapement, weighed only a sixth of an ounce. It appears also that it would be possible so to adjust the recoil of the half-dead pallets that the time would not be affected by any small variation of the force and the arc; since it was found that, when a certain amount of recoil was given the clock gamed instead of losing, under an increase of arc due to an increase of clock-weight. And if the force were kept constant by a train remontoire, such as will be described hereafter, there would in fact be nothing capable of altering the arc or the time. But on account of the small depth of intersection of the circles of the pins and the pallets, on which its action depends, this escapement requires very careful adjustment of the pallets, except where they are on a large scale; and considering the superior qualities of the corresponding gravity escapement, it is not likely to be used, except perhaps in clocks required to go a long time, in which economy of force is a matter of consequence. The pallets should be connected with the pendulum by a spring fork (which indeed is advisable in the common dead escapement with a heavy pendulum, especially the pin-wheel escapement), to prevent the risk of their driving backwards against the scape-wheel when it is not in motion, as it will not clear itself. The distance of the centres should be not less than 25 times the radius of the circle of the edges of the impulse pins.
Fig. 8.—Denison's Three-Legged Escapement.
Detached Escapements.
In all the escapements hitherto described the pallets are never out of moving contact with the scape-wheel, and there have been several contrivances for keeping them detached except during the impulse and at the moment of passing a click which is to release the wheel to give the impulse. This is an imitation of the chronometer escapement in watches which is sometimes called the "detached." There are only two of such contrivances which appear worth special notice. One was proposed by Sir G. Airy in vol. ii. of the Cambridge Transactions, but not executed (so far as we know) till a few years ago in the standard sidereal clock at Greenwich, which is reported to go extremely well. Suppose a dead escapement consisting of a single pallet only, say the right hand one of the pin-wheel escapement (fig. 6), for the Greenwich clock has a pin escapement, and that the wheel is locked generally by a spring detent hooking into any one of its teeth, and capable of being lifted or pushed aside by the pendulum, i.e., by a pin somewhere on the single pallet as it passes to the right, but also capable of being passed without being lifted as the pendulum goes to the left. We shall see afterwards how this is done, in the article Watches. Then as the pendulum goes .to the right, it first lifts the detent at about 1º before zero, and then a tooth or a pin drops on to the pallet and gives the impulse, exactly as in the dead pin-wheel escapement, and with exactly the same amount of friction, substituting only for the dead friction the resistance and friction of passing the detent one way and lifting it the other.
Fig. 9.—
A different escapement on the same principle but involving less friction was adopted by Sir E. Beckett in a clock described in the later editions of his book as having gone for above ten years very satisfactorily, except that, like all direct impulse escapements, including Sir G. Airy's, it must vary with the force of the clock train, due to different states of the oil. The scape-wheel (fig. 9) is five-legged, and has five sharp-edged pins which give the impulse to the hard steel pallet P whenever it passes to the right, provided the wheel is then free to move. It is stopped by the detent DEF, which turns on a pivot F, not in the pendulum crutch, as it looks in the drawing, but on the clock-frame. When the pendulum going to the right arrives at the position here drawn, the click CE on the crutch pushes the detent aside and so unlocks the wheel, which then gives the impulse, moving through 72º until another tooth arrives at the detent and is stopped, the click having then got far beyond it. When the pendulum returns the click lightly trips over the top of the detent. Here there is practically no friction in giving the impulse, as it is directly across the line of centres, as in the three-legged dead escapement, and the friction of passing
