The gravity escapement requires a heavier weight or force to operate the train than a dead beat escapement, because it must be strong enough to be sure of lifting the pallets quickly and firmly, and also because the escape wheel having but six teeth necessitates the use of another wheel and pinion between the escape and center and consequently the train is geared back more than it would be for a dead beat escapement, with the seconds hand mounted on the escape wheel arbor. But with this form of escapement the superfluous force does not work the pendulum and it does no harm if the train is good enough not to waste power in getting over rough places left in cutting the teeth of the wheels or any jamming from those which have unequal widths or spaces. For this reason a high numbered train is better than a low numbered one, as these defects are greater on the teeth of a low numbered train and any defect in such cases will show itself.
In the gravity escapement the escape wheel must have a little run at the pallets before it begins to lift them and in order to do this the banking pins, M, M¹ for the pallet arms to rest on, should hold them just clear of the lifting pins or leaves of the escape wheel. The escape wheel should be as light as possible, for every blow heard in the machine means a loss of power and wear of parts. Of course, in an escapement a sudden stop is expected, but the light wheel will reduce it to a minimum if the fan is large enough. Particular attention should therefore be given to the length of this fan and if the stop of the escape wheel seems too abrupt, the fan should be lengthened.
Fig. 51.
Figs. [50] and [51] show the same escapement with a four-legged wheel instead of the double three-legged. In this case, where there is but one wheel, the pallets must of necessity work on opposite sides of the wheel and hence they are not planted in the same plane with each other, but are placed as close to each side of the wheel as is practicable.
To lay out this escapement, draw the circle of the escape wheel as before, make your line of centers and mark off on the circle 67½° on each side of the line of centers and draw radii to these points, which will indicate the approximate position of the stops. Tangents to these radii, meeting above the wheel on the line of centers will give the theoretical point of the suspension. One set of the lifting pins is planted on radii to the acting faces of the teeth of the escape wheel. The opposite set, on the other side of the wheel, is placed midway between the first set. This secures the lifting at the line of centers. The wheel turns 45° at each beat and its arbor likewise carries a fly.
In case the locking is not secure, the stops may be shifted a little up or down, care being taken to keep them 135° apart. In this way a draw may be given to the locking of the scape wheel arms similar to the draw of the pallets in a detached lever escapement and thus any desired resistance to unlocking may be secured. The stops in either escapement are generally made of steel and it is of the utmost importance that the arms of the escape wheel should leave them without imparting the least suspension of an impulse. Therefore, the stops and the ends of the arms should be cut away (backed off) to rather a sharp angle to insure clearance when the arms are leaving the stops. It is also of equal importance that the legs of the wheels should fall on the stops dead true. The fit of each of the legs should be examined on both stops with a powerful eye-glass, so that they should be correct and also see that when the unlocking takes place the wheel is absolutely free to turn.