CHAPTER VI

THE DAMPING SYSTEM OF THE SPERRY COMPASS

In the Sperry gyro-compass the damping system adopted is mechanically of a very different nature from that used in the early Anschütz, although the theoretical principle of action in both cases is the same. The Sperry method rules out the employment of air or other fluid in any shape or form as a means of generating, applying, or transmitting the damping force, the reason being that if air or other fluid is relied upon, the damping force--or so the Sperry Company holds--will not act in strict unison with the oscillations, but will invariably lag behind.

The details of the Sperry method are indicated in a diagrammatic manner in Fig. 19. As in the Anschütz compass, the spinning wheel revolves in a casing which, being provided with trunnions E F, takes the place of the inner horizontal supporting ring of our elementary gyroscope. Since no blowing action is required of the wheel in this compass the casing, in order to reduce the expenditure of power required to drive the wheel, is exhausted of air until a vacuum of not less than 26 in. is registered on a gauge which forms a permanent fixture on the casing. The exhaustion is effected by attaching a hand-operated vacuum pump to a nipple on the casing. The vacuum produced at one exhaustion remains effective for at least a month under proper treatment. That it is very well worth while exhausting the casing, if the general design of the compass permits it, is shown by the fact that in the 1910 Anschütz compass over 95 per cent. of the work done by the motor driving the spinning wheel was spent against windage and air friction.

The outer ring G (Fig. 19) within which the casing is carried is, as before, mounted on a vertical axis H J. A second outer ring K--or “phantom ring,” as it is called--surrounds the ring G, and is mounted co-axially with it. While the ring G, as before, moves along with the wheel and its casing relatively to the square frame under the influence of the directive force, the ring K is caused to follow it up in exact agreement by means of a small electric motor, the pinion of which engages with a gear wheel L on the upper trunnion, the current to the motor being automatically controlled by the movement of the ring G. The compass card may be regarded as attached directly to the top face of the gear wheel. A second pinion gearing with the wheel L can be arranged to transmit the reading of the card to any number of repeater compasses stationed elsewhere.

In all our preceding illustrations we have shown the pendulous weight S as being attached by a stirrup directly on to the inner horizontal ring or its equivalent, the wheel casing, so as to move in rigid connection therewith. In the early Anschütz compass the design definitely reproduces this arrangement, but in the Sperry compass matters are otherwise. The pendulous weight S (Fig. 19) is carried on a stirrup, which is forked at each end so as to span the rings G K, and which is free to swing on pivots M N fixed on the phantom ring K. The pivots M N are exactly in line with the pivots E F, and as the phantom ring K and the ring G always move in unison, the two sets of pivots remain at all times collinear.

So far the arrangement of parts is exactly similar to that which would be obtained in our simple gyro-pendulum system if the stirrup of the pendulous weight were not fixed rigidly to the inner horizontal ring, but were swung freely on the pivots E F. It can be brought into complete identity with the arrangement of our simple system if the pendulous weight S (Fig. 19) or “bail,” as it is called by the makers, be provided with a pin at its mid point to engage with a hole in the periphery of the casing. The system, as thus arranged, would merely be a distinctly complicated mechanical variation of our simple gyro-pendulum arrangement, and, as a compass, would be open to the same practical objection, namely, the persistence with which any oscillation of the axle, once set up, would continue. The vibrations would, in fact, be quite undamped.

The generation and application of a satisfactory damping force is accomplished in a very simple, yet beautiful and really ingenious, manner by displacing the pin connecting the bail and the casing from the mid position to some position lying eastwards of the vertical axis H J, as shown at Q.

In order to follow the action of this arrangement, let us consider a disc (Fig. 20) swung on a horizontal axis E F on which there is also swung a weighted stirrup S, the stirrup and disc being connected by a pin Q. Let this system of parts be held horizontal, and in the first instance let the pin, as shown in the upper view, be arranged on the centre line of the disc. Then the weight W of the stirrup applies to the disc three forces, namely, a force W + _w_ acting downwards at the end of the pin, and two upward forces P equal to each other at the pivots E F, as will readily be seen by considering the stirrup as a lever fulcrumed at the disc end of the pin. The only force tending to turn the disc is the force W + _w_ acting about the axis E F.

Let, now, the pin be situated excentrically relatively to the axis H J of the disc, as shown in the lower view. The forces applied to the disc by the weight W of the stirrup are again three in number. The force applied at the end of the pin is, as before, W + _w_, but at the pivot E the force applied falls to P - _p_, while the force at the pivot F rises to P + _p_. These forces apply turning moments to the disc. About the axis E F the turning moment, as before, is that of the force W + _w_. This force, owing to the displacement of the pin, has now, in addition, a turning moment about the axis H J in the direction of the arrow R. The upward force P - _p_ on the pivot E tends to turn the disc in the same direction, while the force P + _p_ on the pivot F tends to turn it in the opposed direction. These three turning moments about the axis H J exactly balance each other, as can be confirmed by working out their values. Thus there is no net alteration produced by shifting the pin from its central position, for in all positions of the pin the only effective moment applied by the weight W to the disc is that of the force W + _w_ about the axis E F.

Now, in the Sperry system the forces P - _p_ and P + _p_ are not allowed to act on the trunnions E F of the wheel casing, but are borne by the phantom ring K (Fig. 19) whence they are transmitted back to form part of the load on the follow-up motor. Acting on the casing, therefore, there remains only the force W + _w_. This force exerts a turning moment about the axis E F strictly equal to that produced by a directly attached pendulous weight. In addition, it exerts a turning moment about the axis H J.

As actually constructed, the wheel casing and bail in the Sperry compass cannot be placed in the position shown in Fig. 20, for the forks at the end of the bail permit the bail, and therefore the casing, to swing only through a small angle from the vertical position. It is clear, however, that if the parts could be moved into the horizontal position the moment of the force W + _w_ about the axis H J would be a maximum, just as in this position the moment of the same force about the axis E F is a maximum. It is also clear that when the parts are in the vertical position the force W + _w_ has no turning moment about either axis. In an intermediate position (Fig. 21) the moment of the force W + _w_ about the axis E F is proportional to _a c_--that is, to the component of _a b_ perpendicular to the plane of the casing or disc--just as it would be if the pin were central or if the bail formed a rigid part of the casing. The moment of W + _w_ about the axis H J is likewise proportional to the component _a c_. As the value of the component _a c_, at least for small angles of swing, is proportional to the angle of swing θ, the net result of making the pin excentric and introducing a phantom ring is to apply to the casing when it is deflected (1) the ordinary moment of the weight W about the axis E F, and (2) an additional moment about the axis H J. This latter moment is proportional to the angle of swing θ, and in the position shown in Fig. 21 tends to turn the casing in the direction of the arrow R. If the deflection is towards the other side of the axis E F, the moment applied about H J will clearly tend to rotate the casing in the opposite direction, as shown at T. Comparing Figs. 21 and 16, it will be seen that the excentricity of the pin in the Sperry compass secures exactly the same result as the air blast reaction produces in the Anschütz compass.

It is to be noted that the excentric pin in the Sperry compass is displaced towards the east when the axle is resting on the meridian. If it were displaced towards the west the force applied about the vertical axis would be reversed in its effect, and would tend to increase the oscillations of the sensitive element and not to damp them, as is the intention.