The Gyroscopic Compass: A Non-Mathematical Treatment

by T. W. Chalmers

Attached to the spider A—and therefore forming a portion of the sensitive element—is a sheet metal annular casing of the section shown at E E or F F. The gyros are enclosed within this casing. Ventilating tubes and baffles G are provided at points on the casing between each pair of gyros. The compass card—in the form really of a ring—is attached to the casing at H. Regarding the casing and the spider A as the equivalent of the horizontal ring shown in our diagram ([Fig. 41]), it will be noticed that the gyro casings are not fixed rigidly to it, but are really mounted on ball bearings surrounding their stalks, so that they may rotate about a vertical axis relatively to the rest of the sensitive element. As shown in the plan view, however, the casing of the gyro K is connected by two springs J to the rest of the sensitive element, so that in whichever way the gyro turns on the ball bearings it applies through one or other of the springs a force in the same direction to the annular casing, etc. The gyro is therefore substantially, though not actually, rigidly connected to the rest of the sensitive element, the springs being introduced to provide a yielding connection which will prevent the full force of a sudden turn of the ship from being thrown all at once on to the gyro. The gyros L M are similarly connected to the rest of the sensitive element, but in their case one pair of springs is made to serve both gyros by employing links and a bell-crank lever. The springs, it may be remarked, undoubtedly do play a part in the transmission of the directive force from the gyros to the card and in the avoidance of the quadrantal error. But their presence is not essential to the fundamental principle of action of the compass.

The damping system of the 1912 design is of a very simple nature, and represents a great improvement over the air blast method previously used. Although the wheels are not required to act as blowers, their casings are not exhausted of air. The casings, in fact, are perforated with four large holes on each side, the cooling effect of the circulating air being regarded as of more value in practice than the saving of power which would result if the wheels were run in an exhausted atmosphere.

The damping force is supplied by the weight of a body of oil contained within a trough N extending right round the foot of the annular casing containing the gyros. This trough, a circle as seen in plan, is blocked by eight bulkheads, one each below the north and south points of the compass card, and the others equally spaced round the trough. Through each bulkhead a short pipe passes, so that the oil in the trough may flow from one compartment to another. With the exception, however, of the north and south bulkhead pipes, which are quite free in the bore, the passage of the oil is restricted by means of a wire partially filling the bores of the pipes. By varying the size of wire used, the restriction to the flow of the oil from one compartment to the others, and therefore the rate at which the oil will flow when the sensitive element tilts, can be regulated to give the degree of damping required or to suit any change of viscosity in a fresh supply of oil.

The system in principle has much in common with the Brown method of damping. Should the compass card suffer an easterly deflection, the north point of the card will, as we know, tend to rise under the influence of the earth’s rotation, and will continue to rise until the turning moment applied by the deflected pendulum weight precesses the card back to the meridian, whereafter the north point of the card passing over towards the west will begin to fall towards the horizontal plane, and then, descending still farther, it will once more come back to the meridian. During this compound motion the oil in the trough flows backwards and forwards, accumulating below the south point of the card when the north point of the card is rising and gathering below the north point when the north point is falling. In other words, there is an excess weight of oil below the southern point of the card throughout the complete half-swing from east to west, the maximum excess occurring when the card is crossing the meridian. On the half-swing from west to east the excess weight of oil is below the northern point of the card, the maximum excess occurring, as before, when the card is crossing the meridian. The excess weight of oil at all times thus tends to increase the rise or dip of the north point of the card above or below the horizontal plane, whereas the pendulum weight at all times tends to diminish such rise or dip. Hence the excess weight of oil tries to precess the card in the direction opposed to that in which the pendulum weight is precessing it. The vibration of the card in this compass, as in the Brown design, is therefore damped by the generation of a counter-precessional tendency, and not, as in the early Anschütz and the Sperry designs, by precessing the sensitive element in the direction required to reduce the angle by which the pendulum weight is tilted away from the plumb line.

From what we have already said regarding the Brown system of damping, it will readily be inferred that there is no latitude error in the Anschütz 1912 compass. The damping force is equivalent to a reduction in the weight of the pendulum “bob,” and is not applied directly to reduce the tilt of the bob. The tilt of the pendulum weight required in north or south latitudes to provide the appropriate rate of westerly or easterly precession is not opposed by the damping force called into play by such tilt. Instead, the damping force merely makes the bob lighter, so that the tilt has to be carried farther before the effective weight of the bob can balance the tilting action of the earth’s rotation. The balance will be automatically struck when the moment of the effective weight of the bob is just sufficient to generate the required rate of westerly or easterly precession appropriate to the latitude.

The arrangement of the three gyros at the corners of an equilateral triangle and the general form given to the annular casing and the rest of the sensitive element results in the distribution of the mass of the sensitive element in a very uniform manner around the vertical axis. There is no excessive concentration of the mass towards the east and west plane, and as a result it is unnecessary to add compensator weights to this compass in order to avoid the effects of centrifugal force during quadrantal rolling.

Fig. 51. Sectional Elevation of Anschütz (1912) Compass.

The gyro-wheels are made of a special quality of nickel steel, and are mounted on axles of the de Laval type—that is to say, they are tapered and made of very small diameter (about 0.15 in. at the parallel ends)—in order that they may yield a little should the centre of gravity of the wheel not be truly coincident with the centre line of the shaft. The wheels are 5 in. in diameter, weigh 5 lb. 2 oz., and run at 20,000 revolutions per minute. The motors are of the squirrel-cage type with the rotor windings fixed to the wheels inside a recess concentric with the axle. The field coils are fixed relatively to the gyro casing. It is of interest to note that when the gyro-wheel is being run up to its full speed—an operation taking about five minutes to complete—the axle passes through three critical speeds. These speeds are approximately 7000, 11,000, and 14,000 revolutions, and are believed to be associated, the first with one end of the axle, the second with the other end, and the third with a combined action at both ends. During the period of running up the gyros the starting current is, of course, heavier than the current taken to drive the wheels at the top speed, and as a result a considerable temperature is developed in the wheels and their casings. When, however, the gyros have been running for some time at the top speed the temperature drops, and throughout the compass remains fairly constant at about 150 deg. Fahr. The viscosity of the damping oil, on the constancy of which the constancy of the damping force depends, is therefore less affected by external atmospheric changes of temperature than might be expected. The oil used is a mineral one. It serves not only to damp the vibrations of the card, but also to lubricate the gyro-axles. To this end, as shown in [Fig. 51], pipes are led down from each end of each axle to dip into the oil trough, the flow of oil being induced by means of wicks inside the pipes.

The method adopted for the transmission of the readings from the master compass to the repeaters is of considerable interest. The bowl C containing the mercury and the float is surrounded by two semi-cylindrical strips P Q of silver-plated brass. At one of the gaps between these strips the two abutting edges are faced with platinum. The gap between these platinum faces is 0.11 in. in width, and into it there is inserted, as shown in the plan view, a platinum-iridium ball R, measuring 0.095 in. in diameter.