Anything that spins can technically be considered a gyro. There are two forces associated with the gyro that must be understood: gyroscopic inertia, or rigidity in space, and precession.
Gyroscopic inertia is the natural tendency of any rotating body, spinning at a constant speed, to preserve its plane of rotation. The body will continue to spin in this manner until some outside force is applied to change the plane of rotation. The second force, precession, occurs when outside pressure is applied to the spinning body. Precession causes the body to move in a direction nearly perpendicular to the application of the applied force. The function ofthe gyro is also dependent on having three axes of freedom, namely, spinning, vertical, and horizontal. This allows the gyro to move in any manner without interference.
The gyro cannot become a usable compass, however, without utilizing the two natural constant forces of the earth: the earth's rotation and the force of gravity. To make a gyro a compass, a weight is attached to the frame supporting the spinning element of the gyro. As the earth rotates, the gyro has a tendency to remain in its original plane of rotation. The force of gravity acts upon the weight, forcing the axis of the spinning element to precess toward an alignment with the geographic meridian, north and south. It should be noted that gyros are not necessarily north seeking. Depending on the manufacturer, gyros can use one of several methods to apply a weight, which creates the precessive forces. One way is to utilize a ballistic which uses a free-flowing heavy liquid such as mercury or silicone within the element. This is known as a ballistic gyro. Another method is to use a solid weight suspended from the gyro frame. This is the pendulous style gyro.
Again the construction of the gyro varies with its manufacturer, but generally all gyros have five major parts. Commonly known as the sensitive element, contains the spinning rotor, the electric motor that drives the rotor, the rotor housing, and the supportive frame. The second part, containing the ballistic or pendulous weight, is known as the controlling element. The third element of the gyro is composed of the frames and mechanisms that shadow the movement of the sensitive element without interfering with it. This element is known as the phantom element and also contains the compass card from which the heading off the master compass can be read. The fourth is a supporting element which holds all the elements of the master gyro. This part contains the motor that drives the phantom element to follow the movement of the sensitive element and a transmission system that sends compass signals to the repeater system. This element is known as the spider element. There is the compass binnacle itself which contains all the parts ofthe master gyrocompass. This is more accessible in older units, but modern binnacle housings are sealed to prevent tampering and maintain shockproof supports as well as an inert atmosphere to reduce rotor friction.
The master gyro is operated by an electronic control device which regulates the power systems that operate the various elements of the master gyro including the rotor, phantom, azimuth, and transmission systems (see the illustration at the beginning of this chapter). There is also a separate transmission amplifier and distribution control system which receives signals from the master gyro transmitter and distributes this signal to all of the vessel's repeaters.
