THE AUTOMATIC CONTROL SYSTEM
Automatic control now appears in many fields and involves many control instruments and apparatus. Their usage embraces a great variety of machines, plants and processes.
The essential parts of a basic automatic control arrange¬ment are: (a) a controlled condition, e. g., temperature; (b) a device for measuring the value of this condition, i. e., a measur¬ing unit; (c) apparatus capable of effecting a change in the controlled condition, i.e., a regulating unit, and (d) means for operating the regulating unit in response to the measuring unit, i.e., a controlling unit. The fundamental feature of this basic arrangement is the dependence of the regulation of the measurement. We have a cyclic system consisting of a number of stages linked together to form a continuous chain. Although the controlling unit operates the plant, it is the plant which actuates the controlling unit. The performance of any control¬ling instrument is, therefore, dependent on the particular sys¬tem of which it forms a part.
A simple example, in which the measurement, automatic control and regulation are incorporated in one housing, can be found in thermostatic control of motor car engine-cooling water. In this case the temperature measurement is effected by a sen¬sitive capsule attached to a valve which regulates the amount of water allowed to pass through the radiator. This produces the basic closed-loop system. The resultant cooling water tem¬perature depends on factors, such as the engine and radiator performance.
Here is another example of automatic control and regula¬tion. It is a steam-heated ventilating plant supplying air to an enclosed chamber, the temperature of which is controlled by automatic regulation of the steam supply. In this case the controlling instrument incorporates part of the measuring unit but imposes control action on a separate power-operated steam valve which forms the regulating unit. The resultant tempera¬ture again depends on the plant design but can be adjusted to the desired value by a setting in the controlling instrument which determines the relationship between the measurement and the control action.
Machine parts are held together by parts: (a) working in tension, (b) working in shear, (c) creating friction, and (d) using both shear and friction forces.
Types of Fastenings. – All fastenings can be divided into two classes — disconnectable fastenings and permanent joints.
Disconnectable fastenings, in turn, are effected by: (a) bolts (Fig. 4 a, b) and screws, (b) wedges, (c) dowel pins (Fig. 4 c, d; Fig. 5 a), (d) keys.
Permanent joints are obtained by means of: (a) press fits, (b) shrink fits, (c) rivets, (d) welding, brazing, and soldering and (e) casting.
Forms of Threads. – Screw fastenings are used for holding two or more machine parts together or for adjusting one part with relation to another. In screw fastenings the threads are made in several forms but are always of triangular-type single thread.
Screw threads are made right-hand and left-hand.
TUBING, TUBE CONNECTORS, AND FLEXIBLE HOSE
The various units of a hydraulic system are connected with some form of tubing or flexible hose.
Tubes are joined by means of tube connectors, usually of the same material.
Tubing. – Some of the factors governing the selection of tubing are corrosion, temperature, weight, mechanical strains, abuse, and pressures.
Because of its light weight, aluminum-alloy tubing and fittings are used wherever possible.
Fittings are designed to withstand the bursting pressure that the tube of maximum wall thickness will withstand, assuming that the tube and fittings are of similar materials. For general heighpressure installations steel tubes and fittings are recommended.
Flexible Hose. – Flexible hose is used to connect hydraulic units between stationary and moving parts. Hoses are made up of varying layers of synthetic rubber, fabric, and wi