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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
In installing flexible hose care must be taken that the hose is free from twists. Under pressure a twist will rotate the hose and loosen the connection, causing failure.
CLASSIFICATION OF CONTROL
There are many forms of action which a controller may impose on the regulating unit in relation to the response of the measuring unit. There are also various relationships which may exist between the control action and the corresponding potential value. The two together determine the nature of the control. There are four important types of control which may be regarded as basic, namely: “On-Off”, Proportional, Integral (“Floating”) and Derivative (“Rate-of-Change”).
The simplest form of control is one in which only two settings of the regulating unit are available, one leading to a higher value and the other to a lower value than the one desired. The control action is consequently limited to alternat¬ing two potential values, thus giving rise to a continuous oscillation of the controlled condition.
Such control is known as “On-Off” Control and is widely used in laboratory, domestic, and industrial applications, well-known examples being incubators, refrigerators and air compressors.
In the case of “On-Off” Control, little adjustment is avail¬able beyond that which determines the desired value setting.
Geometrically pins can be divided into cylindrical pins, called straight pins, and conical or taper pins (Fig. 5). Dynamically pins can be classified as those used only to locate the relative position of two parts when there is little or no force acting upon the pin, and those that fasten two or more parts together and are subjected to considerable stresses, which are mostly in shear but sometimes in bending. Locating pins are called dowel pins or simply dowels. A connecting pin, like a dowel, may be used either as a permanent connection or as a fulcrum for a movable joint.
General Remarks. – Riveting has been the standard method of joining plates and structural parts before welding began to replace it with increasing rapidity.
Rivets. – A rivet is a round bar consisting of an upset end called the head, and a long part called the shank. The rivet blank is heated to a red glow, inserted into one of the holes, and, while the head is held firmly against the plate by a heavy sledge, the projecting end is formed into a second head, called the point, by means of a hand hammer and set or by a press.
Button heads are used for small rivets which are driven cold; pan heads are used chiefly in ship work; countersunk heads are used only in special cases, chiefly in structural work and below the waterline in ships; the countersunk points weaken the plate so much that, they should be used only when unavoidable; the others, including button heads, are used in boiler and structural work.
A Bearing is a machine part which supports a moving part and confines its motion. That part of a shaft which rotates in a bearing is called a journal. Bearings in which one rubbing surface slides over another are called plain bearings and may be divided into two classes: Those with a continuous rotary motion and those with an intermittent motion. To the first class belong journal bearings, which carry a load acting at right angles to the shaft axis, and thrust bearings, which take a load acting in the direction of the shaft axis. To the second class belong bearings of parts having a rocking motion, as wrist pins, or a liner reciprocating motion, as crossheads.
Bearings with a continuous rotary motion form the great majority of all bearings.
Bearing Failures. – The failure or need of replacement of a bearing with sliding contact may be due to:
PRESS, SHRINK, AND TRICTION JOINTS
THE ELECTRIC BELL
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