ПОЗНАВАТЕЛЬНОЕ

Оси и плоскости тела человека - Тело человека состоит из определенных топографических частей и участков, в которых расположены органы, мышцы, сосуды, нервы и т.д.

Отёска стен и прирубка косяков - Когда на доме не достаёт окон и дверей, красивое высокое крыльцо ещё только в воображении, приходится подниматься с улицы в дом по трапу.

Дифференциальные уравнения второго порядка (модель рынка с прогнозируемыми ценами) - В простых моделях рынка спрос и предложение обычно полагают зависящими только от текущей цены на товар.

Text 8: Industrial Engineering

One distinction between a mechanical engineer and an industrial engineer is that the former deals with individual machines while the latter deals with machines in combination as part of a system. Industrial engineering is concerned with problems such as manufacturing processes and plant layout.

Industrial engineers must make the most efficient use of plant and equipment to achieve the highest possible degree of productivity. Originally this was conceived entirely in the mechanical terms of what machines could do and how they should be arranged. Now it is known that the effectiveness of the workers in the system must also be considered. The industrial engineer is therefore often involved in labor relations.

Industrial engineering in practice if not in name was born at the beginning of the machine age. People like Newcomen, Watt, and Pickard had to be involved not only in the invention of machines but in their application and installation. Throughout the eighteenth and nineteenth centuries the use of machines for all kinds of manufacturing (beginning in most countries with textiles) multiplied many times. We have mentioned the belt drives common in factories where steam engines powered other machines. These dangerous belts are an example of the problems with which industrial engineers dealt and the solutions they evolved.

A major advance in twentieth century manufacturing was the development of mass production techniques. Mass production refers to manufacturing processes in which an assembly line, usually a conveyor belt, moves the product to stations where each worker performs a limited number of operations until the product is assembled. In the automobile assembly plant such systems have reached a highly-developed form. A complex system of conveyer belts and chain drives moves car parts to workers who perform the thousands of necessary assembling tasks.

Mass production increases efficiency and productivity to a point beyond which the monotony of repeating an operation over and over slows down the workers. Many ways have been tried to increase productivity on assembly lines: some of them are as superficial as piping music into the plant or painting the industrial apparatus in bright colors; others entail giving workers more variety in their tasks and more responsibility for the product. Some automobile companies have experimented with giving individual workers complete responsibility for assembling an entire car; this obviously requires an extraordinarily high degree of skill.

These human factors are important considerations for industrial engineers who must try to balance an efficient system of manufacturing with the complex needs of workers.

Text 9: Bearings

A vital mechanism for reducing friction is the bearing which basically is a device that bears the friction of parts in motion. Often one of the parts will be moving and the other will be stationary. Logs used to move heavy stones in early times were the primitive form of a bearing. They were efficient because they changed sliding friction to rolling friction, thereby decreasing the effort necessary to move the stones.

Reducing friction between the parts of a machine is the principal purpose of bearings. Different types have been designed for use at various points of contact to fit the kinds of motion at work. Probably the most familiar are ball bearings which are used in many machines. Small balls are fitted into a cage, a container that separates them. Cage and bearings are then sealed, often in a lubricant, between rings which are called races. The entire assembly is a ball bearing.

Another familiar type is the roller bearing, a modern version of the logs that were used as primitive bearings. Roller bearings contain small cylinders on which the bearing races can roll. They are usually fitted with the same kind of holding cage and races as ball bearings. In order to sustain pressure from different directions, bearing rollers are sometimes tapered or shaped like cut-off cones and set at an angle to the races. A variation is the needle bearing with cylinders of very small diameter. Needle bearings need not be contained in a cage or between races. Their advantage is greater load-carrying capacity with more friction than ball bearings of comparable size.

A modern development is the non-contact bearing in which there is contact between the machine parts only at rest; when in motion they are separated by a thin layer of gas or fluid. This prevents wear between the moving parts. Non-contact bearings have been developed for such complex and sophisticated systems as missile guidance. The possibility of using cushions of compressed air in transportation systems has been discussed frequently in recent years.

Text 10: Machine Components: Linkage, Spring, Ratchet

Another kind of mechanism is known as a linkage; it is a series of at least three rods or solid links that are connected by joints that permit the links to pivot. When one link is fixed the other links can move only in paths that are predetermined. Like cams, linkages are used to change the direction of motion, to transmit different kinds of motion, or to provide variations in timing in different parts of a cycle by varying the lengths of the links in relation to each other.

The spring is a mechanism that is used in a wide variety of machines; it is frequently an elastic helical coil that returns to its original shape after being distorted. Springs are essential components in watches; in some cam mechanisms they hold the follower in place; they are found in scales and they help to cushion an automobile ride. There are many variations on the basic coiled or spiral spring, including the leaf spring which is made of strips of elastic material and springs that depend on the compression and expansion of air.

A ratchet is another paired mechanism consisting of a wheel with teeth and a pawl which drops into the spaces between the teeth. The ratchet mechanism is used to prevent a motion from being reversed or to change reciprocating into rotary motion.

This is a brief introduction to the complex world of machine components. The infinite number of combinations and variations in which these mechanisms can be combined is the heart of the work of a mechanical engineer.