In studying the construction of machinery it is also necessary to have an understanding of its basic principles.There are a number of parts or assembles such as gears, brakes and clutches that are common to machines of different types.
Wheel and axle. A wheel and axle is specialized type of lever. For example consider a wheel three feet in diameter rigidly fastened to a six inch axle, revolving at ten revolutions and the further the point is from the centre, the larger the circle will be and the faster the point must move all mechanical devices with a fixed amount of power, greater speed means less force. This wheel may be considered to be a member of the third class lever.
Gears. If the axle is locked to two wheel of different size it will deliver different proportions of power and speed to their circumferences because of different leverage ratios. If the wheel edges are notched into teeth which can be meshed with other toothed wheels they are called gears and have almost unlimited capacity for changing the power speed relationship between the input and output shaft. Gears may be connected to the source of power by means of shaft or axles or they may revolve freely on shafts that simply hold them in proper position. A rigid connection can be made to the shaft by means of keys or splints.
Bearings. Where the gear turns upon the shaft or the shaft turns within it supports provision must be made to reduce friction and wear. This may be made by inserting a bushing or ball bearings.
Transmission. A transmission is a box of gears and shafts which provides a change in the speed power ratio. It may be a single speed or sliding gear type in which several ratios are available.
Brakes. A brake is a device for slowing, stopping or holding an object. A friction performs all three functions but a tooth or jaw brake is intended only to hold.
Text 4 Scrapers
The scraper is associated with bulk excavation of earth–works. It consists of a bowl mounted on wheels with pneumatic tyres and may be towed by means of a tractor or contains its own power unit. The scraper bowls varies in capacity but for road construction capacities of 4–12 yd3 would be normal.
The machine is wholly operated by one machine operator who will
control the power or towing unit and manipulate the gears to control the operation of the scraper bowl. The tractor or power unit used with the scraper bowls may be fitted with caterpillar tracks or wheels fitted with pneumatic tyres. The machine is capable of excavating all normal types of earth.
The scraper is a most suitable machine where excavating and depositing of the earth can be carried out on the site.
The scraper can dig, haul, and spread in a single normal working cycle. It can work alone if necessary, but production is usually increased if it is assisted by other machines. It works in thin layers both in the cut and on the dump, without limit as to the number of layers, so that its efficiency is not particularly affected by depth of cut or height of fill.
The tasks normally associated with the scraper are: excavation on the cut-and-fill principle; excavation and stockpiling; inimical levelling and grading of the earthwork: formation of embankments; excavating borrow pits. A scraper has three basic operating parts: the bowl, the apron, and the ejector.
The bowl is a box with rigid sides, with the apron as a movable front, and the ejector as a movable back. The forward edge of the bowl is fitted with cutting edges. The cutting edges are made of wear-resistant steel and are bolted to the bottom of the bowl.
The apron forms the forward section and a variable amount of the bottom of the bowl assembly. When closed it rest at the cutting edges. The apron is hydraulically controlled by a lever in the operator's cab. When the apron is lifted, it moves upward and forward far enough to leave the whole front of the bowl open.
The ejector is the rear wall of the bowl. The most common ejector is
hydraulically controlled and moves horizontally, forcing the load out the bowl. It is supported by rollers riding on the floor and on tracks welded to the
sides of the bowl.
The front of the bowl contains an interchangeable cutting blade and a
movable apron to open or close the front of the bowl. Inside the bowl is
a movable tailgate or ejector gate. All are controlled by the machine operator.
The output of a machine will depend upon: the capacity of the scraper bowl; the nature of the earth; the skill of the machine operator; the total distance to be covered by the machine.
Зачет
К зачету допускаются студенты, выполнившие контрольную работу № 1
(контрольная работа выполняется письменно и защищается в устной форме) и сдавшие тексты учебника или учебных пособий по английскому языку (устная форма ответа) по профилю вуза в объеме 5 с. за каждый семестр.
Для получения зачета студент должен уметь прочитать со словарем незнакомый текст на английском языке, содержащий изученный грамматический материал.
Форма проверки — письменный перевод. Норма перевода — 600-800 печатных знаков в час письменно со словарем на бумажном носителе.
Экзамен
К экзамену по английскому языку допускаются студенты, имеющие зачет за I семестр, выполнившие письменную контрольную работу № 2 и сдавшие учебный материал по чтению (устная форма ответа) за 2 семестр в объеме 5 с.
На экзамене по английскому языку проверяются умения: читать со словарем текст по специальности вуза.
Форма проверки понимания — письменный перевод. Норма перевода - 1000 печатных знаков в час письменно со словарем на бумажном носителе.