2.6. Designing of assembly units.
In agricultural machine building assembly is understood as a set of operation, performed in a definite succession and designed for uniting of details, sets and units into functional units. In agricultural machine-building share of time for assembly is rather high and composes 20A30% of total time for production of machine. Assembleability of agricultural machine depends цроп two factors:
1)quality of decision at the stage of designing choice of shape of
details, accuracy of their production and method of their assembling;
2)quality of technology at the stage of production of details.
Translate the following words Into Russian:
Set of, definite succession, to unite, share of time, total time, quality of decision, accuracy, their production.
Give English equivalents for the following :
Сборка, совокупность, доля времени, собираемость сельхозмашины, зависит, качество, точность, способ соединения.
Subjects for discussion and questions to the text:
1.How is assembly understood in agricultural machine building?
2.How many percent of total time for the production of a machine
does the assembly take?
3.What factors does assembleability of agricultural machine de
pend upon?
2.6.1. Technological demands for designing of assembly units.
The main rule of designing assembly units (AE) is a distinct systemic division of a product into hierarchical groups. Assembly diagram is a result of this work. In designing joints, units and sets, contained in assembly diagram, conditions of full interchangeability, modular principle of assembly, possibilities of mechanization and automation of assembly works are to be provided.
Full interchangeability of details and units is obtained by specification of required toleVances and maximum deviations of shape (non-parallelism, non-perpendicularity, etc.). Types of fits are chosen according to the conditions of work of a connection. The required class of accuracy is determined by dimensional analysis, aimed at testing serviceability of a connection at extreme values of clearance (interference). Sometimes according to the conditions 3f work, clearance (interference) must be held in more narrow limits, than those, occurring at performing dimensions according to the 1-st class accuracy.So in press fits with accuracy of usual classes and unfavorable combination of dimensions (hole, manufactured to a positive tolerance,shaft, manufactured to the smallest positive tolerance) a received connection is not strong enough. In the opposite case (hole to nominal value, shaft to maximum positive tolerance), there are extensive stresses in connection details.
During installation of a piston pin into aluminum alloyed pistons due to high value of coefficient of linear expansion of aluminum alloys the initial (cold) clearance between pin and piston bosses sharply increases at working temperatures, which leads to the danger of breaking a connection. This fact makes to put rollers into the holes of bosses with initial interference, which disappears while heating a piston and is replaced by a clearance of required value. As calculations show, for pin diameter and holes, such narrow tolerance zone is required to be admitted, that is not provided even by accuracy of the first class.
In these cases selective assembly is applied. According to the value of deviation from nominal details are divided into some groups. In assembly only those details are grouped, which in combination with each other provide the required value of clearance (by interference). Naturally in this case principle of interchangeability is violated. Necessity of preliminary division of details into dimensional groups makes production process more complex and slow.
For such kinds of connections it is expedient to introduce heightened (precision or zero) class of accuracy. Modern methods of finishing (precision grinding of shafts, calibrated pulling and handing of holes) allow to get dimension with 0,5-1 mkm. accuracy, which is enough for connection, received now by method of selective assembly. Increasing of machining cost is quite paid off by simplification and reduction of price of assembly.
Selective assembly is also applied in completing of teat cup liner of milking machines. Division into groups is performed according to dimensions. Naturally, in this case tolerances are measured not in mkm, but in mm, and increase of product cos.: is not considerable, as only expenses for cost of completing are increased.
Interchangeability also means exclusion of adjustment and installation of details according to the place. Adjustment requires application of met-alworking operations and complementary machining, which violates rhythm of assembly, reduces quality of it and deprives construction of interchangeability. As a rule, adjustment works are very labor consuming. Preliminary, sometimes-repeated assembly of units, measurements, testing of work of unit and consequent disassembly for introduction of alterations are required. Each assembly-disassembly is connected with flusning of details.
Give Russian counterparts for:
Is a distinct, division, hierarchical, joints, units and sets, interchangeability, tolerances, fits, aimed at testing, service ability, clearances, interference, shaft, extensive stress, installation, a piston pin, aluminium, a; loyed pistons, linear extension, rollers, bosses, tolerance zone, admitted, provided, by accuracy, deviation, principle of inter change ability, violated, complex and slow, it is expedient, heightened, class, calibrated, paid off, reduction of price, milking machines, tolerances, expenses, cost, exclusion, adjustment, adjustment works are very labour consuming, preliminary, alterations, assembly-disassembly, flushing.
Find English counterparts for the following :
Иерархические группы, схема, сборка, соединение, узел, агрегат, выдержаны, взаимозаменяемость, принцип, механизация, автоматизация, допуск, предельное отклонение, параллельность, перпендикулярность, посадка, класс точности, анализ, работоспособность, зазор, натяг, узкий, предел, пресс, неблагоприятный, отверстие, сочетание, вал, номинал, напряжение, поршневой палец, поршень, алюминий, сплав, следствие, коэффициент, установка, линейный, первоначальный, бобылка, в связи, разбивание, заставить, первоначальный, натяг, нагрев, исчезает, расчет, диаметр, обеспечение, селективная сборка, отклонение, сочетание, естественно, нарушение, предварительный, размерный, осложняет и замедляет производственный процесс, целесообразно, повышение, прецизионный, шлифование, калибрующее, протягивание, упрощение, удешевление сборки, комплектование, доильные аппараты, микрон, незначительно, стоимость, комплектование, исключение, поделочный, установка, слесарная операция, станочная обработка, сборка-разборка, внесение, исправление, промывка.
Subjects for discussion and questions to the text:
1.Formulate the main rule of designing assembly units!
2.How is full interchangeability of details and units obtained?
3.What does the type of fits depend on?
4.What is the aim of dimensional analysis?
5.When must clearances be held in more narrow limits than the 1-st
class accuracy"1
6.Speak about the cases when a received connection is not strong
enough and there are extensive stresses in connection details.
7.Give the examples of what can lead to the danger of breaking a
connection.
8.What is the tolerance zone for a pin diameter and holes?
9.When is the selection assembly applied?
10.Why are details devided into groups?
11.When is a heightened class of accuracy introduced?
12.What is the accuracy of demensions/eceived by the method of
selective assembly?
13.Can selective assembly be applied in completing of teat cup liner
of milking machines?
14.What does interchangeability mean?
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