2.3.2. Dimension - similar Series.
Indexes of dimension-similar series depend upon geometric sizes of machine and parameters'of operating processes. In order to keep full similarity of machines with different sizes the following items are required to be observed: geometric similarity and operating process similarity, i.e., to provide similarity of parameters of energetic and power strength of machine and its parts.
Similarity criteria have been elaborated for the majority of types of machines and operating processes. For example, the main conditions for ICE similarity are the following: equality of middle effective pressure PC, depending upon pressure and temperature of mixture at suction, equality of middle speed of a piston V=Sn/SO (where S- piston stroke, n-engine rotational speed) or product equality Dn (where D- piston diameter, which in geometrically similar engines in connected with piston stoke by a relationship S/D=const).
In general similarity criterion is determined as V(Pc,Pn)=const.
If general criterion is the same, then all similar ICE possess the same efficiency, fuel rate, thermal stress, bearing loads (fig.2.9.).
Fig 2,9.Dimension similar series for piston ICE
The main requirement for dimension-similar designs is as follows: not geometric parameters, but main characteristics (power, productivity) must be put into basis of their creation. Inevitable alteration of specific indexes (for, example, engine capacity of ICE) and of mechanical ones (for example, construction rigidity) of geometrically similar machines must be taken into account.
Memorize the following words and give their Russian equivalents :
Indexes, dimension-similar, depend upon, operating processes, in order to, keep, similarity, sizes, items, required, observed, to provide, power, strength, criteria, elaborated, majority, ice, equality-, effective, pressure, depending, speed, a piston, rotational speed, equality, engines, connected, determined, possess, efficiency, bearing, loads, requirement, dimension-similar designs, power, productivity, inevitable, alteration, specific, capacity, construction rigidity, must, taken into account.
Retell the text using English counterparts of the following expressions :
Показатель, размерно-подобный, сохранение, подобие, необходимо, соблюсти, обеспечить, энергетическая надежность, критерий, разработать, большинство, условие, равенство, средний, эффективный, смесь, поршень, произведение, диаметр, связанный, соотношение, обобщенный вид, коэффициент, удельный расход, подшипник, требование, конструкция, построение, следует, учитывать, неизбежное, изменение удельных показателей
Questions to the text:
1.What do dimension-simular series depend upon?
2.What must be observed in order to keep full similarity of machines
with different sizes?
3.What criteria are similar for the majority of tipes of machines?
Give examples.
4.What is the formular of the similarity creterion?
5.What conditions are the main ones for ICE. simularity.
6.What Is the main requrement for demension - similar designs.
7.Why must the alteration of specific indexes of geometrically similar
machines be taken into account?
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