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[ pdf ] Hydraulic and Pneumatic Actuators and their Application Areas

Hydraulic and Pneumatic Actuators and their Application Areas Download
Topic under  Hydraulic cylinder
Source: www14.informatik.tu-muenchen.de 
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Last download on: Mon Oct 16, 2017 08:13:40 AM
Short Desciption:
The telescopic hydraulic cylinder. 3.2.2 Volumetric hydraulic actuator. Volumetric hydraulic actuator is the set of volumetric hydromachines, ...

The Laws The Work Of Hydraulic Actuators Is Based

Liquid is the physical body possessing fluidity. Fluidity is understood as easy mobility of particles, i.e.(that is) ability beyond all bounds to be deformed and come in movement under action of insignificant forces. Due to fluidity liquids practically without efforts get the form given by space, and in balanced condition do not perceive tangents strain. Liquids show resistance to shift in direct dependence on speed of shift deformation and to compression. They donít practically resist to stretching except in cases under special conditions, for example at all-round negative pressure or at attempt to tear off the piston from liquid in the cylinder. However liquids considered technically pure, in practical conditions contain small impurity and dissolved air, therefore they are not capable to maintain even insignificant stretching strain. Taking into account mechanical properties we can distinguish two types of liquid: lowcompressible (dropping) liquids which insignificantly change the volume when temperature and pressure change, and compressed (gaseous). The difference of mechanical properties of these two types of liquid is insignificant, if it is possible to neglect compressibility of gases namely when differences of temperatures and pressure are small, and speed of movement less than 70 km/s. In these cases the laws received for drop liquids, can be frequently fair for gases. From physics approach the difference between these kinds of liquid is significant. Drop liquids have quite certain volume and consequently fill in only that part of space which is equal to their volume. They can form a boundary free surface, and at insignificant volumes are capable to keep the drop shape. The phenomenon arising in the moving liquid when pressure decline up to elasticity of saturated vapor and an ambient temperature is named cavitation. Cavitation is accompanied by formation of steam-to-gas bulbs that move with a stream of liquid in area with higher pressure, slam and radiate shock wave. Slamming bulbs near borders of the current can course the destruction of firm surfaces. Cavitation can arise at low pressure in pipelines, pumps - everywhere where the stream of liquid is exposed to bends, to narrowings expansion (valves, throttles, etc.) followed by. As a rule, cavitation is the undesirable phenomenon, and should be eliminated from hydrosystems because cavitation can course erosion of surfaces, destruction of elements of hydromachines and hydrosystems. It can increase the resistance of pipelines and sharp by reduce efficiency of hydromachines. Boiling and cavitation breaks homogeneity of liquids and limits a scope of the laws based on homogeneous nature of the liquid medium. Silting of narrow cracks and backlashes owing to adsorption (adjournment) of polar-acting molecules of liquid on their walls is called obliteration. The layer of liquid formed at walls gets properties of quasisolid state which viscosity differs from viscosity of liquid. As a result, the part of a boundary layer sticks to a surface of a crack. The thickness of this layer for oils is 4...5 microns, which can essentially reduce a cross-section of slot-hole channels and backlashes or even overgrow them completely. In fluid dynamics, Bernoulliís equation describes the behavior of fluid moving along the streamline. There are typically two different formulations of the equations; one applies to incompressible flow and the other applies to compressible flow.
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