3 - Technologies

3.2 - Working clearance and arithmetic roughness

By combination of running clearance and arithmetic roughness, we mean:

- The working clearance between the parts in movement in relation to each other is defined by generally standard machining tolerances. This clearance exists and is necessary.

- The surface of a machined part is always more or less rough, so sometimes a running in period is required between two elements operating in relation to each other. This roughness can be measured mechanically and expressed in microns to give the arithmetic roughness. It can also be measured hydraulically to give a charge loss coefficient.

In article A 1870 entitled “Mécanique des fluids” (Fluid mechanics) by Mr. Jean Gosse, PhD and Honorary Professor at the Conservatoire National des Arts et Metiers published in April 1996 in “Techniques de l’Ingenieur” (Engineering Technology) paragraph 7.54, under the title “Influence de la Rugosité de la surface” (Effect of surface roughness), there is a relationship between the height of the rough patches and the thickness of the viscous sublayer in the fluid which has been confirmed experimentally. Friction is increased by roughness, so that over a certain roughness the viscous sublayer ceases to exist. Section 9 of the article deals with charge losses.

It is therefore possible to create turbulence in a narrow passage so that it forms the required seal. The narrow passage is created by the operating play and turbulence by the roughness which causes charge loss.

Figure 3.2 - Working clearance and arithmetic roughness	Thus, by way of example, when the working clearance (ψ) is 0.02 mm and the arithmetic roughness (Ra) is 0.2 mm, turbulence generates charge losses which may be sufficient to create the required seal.
Figure 3.3 - Grooves	Arithmetic roughness can be achieved using micro-grooves with rough sides. The micro-grooves are arranged perpendicular to the direction of leaks and in parallel to each other.

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