Roughness on high loaded surfaces

Side effects tribological friction

The roughness on surfaces is caused by the processing of different materials during sawing, cutting, punching and other mechanical processes. If you saw a piece of wood to burn it later in the fireplace, the roughness of the interface is basically irrelevant. The function and energy efficiency of the wood - it is just supposed to burn and provide heat - is not affected. It is a completely different story with components like gear wheels, gears or roller bearings where machine parts are permanently in motion and are continuously in surface contact. If the individual parts are not as smooth as possible and without resistance, "there is a grating sound", or you have, so to speak "grit in the gears". Ultimately, you may be looking at a total write-off and thus an extensive repair or new purchase.

What is the significance of roughness in tribology?

Roughness is a term from the field of surface physics. For tribology, roughness means that two items that are in relative motion to one another cause more friction losses, the rougher their surfaces. The phenomenon is particularly significant in moving machine parts like gears and roller bearings. Roughness leads to high operating temperatures, faster wear in individual elements and in the worst case, if the incorrect lubricants are used and poor maintenance, a total write-off.

How can roughness be prevented?

Depending on the material used, various methods are used nowadays to avoid roughness on surfaces in modern manufacturing technology. These include polishing and electro-polishing, grinding and pickling processes, etching, sandblasting and honing. However, a certain level of roughness always remains, even if only on a nano-scale. It is not discernible to the naked eye or to tactile scanning. The appropriate lubricants and regular inspection or maintenance also help to cushion the effects of roughness on surfaces.

What form deviations in a surface are decisive in terms of roughness?

The DIN standard 4760 divides form deviations in surfaces into six classes, where the values of the first four classes may overlap:

1. class: form deviations
2. class: waviness
3. class: roughness through grooves
4. class: roughness through scoring
5. class: roughness in the micro-structure
6. class: lattice structure of the particular material

Roughness in the classes 1 to 4 can usually be identified visually and by touch on the surface. In the higher classes 5 and 6, the use of a microscope or electron microscope is necessary to examine the nature of the surface or detect errors.

How do you measure roughness and what sort of devices exist for measuring roughness?

There are a wide range of devices for detecting the roughness of surfaces, holes and grooves and usually they are quite easy to handle. The devices measure the average roughness (unit: Rz) and the average roughness value (unit: Ra) in micrometres (µm) and are able to transfer the values for further analysis directly to a PC or a notebook. Depending on the work piece, the measuring devices must meet the standards DIN 4762, 4768, 4771 or 4775 to be sure that unadulterated results are available after the test.

Normal commercial measuring devices basically work with three methods. The manual methods include, for example, the Rugotest, in which a visual or tactile comparison is carried out on the basis of sample surfaces by means of touch. In profile-based methods such as a diamond stylus method, a diamond is run at a constant speed over the surface to measure the roughness. Thirdly, there are area-based methods, in which the measurement is carried out using visual methods, for example by confocal microscopy or conoscopic holography.

Can the roughness also be determined without measuring devices?

To a limited extent, the roughness of a surface can be seen with the naked eye seen or felt with the fingers. Whether this test is sufficient depends on the material used and the intended use of the relevant machine parts. However, real certainty on the suitability of a surface for the planned operation can only be offered by measuring instruments that can detect even the finest of irregularities.

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