Roughness on heavily stressed surfaces

Side effects of tribological friction forces

The roughness of 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 surface is basically irrelevant. The function and energy efficiency of the piece of wood - it should simply burn and provide heat - is not affected. The situation is completely different for workpieces such as gears, gearboxes or roller bearings, where machine parts are constantly in motion and in continuous surface contact. If the individual parts are not as smooth and resistance-free as possible, they will "grind" or you will have "sand in the gears", so to speak. In the end, you may end up with a total loss and therefore an extensive repair or new purchase.

What is the significance of roughness in tribology?

Roughness is a term from the field of surface physics. In tribology (friction theory), roughness means that the rougher the surfaces of two objects in relative motion to each other, the more friction losses they cause. This phenomenon is particularly significant for moving machine parts such as gears and roller bearings. Roughness leads to high operating temperatures, faster wear of the individual elements and, in the worst case, to total failure if the wrong lubricants are used and maintenance is inadequate.

How can roughness be prevented?

Depending on the material used, various processes are used today in modern production technology to avoid roughness on surfaces. These include polishing and electropolishing, grinding and pickling, etching, sandblasting and honing. However, a certain degree of roughness always remains, even if perhaps only in the nano range, which is not recognisable to the naked eye or by touch. Appropriate lubricants and regular inspection and maintenance help to minimise the effects of surface roughness.

Which shape deviations of a surface are decisive for roughness?

The DIN 4760 standard divides shape deviations of surfaces into six classes, whereby the values of the first four classes can overlap:

    1st class: shape deviations
    2nd class: Waviness
    3rd class: Roughness due to grooves
    4th class: Roughness due to scoring
    5th class: Roughness in the microstructure
    6th class: Lattice structure of the respective material

Roughness of classes 1 to 4 can normally be recognised visually and haptically on the surface. For the higher classes 5 and 6, the use of a microscope or electron microscope is necessary to check the condition of the surface or to be able to detect defects.

How do you measure roughness and what roughness measuring devices are available?

There is a wide range of measuring devices for measuring the roughness of surfaces, holes and grooves. They are generally quite easy to use. The devices measure the average roughness depth (unit: Rz) and the average roughness value (unit: Ra) in micrometres (µm) and are able to transfer the values directly to a PC or notebook for more precise analysis. Depending on the workpiece, the measuring devices should comply with the DIN 4762, 4768, 4771 or 4775 standards in order to ensure that the results are unadulterated after the test.
Commercially available measuring devices basically work with three methods. The manual methods include, for example, the rugotest, in which the comparison is carried out visually or haptically by touching sample surfaces. In profile-based methods such as the stylus method, a diamond is moved over the surface at a constant speed to measure the roughness. Thirdly, there are surface-based methods in which the measurement is carried out using optical processes, for example 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 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, only measuring devices that can recognise even the finest unevenness offer real certainty about the suitability of a surface for the planned work process.