Stick-slip effect

Stick-slip effect (from English stick ' to stick' and slip ' to slide') or also stick-slip effect or (self-excited) frictional vibration describes the jerk- sliding of solid bodies moving against one another . Well-known examples are the sound generation of stringed instruments , the chirping of insects, the squeaking of wall panels , creaking doors, screeching brakes or tires, rattling wiper and a latex - balloon rubbelnde or the rim of a drinking glass in vibration versetzende, wet fingertip (see glass harp ).

root cause

The effect can occur when the static friction is noticeably greater than the sliding friction :

${\displaystyle {\begin{aligned}F_{HR}\gg F_{GR}\\\Leftrightarrow \mu _{HR}\gg \mu _{GR}\end{aligned}}}$

With

• ${\displaystyle F}$ for the respective force
• ${\displaystyle \mu }$for the respective coefficient of friction .

At the same time, dampened, coupled parts of the surface exert a rapid sequence of movements: sticking, tensioning, separating and sliding ( see below: mechanism ).

The effect disappears as soon as the friction partners are completely separated by the lubricant ( hydrodynamic sliding friction ).

Viscoelastic materials (e.g. polymer melts ) show a similar effect when their viscous flow changes into sliding under the high shear forces on the tool (e.g. a nozzle).

mechanism

Model with a stick-slip effect

Let V be a linear drive (crank with threaded spindle ), R symbolize a spring constant and M the mass lying on a plate .

The drive V leads to the fact that the spring  R is tensioned until the spring force exceeds the static friction force of the mass M on the plate (disconnect as soon as ) and sets it in motion (sliding down for as long ). ${\displaystyle F_{F}\geq F_{HR}}$ ${\displaystyle F_{F}\geq F_{GR}}$

Soon the speed of the mass exceeds the speed of the drive, whereby the spring relaxes again and the spring force decreases. Due to its inertia , the mass moves a little beyond the point at which the spring force is equal to the sliding friction force, and then stops (sticking as soon as ). After stopping, the drive first catches up this piece and then increases the spring tension again up to the static friction limit (tensioning, as long as ). Then the cycle starts all over again. ${\displaystyle F_{F}\leq F_{GR}}$ ${\displaystyle F_{F}\leq F_{HR}}$

see also: two-position controller with hysteresis

Range of variation

In plate tectonics , relatively short sliding phases (the earthquakes ) are separated by long pauses, because the friction is high in relation to the stiffness of the spring and the drive is slow.

With a string instrument , the friction is small and the spatially distributed mass and spring - the string is both at the same time - is capable of vibrations at their natural frequency . A clean sound is created when the glide phase is relatively long.

The instability leading to the effect is increased when the deformation dynamically presses the surfaces rubbing against one another when the spring is tensioned, while the relaxation reduces the pressure. An example is pushing the fork over the plate.

In the case of the drawer effect, the influence of geometry dominates ; in its pure form it has nothing to do with the stick-slip effect.

Effects and countermeasures

The stick-slip effect is often undesirable in technical applications. It generates noise and structure-borne noise , which is often perceived as unpleasant (see Noise, Vibration, Harshness ) and can lead to increased wear and tear and material fatigue. He can also perform the smallest movements, such. B. in precision machine tools , completely prevent.

Include countermeasures

• reducing the difference between sliding and static friction, often by reducing the overall friction, such as through lubrication
• an increase in the rigidity of the drive or the body itself
• Reduction of the masses involved
• greater attenuation
• a geometry that reduces instability.

literature

• FP Bowden, D. Tabor: The Friction and Lubrication of Solids , Oxford University Press, 2001, 424 p, ISBN 0-19-850777-1 .
• NM Kinkaid, OM O'Reilly, P. Papaclopoulos: Automotive disc brake squeal - Journal of sound and vibration, 2003, v. 267, Issue 1, pp. 105-166.
• K. Magnus, K. Popp: Vibrations: an introduction to physical principles and the theoretical treatment of vibration problems . Stuttgart, Teubner, 2005, 400 pp.
• Bo NJ Persson: Sliding Friction. Physical Principles and Applications . Springer, 2002, ISBN 3-540-67192-7 .
• Valentin L. Popov: Contact Mechanics and Friction. A text and application book from nanotribology to numerical simulation , Springer-Verlag, 2009, 328 p., ISBN 978-3-540-88836-9 .
• Ernest Rabinowicz: Friction and Wear of Materials . Wiley-Interscience, 1995, ISBN 0-471-83084-4 .

Individual evidence

1. ↑ CD Han, RR Lamonte: A study of polymer melt flow instabilities in extrusion in Polymer Engineering & Science 1971 , Volume 11, Issue 5, pp. 385-394 doi : 10.1002 / pen.760110507