Stick-Slip Effect

Definition and Classification within Dynamic Seals

The stick-slip effect describes a jerky stop-and-go motion between two contact partners. Stick means that the contact surfaces adhere (no motion). Slip means that they slide (motion sets in). In sealing technology, this primarily affects dynamic seals — that is, seals with relative motion, for example rod seals and piston seals in hydraulic and pneumatic cylinders.

Stick-slip becomes practically relevant primarily when very slow motion is required, when starting up after standstill, or with short strokes (oscillation). Motion is then often uneven. This can cause vibrations, noise (e.g., squeaking), pressure peaks, poorer positioning accuracy, and increased wear. In controlled axes, this frequently appears as control deviation, although the setpoint is constant.

Static Friction, Sliding Friction, and Breakaway Force

The mechanism depends on two types of friction: static friction (friction at standstill) and sliding friction (friction during motion). In many friction pairings, static friction is higher than sliding friction. Therefore, a small drive force is often not yet sufficient to set a seal in motion.

The breakaway force (also start-up force) is the force threshold that must be exceeded so that adhesion turns into sliding. Until then, force builds up — for example through elastic deformation of seal, guide, or structure. As soon as the breakaway force is reached, motion sets in abruptly, while friction at the same time drops to the level of sliding friction. Precisely this drop frequently leads to the jerk, because stored energy is released. Afterward, motion can subside again, adhere anew, and the cycle starts over.

Mechanism: Why Stick-Slip Arises (Tribology and System Dynamics)

Stick-slip arises from the interplay of tribology (friction and lubrication in the contact) and system dynamics (stiffness, damping, and stored energy in the drive system). In the stick phase, energy is stored in the system. This happens, for example, through:

• elastic deformation of seal and components,
• compressibility and volume changes of the fluid,
• control and drive influences (valves, servo controllers, friction compensation).

When the friction coefficient drops strongly at very low speed, motion becomes unstable: after breakaway, the required force is suddenly smaller than the force currently applied, whereby the system briefly accelerates until it returns to a range in which it adheres again or brakes strongly. Stick-slip is therefore more likely when stiffness is low and/or damping is low and when friction is markedly speed-dependent.

Stribeck Curve and Friction Regimes (Boundary, Mixed, Fluid Friction)

A common model is the Stribeck curve. It describes how the friction coefficient changes with a lubrication parameter that is broadly linked to speed, viscosity of the medium, and contact pressure. For dynamic seals, it is important in which friction regime work is being done:

At very low speed, the lubricating film is often not yet load-bearing. Then boundary or mixed friction dominates, and the friction coefficient can drop noticeably with rising speed. Precisely this drop in friction promotes stick-slip, because it amplifies the jump from too much force in the system to too little friction at the moment of start-up.

Stiction (Start-Up Adhesion) as a Special Case after Standstill

Stiction means start-up adhesion. What is meant is an intensified adhesion after standstill, because the lubricating film can be locally displaced or interrupted. As a result, the breakaway force rises. Stiction is therefore often the concrete trigger that sets stick-slip in motion, particularly when motion subsequently takes place in creep mode. The term is narrower than stick-slip, because it emphasizes the start state, while stick-slip describes the repeated stick-slip cycle.

Influencing Factors with Hydraulic and Pneumatic Seals

In sealing technology, stick-slip rarely depends on a single parameter. Frequently, a combination of material, geometry, mating surface, and operating conditions acts. Particularly susceptible are applications with low speed, long standstill periods, short strokes, or varying temperatures, because the lubrication state changes quickly there.

A compact classification of important influencing factors helps in root cause analysis:

Material and Seal Geometry (Preload, Contact Area)

Material and geometry determine how high friction and how large the breakaway force become. A higher preload increases contact pressure and thereby frequently static friction. Furthermore, many sealing materials are viscoelastic — that is, they deform in a time-dependent way. After standstill, this can change the contact conditions and increase the start-up force.

Low-friction material concepts, for example PTFE-based compounds, can lower friction and, above all, reduce the difference between static and sliding friction. For the stick-slip tendency, precisely this difference is decisive, because it determines how strongly the force jump becomes at the transition to sliding.

Mating Surface and Roughness (Lubricating Film Load Capacity)

The mating surface influences whether a lubricating film can build up stably. A surface that is too smooth can promote adhesion — that is, sticking in the contact. A surface that is too rough can repeatedly tear open the lubricating film, which intensifies friction fluctuations and wear.

In many cylinder applications, a defined surface structure is therefore favorable — for example a suitable honing pattern. It can store lubricant and support film build-up. Decisive is less a single roughness value than the functional structure that matches the sealing system and medium.

Detecting and Reducing Stick-Slip (Practical Checklist)

Stick-slip can usually first be recognized in the motion behavior. When a cylinder judders despite a constant control signal, this is a strong indication. In fluid technology, pressure fluctuations frequently appear alongside, because volume flow and motion are not uniform. With precise positioning tasks, poorer repeatability also shows, particularly in creep mode or when approaching end positions.

Symptoms in Operation (Diagnostic Indications)

Frequent indications are:

• Stop-and-go motion at low speed or during start-up
• Vibrations at the cylinder, the structure, or in the measurement signal
• Noises such as squeaking or knocking
• Pressure peaks or pulsating pressure profiles
• Control deviations and poorer positioning accuracy

When these effects occur primarily after standstill or with short strokes, they match the stick-slip mechanism well.

Measures (Material, Surface, Lubrication, System/Control)

Effective countermeasures usually follow two principles: reduce breakaway force and reduce friction fluctuations, so that the transition from sticking to sliding becomes less abrupt. In practice, a combination of seal design, surface, medium, and system parameters is often required.

In many cases, a short test pays off whether stick-slip disappears at somewhat higher speed. This indicates a lubrication state that only becomes stable at higher relative speed. If the effect persists, the cause frequently lies in contact pressure, surface, or system dynamics.

In the end, stick-slip is a system topic. When requirements for smooth running and positioning accuracy are high, specialized technical consultation on the design of seal, surface, and drive is often sensible.