Stiction is the resistance to starting relative motion between contacting surfaces. In the simple friction model used here, the maximum static resistance is μsN. Once sliding starts, resistance is represented by μkN, which is commonly lower. Compare those forces and the available applied-force margin below. The calculator shows force thresholds; it does not predict a moving system’s vibration or control-loop response.
Stiction Interactive Calculator
Vary normal load, static and kinetic friction coefficients, and maximum applied force. The animation ramps the applied force so the breakaway threshold and lower sliding resistance are visible.
Equation Used
- Rigid block on a level surface with constant normal load.
- User-supplied constant coefficients; no speed, dwell, temperature or lubrication dependence.
- No acceleration, mass, compliance, Stribeck curve or servo dynamics.
- Animated position is explanatory, not predicted displacement.
The four inputs determine breakaway force, sliding resistance, their difference, and the applied-force margin. The block stays in one position while the force arrows change; displacement is not calculated.
How stiction works
While the block is stationary, static friction balances the applied tangential force up to a limiting value. It is not always equal to μsN: that product is the threshold available before sliding. Above it, this illustration shows the chosen kinetic resistance μkN.
The difference between the two values helps explain why a contact may resist starting and then require less force to keep sliding. Whether a machine repeatedly sticks and slips also depends on its mass, stiffness, damping, driving motion and control system. Those dynamics are outside this force comparison.
What each input means
- Normal load: the force pressing the surfaces together, perpendicular to their contact.
- Static coefficient: the selected ratio of maximum static resistance to normal force.
- Kinetic coefficient: the selected ratio of sliding resistance to normal force.
- Applied force: the maximum tangential force reached during the animated ramp.
Real coefficients depend on the contact and operating conditions. This calculator holds them constant; it does not turn a material name, lubricant, dwell time or surface finish into a friction coefficient.
Where breakaway friction matters
- Valves and dampers: a drive must overcome resistance before the controlled part begins moving.
- Linear slides: starting force can differ from the force needed during continuous travel.
- Sealed cylinders: seals contribute to the difference between initial and running resistance.
- Positioning systems: friction near rest can affect small moves and reversals.
These are contexts for understanding breakaway, not diagnoses of a particular machine. A force threshold alone cannot identify the cause of hunting, predict positioning error, or establish a maintenance interval.
The force equations
Fs = μsN; Fk = μkN; force drop = Fs − Fk; breakaway margin = Fapplied − Fs.
N is normal force, μs and μk are dimensionless coefficients, and all force outputs use the same units as N. With inputs in newtons, the results are in newtons.
A positive margin means the selected maximum applied force exceeds the modeled static threshold. A negative margin means it falls short. Neither result gives acceleration or stopping distance. The force drop is a comparison between two resistance values, not a predicted sudden displacement.
Worked example: comparing assumed friction coefficients
Consider a hypothetical contact with N = 180 N, μk = 0.04 and a maximum applied force of 20 N. Compare three assumed static coefficients: 0.05, 0.10 and 0.20. These are example inputs, not measured lubricant or bearing specifications.
- Breakaway forces are 0.05 × 180 = 9 N, 0.10 × 180 = 18 N and 0.20 × 180 = 36 N.
- Sliding resistance is unchanged at 0.04 × 180 = 7.2 N. The corresponding force drops are 1.8 N, 10.8 N and 28.8 N.
- The 20 N applied-force margins are 11 N, 2 N and −16 N. The selected force exceeds the first two thresholds but not the third.
The comparison isolates the effect of the static coefficient. It does not predict a servo’s overshoot, establish a lubrication schedule, or describe a tested product.
Comparing friction approaches
Sliding contacts, rolling guides and non-contact supports have different friction mechanisms. A single assumed coefficient is useful for this elementary force comparison, but it does not provide universal performance rankings.
| Approach | What to evaluate |
|---|---|
| Sliding contact | Contact conditions, normal load, lubrication and measured starting versus running resistance. |
| Rolling guide | Preload, seals, alignment and the guide’s specified or measured resistance. |
| Non-contact support | Support forces, supply requirements, stiffness and losses elsewhere in the mechanism. |
Compare actual component data under the intended conditions. The calculator does not estimate component cost, service life or positioning resolution.
Frequently asked questions
Why does the block stay still in the animation?
The animation compares force arrows during a ramp. Mass, elapsed time and a motion equation are not inputs, so the block position does not represent calculated travel.
Is static friction always equal to μsN?
No. Below breakaway, static friction balances the applied tangential force. μsN is the modeled maximum static resistance.
What is the difference between force drop and breakaway margin?
Force drop is μsN minus μkN. Breakaway margin is the selected maximum applied force minus μsN. One compares friction values; the other compares the drive force with the static threshold.
Where should the friction coefficients come from?
Use measurements or applicable contact data for the actual surfaces and operating conditions. The calculator does not assign coefficients from a material name or lubricant.
Why can the displayed force drop be negative?
The inputs allow μk to exceed μs. In that case the arithmetic difference is negative and there is no modeled drop in resistance after breakaway. Check whether those coefficients describe the intended contact.
Does this calculation include dwell time or lubricant condition?
No. Both coefficients remain constant. Effects of dwell, temperature, speed and lubrication must be reflected in suitable input data or a more detailed model.
Can this predict stick-slip or tune a control loop?
No. The force thresholds can help explain breakaway, but this calculator has no mass, compliance, damping or feedback dynamics. It cannot predict oscillation, overshoot or settling time.
References
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