Friction in hydraulic cylinders
Simscape / Fluids / Hydraulics (Isothermal) / Hydraulic Cylinders
The Cylinder Friction block simulates friction in the contact between moving bodies in hydraulic cylinders. Use this block as a building block in combination with double- and single-acting cylinders to develop a cylinder model with friction. The block simulates the friction force as a function of relative velocity and pressure, and assumes this force is the sum of the Stribeck, Coulomb, and viscous components. The Coulomb friction force consists of the preload force, which is caused by the seal squeeze during assembly, and the force proportional to the pressure. The sum of the Coulomb and Stribeck friction forces at zero velocity is the breakaway friction force. For more information, see the Translational Friction block reference page.
The block approximates the friction force as
|fcfr||Coulomb friction coefficient|
|pA, pB||Pressures in cylinder chambers|
|Kbrk||Breakaway friction force increase coefficient|
|v||Relative velocity in the contact|
|fvfr||Viscous friction coefficient|
To avoid discontinuity at v = 0, the block introduces a small region, |v| ≤ vth, around zero velocity where the friction force is linearly proportional to velocity
R and C are mechanical translational conserving ports associated with the rod and case, respectively. A and B are hydraulic conserving ports that you connect to ports A and B of the cylinder block, as shown in this illustration. The force generated by the block always opposes relative motion between the rod and the case.
To set the priority and initial target values for the block variables prior to simulation, use the Initial Targets section in the block dialog box or Property Inspector. For more information, see Set Priority and Initial Target for Block Variables.
Nominal values provide a way to specify the expected magnitude of a variable in a model. Using system scaling based on nominal values increases the simulation robustness. Nominal values can come from different sources, one of which is the Nominal Values section in the block dialog box or Property Inspector. For more information, see Modify Nominal Values for a Block Variable.
A — Cylinder inlet
Hydraulic conserving port connected to the cylinder inlet.
B — Cylinder outlet
Hydraulic conserving port connected to the cylinder outlet.
R — Cylinder rod
Mechanical translational conserving port associated with the cylinder rod.
C — Cylinder clamping structure
Mechanical translational conserving port associated with the cylinder clamping structure.
Preload force — Preload force caused by seal squeeze during assembly
10 N (default) | nonnegative scalar
Preload force caused by the seal squeeze during assembly.
Coulomb friction force coefficient — Proportionality coefficient for Coulomb friction force
1e-6 N/Pa (default) | nonnegative scalar
Coulomb friction coefficient. This value defines the proportionality between the Coulomb friction force and the pressure in cylinder chambers.
Breakaway friction increase coefficient — Friction force increase over Coulomb friction
1 (default) | scalar greater than or equal to 1
Friction force increase over the Coulomb friction. The Coulomb friction force, multiplied by this coefficient, is breakaway friction force.
Viscous friction coefficient — Proportionality coefficient between viscous friction force and relative velocity
100 N/(m/s) (default) | nonnegative scalar
Proportionality coefficient between the viscous friction force and the relative velocity.
Transition approximation coefficient — Approximation of transition between breakaway and Coulomb frictions
10 s/m (default) | positive scalar
Value of the coefficient cv, which the
block uses for the approximation of the transition between the breakaway and the Coulomb
frictions. Assign this value based on the following considerations: the Stribeck
friction component reaches approximately 5% of its steady-state value at velocity
3/cv, and 2% at velocity
4/cv, which makes it
possible to develop an approximate relationship
vmin is the relative velocity at which
friction force has its minimum value. The default value,
corresponds to a minimum friction at a velocity of about
Linear region velocity threshold — Small vicinity near zero velocity where friction force is linearly proportional to relative velocity
1e-4 m/s (default) | positive scalar
Small vicinity near zero velocity where the friction force is linearly proportional
to the relative velocity. It is recommended that you use values in the range between
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Introduced in R2006b