Cylinder Friction (IL)
Piston friction in a hydraulic cylinder in an isothermal liquid network.
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Description
The Cylinder Friction (IL) block simulates the friction of a piston in a hydraulic cylinder in an isothermal liquid network. The total friction is a combination of Stribeck, Coulomb and viscosity effects. The Coulomb component of friction includes the initial force generated by the seal and the effect of pressure in the cylinder.
Stribeck and viscous friction forces
,
where
-
- value of parameter Breakaway to Coulomb friction force ratio, ;
-
- cylinder piston displacement speed, ;
-
- static torque threshold speed; , where is the value of the Breakaway friction velocity parameter. The block sets the transition range from 0 to the threshold static torque velocity to ensure smooth modelling of the friction force;
-
- dry (Coulomb) friction.
The viscous friction force depends on the Viscous friction coefficient and is proportional to the piston speed.
.
Coulomb friction force
Coulomb friction force is a force directed opposite to the velocity of a body. The motion of a piston creates a radial stress inside a stationary cylinder body, which increases as the internal fluid in the cylinder is compressed. The radial stress is normal to the direction of motion of the piston and results in a Coulomb friction force that opposes the motion of the piston.
The Coulomb friction force is
,
where is the relative velocity between ports R and C, . - is the threshold velocity for the Coulomb force,
.
The threshold velocity for the Coulomb force is a different value to the threshold velocity for the static force that the block uses to calculate the Stribeck friction force, although they both act as a threshold region to ensure smooth modelling of the Coulomb force and the Stribeck force.
- force contribution from seal preloading and cylinder pressure,
where
-
- is the value of the Preload force parameter;
-
- is the value of the Coulomb friction force coefficient parameter;
-
and are the pressures at the cylinder ports A and B respectively. These values are overpressure relative to ambient pressure. These values can be set as atmospheric or other user-defined values using the Environment pressure specification parameter.
Ports
A - inlet or outlet port
isothermal liquid
The isothermal liquid port corresponds to inlet or outlet. This unit has no internal directionality.
B - inlet or outlet port
isothermal liquid
The isothermal liquid port corresponds to inlet or outlet. This unit has no internal directionality.
R - stem
translational mechanics
A mechanical progressive port corresponding to a rod, that is, a moving body.
C is the body
translational mechanics
A mechanical progressive port corresponding to a hull, that is, a stationary body.
Parameters
Actuator
Same fluid on both sides - option to simulate the same fluid on both sides of the block
on (by default) | off
Whether the same fluid is modelled on both sides of the block. If the parameter is checked, the fluid properties are propagated through the block. If unchecked, the chambers of the block are connected to isolated networks of fluids with different properties.
Main
Breakaway to Coulomb friction force ratio - ratio of breakaway force to Coulomb friction force
1.2 (by default) | positive scalar
Breakaway to Coulomb friction force ratio .
Breakaway friction velocity - threshold of motion against the friction force
0.1 m/s (by default) | `positive scalar'.
Threshold value for movement against friction velocity.
Preload force - initial cylinder force generated during seal assembly, N
20 N (By default) | scalar
The initial force in the cylinder due to the seal assembly. The block uses this parameter to calculate the Coulomb friction force.
Coulomb friction force coefficient - Coulomb friction force coefficient
1e-6 N/Pa (by default)| `positive scalar'.
Coulomb friction force coefficient. The block uses this parameter to calculate the Coulomb friction force.
Viscous friction coefficient - viscous friction coefficient
100 N/(s*m) (by default) | positive scalar.
Viscous friction coefficient. This parameter is used to calculate the viscous friction force.
Environment pressure specification - method of specifying the ambient pressure
Atmospheric pressure (by default) | Specified pressure.
Specifies the ambient pressure specification method for Coulomb friction calculations:
-
Atmospheric pressure- atmospheric pressure. -
Specified pressure- the exact value specified in the Environment pressure parameter.
Environment pressure - ambient pressure
0.101325 MPa (by default).
User-defined pressure value for Coulomb friction calculation.
Dependencies
Available when Environment pressure specification is set to Specified pressure.