ClutchClutch based on Coulomb friction 
This information is part of the Modelica Standard Library maintained by the Modelica Association.
This component models a clutch, i.e., a component with two flanges where friction is present between the two flanges and these flanges are pressed together via a normal force. The normal force fn has to be provided as input signal f_normalized in a normalized form (0 ≤ f_normalized ≤ 1), fn = fn_max*f_normalized, where fn_max has to be provided as parameter. Friction in the clutch is modelled in the following way:
When the relative angular velocity is not zero, the friction torque is a function of the velocity dependent friction coefficient mue(w_rel) , of the normal force "fn", and of a geometry constant "cgeo" which takes into account the geometry of the device and the assumptions on the friction distributions:
frictional_torque = cgeo * mue(w_rel) * fn
Typical values of coefficients of friction:
dry operation : mue = 0.2 .. 0.4 operating in oil: mue = 0.05 .. 0.1
When plates are pressed together, where ri is the inner radius, ro is the outer radius and N is the number of friction interfaces, the geometry constant is calculated in the following way under the assumption of a uniform rate of wear at the interfaces:
cgeo = N*(r0 + ri)/2
The positive part of the friction characteristic mue(w_rel), w_rel >= 0, is defined via table mue_pos (first column = w_rel, second column = mue). Currently, only linear interpolation in the table is supported.
When the relative angular velocity becomes zero, the elements connected by the friction element become stuck, i.e., the relative angle remains constant. In this phase the friction torque is calculated from a torque balance due to the requirement, that the relative acceleration shall be zero. The elements begin to slide when the friction torque exceeds a threshold value, called the maximum static friction torque, computed via:
frictional_torque = peak * cgeo * mue(w_rel=0) * fn (peak >= 1)
This procedure is implemented in a "clean" way by state events and leads to continuous/discrete systems of equations if friction elements are dynamically coupled. The method is described in (see also a short sketch in UsersGuide.ModelingOfFriction):
More precise friction models take into account the elasticity of the material when the two elements are "stuck", as well as other effects, like hysteresis. This has the advantage that the friction element can be completely described by a differential equation without events. The drawback is that the system becomes stiff (about 1020 times slower simulation) and that more material constants have to be supplied which requires more sophisticated identification. For more details, see the following references, especially (Armstrong and Canudas de Wit 1996):
See also the discussion State Selection in the User's Guide of the Rotational library.
phi_nominal 
Value: 1e4 Type: Angle (rad) Description: Nominal value of phi_rel (used for scaling) 

stateSelect 
Value: StateSelect.prefer Type: StateSelect Description: Priority to use phi_rel and w_rel as states 
w_small 
Value: 1.0e10 Type: AngularVelocity (rad/s) Description: Relative angular velocity near to zero if jumps due to a reinit(..) of the velocity can occur (set to low value only if such impulses can occur) 
useHeatPort 
Value: false Type: Boolean Description: =true, if heatPort is enabled 
mue_pos 
Value: [0, 0.5] Type: Real[:,2] Description: [w,mue] positive sliding friction coefficient (w_rel>=0) 
peak 
Value: 1 Type: Real Description: Peak for maximum value of mue at w==0 (mue0_max = peak*mue_pos[1,2]) 
cgeo 
Value: 1 Type: Real Description: Geometry constant containing friction distribution assumption 
fn_max 
Value: Type: Force (N) Description: Maximum normal force 
flange_a 
Type: Flange_a Description: Left flange of compliant 1dim. rotational component 


flange_b 
Type: Flange_b Description: Right flange of compliant 1dim. rotational component 

heatPort 
Type: HeatPort_a Description: Optional port to which dissipated losses are transported in form of heat 

f_normalized 
Type: RealInput Description: Normalized force signal 0..1 (normal force = fn_max*f_normalized; clutch is engaged if > 0) 
Modelica.Mechanics.Rotational.Examples Drive train with clutch and brake 

Modelica.Mechanics.Rotational.Examples Drive train with 3 dynamically coupled clutches 

Modelica.Mechanics.Rotational.Examples Demonstrate the modeling of heat losses 

Modelica.Mechanics.Rotational.Examples Simple Gearshift 