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Disk Friction Clutch

Disk friction clutch or brake with kinetic and static friction and controllable pressure




A friction clutch transfers torque between two driveline axes by coupling them with friction. The Disk Friction Clutch block models a friction clutch based on disks in contact, with kinetic friction and static (locking) friction acting on the two axes. For model details, see Disk Friction Clutch Model.


B and F are rotational conserving ports representing, respectively, the clutch input (base) and output (follower) driveshaft axes. The clutch motion is measured as the slip ω = ωFωB, the angular velocity of follower relative to base.

The clutch requires a physical signal input P that represents the hydraulic pressure (in pascals) modulating the applied kinetic friction. This signal should be positive or zero. A signal P less than zero is interpreted as zero.

Dialog Box and Parameters


Force action region

Select how to model the clutch friction geometry. The default is Define effective radius.

  • Define effective radius — Model friction geometry in terms of disk radius.

     Effective Radius

  • Define annular region — Model friction geometry in terms of annulus dimensions. If you select this option, the panel changes from its default.

     Annular Region

Number of friction surfaces

The number N of friction-generating contact surfaces inside the clutch. The default is 4.

Engagement piston area

The effective area A of the clutch piston when the piston is applying pressure across the clutch. The default is 0.001.

From the drop-down list, choose units. The default is meters-squared (m^2).


Friction model

Select how to model the dimensionless Coulomb kinetic friction coefficient kK across the clutch when the clutch is slipping. The default is Fixed kinetic friction coefficient.

  • Fixed kinetic friction coefficient — Model Coulomb kinetic friction in terms of a constant kinetic friction coefficient.

     Fixed Kinetic Friction Coefficient

  • Table lookup kinetic friction coefficient — Model Coulomb kinetic friction in terms of a kinetic friction coefficient lookup function defined at discrete relative velocity values. If you select this option, the panel changes from its default.

     Table Lookup Kinetic Friction Coefficient

Static friction coefficient

Dimensionless Coulomb static friction coefficient kS applied to the normal force across the clutch when the clutch is locked. Must be larger than kK. The default is 0.35.

De-rating factor

Dimensionless de-rating factor D that accounts for clutch disk wear by proportionately reducing clutch friction. The default is 1.

Clutch velocity tolerance

The minimum relative angular speed ωTol above which the clutch cannot lock. Below this speed, the clutch can lock. The default is 1e-3.

From the drop-down list, choose units. The default is radians/second (rad/s).

Engagement threshold pressure

The minimum pressure Pth needed to engage the clutch. This lower bound applies to the physical signal input pressure P. If P falls below this value, the clutch applies no pressure. The default is 100.

From the drop-down list, choose units. The default is pascals (Pa).

Viscous Drag

Viscous drag torque coefficient

Viscous friction coefficient μ applied to the relative slip ω between the base and follower axes. The default is 0.

From the drop-down list, choose units. The default is newton-meters/(radians/second) (N*m/(rad/s)).

Initial Conditions

Initial state

From the drop-down list, choose whether the clutch starts simulation in a locked or unlocked state. The default is Unlocked.

Disk Friction Clutch Model

The Disk Friction Clutch is based on the Fundamental Friction Clutch. For the complete friction clutch model, consult the Fundamental Friction Clutch block reference page. This section discusses the simplified model implemented in the Disk Friction Clutch.

When you apply a pressure signal above threshold (PPth), the Disk Friction Clutch block can apply two kinds of friction to the driveline motion, kinetic and static. The clutch applies kinetic friction torque only when one driveline axis is spinning relative to the other driveline axis. The clutch applies static friction torque when the two driveline axes lock and spin together. The block iterates through multistep testing to determine when to lock and unlock the clutch.

Clutch Variable, State, and Mode Summary

This table summarizes the clutch variables.

Clutch Variables

ωRelative angular velocity (slip)ωFωB
ωTolSlip tolerance for clutch lockingSee the following model
P, PthClutch pressure and thresholdInput pressure applied to clutch discs;
threshold clutch pressure: Pth, P > 0.
PfricClutch friction capacitymax[(PPth), 0]
DClutch de-rating factorSee the following model
NNumber of friction surfacesSee the following model
AEngagement surface areaSee the following model
reffEffective torque radiusEffective moment arm of clutch friction force
kKKinetic friction coefficientDimensionless coefficient of kinetic friction of clutch discs. Function of ω.
kSStatic friction coefficientDimensionless coefficient of static friction of clutch discs.
μViscous drag coefficientSee the following model
τKKinetic friction torqueSee the following model
τSStatic friction torque limit(static friction peak factor)·(kinetic friction torque for ω → 0)
(See the following model)

Relation to Fundamental Friction Clutch

Instead of requiring the kinetic and static friction limit torques as input signals, the Disk Friction Clutch calculates the kinetic and static friction from the clutch parameters and the input pressure signal P.

Kinetic Friction

The kinetic friction torque is the positive sum of viscous drag and surface contact friction torques:

τK = μω + τcontact .

(The kinetic friction torque opposes the relative slip and is applied with an overall minus sign.) The contact friction is a product of six factors:

τcontact = kK·D·N·reff·Pfric·A ≥ 0 .

You specify the kinetic friction coefficient kK as either a constant or a tabulated discrete function of relative angular velocity ω. The tabulated function is assumed to be symmetric for positive and negative values of the relative angular velocity, so that you need to specify kK for positive values of ω only.

The clutch applies a normal force from its piston as the product of the clutch friction capacity Pfric and engagement surface area A, on each of N friction surfaces. The pressure signal P should be nonnegative. If P is less than Pth, the clutch applies no friction at all.

The effective torque radius reff is the effective radius, measured from the driveline axis, at which the kinetic friction forces are applied at the frictional surfaces. It is related to the geometry of the friction surface by:

ro and ri are the outer and inner radii, respectively, of the friction surface, modeled as an annular disk.

The clutch de-rating factor D accounts for clutch wear. For a new clutch, D is one. For a clutch approaching a "uniform wear" state:

Static Friction

The static friction limit is related to the kinetic friction, setting ω to zero and replacing the kinetic with the static friction coefficient:

τS = kS·D·N·reff·Pfric·A ≥ 0 .

kS > kK, so that the torque τ needed across the clutch to unlock it by overcoming static friction is larger than the kinetic friction at the instant of unlocking, when ω = 0.

The static friction torque range or limits are then defined symmetrically as:

τSτS+ = –τS .

Wait State: Locking and Unlocking

The Wait state of the Disk Friction Clutch is identical to the Wait state of the Fundamental Friction Clutch, with the replacement of the positive kinetic friction condition (τK > 0) by the positive clutch friction capacity condition (PPth).

Power Dissipated by the Clutch

The power dissipated by the clutch is |ω·τK|. The clutch dissipates power only if it is both slipping (ω ≠ 0) and applying kinetic friction (τK > 0).

See Also

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