Disk Friction Clutch

Friction clutch with disk plates that engage when plate pressure exceeds threshold




This block represents a friction clutch with two flat friction plate sets that come into contact in order to engage. The clutch engages when the applied plate pressure exceeds an engagement threshold pressure. Once engaged, the plates experience frictional torques that enable them to transmit power between the base and follower driveshafts.

The clutch can be bidirectional or unidirectional. A bidirectional clutch can slip in the positive and negative directions. A unidirectional clutch can slip only in the positive direction. The slip direction is positive if the follower shaft spins faster than the base shaft and negative if it slips slower. The block defines the slip velocity as the difference



  • ω is the slip velocity.

  • ωF is the angular velocity of the follower driveshaft.

  • ωB is the angular velocity of the base driveshaft.

The block provides a physical signal input port (P) for the applied pressure between the clutch plates. The applied pressure must be greater than or equal to zero and has units of Pascals. If the input signal falls below zero, the block treats the plate pressure 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

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

Engagement piston area

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).


Slip directions the clutch allows between its plates. A bidirectional clutch allows positive and negative slip velocities. A unidirectional clutch allows only positive slip velocities. The default setting is Bidirectional.

The unidirectional clutch is equivalent to a friction clutch connected in parallel to a one-way clutch that disengages only when the slip velocity becomes positive. To model a unidirectional clutch with slip in the negative direction, reverse the base and follower port connections.


Friction model

Modeling approach for the kinetic friction coefficient. You can treat the coefficient as a constant or as a function of the clutch slip velocity in a lookup table format. The default setting is Fixed kinetic friction coefficient.

  • Fixed kinetic friction coefficient — Model the kinetic friction coefficient as a constant.

     Fixed Kinetic Friction Coefficient

  • Table lookup kinetic friction coefficient — Model the kinetic friction coefficient as a function of the clutch slip velocity that you provide in a lookup table format.

     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 value is 0.35.

De-rating factor

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

Clutch velocity tolerance

Maximum slip velocity at which the clutch can lock. The slip velocity is the signed difference between the base and follower shaft angular velocities, that is, w=wFwB. The clutch locks if the actual slip velocity falls below the velocity tolerance and if other conditions are present—i.e., if the kinetic friction torque is nonzero and if the transferred torque is within the static friction torque limits. The default value is 0.001 rad/s.

Engagement threshold pressure

Minimum pressure Pth at which the clutch engages. If the pressure input signal falls below this threshold, the clutch automatically disengages. The default value is 100.

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

Clutch state at the start of simulation. The clutch can be in one of two states, locked and unlocked. A locked clutch constrains the base and follower shafts to spin at the same velocity, i.e., as a single unit. An unlocked clutch allows the two shafts to spin at different velocities, resulting in slip between the clutch plates. The default setting 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:

D34(ro+ ri)2ro2+rori+ ri2

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).


These Simscape™ Driveline™ example models contain working examples of disk friction clutches that change gear couplings:


  • B — Rotational conserving port that represents the base driveshaft

  • F — Rotational conserving port that represents the follower driveshaft

  • P — Physical signal input port for the applied pressure between the clutch plates

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