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.
Select how to model the clutch friction geometry. The default
Define effective radius.
Number N of friction-generating contact surfaces
inside the clutch. The default is
Effective area A of the clutch piston when
the piston is applying pressure across the clutch. The default is
From the drop-down list, choose units. The default is meters-squared
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
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.
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
kinetic friction coefficient.
Fixed kinetic friction coefficient —
Model the kinetic friction coefficient as a constant.
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.
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
Dimensionless de-rating factor D that accounts
for clutch disk wear by proportionately reducing clutch friction.
The default value is
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, .
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
Minimum pressure Pth at
which the clutch engages. If the pressure input signal falls below
this threshold, the clutch automatically disengages. The default value
Viscous friction coefficient μ applied
to the relative slip ω between the base and
follower axes. The default is
From the drop-down list, choose units. The default is newton-meters/(radians/second)
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
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 (P ≥ Pth), 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.
This table summarizes the clutch variables.
|ω||Relative angular velocity (slip)||ωF – ωB|
|ωTol||Slip tolerance for clutch locking||See the following model|
|P, Pth||Clutch pressure and threshold||Input pressure applied to clutch discs; |
threshold clutch pressure: Pth, P > 0.
|Pfric||Clutch friction capacity||max[(P – Pth), 0]|
|D||Clutch de-rating factor||See the following model|
|N||Number of friction surfaces||See the following model|
|A||Engagement surface area||See the following model|
|reff||Effective torque radius||Effective moment arm of clutch friction force|
|kK||Kinetic friction coefficient||Dimensionless coefficient of kinetic friction of clutch discs. Function of ω.|
|kS||Static friction coefficient||Dimensionless coefficient of static friction of clutch discs.|
|μ||Viscous drag coefficient||See the following model|
|τK||Kinetic friction torque||See the following model|
|τS||Static friction torque limit||(static friction peak factor)·(kinetic friction torque
for ω → 0)|
(See the following model)
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.
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:
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:
τS ≡ τS+ = –τS– .
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 (P ≥ Pth).
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