Frictional brake with two pivoted shoes diametrically positioned about a rotating drum

Brakes & Detents/Rotational

The block represents a frictional brake with two pivoted rigid shoes that press against a rotating drum to produce a braking action. The rigid shoes sit inside or outside the rotating drum in a diametrically opposed configuration. A positive actuating force causes the rigid shoes to press against the rotating drum. Viscous and contact friction between the drum and the rigid shoe surfaces cause the rotating drum to decelerate. Double-shoe brakes provide high braking torque with small actuator deflections in applications that include motor vehicles and some heavy machinery. The model employs a simple parameterization with readily accessible brake geometry and friction parameters.

In this schematic, a) represents an internal double-shoe brake,
and b) represents an external double-shoe brake. In both configurations,
a positive actuation force *F* brings the shoe and
drum friction surfaces into contact. The result is a friction torque
that causes deceleration of the rotating drum. Zero and negative forces
do not bring the shoe and drum friction surfaces into contact and
produce zero braking torque.

The model uses the long-shoe approximation. Contact angles smaller than 45° produce less accurate results. The equations for the friction torque that the leading and trailing shoes develop are:

$${T}_{LS}=\frac{c\mu {p}_{a}{r}_{D}{}^{2}\left(\mathrm{cos}{\theta}_{sb}-\mathrm{cos}{\theta}_{s}\right)}{\mathrm{sin}{\theta}_{a}},$$

$${T}_{TS}=\frac{c\mu {p}_{b}{r}_{D}{}^{2}\left(\mathrm{cos}{\theta}_{sb}-\mathrm{cos}{\theta}_{s}\right)}{\mathrm{sin}{\theta}_{a}},$$

$$c={r}_{a}+{r}_{p}\mathrm{cos}{\theta}_{p},$$

where for $$0\le {\theta}_{s}\le \raisebox{1ex}{$\pi $}\!\left/ \!\raisebox{-1ex}{$2$}\right.$$,

$${\theta}_{a}={\theta}_{s},$$

and for $${\theta}_{s}\ge \raisebox{1ex}{$\pi $}\!\left/ \!\raisebox{-1ex}{$2$}\right.$$,

$${\theta}_{a}=\frac{\pi}{2}.$$

The parameters in the friction torque equations are:

Parameter | Description |
---|---|

T_{LS} | Brake torque the leading shoe develops |

T_{TS} | Brake torque the trailing shoe develops |

μ | Effective contact friction coefficient |

p_{a} | Maximum linear pressure in the leading shoe-drum contact |

p_{b} | Maximum linear pressure in the trailing shoe-drum contact |

r_{D} | Drum radius |

θ_{sb} | Shoe beginning angle |

θ_{s} | Shoe span angle |

θ_{a} | Angle from hinge pin to maximum pressure point |

c | Arm length of the cylinder force with respect to the hinge pin |

r_{p} | Pin location radius |

θ_{p} | Hinge pin location angle |

r_{a} | Actuator location radius |

The model assumes that only Coulomb friction acts at the shoe-drum surface contact. Zero relative velocity between the drum and the shoes produces zero Coulomb friction. To avoid discontinuity at zero relative velocity, the friction coefficient formula employs this hyperbolic function

$$\mu ={\mu}_{Coulomb}\mathrm{tanh}\left(\frac{4{\omega}_{shaft}}{{\omega}_{threshold}}\right),$$

where:

Parameter | Description |
---|---|

μ | Effective contact friction coefficient |

μ_{Coulomb} | Contact friction coefficient |

ω_{shaft} | Shaft velocity |

ω_{threshold} | Angular velocity threshold |

Balancing the moments that act on each shoe with respect to the pin yields the pressure acting at the shoe-drum surface contact. The equations for determining the balance of moments for the leading shoe are

$$F=\frac{{M}_{N}-{M}_{F}}{c},$$

$${M}_{N}=\frac{{p}_{a}{r}_{p}{r}_{D}}{\mathrm{sin}{\theta}_{a}}\left(\frac{1}{2}\left[{\theta}_{s}-{\theta}_{sb}\right]-\frac{1}{4}\left[\mathrm{sin}2{\theta}_{s}-\mathrm{sin}2{\theta}_{sb}\right]\right),$$

and

$${M}_{F}=\frac{\mu {p}_{a}{r}_{D}}{\mathrm{sin}{\theta}_{a}}\left({r}_{D}\left[\mathrm{cos}{\theta}_{sb}-\mathrm{cos}{\theta}_{s}\right]+\frac{{r}_{p}}{4}\left[\mathrm{cos}2{\theta}_{s}-\mathrm{cos}2{\theta}_{sb}\right]\right),$$

where:

Parameter | Description |
---|---|

F | Actuation Force |

M_{N} | Moment acting on the leading shoe due to normal force |

M_{F} | Moment acting on the leading shoe due to friction force |

c | Arm length of the cylinder force with respect to the hinge pin |

p_{a} | Maximum linear pressure at the shoe-drum contact surface |

r_{p} | Pin location radius |

θ_{p} | Hinge pin location angle |

r_{a} | Actuator location radius |

The model does not simulate self-locking brakes. If brake geometry and friction parameters
cause a self-locking condition, the model produces a simulation error. A brake
self-locks if the friction moment exceeds the moment due to normal forces, that is, when
*M _{F}* >

The balance of moments for the trailing shoe is

$$F=\frac{{M}_{N}+{M}_{F}}{c}.$$

The net braking torque is

$$T={T}_{LS}+{T}_{TS}+{\mu}_{visc}*{\omega}_{shaft},$$

where *μ _{visc}* is the viscous friction
coefficient.

Use the **Variables** tab to set the priority
and initial target values for the block variables before simulating.
For more information, see Set Priority and Initial Target for Block Variables (Simscape).

Unlike block parameters, variables do not have conditional visibility.
The **Variables** tab lists all the existing
block variables. If a variable is not used in the set of equations
corresponding to the selected block configuration, the values specified
for this variable are ignored.

The brake uses the long-shoe approximation.

The brake geometry does not self-lock.

The model does not account for actuator flow consumption.

You can model the effects of heat flow and temperature change
through an optional thermal conserving port. By default, the thermal
port is hidden. To expose the thermal port, right-click the block
in your model and, from the context menu, select **Simscape** > **Block
choices**. Select a variant that includes a thermal port.
Specify the associated thermal parameters for the component.

`F`

Physical signal port that represents the brake actuating force

`S`

Rotational conserving port that represents the rotating drum shaft

`H`

Thermal conserving port. The thermal port is optional and is hidden by default. To expose the port, select a variant that includes a thermal port.

**Drum radius**Radius of the drum contact surface. The parameter must be greater than zero. The default value is

`150`

`mm`

.**Actuator location radius**Distance between the drum center and the force line of action. The parameter must be greater than zero. The default value is

`100`

`mm`

.**Pin location radius**Distance between the hinge pin and drum centers. The parameter must be greater than zero. The default value is

`125`

`mm`

.**Pin location angle**Angular coordinate of the hinge pin location from the brake symmetry axis. The parameter must be greater than or equal to zero. The default value is

`15`

`deg`

.**Shoe beginning angle**Angle between the hinge pin and the beginning of the friction material linen of the shoe. The value of the parameter must be in the range 0 ≤ θ

_{sb}≤ (π-pin location angle). The default value is`5`

`deg`

.**Shoe span angle**Angle between the beginning and the end of the friction material linen on the shoe. The value of the parameter must be in the range 0 < θ

_{sb}≤ (π -pin location angle - shoe beginning angle). The default value is`120`

`deg`

.

**Viscous friction coefficient**Value of the viscous friction coefficient at the contact surface. The parameter must be greater than or equal to zero. The default value is

`.01`

`n*m/(rad/s)`

.**Temperature**Array of temperatures used to construct a 1-D temperature-efficiency lookup table. The array values must increase left to right. The default value is

`[280.0, 300.0, 320.0]`

`K`

.This parameter is visible only when you select a block variant that includes a thermal port.

**Contact friction coefficient**Value of the Coulomb friction coefficient at the belt-drum contact surface. The value is greater than zero. Unless you select a block variant that includes a thermal port, the default value is

`0.3`

.If you select a block variant that includes a thermal port, you specify this parameter as array. The array is the same size as the array for the

**Temperature**parameter. The default value for the thermal variant is`[0.1, 0.05, 0.03]`

.**Angular velocity threshold**Angular velocity at which the contact friction coefficient practically reaches its steady-state value. The parameter must be greater than zero. The default value is

`0.01`

`rad/s`

.

**Thermal mass**Thermal energy required to change the component temperature by a single degree. The greater the thermal mass, the more resistant the component is to temperature change. This parameter is enabled only if you select a block variant that includes a thermal port. The default value is

`50`

`kJ/K`

.