Rack and pinion gear coupling translational and rotational motion, with adjustable pinion radius and friction losses

Simscape / Driveline / Gears / Rotational-Translational

The Rack & Pinion block represents rack and pinion gear that converts between
translational and rotational motion. The rotational-translational gear constrains the
pinion (P) and rack (R) to, respectively, rotate and translate together in a fixed ratio
that you specify. You can choose whether the rack axis translates in a positive or
negative direction, as the pinion rotates in a positive direction, by using the
**Rack direction** parameter.

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.

R_{RP} | Rack-pinion gear ratio |

ω_{P} | Angular velocity of the pinion shaft |

v_{R} | Translational velocity of the rack |

r_{P} | Effective radius of the pinion |

N_{P} | Number of teeth on the pinion |

x_{R} | Rack tooth spacing |

τ_{P} | Pinion shaft torque |

F_{R} | Rack force |

F_{loss} | Total loss force |

F_{Coul} | Friction force |

η | Torque transfer efficiency |

p_{th} | Power threshold |

μ_{P} | Viscous friction coefficient for the pinion shaft |

μ_{R} | Viscous friction coefficient for the rack motion |

Rack & Pinion imposes one kinematic constraint on the two connected axes:

*ω*_{P} =
*R*_{RP}*v*_{R}
.

The transmission ratio is:

*R*_{RP} = 1 /
*r*_{P} =
*ω*_{P} /
*v*_{N} = ± 2*π*
/
*N*_{P}*v*_{R}
.

The two degrees of freedom are reduced to one independent degree of freedom. The forward-transfer gear pair convention is (1,2) = (P,R).

The torque-force transfer is:

*R*_{RP}*τ*_{P}
+ *F*_{R} –
*F*_{loss} = 0 ,

with *F*_{loss} = 0 in the ideal case.

In the nonideal case, *F*_{loss} ≠ 0. For general considerations on nonideal gear modeling, see Model Gears with Losses.

In a nonideal pinion-rack pair (P,R), the angular velocity and geometric constraints are unchanged. But the transferred torque, force, and power are reduced by:

Coulomb friction between teeth surfaces on P and R, characterized by constant efficiency

*η*Viscous coupling of driveshafts with bearings, parametrized by viscous friction coefficients

*μ*

The efficiency *η* of meshing between pinion and rack is fully
active only if the transmitted power is greater than the power threshold.

If the power is less than the threshold, the actual efficiency is automatically regularized to unity at zero velocity.

Efficiency is assumed equal for both the forward and reverse power flow.

The viscous friction coefficients *μ*_{P} and
*μ*_{R} control the viscous friction torque
and force experienced by the rack and pinion from lubricated, nonideal bearings. The
viscous friction torque on the pinion axis is
–*μ*_{P}*ω*_{P}.
The viscous friction force on the rack motion is
–*μ*_{R}*v*_{R}.

Gear inertia is assumed negligible.

Gears are treated as rigid components.

Coulomb friction slows down simulation. See Adjust Model Fidelity.

Port | Description |
---|---|

P | Rotational conserving port representing the pinion |

R | Translational conserving port representing the rack |

H | Thermal conserving port for modeling heat transfer |

P is a rotational conserving port. R is a translational conserving port. They represent the pinion and the rack, respectively.

**Parameterize by**Select how to parameterize the rack and pinion gear. The default is

`Pinion radius`

.`Pinion radius`

— Gear ratio is defined by the effective radius of the pinion.**Pinion radius**Effective radius of the pinion

*r*_{P}. Must be greater than zero. The default is`100`

.From the drop-down list, choose units. The default is millimeters (

`mm`

).

`Tooth parameters`

— Gear ratio is defined by the number of teeth on the pinion gear and the rack tooth spacing. If you select this option, the panel changes from its default.

**Rack direction**Choose whether the rack axis translates in a positive or negative direction when the pinion rotates in a positive direction. The default is

`Positive for positive pinion rotation`

.

Parameters for meshing and friction losses vary with the block variant chosen—one with a thermal port for thermal modeling and one without it.

**Pinion rotational viscous friction coefficient**Viscous friction coefficient

*μ*_{P}for the pinion shaft. The default is`0`

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

`N*m/(rad/s)`

).**Rack translational viscous friction coefficient**Viscous friction coefficient

*μ*_{R}for the rack motion. The default is`0`

.From the drop-down list, choose units. The default is newton/(meters/second) (

`N/(m/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. The default value is

`50`

J/K.**Initial temperature**Component temperature at the start of simulation. The initial temperature alters the component efficiency according to an efficiency vector that you specify, affecting the starting meshing or friction losses. The default value is

`300`

K.

For optimal simulation performance, use the **Meshing Losses** > **Friction model** parameter default setting, ```
No meshing losses - Suitable
for HIL simulation
```

.