Alibre Design

Chapter 19 - Alibre MotionForces External to the SystemThe law of Conservation of Momentum states that in the absence of any externalforces, the total momentum of a system is conserved. For simulation purposes, the onlyways to add external forces or torques to a system are:• Enabling Gravity: When it is enabled, gravity acts on the entire system, and aconstant magnitude and direction is used throughout (you can specify thecomponents of gravity, or choose the default value of 9.81 m/s 2 in the directionof the negative Y-Axis).• Constraining a Part to a “Fixed” or “Anchored” Part, or a Geometry Feature(Planes, Axes) within the main Assembly.Forces Internal to the SystemInternal forces can change the configuration of your assembled designs, but on theirown they cannot influence the total momentum, either linear or angular, of the system.• Constraints between “Moving Parts” can be considered as entirely internal tothe system.Note: You don’t need to explicitly distinguish between these in any way - AlibreMotion will simulate the Assembly as you have designed it.Physical Elements: Motors and Actuators, Springs and DampersOne step in the process of producing a simulation is specifying any physical elementsthat transmit, produce, or modify forces and torques. The Assembly constraints withinyour designs specify where physical elements can be placed, and there may be timesthat you add constraints to your design solely for the purpose of adding a physicalelement such as a spring. An enormous number of real-world interactions can beaccurately modeled by using combinations of these. Depending on the type ofconstraint and the details of its geometry, you may be able to add some or all of thefollowing:• Motor (or Rotary Actuator): Produces a torque or rotational motion. Forinstance, a car engine, a winch, or a DVD player motor.• Rotary Spring: Produces a torque opposing rotational displacement. Real-worldequivalents include a door hinge-spring and a coiled watch-spring.• Rotary Damper: Produces a torque opposing rotational motion, for instance acar clutch, or an electric generator. They can also be used to model “wet”friction.535