Alibre Design

Chapter 19 - Alibre MotionAlibre Motion needs to know the mass and the moments of inertia of a part in order tosimulate it. These are calculated by Alibre Motion from the density of the Part, and thevolume and shape of its design. Be sure to verify the correct material density has beenspecified for each part, whether you are designing new parts, or importing existingones.Group Non-Independent Parts into Sub-AssembliesThe efficiency of a simulator is always dependent on the number of moving parts it hasto analyse and compute. In designs with Parts that are locked together and cannotmove independently of one another, simulation will be more efficient if they aregrouped into sub-assemblies, that the simulator can treat as one object. No loss ofsimulation accuracy is to be expected in these cases, as the physical characteristics of asub-assembly are calculated from its constituent parts. However in many cases,especially where there are numerous constraints and parts, the benefits in performanceand efficiency can be significant.Include Nuts and Bolts In Sub-AssembliesAn extension of the last point with regard to small items, whose dynamics are rarely ofinterest, is that it is usually best to include them in a subassembly with one of the partsthey secure. For instance, you might want to “attach” bolts to one part, the nuts to theother, and then constrain those two sub-assemblies together as another sub-assembly.Alibre Motion will then only need to solve the dynamics of one part, rather than many.Ensure The Constraints are CorrectAlibre Motion uses Automatic Constraint Mapping (ACM) to define the ways that Partsand Sub-Assemblies can interact. Ensuring that the Assembly moves as required beforeactivating Motion goes a long way towards ensuring the expected behaviour fromSimulations. Also, be careful not to over-constrain the Assembly, as this may causelocking.Build Realistic PartsNot just the mass, but also the shape and size of a part determines its moments ofinertia, which influence the way it moves when acted on by a force or torque. Forinstance, a basketball has larger moments of inertia than an identical-weight soccerballhas, and because of this, accelerates more slowly when rolling downhill. (Of course,if you drop them straight down, neglecting air-resistance, they would accelerateidentically). Similarly the center-of-mass must be in the right place to produce realisticbehaviour.Use Realistic Constraints552