Langford Analytic · Knowledge Base
Mechanisms, Motion & Multibody Dynamics
Multibody analysis explains how loads are generated by motion. Structural FEA explains what those loads do to the structure. This section bridges the gap between system motion and structural loads — explaining how moving mechanical systems generate the forces, moments and interface loads subsequently used within structural FEA — from multibody dynamics fundamentals and kinematics vs dynamics through degrees of freedom, joints, linkages, mass properties and inertia tensors, actuators, springs and dampers, inertial loads, contact and clearance, rotating systems, rigid vs flexible bodies, mechanism load extraction for FEA, suspension and landing gear load paths, deployment transients, multibody–FEA co-simulation, test correlation, sensitivity and uncertainty to the complete workflow from system motion to defensible structural load set.
Fundamentals
Multibody Dynamics FundamentalsHow mechanisms made from bodies, joints, constraints and force elements generate the loads that subsequently drive structural FEA — and why a multibody model is a load-generation model, not a structural model.Kinematics vs DynamicsWhy knowing where a mechanism is does not tell you the load — and why a dynamic analysis is required whenever a mechanism accelerates.Degrees of Freedom & Mechanical ConstraintHow joints remove relative degrees of freedom from connected bodies, why over-constraint produces fictitious reactions, and why a numerical constraint must represent a physical restriction.Joints, Hinges, Sliders & Mechanical ConnectionsWhy an ideal joint defines motion but a real bearing, pin and housing must also carry the resulting load — and how idealisation choices affect the interface loads passed to structural FEA.Linkages & Mechanism GeometryHow linkages, bellcranks, toggles and cams transform actuator force and motion — and why the critical structural position may not be the position of maximum actuator force.
Mass, Actuation & Dynamic Load
Mass Properties, Centre of Gravity & Inertia TensorsWhy correct total mass with incorrect mass distribution still produces wrong dynamic loads — and how the inertia tensor, not just the mass, governs rotational dynamics.Actuators, Springs & DampersHow prescribed motion, force-controlled actuators, springs and dampers drive mechanism loads — and why modelling a force-controlled actuator as prescribed motion produces reactions that reflect the analyst's assumption, not the physical behaviour.Inertial Loads in Moving Mechanical SystemsHow translational and rotational inertia, centripetal and tangential acceleration, and gravity combine to produce structural loads in moving systems — and why the acceleration history matters, not just its peak.Contact, Stops, Clearance & BacklashHow clearance, backlash, stops and intermittent contact turn smooth loading into transient impact — and why small clearance can have a large dynamic effect.Rotating Systems & Gyroscopic EffectsHow rotating mass generates centrifugal loads, imbalance forces and gyroscopic moments that have no static equivalent — and why replacing a rotating component with a static mass can eliminate the loads that govern its support structure.
Flexible Systems & Structural Load Extraction
Rigid-Body vs Flexible-Body Multibody ModelsWhen a rigid-body idealisation is sufficient, when structural flexibility must be represented in the multibody model, and how flexible-body inclusion changes the interface loads passed to structural FEA.Extracting Mechanism Loads for Structural FEAThe complete workflow for extracting time-correlated, six-component interface force and moment histories from a multibody model and transforming them into structural FE load cases — and the errors that arise when peak values are extracted without regard for simultaneity.Suspension, Landing Gear & Articulated Load PathsHow articulated mechanical systems — suspension linkages, landing gear struts, deployable supports — distribute load through a travel-dependent path, and why the load path must be traced through the full range of motion rather than at a single position.Deployment & Transient MechanismsHow deployable mechanisms — doors, folding wings, antenna deployments, retractable systems — generate transient loads during release, acceleration, damping, end-stop impact and latching, and why the transient often governs structural design rather than the steady deployed state.Multibody–FEA Co-SimulationWhen and how to couple a multibody system model with a detailed structural FEA model — one-way load extraction, one-way preloading, two-way force–displacement exchange and fully coupled solution — and why the coupling method must follow the physics, not the software capability.
Correlation & Substantiation
Multibody Test CorrelationHow to correlate a multibody dynamics model against measured test data — position, velocity, acceleration, actuator force, joint load, displacement and timing — and why the correlation hierarchy runs from motion first to loads second.Sensitivity, Uncertainty & Model CredibilityHow uncertainty in mass, inertia, friction, joint stiffness, damping, actuator force, clearance and initial position propagates through a multibody model to the interface loads, and why treating uncertain parameters as known constants produces precise but inaccurate load predictions.From System Motion to Defensible Structural Load SetThe complete chain from operating scenario through system configuration, mass properties, constraints, actuation, motion, interface reactions, time histories, critical events and structural load cases to FEA, test correlation and engineering conclusion — the end-to-end process that produces a defensible structural load set.