Assembly Modeling: Mates, Motion, and Interference Detection

Objective. Students will assemble a multi-component mechanical assembly using standard, advanced, and mechanical mates, then run interference detection and basic motion analysis to confirm proper function.

Assembly Modeling in SolidWorks

An assembly file (.SLDASM) brings individual part files (.SLDPRT) together into a working mechanical system. Rather than merging geometry permanently, SolidWorks keeps each part as a separate file and tracks how they relate through mates — geometric constraints that remove degrees of freedom (DOF) from the assembly. Understanding DOF is the foundation of good assembly practice.

A rigid body in 3-D space has six degrees of freedom: three translational (X, Y, Z) and three rotational (Rx, Ry, Rz). Fully constraining every DOF produces a fully defined assembly. Leaving one intentional DOF — for example, rotation about a pin axis — produces the motion the design requires. Leaving unintended DOF is a modeling error that will cause interference problems and invalid motion studies.

Types of Mates

Standard mates handle the most common geometric relationships.

Coincident — two planar faces, edges, or points share the same location and orientation.
Concentric — two cylindrical or conical surfaces share the same axis. Use this for pins in holes.
Parallel / Perpendicular / Angle — constrain angular relationships between faces or edges.
Distance — fixes a gap between two entities.
Tangent — forces a curved surface to touch another surface at one point or line.

Advanced mates add profile-based or path-driven constraints.

Limit (Distance or Angle) — allows a component to move within a specified range. For the marine clevis-and-pin assembly in today's lab, a Limit mate on the clevis jaw opening keeps the pin from traveling beyond its physical stop without locking it in one position.
Symmetric — keeps two components equidistant from a plane.
Width — centers a tab inside a slot.

Mechanical mates represent machine-element relationships: gear ratios, rack-and-pinion, cam follower, screw lead, and universal joint. These are especially useful in manufacturing and marine drive-system layouts.

Marine Clevis-and-Pin Assembly

A clevis is a U-shaped bracket through which a pin passes to create a pivot joint — common in rigging hardware, steering linkages, and throttle-cable ends on marine engines. To assemble it correctly:

1. Fix (anchor) the clevis body as the base component — right-click it in the FeatureManager and choose Fix.

2. Add a Concentric mate between the pin's cylindrical surface and one clevis hole.

3. Add a Coincident mate between the pin end face and the outer face of the clevis jaw to locate it axially.

4. Apply a Limit Distance mate to the pin's travel if a cotter hole must remain accessible — this preserves one translational DOF within bounds.

5. Drag the pin to confirm it slides only within the allowed range.

After all mates are placed, open Tools > Evaluate > Check and review the DOF count in the status bar. A correct clevis pivot shows one rotational DOF (rotation about the pin axis) if the assembly is meant to pivot, or zero DOF if it is a static bracket.

Interference Detection

Interference Detection (Tools > Evaluate > Interference Detection) computes the volumetric overlap between selected components. It reports each interference as a colored volume with the names of the two parts involved.

When you receive the pre-built pump housing assembly:

1. Click Calculate to run the check.

2. Read the results list — each row names the two parts and the interference volume.

3. Double-click a result to highlight the clashing region in the viewport.

4. Open the offending part in its own window (right-click the component > Open Part), measure the feature causing the clash, and edit the dimension to create clearance.

5. Rebuild the assembly (Ctrl+Q) and re-run Interference Detection to confirm zero results.

Common causes of interference in pump housings include bolts that are too long, bearing ODs that exceed bore IDs, and port fittings modeled without thread relief.

Motion Studies

A Motion Study (the tab at the bottom of the graphics area) adds a timeline to an assembly. SolidWorks offers three types: Animation (kinematic, driven by motors and springs), Basic Motion (adds gravity and spring forces with approximate mass), and Motion Analysis (requires SolidWorks Motion add-in; full dynamics with contact and friction). Today's piston-rod study uses the Linear Motor (Animation type) to drive the rod.

Steps:

1. In the Motion Study tab, click Motor.

2. Select Linear Motor, then pick the piston-rod face as the driving face and the cylinder bore axis as the direction.

3. Set the motion function — a constant speed or oscillating sine function produces reciprocation when combined with a Limit mate.

4. Set the study duration (e.g., 4 seconds) and click Calculate.

5. Use Results and Plots to graph displacement vs. time; record peak displacement and period in your lab log.

6. Click Save Animation, choose MP4 format, and export.

Safety in the Lab

CAD lab work is low-hazard but not hazard-free.

Ergonomics: Adjust chair height, monitor distance, and keyboard position before long sessions. Repetitive-strain injuries are a real occupational risk for drafters.
Electrical: Never bypass workstation power strips or use damaged cords. Report damaged equipment to the instructor — do not attempt repair.
File safety: Save and back up work frequently. Losing an assembly session to an unsaved crash wastes time and may require staying after hours in a secured lab — always notify staff before remaining in the building after closing.
Software licensing: Use only school-licensed software on school hardware or a personal machine covered by a student license. Unauthorized software installation on school equipment violates lab policy and may introduce malware.

Connecting to the CSWA Exam

The CSWA exam tests assembly skills directly. Expect questions that ask you to open a supplied assembly, apply specific mates, and read a property — such as the X coordinate of a component's center of mass — from the Mass Properties dialog. Practice reading that dialog and recording values exactly as SolidWorks reports them, because CSWA answers are checked to decimal precision.

Key terms10

Degree of Freedom (DOF)An independent direction of motion — translation or rotation — that a component can move in. A fully unconstrained rigid body has six DOF; mates remove DOF until only the intended motion remains.

MateA geometric constraint applied between two components in a SolidWorks assembly that defines their positional or angular relationship and removes one or more degrees of freedom.

Coincident MateA standard mate that forces two selected planar faces, edges, or points to occupy the same geometric location, making them flush or co-planar.

Concentric MateA standard mate that aligns the axes of two cylindrical, conical, or spherical surfaces so they share one common center axis — the standard way to place a pin in a hole.

Limit MateAn advanced mate that allows a component to translate or rotate freely within a defined minimum and maximum range, preserving intentional motion while enforcing physical stops.

Interference DetectionA SolidWorks evaluation tool that calculates and highlights volumetric overlaps between components in an assembly, identifying geometry that physically cannot occupy the same space.

Motion StudyA SolidWorks timeline-based environment that simulates kinematic or dynamic behavior of an assembly using motors, springs, and gravity, and produces animation and data output.

Linear MotorA motion study driver that applies a prescribed translational displacement, velocity, or acceleration along a selected axis to simulate actuated linear motion such as a piston stroke.

Fully Defined AssemblyAn assembly state in which all components have zero unintended degrees of freedom — every position and orientation is constrained by mates or by the fixed component, except for planned kinematic motion.

ClevisA U-shaped mechanical bracket with aligned holes through which a pin passes to create a pivot or hinge joint; widely used in marine steering, rigging, and throttle-linkage hardware.

In class: Students assemble a five-component marine clevis-and-pin assembly using Coincident, Concentric, and Limit mates, dragging components to verify motion is constrained correctly and no unexpected degrees of freedom remain. · Interference Detection run on a pre-built pump housing assembly provided by the instructor: students identify and document each interference, propose a dimensional fix, and implement the correction in the part file. · Students create a simple linear-motor driven motion study showing a piston rod reciprocating inside a cylinder bore, capture an animation MP4, and record key position data (displacement vs. time) in their lab log.
Materials: SolidWorks 2024 (Assembly and Motion Study modules), pre-built pump housing assembly files, marine clevis part files (instructor-prepared), lab log notebooks, CSWA Exam Guide (Assembly chapter)
Check: Clevis assembly has zero interference and correct degrees of freedom as shown by Interference Detection report and mates list — instructor verifies; motion study animation exported and lab log data table complete — pass requires both deliverables accepted.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.