Complete Tutorial

Simulate kinematic mechanisms in your browser

MechSim is a free, zero-install planar linkage simulator. Drop pins, connect links, and watch your mechanism move — powered by a Newton-Raphson constraint solver.

▶ Open Simulator Read Tutorial ↓

Getting Started

Up and running in 60 seconds

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Load a preset

Click Crank-Slider or Four-Bar in the sidebar to load a working mechanism instantly.

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Press Play

Hit ▶ Play or press Space. The simulator takes a snapshot and animates continuously.

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Reset to edit

Press ↺ Reset to return to the initial snapshot and re-enable all editing tools.

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Three modes: MechSim works as a state machine — Edit → Running → Paused → Edit. You can only modify the mechanism in Edit mode. Pressing Play locks the initial state; Reset always returns to that snapshot.

Toolbar Reference

Every tool explained

The vertical toolbar on the left of the canvas holds all building tools. Click or press the keyboard shortcut.

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Select S

Click pins or links to open properties. Drag grounded non-crank pins to reposition. Drag empty canvas to pan.
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Pin J

Place a free joint anywhere on the canvas. Pins are the joints that connect links.
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Link L

Click pin A then pin B to create a rigid link. Length is fixed at creation distance.
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Ground G

Toggle any pin between free and grounded. Set ω (rad/s) in the props panel to make it a crank pivot.
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Actuator A

Creates a prismatic joint. Can be articulated piston or fixed-axis rail. Supports passive (coupler-driven) mode.
Rail Slider R

Two endpoints define a rigid rail. A slider pin S rides between them. Connect couplers to S for passive mode.
Arc Slider C

Select 3 pins (start → mid → end) to define an arc. Pin S slides along the circular arc.
Circle Slider O

Select center then rim pin. Pin S orbits the full 360° circle. Set speed for continuous rotation.

Keyboard Shortcuts

Work fast

Every tool has a key. Tools are disabled while Running or Paused — press Reset first.

SSelect

JPlace pin

LLink

GGround

AActuator

RRail slider

CArc slider

OCircle slider

SpacePlay / Pause

DelDelete

Ctrl ZUndo

Ctrl YRedo

ScrollZoom

DragPan canvas

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Edit mode only: J L G A R C O are disabled while Running or Paused. Press ↺ Reset to re-enable.

Mechanism Library

Classic linkages you can build

The fundamental mechanisms of planar kinematics, all assemblable from pins and links.

Four-Bar Linkage

Crank, coupler, rocker, ground. Grashof's law governs whether it's a crank-rocker, double-crank, or double-rocker. Foundation of engines, wipers, walking machines.

Crank-Slider

Converts rotation to reciprocating linear motion. The foundation of piston engines and pumps. Built with a grounded crank, coupler, and a fixed-axis slider pin.

Chebyshev Linkage

A four-bar where the coupler midpoint traces an approximate straight line — useful for converting rotation to translation without a prismatic joint.

Rail + Arc + Circle Sliders

Three slider types: straight rail, circular arc (3-point), and full 360° circle. Each slider pin can be passive (coupler-driven) or active (motor speed).

Step-by-Step

Build a four-bar from scratch

The four-bar is the simplest closed kinematic chain. Three links plus the ground.

// Step 1 — Ground pivots Press G (Ground tool) Place two grounded pins — O₂ and O₄ Select O₂ → set ω = 1 rad/s // makes it the crank // Step 2 — Free pins Press J (Pin tool) Place pin A near O₂, pin B near O₄ // Step 3 — Links Press L (Link tool) Connect: O₂ → A // crank arm Connect: A → B // coupler Connect: O₄ → B // rocker // Step 4 — Simulate Press Space → watch it move

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Grashof's Law: For a crank-rocker, shortest + longest ≤ sum of the other two. The preset uses L₂=80, L₃=180, L₄=120, L₁=200 which satisfies 80+200 ≤ 180+120.

Slider types: passive vs active

PassivePosition driven by coupler geometry — NR circle∩line intersection

ActiveMoves at set speed (rad/s or u/s), bounces at limits, drives dependent joints

Active + couplerpinS is fixed anchor — NR resolves downstream passive joints from it

Under the Hood

Newton-Raphson constraint solver

Every link contributes a scalar equation f(q)=0. MechSim assembles the Jacobian and iterates:

The tol and iter inputs in the header control convergence. Lower tol = more precise. Higher iter = handles complex configurations.

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Tuning: For fast preview use tol=0.1, iter=20. For accurate recording use tol=0.001, iter=100. The Levenberg-Marquardt λ=0.1 prevents divergence near singular positions.

Simulation state machine

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LOCKED: NR failed to converge. Usually a link is too short to reach two pins. Fix geometry and press Reset.

FAQ

Common questions

How do I save my mechanism?

Click 💾 Save to download a .mechsim.json file. Click 📂 Open to reload it. No account needed — fully portable.

What does "LOCKED — singular position" mean?

The Newton-Raphson solver couldn't find a valid configuration — usually a link is too short to connect two pins at their current positions. Press ↺ Reset to restore the last valid state, then adjust link lengths.

What's the difference between passive and active sliders?

Passive: position is determined by coupler geometry — NR places the slider at the circle∩line or circle∩circle intersection.

Active: slider moves at a set speed, bounces at limits, and its pinS is treated as a fixed anchor that drives downstream passive joints.

Can I export kinematic data?

Yes. Enable Trace path on any pin. Click ⏺ Record to capture position, velocity, and acceleration at configurable intervals. Export as .csv — timestamps start at zero for easy analysis.

Can an active arc or circle slider drive a passive linear slider?

Yes. Set the arc/circle slider as Active (non-passive, speed≠0). Connect a coupler link from its slider pin to a pin on the passive linear slider. The solver treats the active pinS as a fixed anchor each frame, then resolves the linear slider's position from the coupler constraint.

Why can't I edit while the simulation is running?

When you press Play, MechSim locks the initial snapshot. Edits during simulation would corrupt that snapshot. Press ↺ Reset to return to Edit mode — all tools are re-enabled and the mechanism returns to its initial position.