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As-built joints on the piston linkage

· Fox Bright

The parts are already in place, so tell Fusion how they move. Revolute and slider as-built joints, a sub-assembly you can copy, and limits so the motion stays real.

Fusion's piston linkage sample with two piston sub-assemblies joined to a frame, mid-animation.

Fusion ships a piston linkage under Basic Training 06 Assemblies. The geometry is finished and sitting in position. Nothing moves. This is the case for As-Built Joints: you are not placing parts, you are describing the motion they already have.

Bodies into components

Open the Bodies folder. Joints connect components, not bodies, so select all the bodies (click the first, shift-click the last), right-click, Create Components from Bodies.

The piston is two parts that work together: the outer sleeve and the inner rod. Make a new component called Sub-Assembly and drag both piston components into it. Anything that moves as a unit but is made of several parts gets its own level like this.

Four joints

Press S, find As-Built Joint.

  1. Arm to frame: pick both components, type Revolute, snap to the centre axis of the pin.
  2. Right-click, Repeat As-Built Joint for the second arm pin, and again for the arm-to-piston pin.
  3. Piston rod to piston sleeve: these slide, they do not turn, so the type is Slider. Cylindrical would also let them spin, which you do not want. Pick the axis to slide along.
Fusion As-Built Joint dialog with the piston linkage arm highlighted
An as-built revolute joint between the arm and the frame. The parts stay where they are; only the motion is defined.

Look in the Joints folder: three revolutes at the top level and the slider inside the sub-assembly, because both of its components live there.

Copy the sub-assembly

Right-click Sub-Assembly, Copy. Right-click the top level, Paste New. It lands on top of the original; move it 100 mm across and Capture Position. The slider joint comes along with the copy. The two pins that connect it to the frame do not, so add two more as-built revolute joints.

Animate and limit

Rename the main pivot joint Centre and right-click it, Animate Model. Everything moves, and the piston goes straight through itself, because Fusion does not know about collisions until you tell it.

Drag the rotation handle on the Centre joint and watch: about +20° one way and -35° the other before the piston runs out of travel. Right-click the joint, Edit Joint Limits, minimum -35, maximum 20. Animate again.

Fusion piston linkage with a rotation handle on the centre joint and the joint limits dialog open
Joint limits on the centre pivot: minimum -35°, maximum 20°, measured by dragging until the piston bottomed out.

To drag the mechanism by hand in the viewport, Ground one component (the top arm, say) and capture the position. With a grounded part and limits in place the motion is predictable. Without limits it is still logical, just wrong.