Scissor Lift Mechanism Design

1. Scissor Arms (Links)

• Crossed in X-shaped pairs
• Multiple pivot points per arm
• Connected in series for stages

2. Pivot Pins

• End pivots connect to base/top
• Center pivot where arms cross
• Allow relative rotation

3. Base Platform

• Fixed reference frame
• Mounts bottom pivots
• Provides stability

4. Top Platform

• Moving platform
• Carries the load
• Rises with mechanism extension

Height Relationship:

Design Guidelines:

• N ≥ 2 for significant lift height
• L/W ratio ≥ 4 for stable geometry
• θ_max ≤ 70° to avoid instability

In the spreadsheet, create this table:

Why Aliases?

Aliases let you reference cells by name instead of “B2”, making formulas readable and professional.

Create aliases:

1. Click cell B2

2. Right-click → Properties

3. In “Alias” field, type: ArmLength

4. Click OK

5. Click cell B3

6. Right-click → Properties

7. In “Alias” field, type: ArmWidth

8. Click OK

9. Repeat for all parameters:

   • B4: ArmThickness
   • B5: PinDiameter
   • B6: PinRadius
   • B7: StageCount
   • B8: BaseWidth
   • B9: BaseThickness

Now you can use Spreadsheet.ArmLength anywhere in your model!

Note the PinRadius formula:

Cell B6 contains: =B5/2

This automatically calculates radius from diameter!

Draw horizontal reference:

1. Line tool (press L)

2. Click at origin (0, 0)

3. Move horizontally to the right

4. Click to place endpoint

5. Press Escape

6. Select the line

7. Make it horizontal: Press H key

You now have a horizontal line from the origin!

Dimension the centerline:

1. Distance constraint tool

2. Click the left endpoint of the line

3. Click the right endpoint of the line

4. A dimension dialog appears

5. Click the small ƒx button

6. In the formula editor, type: Spreadsheet.ArmLength

7. Press Enter

Line is now 200mm and parametric!

Convert to construction geometry:

1. Select the line (click on it)

2. Press G key (or click Construction Mode button)

3. The line turns blue/dashed

Draw the crossing pivot:

1. Circle tool (press C)

2. Click somewhere near the middle of the line

3. Move mouse out and click to set size

4. Press Escape

5. Midpoint constraint:

   • Click Midpoint constraint tool
   • Click the circle center
   • Click the centerline (the construction line)
   • Circle center snaps to line midpoint!

6. Radius dimension:

   • Radius constraint tool
   • Click the circle
   • Click ƒx button
   • Type: Spreadsheet.PinRadius
   • Enter

Center hole is perfectly positioned and parametric!

Draw the base connection:

1. Circle tool

2. Click near the left end of the centerline

3. Click to set size

4. Press Escape

5. Coincident constraint:

   • Coincident tool
   • Click circle center
   • Click left endpoint of centerline
   • Circle locks to origin!

6. Radius:

   • Radius constraint
   • Click the circle
   • ƒx → Spreadsheet.PinRadius
   • Enter

Left hole positioned at origin!

Draw the top platform connection:

1. Circle tool

2. Click near the right end of centerline

3. Click to set size

4. Press Escape

5. Coincident constraint:

   • Circle center → Right endpoint of centerline

6. Radius:

   • ƒx → Spreadsheet.PinRadius

All three holes are now positioned and dimensioned!

Draw the structural beam:

1. Rectangle tool

2. Draw a rectangle that encompasses all three circles

3. Should extend past the leftmost and rightmost circles

4. Press Escape

Don’t worry about exact positioning yet. Constraints will fix it!

Center the rectangle about centerline:

1. Click Symmetric constraint tool

2. Click the top edge of the rectangle

3. Click the bottom edge of the rectangle

4. Click the horizontal centerline (construction line)

5. Press Escape

The rectangle is now perfectly centered vertically!

Set the parametric width:

1. Distance constraint tool

2. Click the top edge of rectangle

3. Click the bottom edge of rectangle

4. ƒx button

5. Type: Spreadsheet.ArmWidth

6. Enter

Arm width is now 30mm and parametric!

The rectangle length is automatically constrained!

Because the rectangle corners will be coincident with or near the end hole centers (which are already positioned at the centerline endpoints), the rectangle length is implicitly defined.

If the solver shows degrees of freedom, add:

• Coincident: Rectangle left edge → passes through left hole center
• Coincident: Rectangle right edge → passes through right hole center

Or simply ensure the rectangle extends to the endpoints.

1. Select the sketch in the tree (under ScissorArm body)

2. Click Pad tool in Part Design toolbar

3. In the Pad panel (left side):

   • Type: Dimension
   • Click the ƒx button next to Length
   • Type: Spreadsheet.ArmThickness
   • Click OK

4. Click OK to complete the pad

You now have a 3D scissor arm!

Navigate the 3D view:

• Press V then F to fit all
• Rotate view with middle mouse button
• See the rectangular link with three holes!

1. Rectangle tool

2. Draw rectangle roughly centered at origin

3. Press Escape

4. Symmetric constraints:

   • Symmetric: left edge, right edge, Y-axis
   • Symmetric: top edge, bottom edge, X-axis
   • This centers the rectangle perfectly!

5. Width dimension:

   • Distance: left edge to right edge
   • ƒx → Spreadsheet.BaseWidth
   • Enter

6. Depth dimension:

   • Distance: top edge to bottom edge
   • Type: 60 mm (fixed depth)
   • Enter

For a single-stage lift, we need two pin holes for the bottom pivots.

Left pin hole:

1. Circle tool

2. Draw circle on the left side

3. Press Escape

4. Horizontal constraint:

   • Circle center and origin (centers on X-axis)

5. Distance from origin:

   • Distance: origin to circle center
   • Type: 80 mm (horizontal spacing)
   • Enter

6. Radius:

   • ƒx → Spreadsheet.PinRadius

Right pin hole:

1. Draw another circle on the right

2. Horizontal constraint: Circle center and origin

3. Distance from origin: 80 mm

4. Radius: ƒx → Spreadsheet.PinRadius

Or better - use Equal and Symmetric constraints:

1. After creating both circles
2. Equal constraint: Both circles (equal radii)
3. Symmetric constraint: Both circle centers, Y-axis
4. Distance: One circle center to Y-axis = 80 mm

This ensures perfect symmetry!

Verify and complete:

1. Confirm “Fully constrained”
2. Close sketch

1. Switch to Assembly workbench

   Use workbench dropdown (Assembly4 or A2plus recommended)

2. Create new assembly

   Assembly → Create Assembly

3. Add parts

   You need to add:

   • BasePlatform (once)
   • ScissorArm (twice - same part, two instances!)
   • TopPlatform (once)

   Use “Add Part” or drag from tree, adding ScissorArm twice

1. Select BasePlatform in assembly tree

2. Click Fixed constraint button

3. Base is now locked in place (ground link)

Position the left-rising arm:

1. Select the left end hole axis of the first ScissorArm

2. Select the left pin hole axis of the BasePlatform

3. Click Axial Align constraint

The first arm’s left end is now connected to the base!

Position the right-rising arm:

1. Select the left end hole axis of the second ScissorArm

2. Select the right pin hole axis of the BasePlatform

3. Click Axial Align constraint

The second arm’s left end connects to the right side of the base!

This is the critical scissor pivot!

1. Select the center hole axis of first ScissorArm

2. Select the center hole axis of second ScissorArm

3. Click Axial Align constraint

The arms now cross and can pivot relative to each other!

Connect to the top platform:

1. First arm’s right end hole → TopPlatform right pin hole

   • Axial Align constraint

2. Second arm’s right end hole → TopPlatform left pin hole

   • Axial Align constraint

The mechanism is now complete!

Test the scissor action:

Try dragging the top platform or an arm in the assembly view:

• Arms should rotate about their pivots
• Center crossing should move
• Top platform should rise and fall!
• Motion should be smooth and coordinated

1. Open Spreadsheet

   Double-click Spreadsheet in tree

2. Change ArmLength

   • Click cell B2
   • Type: 250
   • Press Enter

3. Recompute

   Press Ctrl+R or click Recompute button

4. Observe changes:

   • All scissor arms are now longer!
   • Maximum lift height increases!
   • Pin holes remain properly positioned!
   • Assembly maintains all constraints!

✅ Success! Your mechanism scales with arm length!

1. Change ArmWidth

   • Cell B3: Type 40
   • Enter

2. Recompute (Ctrl+R)

3. Verify:

   • Scissor arms are now wider (more robust!)
   • Symmetric constraints maintained
   • All holes still centered
   • Assembly still functions

✅ Your arms are parametric!

1. Change PinDiameter

   • Cell B5: Type 10
   • Enter (note: PinRadius updates automatically!)

2. Recompute

3. Verify:

   • All holes in arms are now larger
   • All holes in platforms match
   • Formula-driven PinRadius worked!

✅ Calculated parameters work perfectly!

1. Set BaseWidth = 500
2. Recompute

Platforms expand for wider stance!

Try extreme values:

• ArmLength = 150, ArmWidth = 20 (compact design)
• ArmLength = 300, ArmWidth = 50 (heavy-duty version)

Everything should update cleanly!

1. Add fillets

   Use Fillet tool to round sharp corners (3mm radius)

2. Add chamfers

   Chamfer the edges of holes for easier pin insertion

3. Create the pins

   Design separate pin parts (cylinders with heads)

1. Add a second stage

   Use 4 arms total for double lift height

2. Create linear actuator mount

   Add base geometry for a screw drive or hydraulic cylinder

3. Design safety locks

   Add features for height-locking pins

1. Motion study

   Calculate lift height as function of base spacing angle

2. Mechanical advantage analysis

   Derive force amplification at different heights

3. Multi-stage parametric

   Make StageCount parameter actually control number of stages

“Center hole not at midpoint”

“Rectangle not symmetric”

“PinRadius doesn’t update with PinDiameter”

“Arms don’t scale uniformly”

“Arms don’t cross properly”

“Platform holes don’t match arm holes”

“Can’t add ScissorArm twice”