Design and 3D Printing of Scotch-Yoke Mechanism

1. Concept & Gearing Kinematics

The **Scotch-Yoke** is a classic reciprocating motion mechanism, converting rotary motion into linear sinusoidal motion, or vice versa. The piston or other reciprocating part is directly coupled to a sliding yoke with a slot that engages a pin on the rotating crank. The primary advantage of the Scotch Yoke over standard slider-crank linkages is that it produces pure simple harmonic motion (cosine speed profiles), minimizing high-frequency vibrations in reciprocating systems (such as high-pressure gas compressors).

This project focuses on the conceptual design, tolerance analysis, and additive manufacturing (3D printing) of a working benchtop Scotch-yoke model.

2. SolidWorks CAD Assembly & Clearances

The assembly was modeled in SolidWorks. It consists of:

• Rotating Crank Disk: Fitted with an offset driving pin to slide inside the yoke channel.
• Sliding Yoke Frame: Designed with a precise guide slot. High dimensional tolerances (0.2 mm air gaps) were designed to prevent binding during manual rotation.
• Linear Guide Rods: Double-bearing slots to constrain yoke movement along a single axis.
• Base Plate Support: Integral structure housing the crankshaft bearing journals.

Front view rendering

Isometric view

Yoke-slot close up

3. 3D Printing & Tolerance Verification

Individual parts were exported as STL files and sliced for FDM 3D printing. PLA (Polylactic Acid) was chosen for its dimensional stability and low shrinkage rates.

• Infill Density: 20% grid pattern for structural members, 40% infill for high-stress crankshaft and offset pins.
• Layer Height: 0.2 mm resolution to guarantee smooth contact surface interface inside the yoke guide slot.
• Assembly Tuning: Post-print sanding was performed on mating components to remove print seams, and silicone-based dry lubrication was applied to reduce friction during rotation.

Figure 4: Completed, fully assembled 3D printed PLA working model of the Scotch-Yoke mechanism