Scotch Yoke Mechanism

Scotch Yoke Mechanism:

The Scotch Yoke is a mechanism for converting the linear motion of a slider into rotational 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 part.

The advantages compared to a standard crankshaft and connecting rod setup are:

• High torque output with a small cylinder size.
• Fewer moving parts.
• Smoother operation.
• Higher percentage of the time spent at top dead center (dwell) improving engine efficiency.
• In an engine application, elimination of joint typically served by a wrist pin, and near elimination of piston skirt and cylinder scuffing, as side loading of piston due to sine of connecting rod angle is eliminated.

The disadvantages are:

• Rapid wear of the slot in the yoke caused by sliding friction and high contact pressures.
• Lesser percentage of the time spent at bottom dead center reducing blowdown time for two stroke engines.

The shape of the motion of the piston is a pure sine wave over time given a constant rotational speed.