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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.


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