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How does a Scotch Yoke mechanism Work?

You can also visit a version of this mechanism with dwell in here.

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.


Siva
30 Apr 2013

Everything about scotchyoke mechanism....i want to know

Johann Wegmann Morovic
04 Aug 2013

Dear Gentlemen,Thankyou very much for this article.I´´ve been Reliability RCM,CAD/FEA and Simulation events user.I am astonished with Bourke cycle engine,the best engine never known,with a Scotch Yoke drive in center.20.000 rpm limit.
RCM has resources for enlarging life span,probably ductile cast iron ferrochrome,spheroidal grapfite, or MM´c composite.
My concern is , I am shamed,I am retired and never saw the tecnique to control de direction of rotation.My other concern is
In comparison,between a rhombic drive and scotch yoke ,at the same reliability service,which is the cheapest?Thanks

Johann Wegmann Morovic
11 Aug 2013

Dear Sirs,
1.-¿Cómo controlo el sentido de rotación en Stirling Motores?CW o CCW?
2.-¿Cual es mejor:Rómbico,Yunque Escosés,o slider-Crank,corredera ,cruzeta y biela?Confiabilidad, Tiempo entre Mantenciones. Gracias Johann wegmann

johann wegmann
07 Oct 2013

I am working in Alpha Stirling Engine,reciprocating and opposed pistons,double or simple action,opposed burners, Nichrome,Infrared or Radioisotops generator to replace gas burners definitivelle savibf 20% termal efficiency,alphas need to change kinematics configuration,it doesnt fit with PV diagram,the use of refrigeating expansion valve will dramatically lower cold cylinder lowest temp,the concept of adiabatic engine i developed in 1972, to day is ceramic coated possible and ringless pstons ceramic coated is adiabatic wall,I wiil increase Cmpression Ratio to 10:1,Temp.Ratio will increase.water ref if ceramic doesnt work, is an old usage to keep the liners within melting point.Please dwells both sides is interesting, I´ll change Bourke dnamics for a symetric piston motion,2 power strokes same time,Bye.Thanks.No one drun displacer is suitable to wwwithstand concentrated strength on shell elements,only vertical can run centerd,is for models,World wide Science,I guess, I read the champions design selection,first alpha in-line, secod 90 alpha,probably V configuration.Throtling valve and ceramic will make this engies take off.And my inside cylinders alectric,Infrared or Cerium based radioisotopes, will become Wegmann-Stirling engine.Best Regards.Johann

suresh
01 Feb 2014

""this mechanism is to be high torque to achived so many machines to be used ""

Doaa
15 Apr 2014

i want to know the full dimensions of the skotchyoke mechanism

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