Scotch yoke
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 shape of the motion of the piston is a pure sine wave over time given a constant rotational speed.
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[edit] Applications
This setup is most commonly used in control valve actuators in high pressure oil and gas pipelines.
Although not a common metalworking machine nowadays, a Shaper uses a Scotch yoke which has been adjusted to provide a slow speed forward stroke and a faster return.
It has been used in various internal combustion engines, such as the Bourke engine, SyTech engine,[1] and many hot air engines and steam engines.
[edit] Internal Combustion Engine Uses
The Bourke engine uses a Scotch Yoke which affords it advantages compared to a standard crankshaft and connecting rod. There are fewer moving parts. Under ideal engineering conditions, force is applied directly in the line of travel of the assembly. Smoother operation: as the motion is sinusoidal, the velocity is cosinusoidal and the acceleration is also sinusoidal (assuming constant angular velocity). A higher percentage of the time spent at top dead center (dwell) improving theoretical engine efficiency of constant volume combustion cycles.[2] 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 mitigated. The longer the distance between the piston and the yoke, the less wear occurs, but inertia is increased, making such increases in the piston rod length realistically only suitable for lower RPM (but higher torque) applications.[3][4]
The Scotch Yoke has not be used in most internal combutions engines because of rapid wear of the slot in the yoke caused by sliding friction and high contact pressures. Also, increased heat loss during combustion due to extended dwell at top dead center offsets any constant volume combustion improvements in real engines.[2] In an engine application, lesser percentage of the time spent at bottom dead center reducing blowdown time for two stroke engines, when compared with a conventional piston and crankshaft mechanism. Experiments have shown that extended dwell time will not work well with constant volume combustion Otto Cycle Engines.[2] Gains might be more apparent in Otto Cycle Engines using a stratified direct injection (diesel or similar) cycle to reduce heat losses.[5].
[edit] References
- ^ "The SyTech Scotch Yoke Engine". AutoSpeed. http://www.autospeed.com/cms/A_0948/article.html. Retrieved 2008-07-08.
- ^ a b c "Science Links Japan | Effect of Piston Speed around Top Dead Center on Thermal Efficiency". Sciencelinks.jp. 2009-03-18. http://sciencelinks.jp/j-east/article/200609/000020060906A0236528.php. Retrieved 2011-12-06.
- ^ Bourke Engine Documentary, Published 1968, p50, "Appraising Engine Efficiency" para2
- ^ Bourke Engine Documentary, Published 1968, p51, "Important Factors in Engine Design"
- ^ "Effect of the Ratio Between Connecting-rod Length and Crank Radius on Thermal Efficiency". Science Links Japan. http://sciencelinks.jp/j-east/article/200623/000020062306A0851764.php. Retrieved 2008-07-08.
[edit] External links
- Brock Institute for Advanced Studies: Scotch Yoke
- "Comparing Simple Crank/Slider and Scotch Yoke Mechanisms" by Fred Klingener, The Wolfram Demonstrations Project; Active demo.
- Another demo [1] with Tcl scripting language
- hydrodynamic lubrication of scotch yoke mechanism [2]
- Animation showing working of scotch yoke mechanism used in valve actuators [3] by Vivek Thakar
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