Scotch yoke
The Scotch Yoke (also known as slotted link mechanism[1]) is a
reciprocating motion mechanism, 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 location of the piston
versus time is a sine wave of constant amplitude, and constant
frequency given a constant rotational speed.
Contents
Applications
Internal combustion engine uses
Modifications
References
Animation
External links
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, crude
shapers can use Scotch yokes. Almost all those use a Whitworth
linkage, which gives a slow speed forward cutting stroke and a
faster return.
It has been used in various internal combustion engines, such as
the Bourke engine, SyTech engine,[2] and many hot air engines
and steam engines.
The term scotch yoke continues to be used when the slot in the
yoke is shorter than the diameter of the circle made by the crank
pin. For example, the side rods of a locomotive may have scotch
Comparison of displacement and acceleration for a
yokes to permit vertical motion of intermediate driving
Scotch yoke compared with a crank and slider
axles.[3][4]
What is essentially a Scotch yoke, is used in the Tide-Predicting
Machine No. 2 to generate a sinusoidal motion(Sine functions).
Internal combustion engine uses
Under ideal engineering conditions, force is applied directly in the line of travel
of the assembly. The sinusoidal motion, cosinusoidal velocity, and sinusoidal
acceleration (assuming constant angular velocity) result in smoother operation.
The higher percentage of time spent at top dead centre (dwell) improves
theoretical engine efficiency of constant volume combustion cycles.[5] It allows
the elimination of joints typically served by a wrist pin, and near elimination of
piston skirts 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 that occurs, but greater the inertia, making such
Piston water pump, with a scotch
increases in the piston rod length realistically only suitable for lower RPM (but
yoke connection to its flywheel
higher torque) applications.[6][7]
The Scotch yoke is not used in most internal combustion engines because of the
rapid wear of the slot in the yoke caused by sliding friction and high contact pressures. This is mitigated by a sliding block
between the crank and the slot in the piston rod. Also, increased heat loss during combustion due to extended dwell at top dead
centre offsets any constant volume combustion improvements in real engines.[5] In an engine application, less percent of the time
is spent at bottom dead centre when compared to a conventional piston and crankshaft mechanism, which reduces blowdown time
for two-stroke engines. Experiments have shown that extended dwell time does not work well with constant volume combustion
Otto cycle engines.[5] Gains might be more apparent in Otto cycle engines using a stratified direct injection (diesel or similar)
cycle to reduce heat losses.[8]
Modifications
An improved Scotch yoke, with a means of absorbing sideways thrust, was patented in 1978 by William L. Carlson, Jr., US patent
4075898.[9]
References
1. "ME 700 Mechanisms | EdLabQuip" (http://www.edlabquip.com/catalogue/me-700-mechanisms/).
2. "The SyTech Scotch Yoke Engine" (http://www.autospeed.com/cms/A_0948/article.html). AutoSpeed. Retrieved
2008-07-08.
3. General Construction, Baldwin Gasoline Industrial Locomotives Baldwin Locomotive Works Record (https://book
s.google.com/books?id=jHwiAQAAMAAJ&pg=PA57), No. 74, 1913; pages 7-9. The use of the scotch yoke is
explained page 8.
4. Norman W. Storer, Electric Locomotive, U.S. Patent 991,038 (https://www.google.com/patents/US991038),
granted May 2, 1911. The use of the scotch yoke is discussed on page 2 of the text.
5. "Science Links Japan | Effect of Piston Speed around Top Dead Centre on Thermal Efficiency" (https://web.archiv
e.org/web/20120127050010/http://sciencelinks.jp/j-east/article/200609/000020060906A0236528.php).
Sciencelinks.jp. 2009-03-18. Archived from the original (http://sciencelinks.jp/j-east/article/200609/000020060906
A0236528.php) on 2012-01-27. Retrieved 2011-12-06.
6. Bourke Engine Documentary, Published 1968, p50, "Appraising Engine Efficiency" para2
7. Bourke Engine Documentary, Published 1968, p51, "Important Factors in Engine Design"
8. "Effect of the Ratio Between Connecting-rod Length and Crank Radius on Thermal Efficiency" (https://web.archiv
e.org/web/20080128230935/http://sciencelinks.jp/j-east/article/200623/000020062306A0851764.php). Science
Links Japan. Archived from the original (http://sciencelinks.jp/j-east/article/200623/000020062306A0851764.php)
on 2008-01-28. Retrieved 2008-07-08.
9. "Patent US4075898 - Scotch yoke - Google Patents" (http://www.google.com/patents/US4075898). Retrieved
2013-01-21.
External links
Brock Institute for Advanced Studies: Scotch Yoke (http://www.brockeng.com/mechanism/ScotchYoke.htm)
"Comparing Simple Crank/Slider and Scotch Yoke Mechanisms (http://demonstrations.wolfram.com/ComparingSi
mpleCrankSliderAndScotchYokeMechanisms/)" by Fred Klingener, The Wolfram Demonstrations Project; Active
demo.
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This page was last edited on 15 June 2019, at 19:51 (UTC).
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