EXPERIMENT 4.1 – SCOTCH YOKE MECHANISM
BACKGROUND OF STUDY:
Pure simple harmonic motion is produced by the Scotch Yoke mechanism when driven by an eccentric or crank. Its early application was on steam pumps but it is not suitable for high speeds due to the backlash in the scotch yoke. It is now mainly used in computing type applications where harmonic motion is required. 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. 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 a Scotch yoke. 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, and many hot air engines andsteam engines.
Figure 1.1 Piston water pump, with a scotch yoke connection to its flywheel
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) results in smoother operation. The higher percentage of time spent at top dead center (dwell) improves theoretical engine efficiency of constant volume combustion cycles.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 ofconnecting 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 increases in the piston rod length realistically only suitable for lower RPM (but higher torque) applications 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 center offsets any constant volume combustion improvements in real engines In an engine application, less percent of the time is spent at bottom dead center 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.Gains might be more apparent in Otto Cycle Engines using a stratified direct injection (diesel or similar) cycle to reduce heat losses.
Scotch Yoke Mechanism
1. To demonstrate the action of a simple crank-driven Scotch Yoke mechanism. 2. To determine graphically the relationship between the linear displacement of the scotch yoke and the angular displacement of the crank.
1. The Scotch Yoke mechanism apparatus was put on level surface. 2. The crank initially set to zero degree.
3. The crank is turned clockwise in increment of 10 degrees. 4. The data on measurement of linear displacement, x corresponding to degree turned (angular displacement) are recorded. 5. The steps above are repeated for counter clockwise movement. 6. All data obtained are tabulated and graph of linear displacement, x against crank angle, θ is plotted to represent their relationship.
DATA AND ANALYSIS:
Angle, θ (⁰)
| Linear Displacement (mm)
| Average Linear Displacement, x
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