SCOTH YOLK MECHANISM

THEORY

It is an inversion of double slider crank mechanism

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.fig 1.1 illustrates a simple scotch yolk mechanism

Comparison of scotch yolk with Slider crank mechansim

The advantages compared to a standard crankshaft and connecting rod setup are:

Fewer moving parts. Smoother operation. Higher percentage of the time spent at top dead center (dwell) improving theoretical engine

efficiency of constant volume combustion cycles, though actual gains have not been demonstrated.

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.

Applications

This setup is most commonly used in control valve actuators in high pressure oil and gas pipelines.

It has been used in various internal combustion engines, such as the Bourke engine, SyTech engine, stirling,and many hot air engines and steam engines. Refer to fig 1.2

Experiments have shown that extended dwell time will not work well with constant volume combustion (Otto, Bourke or similar) cycles.Gains might be more apparent using a stratified direct injection (diesel or similar) cycle to reduce heat losses.

FIGURE 1.1

EXPERIMENT

This mechanism is used for converting rotary motion into a reciprocating motion. The inversion is obtained by fixing either the link 1 or link 3. In Fig, link 1 is fixed. In this mechanism, when the link 2 (which corresponds to crank) rotates about B as centre, the link 4 (which corresponds to a frame) reciprocates. The fixed link 1 guides the frame.

X’=ON-OM

X’=R-RCOS(ωT )X’=R(1-COS(ωT ) X’=R(1-COS(θ)

where x’ is the distance of the slider from the zero reading on the scale or the distance of piston from the top dead center.

X=RCOS(ωT )……………….EQ A

Appratus on which Experiment was performed

Sliding Yolk

CRANK

Piston

SKELTON DIAGRAM

X=RCOS(θ)Here x is the distance of slider from the origion or it can be inter preted as the distance of scotch yolk from the crank shaftTakinng time derivative of eq A we get

v=-Rsin(ωT ).ω………………….EQ BTaking time derivative of eq B we get

a=-Rcos(ωt).ω2………………….EQ CSubstituting the valuve of X from eq A into eq C we get

a=-Xω2

WHEN ωis coonstant A is directly proprtional to X i.e acceleration is directky related to distance of scotch yolk slider from asis of rortation.This is the property of SHM.Thus motion of scotch yolk mechanism obeys simmple harmonic motion and is displacemnt time curve is a sine wave.R=3.5cmω is assumed to be 10 rad per sec

θ X ' theoratical=R(1−cos (θ )) X 'Experimental0 0 010 .52 .520 2 130 4.68 3.840 8.1 750 12.5 11.560 17.5 16.570 23 2180 28.9 2790 35 33100 41 39.5110 46.9 45120 52.5 50.5130 57.5 55.5140 61.8 60150 65.3 64160 67.9 66.5170 69.5 67.5180 70 69190 69.5 69.5200 70 67.5210 65.3 65220 61.8 61.5230 57.5 57.5240 52.5 52.9250 46.9 47

260 41 41270 35 39280 28.9 28290 23 22.5300 17.5 17.5310 12.5 12.5320 8.2 8.0330 4.7 4.5340 2.1 2350 .5 .5360 0 0

plot of X’ and θ of Experimental data

PLOT of and x’ and θ of theoratical data

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