SCOTCH YOKE MECHANISM

AIM OF THE PROJECT

To design and manufacture a working model of Scotch yoke mechanism.

To get a practical exposure of machine tools and other manufacturing equipments.

PROBLEM STATEMENT

To convert rotary motion into Reciprocatory motion in a simple harmonic manner.

To build a working model which can be fabricated easily using the available tools and machinery.

INTRODUCTION

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 mechanism is an inversion of the double slider crank mechanism. 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-axis

Other inversions of the double slider crank mechanism include Oldham coupling and elliptical trammel. History

This linkage is being called by a Scotsman in 1869 a "crank and slot-headed sliding rod“ But now it is known as a Scotch yoke because, in America at least, a "Scotch" was a slotted bar that was slipped under a collar on a string of well-drilling tools to support them while a section was being added

In 1940 Russell Bourke applied this mechanism to the internal combustion engine called Bourke 30 engine

SIMPLE HARMONIC MOTION u v

Suppose crankshaft is rotating at an angular velocity ‘Ω’.

If r is the radius of the crank then, Tangential velocity, v= ‘rΩ’.

From the mechanism we have the following relation; Component of tangential velocity in Y-direction is given by;u = Reciprocating velocity of U-Slot.If α is the angle made by the tangential velocity with X-Axis at any point of time,Component of tangential velocity in Y direction is u = rΩsinα.

u = v.sinα

So, velocity of U-Slot= rΩsinα.

As a result, Velocity of U-Slot is a sine function of α.

α

Now as we know, α is directly proportional to time. This implies velocity of U-Slot is a sine function of time. Hence, the motion of U-Slot is a simple harmonic motion.Advantage of SHM

The sinusoidal motion, cosinusoidal velocity, and sinusoidal acceleration (assuming constant angular velocity) results in smoother operation of the mechanism.

ADVANTAGES AND DISADVANTAGESThe 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 centre

(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 centre reducing blow down 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.

RESOURCES USED

Materials DimensionsMild steel plates 1. 50 mm x 5 mm

2. 50 mm x 2.5 mm

Mild Steel Rod 1. φ20 mm

2. φ25 mm

Mild steel hollow pipe φ30 mm (internal) φ34 mm (external)

Mild steel square pipe 25 mm x 25 mm (external) Thickness-2 mm

EQUIPMENT USED1. Lathe Machine2. Drilling machine3. Shaper machine4. Grinding machine5. Power tools6. Power Hacksaw7. Electric arc welding machine

COMPONENTS1.Crank and Handle

Obtained Cylindrical Rods Of Required Dimension Operations: Plain Turning And Parting on Lathe machine

Welded Handle And Crank With Crank-shaft using electric arc welding.

Dimensions: As shown in the following figure

2. U-slot Obtained square cross section pipe of required length

by cutting the long pipe with the power hacksaw Used surface grinding machine to obtain smooth

exterior surface on the pipe Used power cutter to remove one face of the square

pipeDimensions: as shown in the following figure-

3.Yoke (Slider block) Obtained a cylindrical block of required length by

turning and parting on Lathe machine. Converted the cylindrical block into a cuboid of

required dimensions on Shaping Machine. Hole is drilled in the middle of block to accommodate

the crank using the drilling machine.

Dimensions: As shown in the following figure-

4. Foundation Obtained metallic strips of required lengths by

cutting the long bar using the power hacksaw Drilled holes to mount the crankshaft on the proper

metallic strips using drilling machine Welded the metallic strips to get a rigid foundationDimensions: As shown

5. Guides

Obtained metallic strips of required lengths by cutting from long bar using the power hacksaw

Obtained slots in the metallic strips using the power cutter

Dimensions:

6. Piston and piston rod Obtained cylindrical rods of required diameters and

lengths using plain turning and parting on the Lathe machine.

Welded piston to piston rod using electric arc welding Welded the above piston assembly with the U-slotDimensions:

7. Hollow Cylinder

Cut the pipe of required length using power hacksaw Dimensions:

ASSEMBLY PROCEDURE

1.

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, and many hot air engines and steam engines.

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 centre (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 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 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. Also, increased heat loss during combustion due to extended dwell at top dead centre offsets any constant volume combustion improvements in real engines. In an engine application, less percentage of the time is spent at bottom dead centre when compared to a conventional piston and crankshaft mechanism, which reduces blow down 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.