Scotch Yoke Mechanism

Aim of the ProjectTo design and manufacture a working model

of Scotch yoke mechanism.To get a practical exposure of machine tools

and other manufacturing equipments.

Problem StatementTo 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.

Scotch yoke mechanism

The Scotch yoke is a mechanism for converting the linear motion of a slider into rotational motion or vice-versa.

Introduction

Simple Harmonic MotionScotch yoke mechanism converts rotary motion into translatory motion which is simple harmonic in nature.

MATHEMATICAL EXPLAINATION

Suppose crankshaft is rotating at an angular velocity ‘Ω’. u v If r is the radius of the crank then, tangential velocity, v= ‘rΩ’ .

From the mechanism we have the following relation; α

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.sinαSo, velocity of U-Slot= rΩsinα.

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

Resources UsedMaterials Dimensions

Mild steel plates 1. 50 mm x 5 mm2. 50 mm x 2.5 mm

Mild Steel Rod 1. φ20 mm2. 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 Used

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

COMPONENTSManufacturing Procedure

Crank and HandleLathe Machine

Electric arc welding

•Obtained Cylindrical Rods Of Required Dimension •Operations: Plain Turning And Parting

•Welded Handle And Crank With Crank-shaft

Handle

Crankshaft

Crank

Crank and Handle

U-SlotPower Hacksaw

Power tools

• Obtained square pipe of required length

• Used surface grinding machine to obtain smooth surface• Used power cutter to remove one face of the square

pipe

U-Slot

Yoke (Slider Block)Lathe Machine

Shaping machine

Drilling machine

• Obtained a cylindrical block of required length

• Converted the cylindrical block into a cuboid of required dimensions

• Hole is drilled in the middle of block to accommodate the crank

Yoke (Slider Block)

FoundationPower hacksaw

Drilling

Electric arc welding

• Obtained metallic Strips Of Required Lengths

• Welded the metallic strips to get a rigid foundation

• Drilled holes to mount the crankshaft

Foundation

GuidesPower hacksaw

Power cutter

• Obtained metallic strips of required lengths

• Obtained slots in the metallic strips

Guides

Piston and piston rodLathe machine

Welding

Power Hacksaw

• Obtained cylindrical rods of required diameters and lengths using plain turning and parting

• Welded piston to piston rod• Welded the above assembly to U-slot

Hollow Cylinder• Cut the pipe of required length

Piston and piston rod

Hollow Cylinder

ASSEMBLYStep-by-step procedure

Foundation And Crank ShaftThe crank shaft is made to pass

through the holes drilled in the foundation

Constrained the linear motion and rotation in two axes by above step

Welded washers to constrain linear motion along the crank shaft axis

Now, we can rotate the crank by rotating the handle

Guides and FoundationGuides are welded to the

foundationWelding is done carefully

so that the guides are perfectly vertical and are parallel.

Hollow cylinder is welded to the foundation using metallic strips

Now the hollow cylinder is completely constrained

Foundation and hollow cylinder

U-slot and GuidesIn scotch yoke mechanism,

the linear reciprocating motion of U-slot is constrained by the guides.

Bolts (with metallic washers) passing through the slot of the guide are welded to the U-slot

Now the U-slot is constrained to move along the guide.

Yoke, crank and U-slotPin Joint between crank and

yoke:o Crank is made to pass through

the hole drilled in the yokeo This forms a pin joint i.e.; crank

can freely rotate in the hole of the yoke

Slider joint between yoke and U-slot:o Yoke is placed inside the U-slot,

so that it can freely slides inside it, forming a slider joint.

Piston and Cylinder The hollow cylinder is already constrained

(welded) with the foundation.Piston is made to slide inside the hollow

cylinder.This completes the assembly providing

each component only one degree of freedom (except foundation and guides)

Assembled Model

Final Assembly

Challenges and difficulties faced• Incorrect Selection of Crank Length1. The length of the crank was taken

too large.

2. As a result , the radius of revolution of crank was too large which eventually required a longer U-Slot to facilitate reciprocatory motion.

3. Longer guides were subjected to unbalanced dynamic forces and moments which restricted the free translatory motion..

Unbalanced moment

• Using Wood-based Foundation 1. The size of foundation required to keep the system in

equilibrium had to be bigger, bulkier and complex(because of bigger crank). This problem was tackled using big logs of wood.

2. The wood blocks were cut and joined accordingly using nails . This led to formation of weaker joints.

3. While operation , the wooden guides were subjected to large vibrations which made the reciprocatory motion of U-slot unsatisfactory.

4. Relatively larger force was required to rotate the crank manually .

Rejected wooden model

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

• 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.

Bibliography• http://en.wikipedia.org/wiki/Scotch_yoke

•http://www.tecquipment.com/Theory_of_Machines/Engineering-Science/RE300/M6.aspx

• Theory of Machines by R. S. Khurmi.

Thank You