Scotch Yoke Mechanism

Aim of the Project 

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To design and manufacture a working model of Scotch  yoke  yoke mechanism. mechanism. To get a practical exposure of machine tools and other manufacturing equipments.

Aim of the Project 

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To design and manufacture a working model of Scotch  yoke  yoke mechanism. mechanism. To get a practical exposure of machine tools and other manufacturing equipments.

Problem Stat Statement ement 

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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 machiner y.

Scotch yoke mechanism

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

Simple Harmonic Motion

Scotch 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 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.

α 

v 

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 . 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

Resources Used Materials

Dimensions

Mild 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 Used 1. 2. 3. 4. 5. 6. 7.

Lathe Machine Drilling machine Shaper machine Grinding machine Power tools Power Hacksaw Electric arc welding machine

Manufacturing Procedure

Crank and Handle 

Lathe Machine

Handle

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

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Electric arc welding  Welded Handle And Crank With Crank-shaft

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Crankshaft Crank

Crank and Handle

U-Slot 

Power Hacksaw •

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Obtained square pipe of required length

Power tools •

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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 •

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Shaping machine •

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Obtained a cylindrical block of required length

Converted the cylindrical block into a cuboid of required dimensions

Drilling machine •

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

Yoke (Slider Block)

Foundation 

Power hacksaw •

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Drilling •

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Obtained metallic Strips Of Required Lengths

Drilled holes to mount the crankshaft

Electric arc welding •

 Welded the metallic strips to get a rigid foundation

Foundation

Guides 

Power hacksaw •

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Obtained metallic strips of required lengths

Power cutter •

Obtained slots in the metallic strips

Guides

Piston and piston rod 

Lathe machine •

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

 Welding •

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

Hollow Cylinder 

Power Hacksaw •

Cut the pipe of required length

Piston and piston rod

Hollow Cylinder

Step-by-step procedure

Foundation And Crank Shaft 

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The 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 Foundation 

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Guides are welded to the foundation  Welding is done carefully so that the guides are perfectly vertical and are parallel.

Foundation and hollow cylinder 

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Hollow cylinder is  welded to the foundation using metallic strips Now the hollow cylinder is completely  constrained

U-slot and Guides 

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In 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-slot 

Pin Joint between crank and  yoke: o

o

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Crank is made to pass through the hole drilled in the yoke 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 

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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

Challenges and difficulties faced •

Incorrect Selection of Crank Length

1. 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

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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 •

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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/Enginee ring-Science/RE300/M6.aspx •

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Theory of Machines by R. S. Khurmi.