SCOTCH YOKE MECHANISM The Scotch yoke (also known as slotted link mechanism is a reciprocating motion mechanism, 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 location of the piston versus time is a sine wave of constant amplitude, and constant frequency given a constant rotational speed. THEORY:
Scotch yoke is a mechanism used to convert rotary motion into Sliding motion. This mechanism is obtained from an inversion of the double Slider crank chain. Double slider crank chain is a four-bar kinematic chain having 2 sliding Pairs and 2 turning pairs such that two pairs of the same kind are adjacent. The general version of the double slider crank chain is shown in fig. 1. two Die-blocks, P & Q, slide along slots in a frame, and the pins P & Q on the Die-blocks are connected by a link PQ
Scotch Yoke Mechanism
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[sam id=”4″ codes=”true”] This inversion is obtained by fixing one of the sliders. Refer fig. 2. let Slider block ‘p’ be fixed. Link PQ may then rotate relative to die blocks about The pins P & Q. thus link PQ can rotate with pin P as centre, and will therefore Cause the frame to reciprocate along the axis passing through P, slider block ‘Q’ will also reciprocate in its slot. The stroke of the frame will depend upon the length of link PQ and will be double the length PQ.
Description of the model: A model of scotch yoke mechanism is to be used for study. A diagram of the Model is shown in fig .4. The model consist of a slider plate guides G1 and G2 Mounted on the base plate of the model. The slider plate carries a slot AB in which a slider block ‘Q’ as centre, and drives the slider block Q. arm OR. the Arm OR is free to rotate about point ‘O’ as centre, and drives the slider block ‘Q’. The position of point ‘R’ can be changed by selecting one of the four holes provided on the arm OR and shifting the screw to the particular hole. The Crank OR is further rigidly linked to a handle which is used to provide manual Rotation to the link, and thereby impart motion to the mechanism. An extension To the rotating arm OR is provided which carries a pencil to trace the movement Of the arm.
Precaution:
1. Handle the model with care and attention. The model should not be mishandled 2. It should not be dismantled, unless for a specific purpose and then also,only with the permission of the lab I/c. 3. The moving parts of the model must be lubricated as and when necessary. 4. Defect,if any, noticed in the model must be brought to the attention of the Lab I/c immediately. 5. All measurements must be made with sufficient precision, and must be verified by making repeated measurements. An average value of the measurements must be taken to be the true value of the parameter. Experiment: the student is expected to conduct the following: 1. Study the construction of the model of scotch yoke. 2. Study the movement of the slider plate as the drive shaft is rotated. 3. Trace the path of the crank, and record the motion of the slider plate for different sizes of the crank by changing the link OR. Make measurements to determine the diameter of the circular path described by the actual stroke of slider plate. 4. To search animations of this mechanism on the internet and study the same for a better understanding of the principles involved.Result: 1. State the measurements and the derived values along with the actual values. 2. State the animation studied and its source.
INTRODUCTION Multi-operation machine as a research area is motivated by questions that arise in industrial manufacturing, production planning, and computer control. Consider a large automotive garage with specialized shops. A car may require the following work, replace exhaust system, align wheels, and tune up. These three tasks may be carried out in any order. However, since the exhaust system, alignment, and tune-up shops are in different buildings, it is impossible to perform two tasks for a car simultaneously. When there are many cars requiring services at the three shops, it is desirable to construct a service schedule that takes the least amount of total time.
1.1 Scotch Yoke 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.
Figure 1.1 Sectional view of Scotch yoke mechanism
Figure 1.2 Front view of Scotch Yoke Mechanism
1.2 Construction
The scotch yoke mechanism is constructed with iron bars. Here the crank is made in some length and the yoke is also made using the same material. It is noted that the minimum length of the yoke should be double the length of the crank. The crank and yoke is connected with a pin. Iron bars are welded to both sides of the yoke to get the reciprocating motion. The yoke with the iron bars is fixed on the display board with the help of c clamp. Now the crank is welded to the end of the shaft of the motor. Now the pin on the crank is connected to the yoke. The pin used to connect yoke and crank is a bolt.
1.3 Working principle When the power is supplied to the 12v Dc motor, shaft and crank attached to the shaft start rotating. As the crank rotates the pin slides inside the yoke and also moves the yoke forward. When the crank rotates through in clockwise direction the yoke will get a displacement in the forward direction. The maximum displacement will be equal to the length of the crank. When the crank completes the next of rotation the yoke comes back to its initial position. For the next of rotation, yoke moves in the backward direction. When the crank completes a full rotation the yoke moves back to the initial position. For a
complete rotation of crank the yoke moves through a length equal to double the length of the crank. The displacement of the yoke can be controlled by varying the length of the crank.
SCOTCH YOKE MECHANISM 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. The double slider crank mechanism is a mechanism having two sliding pairs and two turning pairs. Scotch yoke mechanism is formed when one of the two sliding pairs in a double slider crank mechanism is fixed. It has got two turning pairs, one sliding pair and a fixed link. CONSTRUCTION The scotch yoke mechanism is constructed with iron bars. Here the crank is made in some length (say 5cm) and the yoke is also made using the same material. It is noted that the minimum length of the yoke should be double the length of the crank. The crank and yoke is connected with a pin. Iron bars are welded to both sides of the yoke to get the reciprocating
motion. The yoke with the iron bars is fixed on the display board with the help of c clamp. Now the crank is welded to the end of the shaft of the motor. Now the pin on the crank is connected to the yoke. The pin used to connect yoke and crank is a bolt. The whole setup displayed in a plywood board. WORKING When the power is supplied to the 12v dc motor, shaft and crank attached to the shaft start rotating. As the crank rotates the pin slides inside the yoke and also moves the yoke forward. When the crank rotates through in clockwise direction the yoke will get a displacement in the forward direction. The maximum displacement will be equal to the length of the crank. When the crank completes the next of rotation the yoke comes back to its initial position. For the next of rotation, yoke moves in the backward direction. When the crank completes a full rotation the yoke moves back to the initial position. For a complete rotation of crank the yoke moves through a length equal to double the length of the crank. The displacement of the yoke can be controlled by varying the length of the crank. ADVANTAGES 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,use of connecting rod is eliminated when compared to slider crank mechanism and thus reducing the vibrations produced on the connecting rod DISADVANTAGES The disadvantages are: •
Rapid wear of the slot in the yoke caused by sliding friction and high contact pressures.
• Increased heat loss during combustion due to extended dwell at top dead center offsets any constant volume combustion improvements in real engines. • Lesser percentage of the time spent at bottom dead center reducing blow down time for two stroke engines, when compared with a conventional piston and crankshaft mechanism.
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, and many hot air engines and steam engines. In internal combustion engines, scotch yoke mechanism is connected to the piston instead of using the slider crank mechanism. It results in elimination of connecting rod which reduces the vibrations caused in the connecting rod. It has got extended dwell times. 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 loss CONCLUSION The scotch yoke mechanism is made and its advantages and disadvantages are discussed. Its motion characteristics are studied. It is concluded that this mechanism is a good choice to convert rotating motion into reciprocating motion because of fewer moving parts and smoother operation. It can be used in direct injection engines like diesel engines. 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. Scotch-yoke mechanism, pictured in Fig. functions in a manner similar to that of the simple crank mechanism except that its linear output motion is sinusoidal. As wheel A, the driver, rotates, the pin or roller bearing at its periphery exerts torque within the closed yoke B; this causes the attached sliding bar to reciprocate, tracing a sinusoidal waveform. Part a shows the sliding bar when the roller is at 270°, and part b shows the sliding bar when the roller is at 0°.
Applications This mechanism 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 and steam engines. 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.
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 MOTIONu
v
Suppose crankshaft is rotating at an angular velocity ‘Ω’. If r is the radius of the crank then,
α x-axis
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. Now as we know,α is directly proportional to time. Thisimplies 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 DISADVANTAGES The 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 Mild steel plates
Mild Steel Rod
Dimensions 1. 50 mm x 5 mm 2. 50 mm x 2.5 mm 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. Lathe Machine 2. Drilling machine 3. Shaper machine 4. Grinding machine 5. Power tools 6. Power Hacksaw 7. Electric arc welding machine COMPONENTS 1. 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 pipe Dimensions: 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 foundation Dimensions: 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-slot
Dimensions:
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.