Scotch-Yoke Mechanism

 

The Scotch-Yoke also known as slotted link mechanism, is a reciprocating motion mechanism, capable of converting the linear motion of a slider into rotational motion, or vice versa directly depending on the application.

A Scotch-yoke mechanism has a yoke frame with opposed parallel tracks engaged by rollers shown in form of block assembly, arranged in order to transmit power between a rotating crankshaft attached to crankpin which are mounted on block assembly and oppositely directed pistons or other reciprocating part mounted on the yoke frame, which move linearly in line with the reciprocating-axis.

The piston or other reciprocating component can be coupled directly with sliding yoke and slot engaged with a pin and the rotating part. The motion of the piston is simple harmonic motion, taking in consideration that it has a constant amplitude, frequency, given at constant rotational speed.

Understanding Better With An Example: -

Scotch-Yoke mechanism or slotted link mechanism is a simple mechanism used for converting circular motion into reciprocating motion or vice versa as we discussed above. The power is supplied to the DC motor, as a result of which shaft and crank attached to the shaft start rotating.

Crank rotating slides the pin inside the yoke which makes the yoke move forward. If the crank is rotating in clockwise direction then the yoke will in forward direction. The maximum displacement equals the length of the crank.

When the crank completes the next part of rotation, the yoke will come back to its initial position. Then the yoke moves in the backward direction. After completing one full rotation by crank, yoke returns to the initial position.

For a complete rotation of crank the yoke moves through a length equal to double the length of the crank as it performs the linear motion twice. It is possible to control the displacement of the yoke by varying the length of the crank. The amount of displacement is based on the requirement of application.

Now, we will look at the diagram and understand exactly how it works by breaking it down step by step. We can see in diagram that connecting rods are labelled #1 and #2 and A,B,C,D represent 4 steps of the motion. To have further insight there is also a visual representation shown besides the diagram.

Position A: - Say position A indicates beginning position of the motion of pin given in the diagram be the initial position of connecting rod (#1) & (#2).

Position B: - The rotating disc, results slot to reciprocate along with it. Thus, changing the position of connecting rod, connecting rod (#1) and connecting rod (#2) both move in upward direction w.r.t to their initial position.

Position C: - As the disc rotates further with time both the connecting rods (#1) & (#2) get to their extreme uppermost position as we see in the diagram.

Position D: - After position C the connecting rods (#1) & (#2) start to move downwards as the disc further rotates and following similar pattern it reaches extreme bottommost position resulting in its initial position A.

After all position (A, B, C, D) the disc, the pin & connecting rods (#1, #2) is reached it completes one full cycle and then further the cycle repeats and the motion continues resulting in reciprocating motion.

Applications Of Scotch-Yoke Mechanism: 

• This setup coupled with some other mechanisms is used in control valve actuators in high-pressure oil and gas pipelines.
• When the slot in the yoke is shorter compared to the diameter of the circle made by the crank pin it is often referred as scotch yoke mechanism still. For example, the side rods of a locomotive may have scotch yokes to permit vertical motion of intermediate driving axles which is used as a basic component in complex machinery. 
• It is used in manufacturing of double hack-saw. 
• Since long time, it has been used in various internal combustion engines, such as the Bourke engine and many hot air engines and steam engines. 
• It is also used in reciprocating pumps to convert rotational motion into reciprocating motion required for piston movement. 
• The basic principle of Scotch yoke, is used in the Tide-Predicting Machine to generate a sinusoidal motion (sine functions) which is used further as deductions are possible through observed data.
• It is used in beam engine pumps to convert rotational motion into reciprocating motion in contrast to that of reciprocating pumps where it is vice versa. 
• It is also commonly used in making toys which have back and forth motion or cyclic motion.
• Nowadays it is not commonly used metalworking machine due to frequent wearing which results in high maintenance cost of machines but still scotch yoke is used in crude shapers.

 Advantages of Scotch-Yoke Mechanism: -

• Most important advantage is easy construction of assembly and its operation.
• It can be used to perform various applications like cutting, sliding, etc. depending upon what application demands.
• It is a direct method to convert Rotary motion to Linear motion or vice versa.
• Requirement of high torque can be satisfied with small size of cylinder using this mechanism as the space occupied is less.
• The scotch-yoke mechanism increases engine as, the amount of time spent in top dead center is responsible for improving engine efficiency.

Disadvantages of Scotch-Yoke Mechanism: -

• The high wear produced due to sliding friction and high contact pressure is a big problem in many applications.
• Least percentage of time spent in bottom center position, to reduce the below time in two stroke engines.
• The Piston motion is a pure sine wave will occur overtime give a constant rotation speed but over a period of time it may differ.
• It requires a guided path so that reciprocating motion of arm is performed properly.

To sum up it is a great tool if used with proper knowledge it can be very advantageous in applications with space restrictions and high torque requirements.

 

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