Scotch Yoke Mechanism : An Overview

The Scotch Yoke is a reciprocating motion mechanism that changes a slider's linear motion into rotating motion or vice versa. It is also known as a slotted link mechanism. A sliding yoke with a slot that contacts a pin on the rotating component is directly connected to the piston or other reciprocating part. The piston's position over time is a simple harmonic motion, or a sine wave with constant amplitude and frequency when the rotating speed is constant. In this blog we will have an overview of Scotch Yoke mechanism.

Introduction

It is a type of reciprocating motion mechanism that transforms rotational motion into linear motion by sliding. When the reciprocating or piston parts are directly attached for yoke sliding that includes a slot. A pin at the reciprocating part is engaged by that. when the crankshaft of an internal combustion engine rotates to transform rotary motion into linear motion. Where the connecting rod and piston should be joined. Because the rotating motion of the Scotch Yoke Mechanism spends more time at its high point than the Piston Part, it is thought to be more efficient.

Concept and Mechanism

The Scotch yoke is a device that reciprocates motion, changing a slider's linear motion into rotating motion or vice versa. A sliding yoke with a slot that contacts a pin on the rotating component is directly connected to the piston or other reciprocating part. Given a constant rotational speed, the piston's motion has the form of a pure sine wave over time. 

Utilizing this device, rotational motion may be changed into a reciprocating motion. Fixing either link 1 or link 3 results in the inversion. Link 1 is fixed in Fig. In this mechanism, the link 4 (which represents a frame) reciprocates when the link 2 (which represents a crank) rotates around the centre B. The frame is guided by fixed link 1.

Working Principle

It is a simple mechanism that transforms the pin's rotating motion into linear motion. When the DC motor is first powered up and the shaft begins to spin, the pin slider within the yoke section is rotated by the crank, which is also moving forward. while the yoke is receiving a displacement moment at forward and the crank is rotating clockwise. The length of the Crank affects the yoke's maximum displacement. At the same moment as the yoke moving fully forward, the crank finished its clockwise revolution. When in this posture, starting the stork's return takes longer. 

The crank will continue to turn after some time has passed in order to return to its starting position. As a result, the yoke moves rearward before returning to its starting position. As a result, the crank must complete one full revolution before the yoke may finish sliding both forward and backward. The Yoke will slide across a space that is equal to twice the length of the Crank by means of the whole rotation of the Crank. The crank length may be changed to adjust the yoke displacement.

Internal Combustion Engines Uses

In a perfect engineering world, the assembly's course of travel is directly subjected to force. Smoother operation is achieved by the sinusoidal motion, cosinusoidal velocity, and sinusoidal acceleration (under the assumption of constant angular velocity). Theoretically, the engine efficiency of constant volume combustion cycles is improved by the greater proportion of time spent at top dead centre (dwell).  As side loading of the piston caused by the sine of the connecting rod angle is minimised, it enables the deletion of joints traditionally served by a wrist pin as well as a nearly complete elimination of piston skirts and cylinder scuffing. Such increases in the piston rod length are realistically only acceptable for lower RPM (but higher torque) applications since the longer the distance between the piston and the yoke, the less wear that occurs but the greater the inertia.

Since the slot in the Scotch yoke quickly wears out due to sliding friction and high contact pressures, most internal combustion engines do not employ them. A sliding block placed between the crank and the piston rod's slot lessens the impact of this. Additionally, any gains to constant volume combustion in real engines are countered by greater heat loss during combustion as a result of prolonged linger at top dead centre. Less time is spent at bottom dead centre in an engine application compared to a traditional piston and crankshaft mechanism, which shortens the blowdown period for two-stroke engines. Extended dwell times do not perform well with Otto cycle engines that use constant volume combustion, according to experiments. Gains might be more noticeable in Otto cycle engines that use stratified direct injection (diesel or a comparable cycle) to cut down on heat losses.

Applications :

The control valve actuators in high-pressure oil and gas pipelines most often use this configuration. Scotch yokes may be used by primitive shapers even though they are not a typical metalworking tool nowadays. A Whitworth linkage, which provides a slow forward cutting stroke and a rapid return, is used by almost all of them.

It has been used in a number of internal combustion engines, including the SyTech and Bourke engines as well as other hot air and steam engines. When the diameter of the circle formed by the crank pin is less than the slot in the yoke, the name "scotch yoke" is still used. For instance, scotch yokes on a locomotive's side rods allow the vertical mobility of intermediate drive axles. In order to create a sinusoidal motion, the Tide-Predicting Machine No. 2 uses what is effectively a Scotch yoke (sine functions).

Advantages

1) Easy to assemble, operate, and create.

2) Used for tasks like cutting and sliding, among others.

3) It is a straightforward technique of converting from rotary to linear.

4) High torque may be produced with a tiny cylinder size.

5) To increase engine efficiency, it spends a lot of time on top dead centre.

Disadvantages

1) Excessive wear brought on by heavy contact pressure and sliding friction.

2) To shorten the time below for a two-stroke engine, provide the percentage of time spent in the bottom centre position.

3) A constant rotation speed is produced throughout time by the piston motion, which is a pure sine wave.

4) For the arm to move in a reciprocating motion, suitable guides are needed.