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Mechanisms

Lesson 1:

What is a mechanism?

A mechanism is a set of components that transmit or transform a movement. Contrary to what occurs with structures, which are sets of elements designed to hold loads without deforming, in mechanisms there is a relative movement between their components.

Below you can see two very simple examples of a structure (left) and a mechanism (right). While the elements of the construction on the left can’t move, the construction on the right can be deformed by moving the red dot:

Think of your bike. A bicycle is an example of a mechanism in which the movement of your legs (input movement) is transformed into the movement of the wheels (output movement). This way, riding a bike we can move at a high speed without too much effort.

As we will see in the next section, input and output movements can be linear or circular.


Types of motion

Linear motion

The motion of a body is said to be linear when the path it follows is a straight line:

When a body moves with linear motion, it does so with a certain speed, which we call linear speed, which can be calculated with the following equation:

\huge v= \frac{s}{t}

Where v represents the speed, s is the distance (or space) travelled and t the time used.

If the distance is entered in kilometers and the time in hours, the result will be the speed in kilometers per hour (km/h). However, the most correct option is to enter the distance in meters and the time in seconds, in which case the speed is expressed in meters per second (m/s).

Example: calculate the speed of a car that needs 2 seconds to travel 18 meters in a straight line.

\large v= \frac{18}{2}=9\ m/s

Circular motion.

We say that the motion of a body is circular when the path that it follows is a circumference:

All circular motion is determined by a radius, as you can see in the animation above. When the radius of the circular motion becomes zero the body begins to rotate on itself. A typical example of a body with circular (or roughly circular) motion is the Earth. The earth experiences a circular movement around the Sun (revolution) and another on itself (rotation).

When a body moves with circular motion, it does so with a certain speed, which in this case is called angular speed. In technology, we are going to use the following equation to calculate the angular speed:

\Large n=\frac{number\ of\ revolutions}{t}

In which n represents angular speed and t the time taken.

In the subject of technology, the time will always be entered in minutes, so that the speed will be obtained in revolutions per minute (rpm).

Example: calculate the angular speed of a carousel that takes 3 minutes to rotate 12 times.

\large n=\frac{12}{3}=4\ rpm

Linear reciprocating motion

A linear reciprocating motion is a type of linear motion that has many applications in technology as we will see throughout the unit. As you can see in the following animation it is a repetitive back and forth linear movement.


Circular reciprocating motion

Although it does not have as many applications as the previous motion, the circular movement can also be repetitive. Perhaps the best known example of this type of movement is that of a pendulum.


Mechanisms classification

The different mechanisms that we are going to study throughout the unit are usually classified according to their input and output movements:

  • If both the input and the output movements are linear, we say that the device is a linear transmission mechanism.
  • If both the input and the output movements are circular, we say that the device is a circular transmission mechanism.
  • If the input movement is different from the output movement (linear-circular or circular-linear) we say that the device is a transforming mechanism.

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