Mechanisms and Motion

Here are questions that you may be asking

  1. What is a Mechanism?
  2. What is a “System”?.
  3. What is a “Mechanical System” ?
  4. What is a “Cam” ?
  5. What are the names of different types of mechanical movement?
  6. What are 1st, 2nd and 3rd class levers?
  7. What is the"Mechanical Advantage" of using levers?

By answering these questions you will understand

  1. How mechanisms can produce different types of motion.
  2. How Engineers make use of cams and different types of mechanical movement in machines.
  3. The difference between 1st, 2nd and 3rd class levers

You may also want to

  1. Make a Mechanism that uses a Cam to convert one type of motion into another type of motion.
  2. Make levers lift loads, and understand the "Mechanical Advantage".
  3. Tell others about your new knowledge of mechanisms.

A GOOD CHALLENGE IS TO MAKE CAMS THAT ARE INTERCHANGABLE ONTO ONE SHAFT. WHY NOT CHALLENGE YOURSELF TO CREATE THE SMOOTHEST MECHANICAL MOVEMENT THAT YOU CAN.

A mechanical System has an input, a process and an output

Mechanisms and Types of motion

Mechanisms are mechanical devices that convert one form of motion into another.
The engineer is concerned with the following types of motion
  • Rotary motion is turning round in a circle, such as a wheel turning.
  • Linear motion is moving in a straight line, such as on a paper trimmer.
  • Reciprocating motion is moving backwards and forwards in a straight line, as in cutting with a saw.
  • Oscillating motion is swinging from side to side, like a pendulum in a clock.
This project is all to do with creating mechanisms.

The Cam is a Mechanism

The cam is a very common mechanism used by engineers in thousands of applications.

In its simplest form the cam can have a shape or profile similar to a "snail". When rotated it can make a "follower" moves in a reciprocating path.

The cam can become really complicated with tricky profiles for the "follower" to follow. More complicated cam mechanisms require the engineer to add other components to ensure that the follower keeps in contact with the profile.

Eccentric Cam

The eccentric cam profile causes a continuous up & down motion of the follower

Scotch Yoke

The Scotch Yoke is a reciprocating  motion mechanism that converts circular motion into linear motion (or vise versa). Compare the output motion from the scotch yoke and the Eccentric Cam (left) - what difference do you notice between the output motion?

Egg/Pear Cam

The egg/Pear cam profile causes a sharp up & down motion with a period of low/no movement of the follower

Quick Return Mechanism

The quick return mechanism turns circular motion into a reciprocating motion - notice how the 'arm' moves quickly to the side when the pin on the circular disk passes closest to the pivot point of the arm.

Drop/Snail Cam

The Drop/Snail cam profile cause a gentle lift of the follower followed by a sudden downward drop - hence the name "drop' cam.

Bespoke Cam

If a product is "Bespoke" it is made to order...and a bespoke cam is exactly that. Sometimes there is not a specific preexisting type of cam that can produce the type of movement needed in the follower and in this case a "bespoke" cam needs to be designed and made to produce this type of movement.

Geneva Drive

The Geneva Drive turns continuous circular motion into intermittent circular motion. The example below is a 6 slot intermittent drive. The driven wheel (on the left) also locks into position between rotation because the shape of the driver prevents the driven from turning.

LEVERS

During your study of the cantilever Bridge in Unit 1, you found out about levers. As you know there are 3 different 'types' of lever - 1st, 2nd and 3rd class. Each class can be identified from the placement of the LOAD, EFFORT and PIVOT. The Load is the object being lifted/moved, the Effort is where the input movement is needed to move the lever and the PIVOT is the point at which the lever pivots or moves. There is a very simple formula to help you remember and identify the 3 classes of Lever FLE/123 - If P(ivot) is in the middle - then it is a 1st class lever, if L(oad) is in the middle then it is a 2nd class and if E(ffort) is in the middle then it is a 3rd class lever.

1st Class lever, Pivot in middle

The first class lever has the Pivot in between the Effort and the Load. The distance from the Fulcrum and the Effort is generally longer in order to gain mechanical advantage to lift the Load. A crowbar, or a pair of scissors are good examples of where this type of lever is used in 'real life'. The mechanical advantage for this type of lever can be greater or less than 1

2nd Class lever, Load in the middle

The second class lever has the Load in between the Effort and the Pivot. The distance from the Fulcrum and the Effort is generally longer in order to gain mechanical advantage to lift the Load. A Wheel Barrow is a good example of where this type of lever is used in 'real life'. The mechanical advantage for this type of lever is always greater than 1.

3rd Class Lever, Effort in the middle

The third class lever has the Effort in between the Load and the Pivot. The distance from the Fulcrum to the Effort is shorter and it therefore requires greater force to lift/move the load - however, the distance moved at the Effort point is magnified (based on the ratio of distance from Effort to Fulcrum and Load to Fulcrum) at the Load. The mechanical advantage for this type of lever is always less than 1.

The Mechanical Advantage of a Lever

A lever is used to reduce the physical effort or mechanical energy required to lift a load.