Mechanisms

Question 1

What are the four main types of motion?

Answer 1

• Linear (moving in a straight line)
• Rotary (moving in a circular motion)
• Reciprocating (moving back and fourth)
• Oscillating (swing back and fourth)

Question 2

What is a mechanism?

Answer 2

A mechanism is simply a device which takes an input motion and force, and outputs a different motion and force.

Question 3

What is the point of a mechanism?

Answer 3

The point of a mechanism is to make the job easier to do. The mechanisms most commonly used in mechanical systems are levers, linkages, cams, gears and pulleys.

Question 4

What is mechanical advantage and how can it be calculated

Answer 4

How much easier a job is to do is called the mechanical advantage. This can be calculated:
MA = Load(N) / Force(N)

Question 5

What is a lever

Answer 5

A lever is a simple mechanism that uses mechanical advantage to make a job easier. There are three classes of lever that are determined by the position of the load, the effort and the fulcrum (the point where the lever pivots).

Question 6

What is a first class lever and what is an example of it?

Answer 6

A first class lever has the load and the effort on opposite sides of the pivot (fulcrum).
The mechanical advantage can be improved my moving the pivot closer to the load.
An example of this is a seesaw.

Question 7

What is a second class lever and what is an example of it?

Answer 7

A second class lever has the load in the middle between the effort and pivot (fulcrum).
The greater the distance between the load and the effort, the greater the mechanical advantage.
An example of this is a wheelbarrow.

Question 8

What is a third class lever and what is an example of it?

Answer 8

A third class lever has the effort in the middle between the pivot (fulcrum) and load.
A third class lever has no mechanical advantage and is used where a small effort is required to move the load. They are usually used to make a job easier, such as picking up small objects with a pair of tweezers.

Question 9

What is a simple linkage?

Answer 9

A simple linkage will change the direction of a force, for example from a pushing to a pulling force. As the pivot is in the middle, there is no mechanical advantage, just a change in direction.

Question 10

What is a bell crank linkage?

Answer 10

A bell crank linkage is used to transfer a force through 90°.
The bell crank is used on a bicycle to operate a traditional braking system. The linkage is used to transfer the force through 90° and force the brake pad against the rim of the wheel.

Question 11

What is a linkage with a mechanical advantage?

Answer 11

A linkage can be used to gain mechanical advantage by moving the pivot point.
A larger input movement can produce a large output force, but it will move a shorter distance.

Question 12

What is a pulley system

Answer 12

A pulley system can be used to make the lifting of loads easier. By using a series of pulleys, it is possible to decrease the amount of effort required to lift a given load.

Question 13

Give an example of a pulley system?

Answer 13

For example, by using a pulley system to lift a load only half the effort (measured in Newtons) is required, but the load only moves half the distance travelled by the rope that is being used to pull.
This trade-off allows much heavier loads to be lifted with relative ease.

Question 14

How can a pulley system be used in a pillar drill?

Answer 14

A pulley system can be used to change the speed of a rotating shaft by using different sized pulleys. They are usually connected using a belt.
Pulley systems are used inside a pillar drill to change the speed of rotation of the chuck to suit different materials / drill size.

Question 15

What are the equations for velocity ratio for pulleys, gears and sprockets?

Answer 15

VR = Driven pulley diameter / Driver pulley diameter
VR = No of teeth on driven gear / No of teeth on driver gear
VR = No of teeth on driven sprocket / No of teeth on driver sprocket
Remember: VR = driven (diameter or no of teeth) / driver (diameter or no of teeth)

Question 16

What is a gear?

Answer 16

Gears are toothed wheels that are connected to a driver and driven shaft.
The driver is the input.
The driven is the output.
It is possible to change the speed of the driven shaft by changing the size of the gear.

Question 17

What happens if gears are connected directly?

Answer 17

Gears connected directly will move in the opposite direction.

Question 18

What are idler gears?

Answer 18

Additional gears added to a gear system to change the direction rotation.

Question 19

What are chain and sprocket systems and how do they differ from pulley systems?

Answer 19

• Chain and sprocket systems use a chain to transfer the rotary motion from the driver to the driven shaft.
• Chain and sprocket systems rotate in the same direction.
• Chain and sprocket systems are similar to pulley systems, but they are stronger due to the teeth.

Question 20

A driver gear has 10 teeth and the driven gear has 20 teeth.
If the driver shaft is rotating at 300rpm, what speed would be driven shaft be rotating at?

Answer 20

20/10 = 2/1
300/2 = 150rpm

Question 21

A driver gear has 20 teeth and the driven gear has 10 teeth.
If the drive shaft is rotating at 200rpm, what speed would be driven shaft be rotating at?

Answer 21

10/20 = 1/2
200/0.5 = 400rpm

Question 22

What is a worm gear?

Answer 22

• Worm gears can be used to transfer motion by 90°. A worm gear only has one ‘tooth’ that works like a screw thread.
• A tuning peg on a guitar is a good example of a worm gear in use.

Question 23

What is a bevel gear?

Answer 23

• Bevel gears also change the direction of rotation by 90° but different sized gears can be used to change the speed of rotation.
• A hand drill uses bevel gears to change the direction of motion. The larger gear on the input allows the output shaft to turn quickly.

Question 24

What is a rack and pinion gear?

Answer 24

• A rack and pinion gear changes a rotary motion into a linear motion. This is usually a fairly small linear movement that goes in one direction and then back again.
• A rack and pinion system can be used to operate a sluice gate. A handle would be added to the rotary shaft and when turned, the sluice gate would go up or down depending on the direction of rotation.

Question 25

What are cams and followers

Answer 25

Rotary cams are used to turn rotary motion into reciprocating or oscillating motion. As the cam rotates, the follower moves depending on the shape of the cam. They are commonly used in car combustion engines to control the valves and in older vehicles the electrical system.

Question 26

How can differently shaped cams produce different types of movement?

Answer 26

Cams can be shaped to produce different types of movement. A pear-shaped cam creates a very sudden movement as the cam rotates to the peak, whereas a circular cam is a more consistent, smoother, movement.