Drawing gear systems

In this chapter, you will learn how to draw gear systems. First you will do some orthographic or two-dimensional (2D) drawings that show the exact sizes and numbers of teeth on the gears. For these types of drawings, you do not have to draw the teeth, so it is much easier.

Then you will write a design brief for some gear systems of your own and produce specifications for the systems. You will learn to use drawing instruments and an isometric grid to draw your gear systems in three dimensions (3D).

Tech2_gr8_ch3_fig1.tif
Figure 1: The back of a tow truck showing the winch that is used to lift and pull cars that have broken down. A winch is a gear that gives a mechanical advantage. You will learn about winches in this chapter.
Tech2_gr8_ch3_fig2.tif
Figure 2: Spur gears with different radiuses. You will learn how to draw technical diagrams of gears when you are given the radius and the number of teeth of each gear. You don't need to draw the teeth!

Draw gears in two dimensions (2d)

When you draw a gear wheel, you show a number of different circle sizes, but you do not show the gear teeth. The specification for the gear wheels and teeth is shown using notes and tables.

Tech3_gr8_ch3_fig3.tif
Figure 3: How to draw a gear wheel with 15 teeth

Figure 3 shows all the important information for a gear wheel:

The pitch circle diameter on this gear is 35,8 mm. The distance around the pitch circle of this gear is the pitch circle circumference, which is:

Circumference = π × D = 3,1428 × 35,8 mm = 112,5 cm.

So the pitch, or the space for each tooth = 112,5 ÷ 15 = 7,5 mm.

Look at Figure 4. This figure shows how to draw a gear wheel.

Tech2_gr8_ch3_fig4.tif
Figure 4: How to draw a gear wheel

Now draw this gear wheel on the grid by following these steps:

Drawing meshing gears

Look at the drawing of the meshing gears in Figure 6. A small driver gear is shown on the left. It is driving a larger driven gear on the right.

Tech2_gr8_ch3_fig6.tif
Figure 6: Meshing gears

Two spur gears will only mesh properly if:

  • the size and shape of their teeth are the same, in other words the pitch and the depth of gear teeth on both gears are the same, and
  • the pitch circle circumferences of the two gears are touching each other.

The line connecting the centres of the two gears is called the centre line. Centre lines are drawn as chain lines, with long and short dashes.

The distance between the gear centres is shown on this drawing as the centre distance. The exact centre distance for two meshing gears is the pitch circle radius of the driver gear plus the pitch circle radius of the driven gear.

Remember: The radius is ½ of the diameter.

If, for example, this driven gear had 15 teeth and a pitch circle diameter of 35 mm, and the driven gear had 30 teeth and a pitch circle diameter of 70 mm, then the centre distance would be:

Centre distance = ½ × 35 mm + ½ × 70 mm = 17,5 mm + 35 mm = 52,5 mm.

How to draw meshing gear systems

Look at the meshing gears in Figure 6 on the previous page. Figure 7 below shows how to draw a diagram of this gear system, which has a 15-tooth driver gear and a 30-tooth driven gear.

Tech2_gr8_ch3_fig7_B.tif
Figure 7

Draw gear systems with the driven gear rotating in the opposite direction of the driver gear

1. Use the steps on the previous page to draw a gear system with 15 teeth on a driver gear with a 36 mm diameter and 30 teeth on a driven gear with a 72 mm diameter. Use the grid paper in Figure 8. The driver gear drawing has been started for you.

Tech2_gr8_ch3_fig8_NoAnswer_LB.tif
Figure 8: Draw the driven gear on the grid.

2. When you have finished your drawing, use arrows to show the direction of rotation of the driven gear if the driver is turning clockwise.

3. Will the driven gear be rotating faster or slower than the driver?


Draw gear systems with the driven gear rotating in the same direction as the driver gear

Do you remember what an idler gear does? It meshes between the driver and the driven gear. The idler does not change the gear ratio. All it does is change the direction of the driven gear. A gear system with an idler can have the driven and the driver gear turning in the same direction.

Tech2_gr8_ch3_fig9.tif
Figure 9

To draw a gear system with an idler, you will need to draw three gears instead of two. But the principle stays the same.

1. Draw the gear system in Figure 9 on the grid paper on the next page.

2. Draw arrows to show which way each gear will turn.

3. Do the driver and driven gears rotate in the same or in opposite directions?


4. If the driver gear rotates at 1 500 rpm, how fast will the driven gear rotate?


Tech2_gr8_ch3_fig10.tif
Figure 10

Homework: draw gear systems with the driven gear rotating faster than the driver gear

Part A: Rotating in opposite directions

1. Draw the gear system shown in Figure 11. The driver gear has 45 teeth and a pitch circle diameter of 107 mm. The driven gear has 15 teeth and a pitch circle diameter of 36 mm. Use the grid paper in Figure 12.

Tech2_gr8_ch3_fig11_B.tif
Figure 11
Tech2_gr8_ch3_fig12.tif
Figure 12: Draw your gear system on this grid paper.

2. What can you say about the speed of the driven gear compared to the driver gear?


3. Does this system change the direction of rotation?


Part B: Rotating in the same direction

1. Add an idler to this gear system as shown in Figure 13. Now draw this new system on the grid paper in Figure 14.

2. Draw arrows on the drawing to show the direction of rotation of each gear.

Tech2_gr8_ch3_fig13_B.tif
Figure 13
Tech2_gr8_ch3_fig14.tif
Figure 14: Draw your gear system with an idler gear on this grid paper.

3. What does the idler do?


Write a design brief with specifications for gears

Gear systems have two important uses:

In this lesson, you will design gear systems that use both these advantages.

A design brief for a gear that gives a mechanical advantage

Look at Figure 15. It shows a winch for a tow truck. Winches are used to pull broken-down cars onto the back of a tow truck.

Tech2_gr8_ch3_fig15.tif
Figure 15: This mechanism is a winch. It is used to pull broken-down cars onto the back of a tow

A problem with this winch

The company using this winch has found that is not powerful enough to pull large vehicles.

The company asked you to improve the winch. They want the winch to pull large vehicles that are three times as heavy as ordinary cars.

The word tow means to pull a car behind a moving truck for a certain distance. Tow trucks can tow cars, but they can also pull cars onto the back of the truck to carry them to the repair shop.

Write a design brief

1. Write a few short, clear sentences that summarise the problem that needs to be solved, as well as the purpose of the proposed solution. Begin your first sentence with the words:

I am going to design ...


2. Write a list of specifications for the new winch solution.

Remember: Specifications are lists of things that your solution must do, and some things that it must not do.


A design for the improved winch

3. Describe how you are going to improve this winch.


4. How will you know that the winch can pull vehicles that are up to three times heavier than an ordinary car?


5. Complete the drawing in Figure 16 to show how you will improve the winch. Draw the driver gear on top of the motor. Then show where you will place the winder, and draw the winder gear. Use a pitch of 7,5 mm and a depth of 5,0 mm for the gear teeth. Label your drawing with the pitch and number of teeth on each of the gear wheels.

Tech2_gr8_ch3_fig16_NoAnswer_LB.tif
Figure 16: Use this grid to show how you will improve the winch.

Write a design brief for a gear that gives a speed advantage

Look at the system shown below. It shows the inside of a wind turbine. The wind turns the propeller and the propeller turns an electric generator to make electricity.

The problem with wind turbines

The blades of wind turbines turn slowly, at about 9 to 19 rpm. But the electric generator that is driven by a wind turbine needs to turn faster. A turbine manufacturer needs a gear system that will make the generator turn at least four times faster than the wind turbine. Can you help?

Tech2_gr8_ch3_fig17.tif
Figure 17: Inside a wind turbine

1. Write a design brief. You need to write a few short, clear sentences that summarise the problem that needs to be solved, and the purpose of the proposed solution. Begin your first sentence with the words:

I am going to design ...


2. Specifications for your solution. Write a list of specifications for the gear system solution.


A design for the improved wind turbine

1. Draw your design on the grid in Figure 18. Your design should show how you will make the driven generator of the wind turbine move four times faster than the driver. Use a pitch of 0,75 cm and a height of 0,50 cm for the gear teeth.

2. Label your drawing with the pitch and number of teeth on each of the gear wheels.

14464.png
Figure 18: Draw your design on this grid.

Draw gears in three dimensions (3d)

Drawing gears in 3D is mostly about drawing circles in 3D. In this activity, you will draw 3D gears on isometric grid paper.

If you follow the instructions step by step, your drawing will be correct.

How to draw an isometric circle

Look at the pictures in Figure 19. They show how to draw a circle on isometric grid paper. This circle has a diameter of 2, so it is nearly the size of a small gear wheel. Below is an outline of how it can be done.

Tech2_gr8_ch3_fig19a.tif
Figure 19 A and B: How to draw an isometric circle
Tech2_gr8_ch3_fig20a.tif
Figure 20 A and B: Draw your own isometric circle on grid B above.

Draw the gear system that you designed for the winch

Look at the picture in Figure 21. Two gears have been drawn in 3D using isometric grid paper. The teeth of the gear are not shown.

1. Use the grid on the next page to help you draw the system you designed for the winch. Draw the gears to the same size as you specified for the winch in section 3.2.

2. Add a table of information to your drawing that gives all the information necessary for someone to make these gears.

Tech2_gr8_ch3_fig21.tif
Figure 21: Two gears drawn in 3D using isometric paper

Draw your gear system for the winch onto the grid in Figure 22:

Tech2_gr8_ch3_fig22.tif
Figure 22: Draw your gear system for the winch in the grid above.

Next week

Next week, you will investigate a type of gear called bevel gears. You will look at the gears on a bicycle and learn about chain and belt drives. Then you will learn how to analyse gear systems using the systems approach.