1. Gears, Chains and Sprockets Unit 6

2. Introduction Motors can only create a set amount of power. Motors can only create a set amount of power. There is only so much power available, the torque and speed of the motor are limited by each other. For example, if a motor is fast, it cannot lift a heavy load. There is only so much power available, the torque and speed of the motor are limited by each other. For example, if a motor is fast, it cannot lift a heavy load. Gears and gear ratios are used to create a balance between torque and speed. Gears and gear ratios are used to create a balance between torque and speed.

3. Gears Toothed wheels that interlock to transmit power without slippage. Toothed wheels that interlock to transmit power without slippage. Have efficiencies of up to 98% in their transmission of energy. Have efficiencies of up to 98% in their transmission of energy. A gear can interlock or “mesh” with any device having the same type of teeth such as a rack. (i.e. rack and pinion). A gear can interlock or “mesh” with any device having the same type of teeth such as a rack. (i.e. rack and pinion). The gear transmitting the force is the input or drive gear while the gear connected to the drive gear is the output or driven gear. The gear transmitting the force is the input or drive gear while the gear connected to the drive gear is the output or driven gear.

4. Gears (continued) Gears control power transmission in 3 ways: 1. Changes the direction in which power is transmitted. 2. Changes the amount of force or torque. 3. Changes the speed of rotation (RPMs). Gears are typically made of plastic or metals for wear, strength and durability.

5. Gear Ratio The most important feature of gears is that gears of unequal size produce a mechanical advantage. The most important feature of gears is that gears of unequal size produce a mechanical advantage. This can change the speed or torque of the second gear. This can change the speed or torque of the second gear.

6. Spur Gears

7. Transfers speed and torque between parallel shafts. Transfers speed and torque between parallel shafts. Advantages : simple, low cost, easy maintenance. Advantages : simple, low cost, easy maintenance. Disadvantages : noisy due to meshing teeth. Disadvantages : noisy due to meshing teeth.

8. Bevel Gears

9. Used to change the output shaft direction. Used to change the output shaft direction. Shafts are not parallel. Instead, they intersect. Shafts are not parallel. Instead, they intersect. Shafts can be at any angle. 90 degrees is the most common. Shafts can be at any angle. 90 degrees is the most common.

10. Worm Gears

11. Transmit power 2 shafts that are at right angles to each other. Transmit power 2 shafts that are at right angles to each other. Used where a large speed reduction or mechanical advantage is required. (i.e. braking or a locking action or heavy lifting). Used where a large speed reduction or mechanical advantage is required. (i.e. braking or a locking action or heavy lifting). A ratio of 300:1 is not uncommon. A ratio of 300:1 is not uncommon.

12. Gear Ratios

13. Calculated by # driven gear teeth/ #driving gear teeth. Calculated by # driven gear teeth/ #driving gear teeth. Driving Gear Driven Gear SpeedTorque SmallLargeDecreaseIncrease LargeSmallIncreaseDecrease

14. Idler Gears Used to reverse the direction of rotation. Gears go in opposite directions. With an idler gear, they go in the same direction. Has no effect on gear ratio. Used to reverse the direction of rotation. Gears go in opposite directions. With an idler gear, they go in the same direction. Has no effect on gear ratio.

15. Compound Gear Ratios More than one gear on the same axle. More than one gear on the same axle.

16. Chain Drives Used where torque is needed to be transferred over longer distances than allowed by the gears. Used where torque is needed to be transferred over longer distances than allowed by the gears.

17. Tasks 1. THINK Phase – notes and keyterms 2. BUILD Phase – construction as per pp. 46-52. 3. AMAZE Phase (Challenge) – pp. 54-55 4. Engineering Notebook – p. 58. Omit the Presentation section. 5. STEM – Science #1, 3; Eng #1; Math #1,2