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Compound Gears Unit 6

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**Introduction 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. Gears and gear ratios are used to create a balance between torque and speed.

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Gears Toothed wheels that interlock to transmit power (torque) without slippage. 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). 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.

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**Gears (continued) Gears control power transmission in 3 ways:**

Changes the direction in which power is transmitted. Changes the amount of force or torque. Changes the speed of rotation (RPMs). Gears of unequal size can be combined to produce a mechanical advantage resulting in a change of speed and torque. Gears are typically made of plastic or metals for wear, strength and durability.

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Spur Gears

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**Spur Gears Transfers speed and torque between parallel shafts.**

Advantages : simple, low cost, easy maintenance. Disadvantages : noisy due to meshing teeth.

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Bevel Gears

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**Bevel Gears Used to change the output shaft direction.**

Shafts are not parallel. Instead, they intersect. Shafts can be at any angle. 90 degrees is the most common.

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Worm Gears

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Worm Gears 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). A ratio of 300:1 is not uncommon.

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Gear Ratio 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. When a smaller gear meshes with a larger gear, the torque applied to the smaller gear is increased. The increase is based on the difference between the radius of each gear.

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**Gear Ratios In this example, the torque is increased 3X.**

For each torque increase, there is an equal speed decrease. Here, by 1/3X.

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Gear Ratios

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**Gear Ratios Calculated by # driven gear teeth/ #driving gear teeth.**

Speed Torque Small Large Decrease Increase

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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.

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

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

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Compound Gears Compound gears are formed when you have more than one gear on the same axle and can affect the overall gear ratio of the system. A compound gear has multiple gear pairs. Each pair has its own gear ratio. The overall gear ratios is found by multiplying the gear ratios of each of the gear pairs.

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In this example, a compound gear ratio of 1:25 is achieved using only 12 and 60 tooth gears. This will give a VEX robot the ability to turn an axle 25 times than normal (25X less torque)

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Tasks Construct a gear box with 3 different gear combinations. Follow the procedure at the link provided below. Learning journal – construct diagrams of your gear configurations and identify what was successful and what was not. Construct a worksheet from the exercise above. Identify on the worksheet the RPM, mass lifted and the gear ratio for each axle. Draw at least 2 conclusions. Assignment – gear ratios Construct a robot to lift a 5 kg mass.

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