Compound Gears Unit 6

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.

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.

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.

Spur Gears

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

Bevel Gears

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.

Worm Gears

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.

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.

Gear Ratios In this example, the torque is increased 3X. For each torque increase, there is an equal speed decrease. Here, by 1/3X.

Gear Ratios

Gear Ratios Calculated by # driven gear teeth/ #driving gear teeth. Speed Torque Small Large Decrease Increase

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.

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

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

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.

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)

Tasks Construct a gear box with 3 different gear combinations. Follow the procedure at the link provided below. http://www.education.rec.ri.cmu.edu/roboticscurriculum/vex_online/lessons/gearbox/mechanical_advantage.html 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.