Compound GearsCompound Gears

Unit 6Unit 6

IntroductionIntroduction

Motors can only create a set amount of Motors can only create a set amount of power. power.

There is only so much power available, There is only so much power available, the torque and speed of the motor are the torque and speed of the motor are limited by each other. For example, if a limited by each other. For example, if a motor is fast, it cannot lift a heavy load.motor is fast, it cannot lift a heavy load.

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

GearsGears Toothed wheels that interlock to transmit Toothed wheels that interlock to transmit

power (torque) without slippage.power (torque) without slippage. Have efficiencies of up to 98% in their Have efficiencies of up to 98% in their

transmission of energy. transmission of energy. A gear can interlock or “mesh” with any A gear can interlock or “mesh” with any

device having the same type of teeth such device having the same type of teeth such as a rack. (i.e. rack and pinion).as a rack. (i.e. rack and pinion).

The gear transmitting the force is the The gear transmitting the force is the input or driveinput or drive gear while the gear gear while the gear connected to the drive gear is the connected to the drive gear is the output output or driven gearor driven gear. .

Gears (continued)Gears (continued)

Gears control power transmission in 3 ways:Gears control power transmission in 3 ways:1.1. Changes the direction in which power is Changes the direction in which power is

transmitted.transmitted.2.2. Changes the amount of force or torque.Changes the amount of force or torque.3.3. Changes the speed of rotation (RPMs).Changes the speed of rotation (RPMs).

Gears of unequal size can be combined to Gears of unequal size can be combined to produce a mechanical advantage resulting produce a mechanical advantage resulting in a change of speed and torque. in a change of speed and torque.

Gears are typically made of plastic or Gears are typically made of plastic or metals for wear, strength and durability. metals for wear, strength and durability.

Spur GearsSpur Gears

Spur GearsSpur Gears

Transfers speed and torque between Transfers speed and torque between parallel shafts.parallel shafts.

Advantages : simple, low cost, easy Advantages : simple, low cost, easy maintenance.maintenance.

Disadvantages : noisy due to Disadvantages : noisy due to meshing teeth. meshing teeth.

Bevel GearsBevel Gears

Bevel GearsBevel Gears

Used to change the output shaft Used to change the output shaft direction. direction.

Shafts are not parallel. Instead, they Shafts are not parallel. Instead, they intersect. intersect.

Shafts can be at any angle. 90 Shafts can be at any angle. 90 degrees is the most common. degrees is the most common.

Worm GearsWorm Gears

Worm GearsWorm Gears

Transmit power 2 shafts that are at Transmit power 2 shafts that are at right angles to each other. right angles to each other.

Used where a large speed reduction Used where a large speed reduction or mechanical advantage is or mechanical advantage is required. (i.e. braking or a locking required. (i.e. braking or a locking action or heavy lifting).action or heavy lifting).

A ratio of 300:1 is not uncommon. A ratio of 300:1 is not uncommon.

Gear RatioGear Ratio

The most important feature of gears is The most important feature of gears is that gears of unequal size produce a that gears of unequal size produce a mechanical advantage.mechanical advantage.

This can change the speed or torque of This can change the speed or torque of the second gear. the second gear.

When a smaller gear meshes with a When a smaller gear meshes with a larger gear, the torque applied to the larger gear, the torque applied to the smaller gear is increased. smaller gear is increased.

The increase is based on the difference The increase is based on the difference between the radius of each gear. 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 RatiosGear Ratios

Gear RatiosGear Ratios

Calculated by # driven gear teeth/ Calculated by # driven gear teeth/ #driving gear teeth. #driving gear teeth.

Driving Driving GearGear

Driven Driven GearGear

Speed Speed TorqueTorque

SmallSmall LargeLarge DecreaseDecrease IncreaseIncrease

LargeLarge SmallSmall IncreaseIncrease DecreaseDecrease

Idler GearsIdler Gears Used to reverse the direction of rotation. Used to reverse the direction of rotation.

Gears go in opposite directions. With an Gears go in opposite directions. With an idler gear, they go in the same direction. idler gear, they go in the same direction. Has no effect on gear ratio.Has no effect on gear ratio.

Chain DrivesChain Drives

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

Compound Gear RatiosCompound Gear Ratios

More than one gear on the same More than one gear on the same axle. 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 Tasks 1.1. Construct a gear box with 3 different gear Construct a gear box with 3 different gear

combinations. Follow the procedure at the link combinations. Follow the procedure at the link provided below. provided below.

http://www.education.rec.ri.cmu.edu/roboticscurriculum/vex_online/lessons/gearbox/mechanical_advantage.html1.1. Learning journal – construct diagrams of your Learning journal – construct diagrams of your

gear configurations and identify what was gear configurations and identify what was successful and what was not. successful and what was not.

2.2. Construct a worksheet from the exercise above. Construct a worksheet from the exercise above. Identify on the worksheet the RPM, mass lifted Identify on the worksheet the RPM, mass lifted and the gear ratio for each axle. Draw at least 2 and the gear ratio for each axle. Draw at least 2 conclusions. conclusions.

3.3. Assignment – gear ratiosAssignment – gear ratios4.4. Construct a robot to lift a 5 kg mass. Construct a robot to lift a 5 kg mass.