LEGO Mindstorms Class:  Lesson 8

Gears:

The LEGO Mindstorms Set comes with all sorts of gears, but up to this point we haven't explored how to use them.    Today we're going to see how gears work, and how they can be useful in building robots and other devices.  Below is a picture of some of the basic gears that come with the Mindstorms set, as well as a definition of some of the different gear types that are included.  The different shaped gears have different purposes, but we'll focus on the spur gears.

Some Different Gear Types



Spur Gears -  Have straight teeth, and their axles are parallel (lined up) with each other.  They are the most common type of gear.

Bevel Gears  - Can change the direction the axle is rotating by an angle (usually 90 degrees)

Rack and Pinion Gears - Combine the motion of a round gear with a flat-toothed platform.  This lets you convert circular motion to straight-line motion.

Worm Gear - This gear consists of a cylinder with a spiral groove (like a screw) that turns.  Each time it turns around once, the round gear moves forward by one tooth.  The cylinder can turn the round gear, but turning the round gear does not move the cylinder.

LEGO Mindstorm Gears (spur gears are underlined in red)


Question 1:  In the pictures below, what types of gears are being used in the hand mixer and the electric mixer?  Write the answers in the spaces below the pictures:
                         

Gear Type:______________________________                             Gear Type________________________________

To make the Mindstorms robot move, motors provide power to turn the wheels.  We use gears for three main reasons:

  1. Changing the speed of the wheels
  2. Changing the power going to the wheels
  3. Change the direction the axles are turning

Let's see how that works.  Gears have "Teeth" that mesh together, so when one gear turns, it makes the other gear turn, too.  Notice that the gears turn in opposite directions.  The driver gear is the one that provides the power, and the follower gear gets turned by the driver.  The gears in the first picture below are the same size, so each time the driver completes one turn, the follower completes one turn.

Two Gears Turn in Opposite Directions

With Three Gears,  the Driver and Follower Turn in the Same Direction

Gearing down and Gearing Up:

When you use two gears of different sizes, you will find that they turn at different rates.  The picture on the left below below show a gear with 8 teeth turning a gear with 24 teeth.  Because the small gear (the driver) goes around three times (8 x 3 = 24) for every one time the follower gear goes around, this means that a wheel attached to the follower axle will go three times more slowly than the driver gear.  Similarly in the picture on the right, for every time the driver gear turns once, the follower gear will go around three times.  Therefore the follower axle is turning three times as fast as the driver axle.

Gearing Down
(8 Tooth Gear Drives a 24 Tooth Gear)

Gearing Up
(24 Tooth Gear Drives an 8 Tooth Gear)

The follower axle turns more slowly than the driver but has more power.

The follower axle turns more quickly than the driver but has less power.

Making a LEGO Gear Chain:

You can take advantage of gear ratios to put together a set of gears where the follower goes much more quickly than the driver (or vice versa).   An example of a gear chain is shown below.  The because the big gears have 40 teeth, and the small gears  have 8 teeth, the small gears turn around 5 times for every turn of the big gear (5 x 8 = 40).  That means, if we are gearing up, each set of gears increases the speed of the axle by a factor of 5.  Since we have geared up twice, we have increased the speed by a factor of 5 x 5 = 25 times!

                      
        Picture 1: A gear train.                                                                Picture 2:  The same gear train, built a different way.

As we said above, turning the rightmost crank in each picture, makes the leftmost crank turn 25 times faster.  There is a trade-off for this increased speed, however.  The faster crank will have a factor of 25 times less power.

Exercise 1:  Build a gear chain similar to the pictures above, and see if you can feel the difference in power that you get when turning each handle.  You can feel this with your hands by trying to hold one handle still while trying to turn the other.  You will notice that one of the handles will be harder to turn than the other one.

Exercise 2: Can you build a gear chain so that the follower gear goes more than 100 times faster than the driver gear?  Try it!


Question 2:   In the picture below, if gear A is turning clockwise, as shown, what direction are each of the other gears going (clockwise or counter-clockwise)?  Which gear is turning the slowest?

Gear A:  Clockwise________________________

Gear B:  ________________________________

Gear C:  ________________________________

Gear D:  ________________________________

Slowest Gear?  ___________________________


Exercise3:  Build a "Gear-Bot"  This is an engineering challenge.  Can you build a robot that uses gear ratios to increase its speed?  Your instructor has a geared robot that you can use as an example.  How fast can you get your robot to go?