Gears and Torque

The Challenge

• To design a vehicle with maximum puling power, in order to pull an opponent’s robot over the middle line.

• To understand the concepts of friction, torque and gear ratio while playing games.

Battle Bots: The Tug of War Game

• After design modifications, tug of war begins

• Winner: The robot that wins the game and costs the least

• Each team is given an identical robot to modify

• Modifications: amount of power provided to motor, robot weight, gear ratio, wheel size

• Each team receives the same amount of points to spend on modifications; every modification costs some points

Lego Mechanisms

gears — motors — physics concepts

gear ratio

torque

normal
force

speed

weight

force of
friction

applied
force

The driver gear is the gear that is receiving the force from the motor. The follower gear is where the wheel or other thing to be turned is attached.

​

Driver vs. Follower

Concept of Gear Ratio

• Sports cars go FAST (have speed), but cannot pull any weight

• Big trucks and tractors can PULL heavy loads (have power), but cannot go fast

2004 Dodge Viper SRT-10
Siromer 204 Tractor

Concept of Gear Ratio

Gears are used for two basic purposes:

• To increase or decrease rotation speed

• To increase or decrease torque

Concept of Gear Ratio

• The smaller gear has 13 teeth; the larger gear has 21 teeth

• Therefore, the gear ratio is 21/13 or 1.62/1 or 1.62.1

• In other words, it takes 1.62 revolutions of the smaller wheel to make the larger wheel turn one revolution

Concept of Gear Ratio

Gearing down

• Small gear drives LARGE gear
• The large gear turns slower

torque

Gearing up
• LARGE gear drives small gear
• The small gear turns faster

torque

​speed

​speed

Concept of Torque

• Torque is an applied force on a lever arm

Where is the torque on your robot?
• The motor on your robot uses a set amount of torque to turn the wheels. The driver gear uses torque to turn the follower gear.

point of rotation

Torque Discussed

• Constant torque from the motor (driver gear), for example 25 N-m

• Force exerted at the point of contact between gears is 25 N

• Torque about center of follower gear is 125 N-m

driver

follower

1 m

5 m

​25N x m = 1m x f
F = 25N
T = 5m x 25N
T = 125N x m
T_out = G.R. x T_driver

Concept of Force (and Motion)

Concept of Weight and Normal Force

• The weight of an object is not the same as its mass

• The weight is defined as the force exerted by gravity

• For an object to stay on the ground, the force of gravity must be offset by an equal force pushing back—we call this force the normal force— the Earth is pushing back against gravity

​F_N

​F_g

​F_N

​F_g

Concept of Force of Friction

• Friction is the resistance encountered by two touching materials

• No contact = no friction
• Something with a lot of weight makes a lot of friction with the surface of the ground

​F_f = F_N x μ

​F_g

​F_N

​F_f

​F_p

​F_p

​F_N

​F_g

​F_f

Concepts of Work and Power

Concepts of Work and Power

The Game Rules

• At the start:

• Each team receives 50 points

• All robot vehicles come with: large wheels, 1:1 gear ratio, no added weight, 75 power

• Point allocation for robot modifications:

Points

Modification

10

Gear ratio change

5

Smaller wheels

2

More motor power

1

One sheet of weight