Intro to Gears

Gears can be used to do several important things:

1) They can reverse the direction of a motor or drive.

2) They can increase or decrease the turning of a motor. The speed of a motors turning is called RPM- rotations per minute.

3) They can increase or decrease the power of a motor's turning. The power of a motor turning is called Torque.

To understand how gears increase or decrease a motors speed or torque, we need to understand GEAR RATIO'S.

WHAT IS A GEAR RATIO:

A gear ratio defines the relationship between multiple gears.

Gear Ratio= Output gear # teeth / Input gear # teeth

For example, if our motor is attached to a gear with 60 teeth and this gear is then attached to a gear with 20 teeth that drives a wheel, our gear ratio is 60:20, or more accurately 3:1

If you do not want to count a gears teeth (or if they do not exist), gear ratio's can also be determined by measuring the distance between the center of each gear to the point of contact.

For example, if our motor is attached to a gear with a 1" diameter and this gear is connected to a gear with a 2" diameter attached to a wheel,

From the center to edge of our input gear is 0.5"
From the center to edge of our output gear is 1"

Our ratio is 1/0.5 or more accurately 2:1

How does a gear ratio affect speed:

The gear ratio tells us how fast one gear is rotating when compared to another.

If our input gear (10 teeth) is rotating at 5 rpms , and it is connected to our output gear (50 teeth), our output gear will rotate at 1 rpms.

Why?
Our gear ratio is 50:10... or 5:1

If our small gear rotates 1x, our large gear only rotates 1/5. It takes 5 rotations of our small gear to = 1 rotation of our large gear. Thus our large gear is rotating at 1/5 the speed = 1rmp.

What if our gear ratio where 1:3 ?
In this case our input gear is 3x larger as large as our output gear.

If our input gear were rotating at 20rmps.... each rotation, would result in 3 rotations of our output gear. Our output would be 60rpms.

How does gear ratio affect Torque:

First....What is torque?:

Torque is a twisting force- (it doesn't do any 'work' itself- it is simple an application of energy).

Work (or 'stuff') happens, when torque is applied and movement occurs.

"Torque is a force that tends to rotate or turn things. You generate a torque any time you apply a force using a wrench. Tightening the lug nuts on your wheels is a good example. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which tends to turn the lug nut.

English units of torque are pound-inches or pound-feet; the SI unit is the Newton-meter. Notice that the torque units contain a distance and a force. To calculate the torque, you just multiply the force by the distance from the center. In the case of lug nuts, if the wrench is a foot long, and you put 200 pounds of force on it, you are generating 200 pound-feet of torque. If you use a two-foot wrench, you only need to put 100 pounds of force on it to generate the same torque." (howstuffworks.com)

In summary:

Torque equals Force multiplied by Distance

How does gear ratio affect Torque?:

Simply put, torque at work (such as at a wheel) is your motor's torque times your gear ratio.

Motor Torque x gear ratio = torque at the wheel

Lets say we have a 10rmps motor that is capable of 5 oz Torque (we know this from our motor spec.)

Lets say we have 2 gears. Our input gear (attached to our motor) has 10 teeth Our output gear has 50 teeth

Our Gear ratio is 5:1

Motor Torque x gear ratio = torque at the wheel

5oz x 5:1 = 25 oz

What if our gear ratio were 1:3 ?

5oz x 1:3 = 1.6oz

Notes on Finding Gears:

It is fairly easy to understand the basics of gear theory.
Finding, purchasing and building gears systems is another matter.

Check outKHK Gear Supply - the side menu has three button links about basic great geometry's, elements of gear technologies and an appendix.
These pages are VERY helpful for ordering gears as they describe how gears are measured (for example what a pitch diameter is, what types of gears fit, etc.)

The best and only supplier of retail gears online I found was http://www.mcmaster.com/ .
They deliver within 24 hours and are very easy to work with-

IMPORTANT THINGS TO KEEP IN MIND

1: For gears to fit together they need to have the SAME PITCH (they can have any number of teeth).

2: The Pitch Diameter is roughly the Gear Diameter. (The Pitch Diameter and the Pitch are two different measurements.)

3: Many gears come with small tightening screws to mount the gear bore (hole) onto any rod of exactly the same size.

Example:
The following 2 gears from McMAster-Carr will fit together

They have the same pitch (24). One has 12 teeth and is roughly .5" in diameter. One has 24 Teeth and is roughly 1" in diameter.

For more information:

Read the section on
Gears in How Stuff Works

Also see
Pulleys and How Block and Tackle Works

How Stuff Works for the simplest description.
(Especially note gear train and planetary gear examples)

Or:
David H. Cowden's, BASIC GEAR THEORY for a bit more detail.