Gear Basics

 Notes on my work on Gears

How many wheels does this toy car have ? 

There are many, not just four. 

These wheels inside are special wheels. These wheels have teeth on the outside also known as Gear.

Most of the machines around us use gears to perform the intended work. [Clock, Cassette Player, Drill machine, Sugarcane ]

Lets understand more about gears. 

I will pin this wheel. It rotates freely around the pin. This motion can be transferred to another wheel If they are in close contact with each other. But this contact may not always work especially for high speeds. Friction may wear the surface as well over time. 

To take care of this, let us attach this pointed profile to the wheels. 

Now the motion of one wheel can be easily transferred at all speeds. 

A wheel with teeth on the outside is called gear. 

Instead of regular plastic gears, we will use gears laser cut from MDF sheet and some pins. 

Normally gears are driven by a motor but for now we will rotate it by hand.

Let us take this gear and pin it in the center.

With little force I can rotate it 

Let us mark a black line like this. 

I moved it by 360 degrees or one rotation in an anti-clockwise direction. 

Or one rotation again in a clockwise direction. 

Another gear can be placed like this on the right side. 

Rotating gear on the left will not have any effect on the gear on the right. 

Let us place the teeth of one gear in between the teeth of the another gear

This is also known as meshing the gear. 

Let us mark gear on the left as A while gear on the right as B. 

We will place push pins next to the markings. That way we can count the number of rotations.

One rotation of A also results in one rotation of B but with one change. B rotates in the opposite direction. 

This is true even if A is rotated multiple times, slower or faster. 

This is also true when I rotate B. A rotates the same number of turns but in opposite directions. 

The gear A is called driver gear while the gear B is called driven gear. 

How about introducing another identical gear in between ? Can you predict how B will move now ?

Gear B turns in the same direction as that of A.  Number of rotations however remains unchanged. 

Now let us replace middle gear with another one, this time with more teeth. How will gear B react ?

Direction and number of rotations for the gear on the right do not change. 

This is also true when we use middle gear with less teeth.

Middle gear is also known as idler gear -  a gear placed between a driving and a driven gear to transfer motion without change of direction .

Idler gear can be placed anywhere as long as it is meshed properly.

How about introducing one more in between A and B ? 

Everything is the same except the direction of rotation. Can you find some relation between number of gears between driven gear and driver and direction of rotation ?

Now let us look at gears of different teeth. 

This one with 20 teeth and this one with 40 teeth. 

One full rotation of gear C results in only half rotation of gear D.

Tried another way, one full rotation of gear D results in two rotations of gear C.

This observation leads to another important concept - Gear ratio.

This is true even if I rotate gear C faster or slower, clockwise or counter-clockwise.

One time, four times or even 10 times. I can even attach motor to gear A and run it much faster. Ratio still remains the same.

Relationship of two meshing gears does not change. 

Gear ratio is the number of rotations of driver gear to the number of rotations of a driven gear.

We use the word ratio to describe this constant relationship between two values. 

A colon is often used to show a gear ratio:

gear ratio = rotations of a driver gear : rotations of a driven gear

The order of the two numbers in a ratio is very important. 

What will be the gear ratio in this scenario ? It will be 1:2 instead of 2:1.

 In terms of the bicycle, the gear ratio is the ratio of pedal rotations to wheel rotations.

Can you find gear ratios for these pairs ? 

Is there a relationship between the gear ratio and the ratio between teeth ?

Teeth can be used to find the gear ratio. This relationship between the tooth ratio and the gear ratio is very important. It allows us to find a gear ratio without actually having to build a gear train and turn the gears. We can find the ratio of any two gears if we know the number of teeth on each of the gears. 

We can rewrite the gear ratio as teeth on driven gear to teeth of driver gear. 

gear ratio = = rotations of driver gear

rotations of driven gear

teeth of driven gear

teeth of driver gear

So far we have used only single gear meshed with another single gear to increase or decrease the speed of rotation. 

There is another way which we can use to achieve larger ratios - compound gears.

Gear Trains

To create large gear ratios, 

A small gear and a larger gear are glued together, one on top of the other. Gear trains often consist of multiple gears in the train.

One full rotation of large gear on the left side results in 8 rotations of smaller gear on the right side. Other way, 8 rotations of small gear results in one rotation of gear on the left side. 

In the next video, we will discuss transfer of motion across planes, torque as well as other ways to create gears from everyday material.

Thank You.