As we all know, there is hardly any machine that does not have gears for its operation, we also know that not all of them mesh with each other and that there are straight or helical ones among many other types, but do you know exactly why?

In this lesson I leave you a guide, as complete as possible, so that you have a reference where to start when it comes to understanding the gears.

## What is a gear?

A gear is called both a toothed piece that transmits movement to a similar one, and a set of pieces that transmit mechanical movement between them.

Its function is to transmit the circular motion from, normally a motor, to another mechanical element that is located at a distance or position that cannot be carried out directly with an axis.

### Types of gears

Gears provide many transmission solutions thanks to the fact that there are many different types.

**Spur gear:**They are the simplest when it comes to using and manufacturing. They have many advantages and a few cons that I detail here.**Helical gear:**They are an evolution of spur gear. They have several advantages over these but in exchange for greater manufacturing complexity. I explain how they are done here.**Bevel gears:**Perfect for transmitting movement where we have to change the transmission angle, normally 90º.**Worm screw:**It is designed to transmit great efforts and increase the power in the transmission. All detailed is here.**Roller chains:**Everyone knows how a bicycle works. To know its calculations and how it is manufactured, you will find it here.**Zippers:**We could say that it is a spur gear stretched in a straight line. Very useful for moving elements on wheels coupled to a pinion. All the information here.

### Parts of a gear

A gear is divided into several parts, and it is very important to know them since not all gears are compatible with each other due, above all, to the type of tooth.

**Inner diameter (ID):**It is the diameter that we can measure from the base of the teeth.**Outside diameter (OD):**It is the diameter that we can measure on the outside of the gear.**Primitive diameter (PD):**It is the diameter that describes the tangencies of the gears placed together. It cannot be measured, but we can calculate it. It is very important since calculations are made on this measure.**Number of teeth (Z):**It is the number of teeth that make up the gear.**Modulus (M):**It is the relationship between the number of teeth and the primitive diameter. 2 gears of different module, do not work with each other.**Circular pitch (CP):**It is the distance between one tooth and the next.

### Teeth of a gear

The teeth of a gear are those that make the effort to transmit movement. There are 2 types, normal teeth and short teeth. The most important parts are:

**The head:**It is the distance from the original diameter to the outer diameter (De / 2-Dp / 2). It is also known as Adendum. In normal tooth gears, the head is equal to 1 x M. In the short tooth it is 0.75 x M.**The foot:**It is the distance from the inner diameter to the primitive diameter (Dp / 2-Di / 2). It is also known as Dedendum. The normal tooth foot equals 1.25 x M. For the short tooth it is 1 x M.**Length of the tooth or flank (Ld):**It is the depth of the tooth.**Tooth height:**It is the sum of the foot and the head, it is also the difference between the outside and inside diameter divided by 2.

So far the introduction on gears. You can find much more detailed information on each gear above.