AEG Gearbox

How it works and what are the most important parts inside?

An airsoft gearbox is a mechanical shell that houses the inner workings of an electric airsoft gun (AEG) powered by an electric motor and a battery as the power source. It is placed in the lower receiver of the gun and attached through the motor grip and buffer tube bolt at the back of the receiver and a series of pins depending on the gun style. There are nine types of gearboxes found in various models of electric guns each one with slightly different inner working components. These gearboxes typically have three fire modes with variants having up to five, the most common being safety, semi-automatic, and fully automatic firing with variants including 3-round and 6-round bursts. Their main function is to cycle a piston inside the gearbox with the pull of the trigger to push a nozzle that shoots a stream of air through the hop-up and propels airsoft pellets through the barrel.

The most common gearbox used in most electric airsoft guns is the version 2 gearbox. The very first iterations of these gearboxes were created by the Japanese toy company Tokyo Mauri in 1992, later modified by various manufactures around the globe while keeping the Marui specifications the same. These gearboxes are typically found in m4/m16 style airsoft guns and have had various variants through its history. The AK47 replicas are using the V3 gearbox. Typically, any airsoft gearbox has around 20 components and this is what they do:

Internal Parts

Spur Gear

Gears are a set of internal components that are inside your AEG gearbox. Gears transfer energy from the motor to the piston that in turn pulls the spring back and releases it, thus giving your AEG power. The spur gear acts as the intermediary gear between the bevel and sector gear.

Bevel Gear

Bevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped. Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well. The pitch surface of bevel gears is a cone, known as a pitch cone. Bevel gears transfer the energy from linear to vertical power, making it very useful in machines widely used in mechanical settings.

Sector Gear

A Sector gear is a gear that has teeth cut along a section of its circumference only. The remaining portion of the gear is smooth without any teeth. This arrangement may save space, material, and manufacturing costs. Sector gears are used in applications when less than 360° of rotation is required.

Pinion Gear

A pinion is the smaller of two meshed gears in an assembly. Pinions can be either spur or helical type gears, and be either the driving or driven gear, depending on the application. In the V2 gearbox the pinion gear is attached to the shaft of the electrical motor.

Gearbox Case

The Gearbox is the actual case that holds all the parts together. It is the protective casing which surrounds all the components inside, providing physical protection and mechanical support for them. All contemporary airsoft AEGs use a mechanical system inside their gearbox that contains a combination of three gears and a motor to allow a smooth transfer of electrical energy into mechanical energy.

Tappet Plate

The tappet plate is connected to the air nozzle to (via contact with a spring and a nub on the sector gear) enable the air nozzle to cycle back and forth. This motion enables the air nozzle to feed the next BB into the hop-up unit.

Selector Plate

The selector plate is a shifting part that adjusts the engagement between the cut off lever and sector gear relative to the fire selector switch on the automatic electric gun's receiver.

Hop-Up Bucking

The Hop up Bucking is the part in the hop up chamber that eventually makes contact with the bb as soon as it is shot into the inner barrel. Because the bb makes contact with the bucking the bb gets spin and that determines the flight of the bb.

Hop-Up Unit

The hop-up unit on an airsoft gun is a mechanical device that applies backspin to the BB as it exits the barrel. The amount of backspin can be adjusted, which in turn adjusts the range and accuracy of the gun. The hop-up system is usually located inside the gun's body, near the magazine well.

Piston

The piston is a compression part inside the gearbox and is responsible for forcing air through the nozzle when the gun is fired.
The mechanical energy is transferred to the air system when the piston meets the sector gear. The air system turns this mechanical energy into pneumatic energy that shoots the tiny bb out of the gun. It's the combination of these two energies that makes the Airsoft Gun work.

Piston Head

The piston head is responsible for forcing the air inside the cylinder out of the gearbox to propel the BB.

Cylinder

The cylinder is a crucial internal component in airsoft electric guns, or AEGs. As the name implies, the shape of this part is that of a cylindrical tube. Its main function is to determine the primary volume of air used in each operational cycle (or shot) of the airsoft electric gun.

Cylinder Head

This can be a plastic or metal cap that seats inside of the front end of the cylinder on the rear end, with a smaller metal tube protruding from the front end for the air nozzle to seat over top of.

Air Nozzle

Air Nozzles feed rounds into the hop up and channel air from the cylinder through the hop up unit and down the barrel. Most airsoft nozzles are a plastic or aluminum tube with a grooved rear edge to lock into place in the tappet plate.

Spring

The spring inside the gearbox is responsible for forcing the piston forward. The amount of force this spring exerts on the piston in conjunction with the air pressure created inside the cylinder will result in the power it puts behind the BB to propel it.
A spring upgrade will increase your AEG's range slightly but will increase the wear on the internal components and shorten the AEG's lifespan in most cases.
When you pull the trigger, it pushes on the sear so it lifts off the piston. With the piston unlatched, the spring can expand out, pushing the piston forward. This quickly compresses the air in the chamber behind the BB, building up the pressure needed to propel it down the barrel.

Spring Guide

The name says it all, the Spring guide guides the spring and makes sure it stays neatly in place when the gearbox is in operation.

Electric Motor

In every airsoft gun one of the most important parts that keeps the gun running is the motor, this is what powers your gun and helps maintain your velocity and rate of fire (ROF).

A higher speed motor spins at a higher RPM, which in turn increases the cycling speed of the gearbox and the rate of fire of the gun. A higher torque motor spins with more force, which allows the gearbox to turn against heavier resistance from a higher tension spring which can increase the FPS.

Bearings

The bearings (or bushings) are installed in the gearbox housings of the airsoft gun and they set the position of the gears. The bearings also serve for lowering of friction torque moment on the axis of the gears.

Anti-Reversal Latch

The anti-reversal latch is a small part in your gearbox that prevents the gears from unwinding when under tension. As the gears turn, the latch locks in on small grooves in the bevel/umbrella gear preventing it from back-spinning. Sometimes this part can break, causing your gun to perform & sound ghastly.

Trigger

The trigger in AEG's works by simply touching two metal contacts together when you pull the trigger. This closes the electrical circuit and allows the current from the battery to flow through the wires to the motor.

MOSFET

MOSFET is an electronic system in which the most important element is the Metal-Oxide Semiconductor Field-Effect Transistor - a.k.a MOSFET. It can be installed in any electric airsoft gun, after appropriate adaptation.

This system then changes the way the current flows through the individual parts in the airsoft gun. The trigger contacts are only used to tell the switch to open and close, and almost no current goes through the contacts themselves. Greatly improves reliability, and it can improve trigger response and rate of fire.