Gearbox Parts

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Version 2 Gearbox diagram
AEGs and AEPs both use varying combinations of a battery, motor, gears, spring, and piston to create pressurized air and propel BBs down the barrel. BBs are fed into the hopup chamber using a tappet plate and air nozzle.


Traditionally airsoft guns have used NiCd and NiMh batteries for power. In recent years LiPo batteries have become popular because of their smaller size, lower cost, and better discharge patterns. It is important to note that LiPo batteries can not be charged with a NiCd or NiMh charger, but need to be charged with a smart LiPo charger that will balance the individual cells appropriately.
a popular LiPo/NiCd/NiMh smart charger
Turnigy Nanotech LiPo battery packs


Long-type motor, typically seen in M4 platform AEGs
Gearboxes are cycled using electric motors. There are many ways to classify a motor. Each motor will have a defining set of characteristics telling you what applications it is appropriate for. First and foremost there are different motor sizes: long, medium, and short.

Long motors are most common, and are found in almost all M4/M16/HK416 variants. Medium motors are rather rare, and are chiefly seen in KAC PDW clones (Bravo RDW, VFC PDW, DBoys PDW, etc) and are accompanied by a shorter than usual pistol grip. Short motors are typically used in gearboxes with attached motor cages such as AK47s and some MP5s. The only real difference between all these motor types is the length of the central shaft (and where it places the pinion gear). It is possible to modify a longer motor to fit a shorter motor's application.

The second defining characteristic of a motor is the type of magnets it uses. Neodymium magnets are common in aftermarket motors, but weaker (and cheaper) ferrite magnets are most common in OEM motors.

The third and final characteristic of a motor is the TPA count, or turns per armature. The weaker the magnets, the more turns are required to achieve a given amount of torque. Thusly, ferrite motors require higher TPA to achieve the results that a more efficient neo motor can get with fewer TPA.


Sector, bevel, and spur gears
With a few exceptions, AEG gearboxes function using 3 gears (not counting the piston teeth and the pinion gear on the motor). The motor's pinion turns the bevel gear, which then turns the spur gear, which turns the sector gear, which finally pulls back the piston. The portion of the sector gear that meshes with the piston only has teeth around approximately half of its circumference. When the piston is fully retracted the sector gear runs out of teeth. At this point the piston is free to spring back forward (using the main spring's pressure) and fire a BB.

The sector gear has stud on it's side which grabs the back of the tappet plate and retracts the tappet plate (and air nozzle) allowing a BB to be fed into the hopup chamber. The air nozzle and piston return to their resting positions at approximately the same time, meaning that the air nozzle has forced the BB into the hopup bucking by the time that the pressurized air from the piston is being forced down the barrel.

Main Spring[edit]

A pair of AEG main springs

The main spring is the source of power within a gearbox. It is directly related to the speed of the bb as it leaves the gun.

Main springs have a few different ratings. Most are "M" rated. The "M" stands for "Meters per Second" or roughly what the output of the gun should be with that given spring on .20g bbs. So an "M120" should output approximately 120 meters per second with .20g bbs (m/s), an M100 should output approximately 100 m/s with .20g bbs. The "SP" rating is also common among springs. These are more powerful than "M" rated springs, and are supposedly chronoed with .25g bbs rather than .20g bbs. So an "SP120" would result in 120 m/s with .25s, or about 135 m/s with .20g bbs. There are also "%" rated springs. These springs are a little more complicated and use the base power level of TM guns to base their percentage off of. TMs come shooting at approximately 0.9J. So a "200%" spring would be two times 0.9J or 1.8J of output, or about an M130 or SP120.

Springs are not super exact, some may be very hot, others might shoot low. This is largely due to the system they are in. Air leaks can cause a spring to underperform, while good air seal can cause a spring to over perform. Barrel length can also affect power output; shorter barrels have more difficulty reaching higher power levels than longer barrels. Good brands of springs can be reliable, Prometheus, PDI, Systema, Guarder, Madbull are all generally considered to be good brands of spring. SHS is popular because they are cheap, but they are often overpowered for their rating, with M120s often performing like M130s or even M140s. Cheaper springs often can vary between batches, with the fps output being different with the same spring label but in a different batch of springs from the manufacturer.


Tappet Plate[edit]

typical tappet plate
The tappet plate is critical to the feeding of a system. The plate is located between the air nozzle and the sector gear. It is pulled back, which pulls back the air nozzle, allowing a BB to load into the hop up chamber. The tappet is then released and pulled or pushed back forward to create a seal between the air nozzle and bucking.

Tappet plates differ between gearbox versions, so be sure to get the proper tappet plate for the gearbox version you have. Something else to consider is that there are two different specs for tappet plates: long and short tappet plates. Short tappet plates generally feed better than longer tappet plates. However, shorter tappet plates are more prone to failure due to other specs being off or incorrect within a system. SHS, Guarder, King Arms, and the like are short tappet plates, while Lonex and Core are examples of long tappet plates.

Air Nozzle[edit]


Cylinder Head[edit]

Cutoff Lever[edit]

Anti-reversal Latch[edit]

Antireversal latch and spring
The Anti-Reversal Latch, or ARL, is a lever within the gearbox that prevents the gears from spinning backwards. This prevents damage to the tappet plate, piston, and to the gears themselves. Double feeding can also be caused by an ARL that is not functioning properly.

The ARL rides along the toothed portion of the bevel gear, locking into place when the gear stops spinning.The ARL is held in place laterally by holes in each side of the gearbox shell. Pressure is maintained on the bevel gear by a small spring wrapped around the latch. This spring pushes on the gearbox shell to the rear of the latch and forces the latch cam into the bevel gear.

Not all gearboxes need ARLs, a few gear boxes do not even have ARLs, but it's never a bad thing to have one. They are important because most gearboxes have a small amount of overspin with them. In higher power set ups, the spring will cause the system to reset without an ARL. Systems that do not use ARLs are often ones with specialized mosfets that are able to stop the gears at a relaxed position so that the ARL is not necessary.