How do automatic transmission bands work?

If you have an automatic transmission vehicle, you've probably wondered more than once how it makes gear changes, or why the engine doesn't shut down every time you stop. The automatic transmission is very different from the manual one, and much of those differences lie in their parts and the function they have in the system.
One of the most important pieces is the transmission band.

How the automatic transmission band works

The transmission brake band is a steel belt with friction material adhered to the inner surface. It is part of the planetary gearset control along with the friction discs, the unidirectional clutch, and its function is to block the gear elements for a short time, such as the crown, the sun or the drum. Their operation is controlled by a hydraulic unit that allows them to rotate, or blocks their movement. One end of the band is anchored against the transmission case, and the other end is connected with an operating piston of the servo unit, where the hydraulic oil is sent under pressure to tighten the band around the drum and prevent it from turning.

Bands and Planetary Gear set

The band’s state directly impacts the operation of the drum and planetary gear set. This way, an automatic transmission can change gears according to the revolutions per minute (RPM) rate.

The planetary mechanism´s principle of retention and locking is very similar to pressing the brake pedal. Automatic transmission are driven by oil pressure; when the automatic transmission fluid (ATF) reaches the servo mechanism, the piston begins to move under the pressure of the fluid, so that the band squeezes and blocks the elements of the planetary mechanism for a short period; once that time has elapsed, the gear assembly is unlocked and the servo mechanism is turned off. As the fluid pressure equalizes, the band weakens, the spring that moves backwards causes the piston to return to its starting position and the band to return to its free state.

Symptoms and usual faults

The most frequent problems with the planetary gear locking system, or with the transmission brake band are:

• Damage to the friction layer on the band. This damage can be from wear and tear related to use, causing the planetary mechanism to suffer an incomplete blockage. As a result of this situation, during the gear shift process, the driver may experience shaking, or hear noises such as scratches coming from the automatic transmission box.
• Servo Piston breakdown. If one of the servo piston components is worn or broken, the band will not be able to block the set of planetary gears, so at each speed change you will feel jerks or noises from the gearbox.
• Damaged piston stem. Damage to the servo piston stem can cause metal fragments to disburse throughout the transmission. The ATF will distribute these metal fragments to all parts of the transmission causing major damage if not resolved promptly
• Spring wear. If the spring is damaged, as in the previous point, there may be polluting residues from the servo mechanism, in which the band locks the transmission´s moving parts, so that when trying to start the car, it won't not move.

Maintenance services

Band adjustment

It ensures the transmission band's normal operation, for which the clearance between the brake drum and the band is regularly adjusted by means of an adjustment screw, or a fork with a locknut and a torque wrench. If the adjustment screw is located inside the gearbox, it will be necessary to remove the transmission tray to adjust the band.

The revision of the band should be carried out together with the AFT and filters change; it is important to check that the band is not too stretched, as this can accelerate the wear and cause it to break.

Band breakage

If the transmission band has broken, or if the wear is extreme and about to break, it is important to replace it before it causes gear failures that compromise the driver and their companion´s safety.

At Sun Transmission you will find the parts you need to keep your vehicle's transmission in optimal condition. Browse our online store to find all the transmission parts online we have for you, like transmission pan gasket, automatic transmission fluid, transmission coolers, modulator valve transmission, or transmission overhaul kits. 

We invite you to browse our blog, and find all kinds of useful information related to the proper functioning of your vehicle, automatic transmission, electronic power steering and much more. And if you have questions or need more information just contact us today, we will gladly assist you.