What Does Tightening the Bands in a Transmission Do?

by Chris Weis
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The inner workings of automatic transmissions are complex and sophisticated in total, but individual components are easily understood. Bands and clutches must engage and release in a precise and particular order to produce a certain outcome. The various functions that accomplish smooth and timely gear changes have to work in harmony. Band adjustments are done with this end in mind. Transmission technicians use guides that illustrate which clutches and bands engage to produce a certain outcome in a specific transmission. These guides can help determine whether a band adjustment is an appropriate method of solving a particular problem. This once common maintenance practice has all but been eliminated by advances in transmission fluids and materials.

The Band

As the name implies, a transmission band is a steel strap. The strap wraps around the outside of a round drum. Typically, the drum rotates any time the band is not applied. The inside surface of the band is lined with a friction material similar to that of brake shoes. When the band is applied, it grips the drum firmly and halts the rotation of the drum. This action causes the transmission components to interact in different ways for separate tasks. When the friction material on the inside surface of the band becomes worn, the gripping ability is affected. Gear engagements can become indistinct or slow to occur, resulting in a slipping sensation.

The Gap

Although the band surrounds the drum, it stops short of being a complete circle. The gap between the ends of the band is at its widest when the band is disengaged. The drum spins freely in this instance, and transmission fluid fills the clearance between the drum and the band friction surface. The lubrication is meant to prevent wear of the friction material. The spring-like nature of the steel band helps keep the end gap at its widest until the band is applied.

Closing The Gap

Like most physical movements in an automatic transmission, the band is applied by hydraulic pressure. Valves in the transmission govern the timing of fluid pressure on a piston, or servo. When the band is applied, the servo pushes the end gap of the band closed, or nearly so. The circular strap becomes, in effect, a smaller circle that grips the drum. The other end of the band is held stationary by a fixed rod, and this device, or the servo push rod, is the adjustment point. Typically, the threaded rod passes through the transmission case to allow external adjustment of the band end gap.

A Light Touch

A band that is adjusted too tightly will displace the fluid that prevents friction material loss. Constant contact with the spinning drum also occurs, and both the drum and band can become glazed to the point where a partial overhaul is necessary. For those reasons, bands are adjusted in small, precise increments. A nut is affixed to the threaded rod or adjusting screw to secure it in place. The nut is loosened, and the screw is tightened a specific number of turns, or to a designated torque value, depending on the manufacturer's design. Once the desired adjustment is made, the screw is secured in place by tightening the lock nut. Proper adjustments provide component longevity and good shift qualities.

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