The brake band is used to lock a planetary gearset
element to the transmission case so that the element
can act as a reactionary member. The brake band is
made up of the following elements (fig. 4-33):
BAND.The brake band is a circular piece of
spring steel that is rectangular in cross section. Its
inside circumference is lined with a friction material.
The brake band has bosses on each end so that it can be
held and compressed.
DRUM.The drum fits inside of the band and
attaches to the planetary gear-set element and is to be
locked by the band. Its outer surface is machined
smoothly to interact with the friction surface of the
brake band. By pulling the open ends of the band
together, the rotation of the drum stops.
ANCHOR.The anchor firmly attaches one end
of the brake band to the transmission case. A provision
for adjusting the clearance between the band and the
drum is usually provided on the anchor.
SERVO.The servo uses hydraulic pressure to
squeeze the band around the drum. The servo piston is
acted on by hydraulic pressure from the valve body that
is fed through an internal passage through the case. The
servo piston has a seal around it to prevent leakage of
hydraulic pressure and is spring loaded to allow quick
Figure 4-33.Brake band.
release of the band. Some servos use hydraulic
pressure on both sides of their pistons so that they use
hydraulic pressure for both the release and the
application of the band.
The operation of the brake band is as follows (fig.
ReleasedWhen the brake band is released,
there is no hydraulic pressure applied to the servo, and
the drum is free to rotate within the band.
AppliedWhen the brake band is applied,
hydraulic pressure is applied to the servo that, in turn,
tightens the band around the drum. The result is that the
drum is locked in a stationary position, causing an
output change from the planetary gearset.
In the applied circuit of a clutch or band, an
accumulator is used to cushion initial application (fig.
4-35). It temporarily absorbs some of the hydraulic
pressure to cause slower movement of the applied
The hydraulic system of an automatic transmission
serves four basic purposes:
1. Actuates clutches and brake bands by hydraulic
pressure from the hydraulic slave circuits.