Automatic Transmission Bands & Clutches

To get the gear multiplication necessary for acceleration and the gear reduction that allows high vehicle speeds at low engine rpms, manual transmissions use paired sets of meshed gears that rotate on parallel shafts. In contrast, automatic transmissions use planetary gearsets for all forward gears and reverse. A planetary gearset consists of 3 members: a central sun gear, an outer ring gear, and the planetary gears in between. All gears are in constant mesh and most automatic transmissions have at least 2 planetary gearsets. To achieve the desired gear ratios, one planetary member is driven by the engine while another is held stationary, with the remaining member becoming the output. Clutches and bands, applied by hydraulic pressure, are the components used to control the planetary members.

Clutch packs consist of multiple, alternating steel and friction material-lined plates housed in a drum or the transmission case. The friction plates are internally splined to fit into grooves on a hub, while the steel plates have external splines that engage grooves in the drum or case. A piston compresses the plates, locking the hub and drum together when hydraulic fluid pressure is applied. Return springs disengage the piston when pressure is removed. Bands are friction material-lined flexible metal rings that surround and are used to stop rotating clutch drums. One end of the band is attached to the transmission case, while the other is engaged by a hydraulic piston in a servo. The servo contains a spring to disengage the band when hydraulic pressure is removed. Hydraulic pressure in the transmission is developed by a pump driven by the torque converter and then routed to the valve body. Valves or electric solenoids in the valve body direct hydraulic pressure to the appropriate bands or clutch packs to shift gears.

Automatic transmission clutches & bands normally last many thousands of miles, but the friction material on clutch friction plates and bands wears with use and because of this clutches & bands eventually lose their pressure applying and holding capabilities. How a vehicle is used affects clutch and band durability, with a shorter lifespan on a car that is used in stop-and-go city driving with frequent shifting, and a longer lifespan on one that racks up most of its miles on the highway. Slipping between gears, where the engine revs higher without a corresponding increase in speed, is a common symptom of worn clutches & bands, as is fluid that looks and smells burnt. When checking fluid also look for the presence of band and clutch friction material particles. On modern, computer controlled cars, before condemning clutches and bands the electronic control system should be checked for trouble codes and the status of input sensors to ensure solenoid valves are being signaled to shift at the proper times.

Vehicle manufacturers typically engineer a certain amount of designed-in slippage to automatic transmissions to achieve smooth, soft shifts. They accomplish this with shift timing in the valve body that extends time between gears and with the design of the friction material used in the bands and clutch packs. Unfortunately, for performance driving this can result in lazy shifts, poor acceleration, and faster clutch and band wear, which is only made worse when more power is put through the transmission. Retuning the transmission for faster, firmer shifts with a transmission controller, valve body, or valve body kit is a good start, but for best results and the most reliability it should be accompanied by performance bands and clutches.

Along with replacement components, we also offer clutch friction plates lined with high temperature resistant material that has a high coefficient of friction for maximum grip and durability, as well as heavy-duty steel plates that are nitrided to increase hardness and resist wear. We also have hubs and drums made of stronger material like billet steel so you can forget about spline stripping, including components that can hold more plates than OE for increased clamping force. Our performance bands have the same quality friction lining and are wider than OE for increased clamping power. In addition to the above, in this section you’ll also find heavy-duty billet servos that can apply more force to the bands for greater holding capacity.