25.19.
Transmission Shift and Drive Line Features
An automatic transmission gear ratio change is called a shift. Shifting requires the release of one planetary member and the application of a holding device of another, both release and application need to be properly timed. The reaction member of the planetary gear set always tends to turn backward while the gear set is carrying a torque load and the reaction force is proportional to the torque being carried. As the torque load transfers from one planetary member to another, the load on the reaction member changes from reverse to forward direction. Ideally, the holding device should be applied or released at the instant torque reversal occurs. When a holding or driving device becomes worn, it first becomes apparent to the driver when it slips and fails to hold the required torque while it is being applied.
During upshifts, the applied member must have a higher torque capacity than the released member. This is required because engine inertia momentarily increases torque as the engine is slowed to the new speed and this is added to the torque being produced by the engine. During downshift, the engine speed must increase as the shift moves to a lower gear. The application of force must be gradual before the holding force is released to prevent engine run away.
Shifting quality or smoothness is primarily dependent upon the characteristic output torque, which varies during the shift. If one member is released before the second member is applied the transmission momentarily remains in neutral and the engine tends to run away. On the other hand, if application occurs before release, the transmission is momentarily locked in two gears, producing a bump. Good shift quality transfers the load from one member to the next by allowing a slight amount of slippage to occur during application as the new member picks up the torque, which may last for 0.6 second. Longer application, though produces smoother shifts, but reduces the service life of the unit.
Size and clearances in the automatic transmission, which are required for correct operation, are very carefully controlled during manufacturing. Because of the build-up method, one part depends upon the accuracy of several other parts. The correct final axial movement or end clearance is controlled by a selective fit thrust spacer somewhere in the assembly. Automatic transmissions usually use pressure lubricated bushing-type bearings on their main rotating parts. Most of these bearings are babbitt or copper-lead bearing materials on a steel backing. All bushings are installed in bores located in either case, or hub. The front of the gear train is supported on a hub extending in back from the pump cover. Oil transfer rings are located on this hub to minimize leakage as control oil transfers from the stationary hub to a rotating clutch drum. The rear end of the gear train is supported by the rear of the transmission case. Shafts and drum hubs support one another on these two main support locations. The transmission input shaft is splined between the converter turbine and a front clutch hub. On the front, the turbine is supported in a bushing within the torque converter cover or inside the front of the stator shaft. The front clutch hub is mounted with a bushing on the rearward extension of the transmission pump cover. The front of the transmission output shaft rides on a bushing at the rear of the transmission case. The rear of the output shaft is supported by a bushing in the back of the transmission extension. The output shaft extends almost to the input shaft or an intermediate shaft is used between them. Typical bearing locations are shown in Fig. 25.43.
Planetary gear-set members, clutch hubs, and brake drums are splined to these shafts for driving and riding on bushing when they are required to be free turning. Non-rotating clutch and brake parts including oil transfer hub and seal rings in some transmissions are supported by the transmission case to minimise the load on the shafts. Transmission rotating members
Fig. 25.43. Sectional view of automatic transmission showing location of bushings and bearings.
are spaced with thrust bearings, and needle roller bearings are used for high load conditions. These bearings are normally made of babbitt on a steel backing. Sub-assemblies are held together with snap rings and retaining rings. Sub-assemblies which rotate together are conÂnected with drive lugs at their outer edges.
The basic transmission disassembly procedure is to lift the torque converter from the input and stator shafts. Removal of the valve body assembly follows. The front pump assembly is removed to get to the gear train. Major gear train subassemblies can then be removed from the front of the transmission case. The snap rings and retaining rings are to be removed completely for disassembling subassemblies.
