Clutches and Bands in Automatic Transmissions: In-Depth Notes

Learning Objectives

  • 8.1: Identify components of a multiple-disc driving clutch and describe its operation.

  • 8.2: Identify components of a one-way driving clutch and describe its operation.

  • 8.3: Discuss holding clutches and bands.

  • 8.4: Explain shift quality.

Driving Devices

  • Definition: Driving devices are commonly multiple-plate disc clutches.

  • Terminology:

  • Chrysler refers to the first clutch as the front clutch.

  • Ford names it the high-reverse clutch.

  • General Motors and Toyota call it the direct clutch (Toyota often uses C2).

  • Function: Ensure synchronizing shifts in the transmission system.

Multiple-Disc Driving Clutch

Components

  • Drum

  • Hub

  • Lined Plates (discs)

  • Unlined Discs (steels)

  • Pressure Plate

  • Apply Piston

  • Piston Return Springs

  • Clutch Plates and Lining Surface

  • Friction lining materials:

    • Paper

    • Aramid (Kevlar)

Clutch Operation

  • Key Terms:

  • Clutch Pack Apply

  • Pressure Balanced Release

  • Clutch Pack Release

  • Double & Single-Sided Friction Plates

  • Forces Involved

  • Static and Dynamic Friction

  • Garage Shifts

  • Interaction Mechanism: The apply piston uses hydraulic pressure to connect and disconnect the clutch packs.

Friction Plates

  • Features:

  • Groove patterns for fluid expulsion, heat dissipation, and altering friction characteristics during engagement.

  • Differences noted in plate types: smooth plates versus waffle plates concerning heat and engagement speed.

One-Way Driving Clutch

  • Types:

  • Roller Clutch

  • Sprag Clutch

  • Mechanical Diode

  • Functionality: Allows the input to a gear set to overrun during transmission shifts into higher gears.

  • Operation Mechanisms:

  • Roller clutches lock when the inner race rotates fast relative to the outer race.

  • Sprag clutches lock using wedged design principles for speed control.

Holding/Reaction Devices

  • Definition: Serves as a brake to hold planetary gear set members in reaction.

  • Types: Includes multiple-disc clutches, bands, and one-way clutches.

  • Significance: Vital for maintaining stable operations during gear transitions.

Bands

Overview

  • Purpose: Prevent rotation of the associated drum, typically part of the planetary gear train.

  • Types:

  • Rigid bands

  • Flex bands

  • Double-wrap bands

Components

  • Servo components:

  • Cylinder

  • Piston

  • Piston rod/pin

  • Return spring

  • Friction material: Similar to clutch plates, essential for effective engagement.

Shift Quality

  • Definition: Refers to the smoothness of shifts or the overall shift feel.

  • Key Components: Accumulators and wave plates play crucial roles in managing hydraulic pressure and achieving desired shift quality.

  • Mechanism: The accumulator absorbs fluid during pressure build-up to ensure seamless transitions; wave plates cushion shifts.

Summary

  • Power flow: Controlled by driving and reaction devices including multiple-disc clutches, one-way clutches, and bands.

  • Operational Roles: Multiple-disc clutches primarily act as drivers but also serve to hold planetary members; one-way clutches hold planetary members in reaction and can drive multiple-disc clutches. Bands are fundamental in holding members in reaction.

  • Impact of Timing: Timing of applying devices directly influences the shift quality.