Driving and Holding Devices
An automatic transmission is fitted with a number of brake bands and multi-phase clutches. Clutches are used to connect the gear train to the input shaft and band brakes hold a part of the train stationary. In newer designs of gearboxes, band brakes are replaced by clutches to obtain a compact and lighter gearbox. Additionally, it eliminates the need for periodic adjustment of the brake bands to compensate for friction lining wear.A pump driven at engine speed from the fluid converter provides pressurized oil, which is distributed by control valves to the appropriate clutch and brake for actuation of these parts.
Multi-plate Clutches.
Numerous wet type multi-disc clutches are used with automatic transmission and most of these operate on the same principle. Figure 25.44 represents a typical construction of a multi-plate clutch. Two sets of steel plates, inner and outer, are connected alternately by protruding tabs to the hub and drum respectively. The faces of the inner plates are bonded with a friction material having either a hard or comparatively soft texture. A hard facing is made of a cellulose compound, or synthetic fibre, bonded together with a phenol resin to obtain a suitable friction value. A soft facing, which is based on a compound of paper, is more porous and elastic.Paper-based facings normally provide a smoother and quieter operation over a wider range of temperature and pressure.
When the clutch is to be engaged, pressurized oil is supplied through a drilling, in either the casing or the shaft, to the clutch operating cylinder. A number of synthetic rubber "O" rings and square-section, cast iron seals are used to prevent leakage of oil between moving parts and loss of pressure needed to operate the clutch. Torque transmitted by a given multi-plate clutch depends on friction value and operating pressure, therefore one of these is the cause when clutch slip occurs.
When the clutch is disengaged the drag between the plates acts as an energy drain, so suitable arrangement is incorporated to separate the plates. A large clutch release spring retracts the operating piston and in some cases the steel outer plates are slightly dished. Immediately after the disengagement of the clutch, the centrifugal motion of the residual oil in the operating chamber acts on the piston and causes slight drag. This is prevented by releasing the pressure through a check valve. When oil pressure operates the clutch the check valve is held closed. Removal of this pressure allows the ball to move outwards due to the centrifugal effect so that the release hole in the check valve opens.
Some automatic systems use a clutch operated by two pistons, one small and the other large in area. This construction (Fig. 25.45) allows the thrust on the plates to be varied to suit the conditions so that a smoother operation is obtained.
Fig. 25.44. Multi-disc clutch.
Fig. 25.45. Clutch operation with two pistons.
Brake Bands.
Normally two or more brake bands are used in a gearbox to prevent rotation of the gear members. An external contracting, single-band type brake is installed (Fig. 25.46). The band is anchored at one end to the casing to resist the rotational force and is actuated by a hydraulic servo at the free end. For maximum braking torque the anchor should be at a position such that the rotational force of the drum produces a self-wrapping action. The brake band is made of spring steel and is internally lined by bonding to it hard friction material. When the brake is applied this material makes contact with a steel drum, which is a part of the gear train.An adjuster is normally provided at the band anchor to compensate for wear. The screw for this adjuster is often carried through the gearbox casing to simplify maintenance. For adjustment of the band, the lock nut is released and then the screw is tightened to a given clearance or torque setting. The common types of servo actuator used to suit the gearbox, are :
• single-acting
• delay
• double-acting.
Single-acting Servo.
This type of servo actuator (Fig. 25.46) uses a single hydraulic piston installed in a cylinder, which is rigidly mounted on the gear box casing. A synthetic rubber "O" ring prevents fluid leakage past the piston. A spring is installed in the chamber to return the piston. The force exerted by a servo must be sufficient to prevent slip of the drum, so that bands controlling gears having a high torque output are fitted with a servo cylinder, which has either a large area or a high operating pressure.Delay Servo.
When the vehicle is in motion, a sudden engagement of a brake causes a jolt. As harsh changes and jolts should not occur with well-designed automatics, special device like delay servo (Fig. 25.47) is incorporated to minimize this problem. To achieve this delay feature a second (inner) spring and a piston rod are used that can slide through the piston, instead of being rigidly attached as in the single-acting type.When fluid pressure is applied, the initial piston movement compresses the inner spring so that positive contact between the piston and rod is made and the full force is applied to the brake in the normal way. The cushioning action of the inner spring provides a gradual application of the brake. Also it damps the vibration produced by the band during the initial stage of engagement.
Double-acting Servo.
This type servo can direct the fluid pressure to one or both sides of the piston. Fluid under pressure supplied at A (Fig. 25.48B) acts on the small area side of the piston and the servo is applied in the normal way. When an equal fluid pressure is applied to A and B (Fig. 25.48A), the piston moves in the release direction because unequal thrust is produced on the piston by the fluid pressure acting on the side of larger area.A Borg-Warner 35 gearbox uses a double servo to provide a smooth change between second and third gears. In this case application of the brake gives second gear. When a specified speed is reached, a change to third is made by directing fluid from a single line to the appropriate clutch and also to the release side of the servo. In addition to the reduction of the number of hydraulic lines, this layout provides a quick down-change because immediately after fluid is released from one side of the servo, the second gear comes into operation.
Fig. 25.46. Brake band actuation by single-acting servo.
Fig. 25.47. Delay servo.
The majority of automatic gear changes are made whilst the drive is being transmitted through the gear box. If a brake, or clutch, is released before the next gear unit is partly applied, then the engine suddenly speeds up. This condition is called flare up. Overlapping the engagement of the new gear with the disengagement of the old gear prevents it. The period of overlap should not be too large, otherwise a harsh gear change condition, sometimes called tie-up, results because the two gears oppose each other. In this process, each gear tends to drive the output shaft at a speed dictated by its ratio, with the result the gear box partially locks up.
Fig. 25.48. Double-acting servo. A. Brake being released. B. Brake being applied.