54 Basic principle of a hydraulically controlled gearshift

Selecting the drive D range positions the manual valve spool so that line pressure from the pressure regulator valve passes through to the shift valve, throttle valve and governor valve (Fig. 5.5(a)).

Throttle pressure will be introduced to the spring end of the shift valve via the throttle valve. Depressing the accelerator pedal allows the spool valve to move outwards. This increases the valve opening so that a high throttle pressure will be delivered to the shift valve. Conversely, depressing the accelerator pedal partially restricts the flow of fluid and therefore reduces the throttle pressure reaching the shift valve (Fig. 5.5(b)).

At the same time, line pressure enters the governor valve, flows between the wasted region of both primary and secondary spool valves and reacts against the difference in the annular adjacent face areas of each spool valve. Both valves are forced inwards, covering up the two exits from the governor valve housing. As the vehicle moves forwards, the rotation of the governor causes a centrifugal force to act through the mass of each governor valve so that it tends to draw the valve spools outwards in opposition to the hydraulic pressure which is pushing each valve inwards (Fig. 5.5(a)).

With rising output shaft speed, the centrifugal force acting through the primary valve is sufficient to overcome the hydraulic line pressure, which is acting against the shouldered groove face area and will therefore progressively move outwards as the rotational speed increases until the valve borders on an end stop. The opening of the governor valve outlet passage now allows fluid to flow out from the governor, where it is then directed to the large diameter end of the shift valve. This output pressure is known as governor pressure. With even higher rotational output shaft speed (vehicle speed), greater centrifugal force will be imposed on the secondary valve until it is able to overcome the much larger hydraulic inward load imposed on the large shoulder of this valve. The secondary valve will start to move out from the centre of rotation, uncovering the secondary valve outlet passage so that increased governor pressure passes to the shift valve.

This two stage governor valve action enables the governor to be more sensitive at the very low speeds but not oversensitive at the higher speeds (Fig. 5.5(c)). Sensitivity refers to the amount of fluid pressure increase or decrease for a unit change in rotational speed. If there is a large increase or decrease in governor pressure per unit charge in speed, then the governor is sensitive. If there is very little variation in governor pressure with a change in rotational speed (i.e. vehicle speed), then the governor is insensitive and therefore not suitable for signalling speed changes to the hydraulic control systems.

The reason a single stage governor would not perform satisfactorily over the entire output shift speed range is due to the centrifugal force square law: at low speeds the build-up in centrifugal force for a small increase in rotational speed is very small, whereas at higher speeds only a small rise in speed produces a considerable increase in centrifugal force. If the governor has the correct sensitivity at high speed it would be insensitive at low speed or if it has the desired sensitivity at low speed it would be far too responsive to governor pressure changes in the higher speed range.

Once the governor pressure end load (PG x AG) equals the spring and throttle pressure load (FS + PT x At) with rising vehicle speed, any further speed increase will push the shift valve plunger towards the spring end to the position shown in Fig. 5.5(a). The fluid on the applied side of the band brake servo piston will now exhaust (drain) through the shift valve to the inlet side of the oil pump. Simultaneously, line pressure from the manual valve is directed via the shift valve to both the release side of the band servo piston and to the multiplate clutch piston which then energizes the friction plates.

Supply fluid to the spring side of the servo piston (known as the release side), provides a more progressive and controllable transition from one gear change to another which is not possible when relying only on the return spring.

When the vehicle's speed is reduced or the throttle pressure is raised sufficiently, the shift valve plunger will move to the governor pressure end of the valve (Fig. 5.5(a)). The line pressure transmitted to the shift valve is immediately blocked and both the multiplate clutch and the band brake hydraulic feed passages are released of fluid pressure by the middle plunger land uncovering the exhaust part. Simultaneously, as the same middle land covers the right hand exhaust port and uncovers the line pressure passage feeding from the manual valve, fluid will flow to the applied side of the band servo piston, causing the band to contract and so energize the brake.

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