531 Pressure supply and regulating valve

The essential input to the hydraulic control system is fluid pressure generated by a pump and driven by the engine. The pump's output pressure will increase roughly in proportion to the engine's speed. However, the pressure necessary to actuate the various valves and to energize the clutch and band servo pistons will vary under different working conditions. Therefore the fluid pressure generated by the pump is unlikely to suit the many operating requirements. To overcome these difficulties, a pressure regulating valve is used which automatically adjusts the pump's output pressure to match the working requirements at any one time. One of the functions of the pressure regulating valve is to raise the line pressure reaching the clutch and brake when the vehicle is driven hard with large throttle opening to prevent the friction surfaces slipping. Conversely under light loads and with a small throttle opening, a much lower line pressure is adequate to clamp the friction plates or bands. By reducing the line pressure, fierce clutch and brake engagements are eliminated which promotes smooth and gentle gear changes. Power consumption, which is needed to drive the hydraulic pump, is also reduced as actuating pressures are lowered. The pressure regulating valve is normally a spring-loaded spool type valve, that is, a plunger with one or more reduced diameter sections along its length, positioned in a cylinder which has a number of passages intersecting the cylinder walls.

When the engine speed, and correspondingly pump pressure, is low, fluid flows via the inlet port around the wasted section of the plunger and out unrestricted along a passage leading to the manual valve where it is distributed to the various control valves and operating pistons. As the pump pressure builds up with rising engine speed, line pressure is conveyed to the rear face of the plunger and will progressively move the plunger forward against the control spring, causing the middle land to uncover an exhaust port which feeds back to the pump's intake. Hence as the pump output pressure tends to rise, more fluid is passed back to the suction intake of the pump. It therefore regulates the output fluid pressure, known as line pressure, according to the control spring stiffness. To enable the line pressure to be varied to suit the operating conditions, a throttle pressure is introduced to the spring end of the plunger which opposes the line pressure. Increasing the throttle pressure raises line pressure and vice versa.

In addition to the main pressure regulating valve there is a secondary regulating valve which limits the fluid flowing through to the torque converter. Raising the torque converter's fluid pressure increases its torque transmitting capacity which is desirable when driving in low gear or when the engine is delivering its maximum torque.

5.3.2 Speed and load sensing valves

For gear changes to take place effectively at the optimum engine and road speed, taking into account the driver's demands expressed in throttle opening, some means of sensing engine load and vehicle road speed must be provided. Engine output torque is simply monitored by a throttle valve which is linked to the accelerator pedal, either directly or indirectly, via a vacuum diaphragm operated linkage which senses the change in induction depression, which is a measure of the engine load. The amount the accelerator pedal or manifold vacuum alters is relayed to the throttle valve which accordingly raises or lowers the output pressure. This is then referred to as throttle pressure.

Road speed changes are measured by a centrifugal force-sensitive regulating valve which senses transmission output shaft speed and transmits this information in the form of a fluid pressure, referred to as governor pressure, which increases or decreases according to a corresponding variation in road speed. Both throttle pressure and governor pressure are signalled to each gear shift valve so that these may respond to the external operating conditions (i.e. engine torque developed and vehicle speed) by permitting fluid pressure to be either applied or released from the various clutch and brake actuating piston chambers.

Shift valves are of the spool plunger type, taking the form of a cylindrical plunger reduced in diameter in one or more sections so as to divide its length into a number of lands. When operating, these valves shift from side to side and cover or uncover passages leading into the valve body so that different hydraulic circuits are switched on and off under various operating conditions.

The function of a shift valve is to direct the fluid pressure to the various clutch and brake servo pistons to effect gear changes when the appropriate load and speed conditions prevail. Shift valves are controlled by line or throttle pressure, which is introduced into the valve at the spring end, and governor pressure, which is introduced directly to the valve at the opposite end. Generally, the governor valve end is of a larger diameter than the spring end so that there will be a proportionally greater movement response due to governor pressure variation. Sometimes the shift valve plunger at the governor pressure end is referred to as the governor plug.

The position of the shift valve at any instant depends upon the state of balance between the opposing end forces acting on the spool valve end faces.

Spring Throttle Governor load ^ pressure load = pressure load

where FS = Spring load

Ft = Throttle pressure load PT = Throttle pressure Fg = Governor pressure load At = CSA of plunger at throttle pressure end

PG = Governor pressure Ag = CSA of plunger at governor pressure end

Thus increasing or decreasing the spring stiffness or enlarging or reducing the diameter of the spool valve at one end considerably alters the condition when the shift valve moves from one end to the other to redirect line pressure to and from the various clutch and brakes and so produce the necessary gear change.

Each shift valve control spring will have a particular stiffness so that different governor pressures, that is, road speeds, are required to cause either a gear upshift or downshift for a given opposing throttle pressure. Conversely, different engine power outputs will produce different throttle pressures and will alter the governor pressure accordingly when a particular gear shift occurs. Large engine loads (high throttle pressure) will delay gear upshifts whereas light engine load demands (low throttle pressure) and high vehicle speeds (high governor pressure) will produce early upshifts and prevent early downshift.

To improve the quality of the time sequence of up or down gear shift, additional valves and components are included to produce a smooth transition from one gear to the next. Some of these extra devices are described in Section 5.6.

5.3.4 Clutch and brake coupling and hold devices (Figs 5.5 and 2.16)

Silent gear change synchronization is made possible by engaging or locking out various members of the epicyclic gear train gear sets with the engine's power being transmitted continuously. It therefore requires a rapid and accurate gear change which is achieved by utilizing multiplate clutches and band brakes. A gear up- or downshift therefore occurs with the almost simultaneous energizing of one

Fig. 5.5 (a and b) Basic multiplate clutch and band brake transmission hydraulic control system

clutch or brake and a corresponding de-energizing of another clutch or brake.

Multiplate clutch (Figs 2.16 and 5.5) Wet multiplate type clutches are very compact for their torque transmitting and heat dissipating capacity. They are used to lock any two members of a planetary gear set together or to transfer drive from one shaft or member to another quickly and smoothly. The rotating and fixed friction plates can be energized by an annular shaped, hydraulic-ally operated piston either directly or indirectly by a dished washer which acts also as a lever to multiply the operating clamping load. Return springs are used to separate the pairs of rubbing faces when the fluid pressure is released. Wear and adjustment of the friction plate pack is automatically compensated by the piston being free to move further forward (see Chapter 2, Fig. 2.16).

Band brake (Fig. 5.5) This form of brake consists of a friction band encompassing an external drum so that when the brake is applied the band contracts, thereby wrapping itself tightly around the drum until the drum holds. The application of the band is achieved through a double acting stepped servo cylinder and piston. Fluid line pressure is introduced to the small diameter end of the piston to energize the band brake. To release the band, similar line pressure is directed to the spring chamber side of the cylinder. Band release is obtained due to the larger piston area side producing a greater force to free the band. This method of applying and releasing the band enables a more prolonged and controllable energizing and de-energizing action to be achieved. This class of brake is capable of absorbing large torque reactions without occupying very much space, which makes the band brake particularly suitable for low gear high torque output gear sets. Band wear slackness can be taken up by externally adjusting the anchor screw.

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