5718 The governor valve GV

The governor revolving with the transmission output shaft is basically a pressure regulating valve which reduces line pressure to a value that varies with output vehicle speed. This variable pressure is known as governor pressure and is utilized in the control system to effect up and down gear shifts from 1-2 and 2-3 shift valves. Governor pressure opposes shift valve spring force, throttle pressure and kickdown pressure, and the resulting force acting on the governor plug and shift valve determines the vehicle's gear change speeds. The governor drive is achieved through a skew gear meshing with a ring gear mounted on the reverse annular gear carrier which is attached to the output pinion shaft.

The two types of governor valves used for this class of automatic transmission are the ball and pivot flyweight and the plunger and flyweight. These governors are described below.

Plunger and flyweight type governor (Fig. 5.23) Rotation of the governor at low speed causes the governor weight and valve to produce a centrifugal force. This outward force is opposed by an equal and opposite hydraulic force produced by governor pressure acting on the stepped annular area of the governor valve. Because the governor valve is a regulating valve, and will attempt to remain in equilibrium, governor pressure will rise in accordance with the increase in centrifugal force caused by increased rotational speed. As the output shaft speed increases, the governor weight moves outwards (due to the centrifugal force) to a stop in the governor body, when it can move no further. When this occurs, the governor spring located between the weight and the governor valve becomes effective. The force of this spring then combines with centrifugal force of the governor valve to oppose the hydraulic pressure, thus making the pressure less sensitive to output shaft speed variation. Therefore the governor provides two distinct phases of regulation, the first being used for accurate control of the low speed shift points.

Fig. 5.23 Plunger and governor valve

Fig. 5.23 Plunger and governor valve

Ball and pivot flyweight type governor (Fig. 5.24) This type of governor consists of a ball valve controlled by a hinged flyweight and a pressure relief ball valve. Fluid from the oil pump at line pressure is introduced via a restriction into an axial passage formed in the governor drive shaft. When the transmission output shaft stops rotating (vehicle stationary) with the engine idling, fluid pressure forces the governor ball valve off its seat, permitting fluid to escape back to the sump. Rotation of the output shaft as the vehicle accelerates from a standstill causes the flyweight centrifugal force to close the ball valve. Therefore fluid trapped in the governor drive shaft passage, known as governor pressure, has to reach a higher pressure before fluid exhausts through the valve. By these means the line pressure is regulated to a valve that varies with the output shaft and vehicle speed. A pressure relief valve is also included to safeguard the system from excessively high pressure if the governor valve malfunctions.

5.7.19 Hydraulic accumulator (Fig. 5.24) This is a cylinder and spring loaded piston which is used to store a small amount of pressure energy to enable a rapid flow of fluid under pressure to one of the operating components or to absorb and smooth fluctuating fluid delivery. The piston is pushed back when the fluid pressure exceeds the spring load and fluid enters and fills up the space left behind by the displaced piston.

With the transmission in neutral or park, line pressure from the pressure pump enters the accumulator at the opposite end to the spring, thereby displacing the piston and compressing the spring. When the hydraulic control shifts into the second gear phase, line pressure from the 1-2 shift valve is directed to the second gear band servo applied end and the spring end of the accumulator.

When the accumulator spring is compressed, fluid from the supply can flow rapidly to the applied side of the band servo piston. As soon as the servo piston meets resistance (starts to apply its load), the fluid pressure increases and the accumulator piston spring is extended as the piston is pushed back by the spring. This is because there is equal line pressure acting on either side of the accumulator piston and so the spring is able to apply its load and extend. As a result, the supply of fluid is reduced to the applied side of the second gear band servo piston. The accumulator therefore smooths and times the application of the second gear band brake in order to reduce the risk of shock and a jerky operation. In addition, the extra quantity of fluid in the system due to the accumulator leads to a slow rate of release of the servo piston and band.

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