59 The continuously variable belt and pulley transmission

The continuously variable transmission CVT, as used by Ford and Fiat, is based simply on the

Table 5.4 Table of possible faults

Selector position Possible fault

D FC or OWC

Table 5.5 Table of symptoms for a faulty one way clutch

Vehicle response

Fault 0-50 km/h above 50 km/h

Slipping stator Very sluggish Drives normally

No hill start possible

Seized stator Drives normally Loss of power

Severe overheating principle of a belt running between two V-shaped pulleys which is designed so that the effective belt contact diameter settings can be altered to produce a stepless change in the input to output pulley shaft speed.

Van Doorne Transmissie in Holland has been mainly responsible for the development of the steel belt which is the key component in the transmission. At present the steel belt power output capacity is suitable for engine sizes up to 1.6 litres but there does not appear to be any reason why uprated steel belts cannot be developed.

This type of transmission does not suffer from the limitations of the inefficient torque converter which is almost universely used by automatic transmissions incorporating epicyclic gear trains operated by multiplate clutches and band brakes.

5.9.1 Stepless speed ratios (Figs 5.25 and 5.26) The transmission consists basically of a pair of variable width vee-shaped pulleys which are interconnected by a composite steel belt. Each pulley consists of two shallow half cones facing each other and mounted on a shaft, one being rigidly attached to it whereas the other half is free to slide axially on linear ball splines (Fig. 5.25). The variable speed ratios are obtained by increasing or decreasing the effective wrap contact diameter of the belt with the primary input pulley producing a corresponding reduction or enlargement of the secondary output pulley working diameter. The belt variable wrap contact diameter for both primary and secondary pulleys is obtained by the wedge shaped belt being supported between the inclined adjacent walls of the two half pulleys.

When the primary input half pulleys are brought axially closer, the wedge or vee-shaped belt running between them is squeezed and is forced to ride up the tapered walls to a larger diameter. Conversely, since the belt is endless and inextensible, the secondary output half pulleys are compelled to separate, thus permitting the belt wrap to move inwards to a smaller diameter.

Alternatively, drawing the secondary output half pulleys closer to each other enlarges the belt's running diameter at that end. Accordingly it must reduce the primary input pulley wrap diameter at the opposite end. A one to one speed ratio is obtained when both primary and secondary pulleys are working at the same belt diameter (Fig. 5.26). A speed ratio reduction (underdrive) occurs when the primary input pulley operates at a larger belt contact diameter than the secondary output pulley (Fig. 5.26). Conversely, a speed ratio increase (over drive) is achieved if the belt contact with the primary pulley is at a smaller diameter relative to the secondary pulley wrap diameter (Fig. 5.26).

In the case of the Ford Fiesta, the pulleys provide a continuously variable range of ratios from bottom 2.6:1 to a super overdrive top of 0.445:1.

An intermediate gear reduction of about 1.4:1 between the belt output pulley shaft and the final drive crownwheel is also provided so that the transmission can be made to match the engine's power output and the car's design expectation.

5.9.2 Belt design (Figs 5.26 and 5.27) Power is transmitted from the input to the output pulley through a steel belt which resembles a steel necklace of thin trapezoidal plates strung together between two multistrip bands made from flexible high strength steel (Fig. 5.27). There are 300 plates, each plate being roughly 2 mm thick, 25 mm wide and 12 mm deep, so that the total length of the endless belt is approximately 600 mm. Each band is composed of 10 continuous strips 0.18 mm thick. Also made of high strength steel, they fit into location slots on either side of the plates, their purpose being to guide the plates, whereas it is the plates' function to transmit the drive by pushing. Another feature of the plates is that they are embossed in a dimple form to assist in the automatic alignment of the plates as they flex around the pulley.

Contact between the belt plates and pulley is provided by the tapered edges of the plates which match the inclination of the pulley walls. When in drive, both primary and secondary sliding half pulleys are forced against the belt so that different plates are in contact and are wedged between the vee profile of the pulley at any one time.

Consequently, the grip produced between the plates and pulley walls also forces the plates together so that in effect they become a continuous strut which transmits drive in compression (unlike the conventional belt which transfers power under tension (Fig. 5.26)). The function of the non-drive side of the belt, usually referred to as the slack side, is only to return the plate elements back to the beginning of the drive (compressive) side of the pulleys.

The relative movement between the band strips and the plates by this design is very small, therefore frictional losses are low. Nevertheless the transmission efficiency is only 92% with a one to one speed ratio dropping to something like 86% at pull-away, when the speed ratio reduction is 2.6:1.

Fig. 5.25 Section view of a transverse continuously variable transmission

Input Narrow

Input Narrow

Wide

1:1 Pulley ratio 2-6:1 Pulley ratio 0-445 :1 Pulley ratio

Wide

1:1 Pulley ratio 2-6:1 Pulley ratio 0-445 :1 Pulley ratio

Fig. 5.26 Illustration of pulley and belt under- and overdrive speed ratios

Fig. 5.26 Illustration of pulley and belt under- and overdrive speed ratios

Fig. 5.27 Steel belt construction

5.9.3 Hydraulic control system (Fig. 5.28) The speed ratio setting control is achieved by a spur type hydraulic pump and control unit which supplies oil pressure to both primary and secondary sliding pulley servo cylinders (Fig. 5.28). The ratio settings are controlled by the pressure exerted by the larger primary servo cylinder which accordingly moves the sliding half pulley axially inwards or outwards to reduce or increase the output speed setting respectively. This primary cylinder pressure causes the secondary sliding pulley and smaller secondary servo cylinder to move proportionally in the opposite direction against the resistance of both the return spring and the secondary cylinder pressure, this being necessary to provide the correct clamping loads between the belt and pulleys' walls. The cylinder pressure necessary to prevent slippage of the belt varies from around 22 bar for the pull away lowest ratio setting to approximately 8 bar for the highest overdrive setting.

The speed ratio setting and belt clamping load control is achieved via a primary pulley position senser road assembly.

Fig. 5.28 Transaxle continuously variable belt and pulley transmission layout

However, engine and road speed signals are provided by a pair of pitot tubes which sense the rate of fluid movement, this being a measure of speed, be it either under the influence of fluid flow caused by the engine's input or by the output drive relating to vehicle speed.

5.9.4 Epicyclic gear train construction and description (Figs 5.25 and 5.28) Drive in both forward and reverse direction is obtained by a single epicyclic gear train controlled by a forward multiplate clutch and a reverse multiplate brake, both of which are of the wet type

(immersed in oil) (Fig. 5.25). The forward clutch is not only used for engagement of the drive but also to provide an initial power take-up when driving away from rest.

The epicyclic gear train consists of an input planetary carrier, which supports three sets of double planetary gears, and the input forward clutch plates. Surrounding the planetary gears is an internally toothed annulus gear which also supports the rotating reverse brake plates. In the centre of the planetary gears is a sun gear which is attached to the primary pulley drive shaft.

Neutral or park (N or P position) (Fig. 5.28) When neutral or park position is selected, both the multiple clutch and brake are disengaged. This means that the annulus gear and the planetary gears driven by the input planetary carrier are free to revolve around the sun gear without transmitting any power to the primary pulley shaft.

The only additional feature when park position is selected is that a locking pawl is made to engage a ring gear on the secondary pulley shaft, thereby preventing it from rotating and causing the car to creep forward.

Forward drive (D or L position) (Fig. 5.28) Selecting D or L drive energizes the forward clutch so that torque is transmitted from the input engine drive to the right and left hand planetary carriers and planet pins, through the forward clutch clamped drive and driven multiplates. Finally it is transferred by the clutch outer casing to the primary pulley shaft. The forward gear drive is a direct drive causing the planetary gear set to revolve bodily at engine speed with no relative rotational movement of the gears themselves.

Reverse drive (R position) (Fig. 5.28) Selecting reverse gear disengages the forward clutch and energizes the reverse multiplate brake. As a result, the annular gear is held stationary and the input from the engine rotates the planetary carrier (Fig. 5.28).

The forward clockwise rotation of the carrier causes the outer planet gears to rotate on their own axes as they are compelled to roll round the inside of internally toothed annular gear in an anticlockwise direction.

Motion is then transferred from the outer planet gears to the sun gear via the inner planet gears. Because they are forced to rotate clockwise, the meshing sun gear is directionally moved in the opposite sense anticlockwise, that is in the reverse direction to the input drive from the engine.

5.9.5 Performance characteristics (Fig. 5.29) With D drive selected and the car at a standstill with the engine idling, the forward clutch is just sufficiently engaged to produce a small amount of transmission drag (point 1). This tends to make the car creep forwards which can be beneficial when on a slight incline (Fig. 5.29). Opening the throttle slightly fully engages the clutch, causing the car to move positively forwards (point 2). Depressing the accelerator pedal further sets the speed ratio according to the engine speed, road speed and the driver's requirements. The wider the throttle is opened the lower the speed ratio setting will be and the higher the engine speed and vice versa. With a light constant throttle opening at a minimum of about 1700rev/min (point 3) the speed ratio moves up to the greatest possible ratio for a road speed of roughly 65 km/h which can be achieved on a level road. If the throttle is opened still wider (point 4) the speed ratio setting will again change up, but at a higher engine speed. Fully depressing the accelerator pedal will cause the engine speed to rise fairly rapidly (point 5) to about 4500rev/min and will remain at this engine speed until a much higher road speed is attained. If the engine speed still continues to rise the pulley system will continue to change up until maximum road speed (point 6) has been reached somewhere near 5000 rev/min.

Partially reducing the throttle open then causes the pulley combination to move up well into the overdrive speed ratio setting, so that the engine speed decreases with only a small reduction in the car's cruising speed (point 7).

Even more throttle reduction at this road speed causes the pulley combination speed ratio setting to go into what is known as a backout upshift (point 8), where the overdrive speed ratio reaches its maximum limit. Opening the throttle wide again brings about a kickdown downshift (point 9) so that there is a surplus of power for acceleration. A further feature which provides engine braking when driving fast on winding and hilly slopes is through the selection of L range; this changes the form of driving by preventing an upshift when the throttle is eased and in fact causes the pulley combination to move the speed ratio towards an underdrive situation (point 10), where the engine operates between 3000 and 4000 rev/min over an extensive road speed range.

The output torque developed by this continuously variable transmission approaches the ideal

constant power curve (Fig. 5.29) in which the torque produced is inversely proportional to the car's road speed.

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