CVT Transmissions

4844 Transmissions and Drivelines

CVT Transmission

Jake Stewart

There are three different types of CVTs (Continuously variable transmission)

• Pulley based CVT
• Toroidal CVT
• Hydrostatic CVT

Today we are going to look at most used CVT, Pulley type.

Unlike a traditional Automatic Transmission CVTs don't contain gear sets so there are no interlocked toothed wheels. Instead a common CVT is made up of:

• A high power rubber or metal belt
• A driving pulley
• A driven pulley

(Other sensors and microprocessors are used in CVTs but the three main components above are what it needs in order for it to operate)

The CVT is structured with a driver and driven pulley in the shape of a cone facing each other with the metal or rubber V belt around the two.

As the Driver pulley closes up and opening up the Driven pulley the radius of the belt decreases therefore increasing the speed which means the engine is in a high gear. But when the Driven pulley closes up and the Driver pulley opens up increasing the radius of the belt speed decreases which means the engine is in a low gear. When one pulley decreases in radius the other increases in radius and vise verse to keep the belt tight.
As the pulleys open and close moving the belts radius it creates a number of high to low gear ratios.
CVTs can use either

• Hydraulic pressure
• Centrifugal force
• Spring tension
  to create the force necessary to adjust the pulley half's.

The Driver pulley is connected to the crankshaft of the engine.
The Driven pulley is connected to the drive shaft transferring torque to the wheels.
Metal belts are alot more durable than rubber and don't slip enabling the CVT to handle slightly more torque. They are also quieter than the rubber belt.

History:

1490 - The first CVT transmission idea was sketched by Lenoardo DaVinci

1886 - The first CVT made was by Daimler and Benz

1935 - Adiel Dodge receives a CVT copy

1939 - A fully automatic planetary gear set transmission introduced
1958 - Daf produces a CVT in car
1989 - Subaru Just GL U.S sells the very first automobile CVT
2002 - Saturn Vue first Saturn to offer CVT technology
2004 - Ford begins offering a CVT
Early CVTs were limited to the amount of horsepower they could handle which left some people with concern about the long term reliability. Now with the advanced technology the CVTs has been made a lot more robust.
Advantages:

• Much cheaper to build as it contains less components
• Smaller and lighter than the traditional Transmission which is good for small machinery and bikes that cant contain a large Transmission
• Much smoother than other Transmissions
• Better fuel consumption

Disadvantages:

• No faster or reliable than Automatic Transmissions
• Cannot cope with huge engines or a high amount of torque
• User acceptance can be a big disadvantage of the CVT.  Because the CVT allows the engine to rev at and speed the noise coming from the engine can sound odd. The changes in the engine note can sound like a slipping transmission or clutch but is a perfectly normal sound for the CVT. 
• With the traditional Transmission you plant your foot to the floor and the Transmission kicks down a gear giving you instant power, but with the CVT they provide a smoother response taking longer to pick up to speed which some people can think it means the CVT is slower therefore not as good.

Examples of what CVTs are used in:

• Tractors
• Quad bikes
• Motorbikes
• Snow mobiles

Reference pages:

http://www.howstuffworks.com/
http://www.autoshop101.com/
http://www.youtube.com/
http://carsabout.com/
http://www.cvt.co.nz/