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/

Automatic transmission questions

1. What is a fluid coupling? Give an example 
It is a two member drive coupling that are not physically connected but are facing each other enclosed in a shell. They are made up of a Impeller and a Turbine which transfers hydraulic fluid to the transmission. A torque converter is a Fluid coupling without the Stator.

2. What is the name given to a hydraulic coupling that increases torque?
Torque converter

3. Name the main parts of a torque converter
Impeller
Turbine
Stator

4. Explain the operation of each part of the torque converter
Impeller - The engine driven input, spins the hydraulic fluid in such a way to the Turbine
Turbine - Connected to the input of the transmission. The amount of fluid it receives from the Turbine determines how fast/slow it rotates the transmission.
Stator - Located in the centre of the torque converter. Redirect fluid returning from the Turbine before it hits the Impeller

5. Describe how torque is increased by the torque converter
A Stator is used. When fluid leaves the Impeller it is picked up by the Turbine but the fluid that doesn't pass through hits the Turbine and shoots back to the Impeller hitting it in the opposite direction its rotating causing the Impeller to slow down and slowing down the engine so there is potential that has just been lost. A Stator redirects the fluid that the Turbine rejects and because the shape of the vanes it causes the fluid to hit the Impeller in the direction its rotating acting with the Impeller and not against it therefore not loosing potential.

6. Explain the operation of the stator
The Stator is in the centre of the Impeller and Turbine. Its vanes are curved opposite to the Turbine and its job is to redirect the fluid returning before it hits the Impeller so the fluid ends up entering the Turbine in the same way its rotating.

7. What is rotary flow?
This is the circular flow that follows the rotation of the Torque converter.

8. What is coupling point?
Occurs when the speed of the Impeller and Turbine are very similar usually due to the vehicle being driven at a constant speed. This is the coupling point which means the stator must be able to freewheel in a clockwise direction. If this doesn't happen it would reduce the flow of fluid and slow down the vehicle.

9. What is meant by vortex flow?
This is a spiralling flow of the fluid inside the torque converter and continues as long as there is a difference in speed of the Impeller and Turbine.

10. Explain how a one way clutch works in the converter?
A one way clutch is attached to the Stator inside the Torque converter. It allows the Stator to rotate in the same direction as the Turbine. If the Stator tries to rotate in the other direction the one way clutch locks it in place to prevent it from doing so.

11. What is the purpose of a torque converter
To transfer the torque of the engine to the transmission and can either multiply the torque produced from the engine or act as a fluid coupling. It also smooths out engine rotation and can help to absorb vibrations from the engine.

12. Why do some converters have a clutch?
To prevent the Stator from rotating in the wrong direction the one way clutch locks it in place.

13. Explain how a lock up clutch works
A lock up clutch is used to lock Turbine to the converter housing which provides a direct drive from the engine to the transmission like a manual clutch. This reduces fuel consumption and provides a one to one drive.

14. What does a servo do in the transmission?
A servo is a spring loaded assembly that activates and releases the front and rear bands when shifting gears.

15. What is meant by torque multiplication?
The speed different between the Impeller and Turbine.

16. What does a band do in the transmission?
Bands connect and disconnect components in the transmission. They wrap around sections of the gear train and are attached to the housing. Hydraulic cylinders inside the case of the transmission actuate the bands.


Reference pages:
http://www.autoshop101.com/
https://www.google.co.nz/
http://www.howstuffworks.com/
http://moodle.unitec.ac.nz/

Automatic transmission - Mechanical system

In the Automatic transmission mechanical system there are three main components.

• Gear sets (usually planetary) Used to create different speed ratios that the transmission can produce.
  
  - Sun gear, This is the gear which is located in the centre of the gear set and which the other smaller gears rotate around.
  
  - Ring gear, These are the gears around the outside of the gear set.
  
  - Planet carrier, This gear is located between the outer ring gear and inner Sun gear. It connects to both these gears and is mounted on Planetary pinions.
  
  Any of these gears can be held stationary (one at a time) and the other gears will still rotate around the stationary gear.
• Multi plate clutches
  - Used to hold two rotating planetary gear set components. Consists of a forward clutch and a direct clutch. They engauge steel plates and transfer the torque of the engine. Also lets the drive manually change gears at times.
• Band type brakes- Used to hold planetary gear set components to the transmission case. Located around the outer edge of the direct clutch drum. Takes very little space in the in the transmission housing and has a big surface area to ensure a strong holding force.

For our first week in Transmissions and Drivelines we dismantled a Transaxle Toyota Transmission.

Safety and equipment:

• Safety in the workshop is very important. We required Overalls and steel cap boots in case the Transmission or a component was to fall, our feet would be protected. We required the basic tools like socket set, soft face hammer, circlip pliers, spanners and a few extras such as Feeler gauge and Torque wrench.

What we did:

• The very first thing we did was a Visual inspection on the Transmission. We rotated it checking for any missing parts (ie, bolts, nuts), broken parts or damage to the Transmission itself and noted them down for later reference. We found ours was missing a circlip to hold the 5th gear selector hub in place, and a main bolt that splits the Transmission in half was missing.
• Using the gear lever we went through each gear (1st, 2nd, 3rd, 4th, 5th and reverse). Made sure we could select each with ease and none were jammed or crunchy. Reverse was noisy but we found out this is because the reverse gear has Spur cut gears which means the gears are straight cut. Not Helical like 1st, 2nd, 3rd, 4th and 5th which are quieter gears. We ended up locking our Transmission into 5th gear and not being able to select out of it.
• Then following the workbook we took the Transmission apart  made sure to keep all parts we removed together and replace any missing or broken parts for re-installation later on.

Why we did it:

• The visual inspection was carried out to identify any broken/missing part or damage to the Transmission. Which would affect its ability to perform (ie, a missing ball bearing in the fork selector housing would not allow the fork to lock into place and would slip not engaging gear)
• We checked each gear could be selected to help us identify any problems for when we dismantle the Transmission. (ie, if we couldn't select 3rd gear there could be a problem with the fork selector or the selector hub which we would have to fix before re assembling in order to get the Transmission working properly).
• The reason for following the workbook was to make sure we were taking apart the Transmission correctly and not taking any shortcuts. Making sure all parts that were removed were properly put back to ensure the Transmission functions properly. (ie, not putting back a circlip on the 5th gear selector hub would mean it could jump out of place and not lock the gear properly or not lock the gear at all).

Summary:
After pulling our Transmission apart we found that 5th gear fork slipped from the gear lever housing assembly which lead to our Transmission being stuck into gear. We found that all of the internal components were in good working order and the missing parts like circlips were replaced and all our bolts were torqued to the manufactures specs. After re-assembling it our Transmission was no longer stuck in gear and we could select each with ease.