1. Standard Transmissions
Diesel Technology Power Trains

2. Need for a Transmission
Most trucks haul heavy loads (40,000 to 80,000 Lbs or more)
Even though a diesel engine produces high amounts of torque, it is not enough to initially move the load
Must have a way to increase the torque and decrease the speed when starting

3. Purpose of the Transmission
To take the torque developed by the engine and multiply it
To change the speed of the input shaft
To change the direction of the torque entering the input shaft (reverse)

4. Types of Heavy Duty Transmissions
Most common type has two transmissions in one:
Main gear box in the front
Auxiliary gear box in the rear
Other types have a separate main box and auxiliary box connected by a stub driveshaft

5. Main Box Can have 4, 5, or 6 speeds
Torque comes from the clutch discs through the input shaft
Is mounted directly to the engine with a SAE standard bolt pattern
Shares the same physical case with the auxiliary box

6. Auxiliary Box Usually has 2 or 3 speeds
Receives the torque from the main box main shaft
Often is shifted with air pistons
Torque exits from the output shaft to the driveline
Auxiliary box can be separate (Brownie Box)

7. Transmission Gears
Since the main box can be shifted separately from the auxiliary box, the combination can result in 10, 12, 18, or 20 speeds
A 5 speed main box with a 2 speed auxiliary box will result in 10 speeds
A 6 speed main box with a 3 speed auxiliary box will result in 18 speeds
Will also result in 2 or 3 different speeds in reverse

8. Gear Terminology
Drive gear is the gear that transmits the power to another gear
Driven gear is one that receives the power from the drive gear
A drive gear can become a driven gear under the right circumstances

9. Gear Speed and Direction
Gear rotation is expressed in either clockwise (same way as a clock) or counterclockwise (opposite way of a clock)
Can be viewed from front or back
Rotation speed is expressed in revolutions per minute (RPM’s)

10. Gear Teeth Terminology
Drive side of the gear tooth is the side that transmits power to another gear
Coast side is the opposite side of the gear tooth
Most gears are constant mesh as they are always in contact with the other gear

11. Gear Problems Excessive Backlash Climbing
When there is too much space between the gear teeth on the drive or coast side
Will cause excessive wear and tooth chipping
Climbing
When a gear tooth has so much backlash it tries to climb over the driven gear teeth
Usually from bad bearings or bent shafts
Results in rapid gear destruction

12. Gear Problems Bottoming
When the gear teeth hit the root (bottom) of the driven gear
Can cause teeth chipping and excessive wear
Usually from bent shafts or wrong tooth pitch

13. Gear Teeth The top of the tooth is referred to as the Outside Diameter
The inside of the tooth is referred to as the Root Diameter
The diameter of the
middle of the tooth is
called Gear Pitch

14. Gear Pitch All gears that mesh together must have the same gear pitch
You can figure gear pitch by using this formula:
Gear pitch divided by number of teeth
36 gear teeth and pitch diameter is 6”
Gear pitch 6 (36 ÷ 6 = 6)

15. Types of Gears
Spur Gears
Helical Cut Gears

16. Spur Gears
Most common in heavy duty applications because it is a rugged design
Gear teeth go straight across, parallel to the shaft
Only one or two teeth are in contact with the other gear

17. Helical Cut Gears Used on many medium duty transmissions
Have gear teeth that spiral across the face
The cut creates a thrust load and requires a thrust bearing
Two or more gear teeth are in contact with each other at all times

18. Gear Type Comparisons Spur Helical Cut Noisy No thrust load Rugged
Quiet
Thrust load
Less able to accept high torque

19. Gear Ratios
To change the speed of the input shaft, you need to change one or more gear outside diameters
To reduce speed, the drive gear will be smaller than the driven gear

20. Gear Ratios
To keep the speed the same, both gears remain the same size

21. Gear Ratios
To increase the speed, the drive gear will be larger than the driven gear

22. Calculating Gear Ratios
To determine how much faster or slower the ratio will be, use this formula:
Driven Teeth divided by Drive Teeth equals Gear Ratio (to one)
Example 4.11:1
75 driven teeth and 45 drive teeth
1.67:1 Gear Ratio (75 ÷ 45 = 1.67)

23. Gear Ratio Torque
As you increase the gear reduction, the torque will increase by the same amount
A 2:1 gear reduction will increase torque by a factor of 2
As you increase the overdrive, the torque will decrease by the same amount
A 0.8:1 gear overdrive will reduce torque by a factor of 0.8

24. Gear Rotation
A gear turning counterclockwise will turn another gear clockwise
The effect is turning the driven shaft in reverse

25. Gear Rotation
When a third gear is added between two gears, the result is that both the drive and driven will turn the same direction
The middle gear has no effect gear reduction
Middle gear called an Idler Gear

26. That Concludes the Standard Transmission Lecture