Standard TransmissionsStandard Transmissions

Diesel Technology Power TrainsDiesel Technology Power Trains

Need for a TransmissionNeed 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

Purpose of the TransmissionPurpose of the Transmission

1. To take the torque developed by the engine and multiply it

2. To change the speed of the input shaft

3. To change the direction of the torque entering the input shaft (reverse)

Types of Heavy Duty Types of Heavy Duty TransmissionsTransmissions

• 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

Main BoxMain 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

Auxiliary BoxAuxiliary 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)

Transmission GearsTransmission 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

Gear TerminologyGear 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

Gear Speed and DirectionGear 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)

Gear Teeth TerminologyGear 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

Gear ProblemsGear Problems

• Excessive Backlash– 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

Gear ProblemsGear 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

Gear TeethGear 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

Gear PitchGear 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)

Types of GearsTypes of Gears

• Spur Gears

• Helical Cut Gears

Spur GearsSpur 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

Helical Cut GearsHelical 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

Gear Type ComparisonsGear Type Comparisons

• Spur – Noisy– No thrust load– Rugged

• Helical Cut– Quiet– Thrust load– Less able to accept

high torque

Gear RatiosGear 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

Gear RatiosGear Ratios

• To keep the speed the same, both gears remain the same size

Gear RatiosGear Ratios

• To increase the speed, the drive gear will be larger than the driven gear

Calculating Gear RatiosCalculating 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)

Gear Ratio TorqueGear 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

Gear RotationGear Rotation

• A gear turning counterclockwise will turn another gear clockwise

• The effect is turning the driven shaft in reverse

Gear RotationGear 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

That Concludes the Standard That Concludes the Standard Transmission LectureTransmission Lecture