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Standard Transmissions

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Presentation on theme: "Standard Transmissions"— Presentation transcript:

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

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