Chapter 12: Drive Trains

1. Chapter 12: Drive Trains BAE 517 - Lecture 12

2. Drive trains serve the following functions: Transmit power from engine to drive wheels and PTO. A means of smoothly engaging engine power at start-up. Transform engine torque and speed to meet load requirements. Provides means for reversing the direction of travel. Provides a means of smoothly stopping the vehicle.

3. Fig. 12.1 Vehicle drive-train.

4. Clutches Friction clutches are design to absorb energy during slippage. Virtually all vehicle clutches are spring-loaded friction disks.

5. Fig. 12.2 Spring Loaded Dry Clutch

6. Fig. 12.3 Hydraulically Actuated Wet Clutch

7. Torque Transmission Capacity of a Clutch The torque transmission capacity of a clutch can be estimated as, where, Tc is the torque capacity, f is the friction coefficient, Fc is clamping force of clutch, rm is the mean radius of the clutch, and ns is the number of friction surfaces.

8. Torque Transmission Capacity of a Clutch The mean effective radius is approximated as, where, Do is the outer diameter of the clutch disk, and Di is the inside diameter.

9. Table 12.1 Clutch Design Data

10. Heat Generation at Clutch Heat generation, Q, is, where, Ts is the average torque transmitted during slippage, Ns is the average speed difference between the pressure plate and disk, and ts is the slip duration.

11. Heat Generation at Clutch The temperature rise of the clutch can be estimated as, where, mc is the mass of the heat absorbing parts of the clutch, and Cp is the specific heat of the heat absorbing pats of the clutch.

12. Heat Generation at Clutch One of the previous equations can be modified to estimate the rate of heat generation, Er, where, Ac is the combined area of all friction surfaces.

13. Transmission and Load Matching The speed (Na) is related to the speed of the engine (Ne) as, where, Gpt is the overall power train ratio.

14. Transmission and Load Matching Similarly, axle torque (Ta) is related to the engine torque (Te) as, where, ept is the overall power train efficiency.

15. Fig. 12.4a Engine Torque-Speed Curve

16. Fig. 12.4b Power Train Torque-Speed Curve

17. Types of Transmissions Sliding Gear Constant-Mesh Synchromesh Powershift Continuously Variable Transmission (CVT) Hydrokinetic

18. Gear Design Spur or helical gears are meshed between parallel shafts. Spur gears have teeth that are parallel to the shafts, while helical gear teeth are angled with respect to the shafts. Helical gears continually transfer the load from one gear to the other.

19. Gear Design Gear teeth typically have a tooth profile that is �involute� (generated by unwrapping a string from a cylinder). Constant involute profiles generate constant angular velocities.

20. Fig. 12.5 Involute Tooth Profile

21. Fig. 12.6 Gear Tooth Pressure Angle

22. Fig. 12.7 Gear Tooth Terminology

23. Gear Design The center distance of the shafts (Dc) is related to the pitch diameter of the pinion gear (Dp) as, where, G is the overall gear ratio (must be >1).

24. Gear Design The following equation gives the relationship between the tooth size module (m), pitch diameter (DG) and the number of teeth (n),

25. Gear Design For helical gears the modulus, m, must be modified to account for the helix angle, y,

26. Typical Tooth Moduli Transmission Gears � 4 to 5 mm Powershift Planetary Gears � 1.2 to 3.5 mm Spiral Bevel Gears � 8 to 12 mm Final Drive Gears � 5 to 7 mm

27. Gear Design Working pressure angles (fw) are selected for a specific pitch diameter,

28. Transmission Types Sliding Gear � gears slid horizontally on splined shaft to move in or out of mesh. Constant Mesh � gears are in constant mesh, and are free to rotate on one of the shafts, splined collars lock gears to shaft Synchromesh � sliding couplings within the constant mesh transmissions are replaced with synchronizers

29. Constant Mesh Transmission

30. Fig. 12.9a Synchronizer

32. Powershift Transmissions Can be shifted with virtually no interruption in power. Types of powershift transmissions: a) countershaft, and b) planetary. Hydraulic pressure is utilized to actuate the clutches.

33. Fig. 12.10a Countershaft Powershift Transmission(Hi/Lo Shift)

34. Hi/Lo Powershift When neither clutch is engaged, transmission is in neutral. When left clutch is engaged, output shaft turns slower than input. When right clutch is engaged, output shaft turns the same speed as the input. When both clutches are engaged, transmission is in �Park.�

35. Fig. 12.10b Countershaft Powershift Transmission(Reverser)

36. Reverser Powershift When neither clutch is engaged, transmission is in neutral. When left clutch is engaged, output shaft turns opposite direction of the input. When right clutch is engaged, output shaft turns the same direction as the input. When both clutches are engaged, transmission is in �Park.�

37. Fig. 12.11 Planetary Gear Set

38. Planetary Speed Ratios The following equation can be utilized to determine speed and torque ratios for planetary gear sets, where n is the number of gear teeth and N is gear speed.

39. Planetary Torque Relationship The following equation can be utilized to determine the respective torque within the planetary set,

40. Overall Planetary Efficiency The following equation can be utilized to determine the overall planetary efficiency,

41. Compound Planetary Transmissions Contains two set of different size planets, one meshing with the sun and the second meshing with the ring gear. May also include two sun and/or two ring gears.

42. Fig. 12.12 Full Powershift Transmission

43. Table 12.2 Element Engagement and Resulting Gear Ratios

44. Fig. 12.13 Electro-Proportional Pressure Reducing Valve

45. Fig. 12.21a Differential

46. Fig. 12.21b Differential

47. Fig. 12.22 Planetary Final Drive

48. Fig. 12.23 Skid-Steer (Caterpillar)

49. Fig. 12.24 Tractor PTO

50. PTO Categories Type 1: 540 rpm, 35 mm shaft, up to 65 kW Type 2: 1000 rpm, 35 mm shaft, 45 kW to 120 kW Type 1: 1000 rpm, 45 mm shaft, 110 kW to 190 kW

51. Homework Set No. 12 Do problems 12.1, 12.6, 12.9 and 12.12 at the end of Chapter 12 for next Tuesday.