Gearbox Design

Preliminary Analysis & Gear Design

1. Compute the nominal velocity ratio (VR):
2. Specify the trial diametral pitch (Pd) using table (13-3):
3. Pd = ?                                                                 (tpi – teeth per inch)

4. Specify the number of teeth on the pinion (Np):
5. Hint: Minimum number of teeth to avoid interference for pinions having 20° pressure angles is 18

Np = ?

6. Compute the number of teeth on the gear (Ng):
7. NOTE: Remember, Ng must be an integer, so round-off the value to the closest integer

8. Check the new velocity ratio (VR) and verify if the new gear speed (ng) falls within the specified range of 460~465 rpm:
9. &

(rpm)

10. Compute the pitch diameters (dp and dg):
11.                                                                     (in.)

&

(in.)

12. Check whether the center distance (C) is in the specified range (£ 5"):
13.                                                             (in.)

14. Compute the pitch line velocity (n t):
15.                                                             (fpm – feet per minute)

16. Compute the transmitted load (Wt) for the specified power of 3 hp:
17.                                                             (lb.)

18. Specify the gear face width (F):
19.                                                              (in.)

20. Compute the bending stress (s tp) on the pinion teeth:
21.                                                     (kpsi)

Based on a reliability of 99%, the computed stress can be compared directly with the allowable stress shown in fig. (14-2) to find the required Brinell hardness for the pinion teeth and call it ‘x

x = ?                                                                      (BHN)

22. To evaluate the bending stress on the gear teeth (s tg), use the following equation:
23.                                                             (kpsi)

For the value calculated above, find the required Brinell hardness from fig. (14-2) and call it ‘z

z = ?                                                                      (BHN)

24. Compute the contact stress (s c) on the gear pair:
25.                                                     (kpsi)

Compare the computed stress with the allowable contact stress in fig. (14-3) and find the required surface hardness, call this hardness value ‘y’

y = ?                                                                             (BHN)

26. Specify materials for the gears, keeping in mind the fact that pinions are typically made from materials about 16% harder than those used for the gear to ensure optimum performance.

 Stress Brinell Hardness Selection Procedure Bending Stress on Pinion x }Use maximum value for              selecting pinion material}Use maximum value for              selecting gear material Contact Stress on Gear Pair y Bending Stress on Gear z