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)

  3. Specify the number of teeth on the pinion (Np):

    4. Hint: Minimum number of teeth to avoid interference for pinions having 20° pressure angles is 18

    Np = ?
  4. Compute the number of teeth on the gear (Ng):

    5. NOTE: Remember, Ng must be an integer, so round-off the value to the closest integer

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

    6. &

                                                                        (rpm)

  6. Compute the pitch diameters (dp and dg):

    7.                                                                     (in.)

    &

                                                                        (in.)

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

    8.                                                             (in.)

     

  8. Compute the pitch line velocity (n t):

    9.                                                             (fpm – feet per minute)

     

  9. Compute the transmitted load (Wt) for the specified power of 3 hp:

    10.                                                             (lb.)

     

  10. Specify the gear face width (F):

    11.                                                              (in.)

     

  11. Compute the bending stress (s tp) on the pinion teeth:

    12.                                                     (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)

     

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

    13.                                                             (kpsi)

     

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

    z = ?                                                                      (BHN)

     

  13. Compute the contact stress (s c) on the gear pair:

    14.                                                     (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)

     

  14. 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