T-221
SECTION 19 FEATURES OF TOOTH SURFACE CONTACT
Tooth surface contact is critical to noise, vibration, efficiency, strength, wear and life. To
obtain good contact, the designer must give proper consideration to the following features:
- Modifying the Tooth Shape
Improve tooth contact by crowning or relieving.
- Using Higher Precision Gear
Specify higher accuracy by design. Also, specify that the manufacturing process is
to include grinding or lapping.
- Controlling the Accuracy of the Gear Assembly
Specify adequate shaft parallelism and perpendicularity of the gear housing (box or
structure).
Surface contact quality of spur and helical gears can be reasonably controlled and
verified through piece part inspection. However, for the most part, bevel and worm gears
cannot be equally well inspected. Consequently, final inspection of bevel and worm mesh
tooth contact in assembly provides a quality criterion for control. Then, as required, gears
can be axially adjusted to achieve desired contact.
JIS B 1741 classifies surface contact into three levels, as presented in Table 19-1.
The percentage in the above table considers only the effective width and height of teeth.
19.1 Surface Contact Of Spur And Helical Meshes
A check of contact is, typically, only done to verify the accuracy of the installation,
rather than the individual gears. The usual method is
to blue dye the gear teeth and operate for a short
time. This reveals the contact area for inspection and
evaluation.
19.2 Surface Contact Of A Bevel Gear
It is important to check the surface contact of a
bevel gear both during manufacturing and again in final
assembly. The method is to apply a colored dye and
observe the contact area after running. Usually some
load is applied, either the actual or applied braking, to
realize a realistic contact condition. Ideal contact favors
the toe end under no or light load, as shown in Figure 19-
1; and, as load is increased to full load, contact shifts to
Table 19-1 Levels of Gear Surface Contact
Tooth Width Direction
Tooth Height Direction
Cylindrical Gears
Bevel Gears
Worm Gears
Cylindrical Gears
Bevel Gears
Worm Gears
Cylindrical Gears
Bevel Gears
Worm Gears
A
B
C
More than 70%
More than 50%
More than 50%
More than 35%
More than 35%
More than 25%
More than 20%
More than 40%
More than 30%
More than 20%
Levels of Surface Contact
Level
Types of Gear
Fig. 19-1 The Contact Trace on
Central Front End
Heel
(Outer)
End
Toe
(Inner)
End
