The straight bevel gear has straight teeth flanks which are along the surface of the pitch cone from the bottom to the apex. Straight bevel gears can be grouped into the Gleason type and the standard type.
In this section, we discuss the Gleason straight bevel gear. The Gleason Company defined the tooth profile as: whole depth h = 2.188m; top clearance Ca = 0.188m; and working depth hw = 2.000m.
The characteristics are:
¥ Design specified profile shifted gears:
In the Gleason system, the pinion is positive shifted and the gear is negative shifted. The reason is to distribute the proper strength between the two gears. Miter gears, thus, do not need any shifted tooth profile.
¥ The top clearance is designed to be parallel
The outer cone elements of two paired bevel gears are parallel. That is to ensure that the top clearance along the whole tooth is the same. For the standard bevel gears, top clearance is variable. It is smaller at the toe and bigger at the heel.
Table 8-1 shows the minimum number of teeth to prevent undercut in the Gleason system at the shaft angle S = 90º.
Table 8-2 presents equations for designing straight bevel gears in the Gleason system. The meanings of the dimensions and angles are shown in Figure 8-9. All the equations in Table 8-2 can also be applied to bevel gears with any shaft angle.
The straight bevel gear with crowning in the Gleason system is called a Coniflex gear. It is manufactured by a special Gleason "Coniflex" machine. It can successfully eliminate poor tooth wear due to improper mounting and assembly.
The first characteristic of a Gleason straight bevel gear is its profile shifted tooth. From Figure 8-10, we can see the positive tooth profile shift in the pinion. The tooth thickness at the root diameter of a Gleason pinion is larger than that of a standard straight bevel gear.
Table 8-1 The Minimum Numbers of Teeth to Prevent Undercut
| Pressure Angle |
Combination
of Numbers of Teeth Z1 Z2 |
|||||
| (14.5º) | 29/over 29 | 28/Over 29 | 27/Over 31 | 26/Over 35 | 25/Over 40 | 24/Over 57 |
| 20º | 16/Over 16 | 15/Over 17 | 14/Over 20 | 13/Over 30 | - | - |
| (25º) | 13/Over 13 | - | - | - | - | - |
8.5.2. Standard
Straight Bevel Gears
A bevel gear with no profile shifted tooth is a standard
straight bevel gear. The applicable equations are in Table 8-3.
These equations can also be applied to bevel gear sets with
other than 90º shaft angle.
8.5.3 Gleason Spiral Bevel Gears
A spiral bevel gear is one with a spiral tooth flank as
in Figure 8-11. The spiral is generally consistent with the curve
of a cutter with the diameter dc The spiral angle b
is the angle between a generatrix element of the pitch cone and the tooth
flank. The spiral angle just at the tooth flank center is called central
spiral angle bm
In practice, spiral angle means central spiral angle.
All equations in Table 8-6 are dedicated for the
manufacturing method of Spread Blade or of Single Side from Gleason. If a
gear is not cut per the Gleason system, the equations will be different
from these.
The tooth profile of a Gleason spiral bevel gear shown here
has the whole depth h= 1.888m; top clearance Ca= 0.188m; and
working depth hw = 1.700m. These Gleason spiral bevel gears
belong to a stub gear system. This is applicable to gears with m>2.1
Table 8-4 shows the minimum number of teeth to avoid
undercut in the Gleason system with shaft angle S
= 90º and pressure angle an = 20º.
If the number of teeth is less than 12, Table 8-5 is used to
determine the gear sizes.
All equations in Table 8-6 are also applicable to
Gleason bevel gears with any shaft angle. A spiral bevel gear set requires
matching of hands; left-hand and right-hand as a pair.
