# Gear Types and Terminology

**1.1 Types of Gears**

**(1) Parallel Axes Gears**

**1 Spur Gear**

**2 Gear Rack**

**3 Internal Gear**

**4 Helical Gear**

**5 Helical Rack**

**6 Double Helical Gear**

**(2) Intersecting Axes**

**1 Straight Bevel Gear**

**2 Spiral Bevel Gear**

**3 Zerol Bevel Gear**

**(3) Nonparallel and Nonintersecting Axes Gears**

**1 Worm Gear Pair**

**2 Screw Gear (Crossed Helical Gear)**

**(4) Other Special Gears**

**1 Face Gear**

**2 Enveloping Gear Pair**

This gear is a slight deviation from a bevel gear that originated as a special development for the automobile industry. This permitted the drive to the rear axle to be nonintersecting, and thus allowed the auto body to be lowered. It looks very much like the spiral bevel gear. However, it is complicated to design and is the most difficult to produce on a bevel gear generator.

**1.2 Symbols and Terminology**

Symbols and technical words used in this catalog are listed in Table 1.2 to Table 1.4. The formerly used JIS B 0121 and JIS B 0102 Standards were revised to JIS B 0121:1999 and JIS B 0102:1999 conforming to the International Standard Organization (ISO) Standard. In accordance with the revision, we have unified the use of words and symbols conforming to the ISO standard.

Table 1.2 Linear and Circular Dimensions

Terms and Symbols

- Centre distance
- Reference pitch
- Transverse pitch
- Normal pitch
- Axial pitch
- Base pitch
- Transverse base pitch
- Normal base pitch
- Tooth depth
- Addendum
- Dedendum
- Chordal height
- Constant chord height
- Working depth
- Tooth thickness
- Normal tooth thickness
- Transverse tooth thickness
- Crest width
- Base thickness
- Chordal tooth thickness
- Constant chord
- Span measurement over k teeth
- Tooth space
- Tip and root clearance
- Circumferential backlash
- Normal backlash
- Radial backlash
- Axial backlash (Radial play) NOTE 1
- Angular backlash
- Facewidth
- Effective facewidth
- Lead
- Length of path of contact
- Length of approach path
- Length of recess path
- Overlap length
- Reference diameter
- Pitch diameter
- Tip diameter
- Base diameter
- Root diameter
- Center reference diameter
- Inner tip diameter
- Reference radius
- Pitch radius
- Tip radius
- Base radius
- Root radius
- Radius of curvature of tooth profile
- Cone distance
- Back cone distance

* NOTE 1.

“Axial backlash” is not a word defined by JIS.

Table 1.3 Angular Dimensions

**Terms and Symbols**

- Reference pressure angle
- Working pressure angle
- Cutter pressure angle
- Transverse pressure angle
- Normal pressure angle
- Axial pressure angle
- Transverse working pressure angle
- Tip pressure angle
- Normal working pressure angle
- Reference cylinder helix angle
- Pitch cylinder helix angle
- Mean spiral angle NOTE 2
- Tip cylinder helix angle
- Base cylinder helix angle
- Reference cylinder lead angle
- Pitch cylinder lead angle
- Tip cylinder lead angle
- Base cylinder lead angle
- Shaft angle
- Reference cone angle
- Pitch angle NOTE 3
- Tip angle NOTE 4
- Root angle NOTE 5
- Addendum angle
- Dedendum angle
- Transverse angle of transmission
- Overlap angle
- Total angle of transmission
- Tooth thickness half angle
- Tip tooth thickness half angle
- Spacewidth half angle
- Angular pitch of crown gear
- Involute function (Involute
*α*)

NOTE 2. The spiral angle of spiral bevel gears was defined as the helix angle by JIS B 0102

NOTE 3. This must be Pitch Angle, according to JIS B 0102.

NOTE 4. This must be Tip Angle, according to JIS B 0102.

NOTE 5. This must be Root Angle, according to JIS B 0102.

Table 1.4 Others

**Terms and Symbols**

- Number of teeth
- Equivalent number of teeth
- Number of threads, or number of teeth in pinion
- Gear ratio
- Transmission ratio
- Module
- Transverse module
- Normal module
- Axial module
- Diametral pitch
- Transverse contact ratio
- Overlap ratio
- Total contact ratio
- Angular speed
- Tangential speed
- Rotational speed
- Profile shift coefficient
- Normal profile shift coefficient
- Transverse profile shift coefficient
- Center distance modification coefficient
- Tangential force (Circumference)
- Axial force (Thrust)
- Radial force
- Pin diameter
- Ideal pin diameter
- Measurement over rollers (pin)
- Pressure angle at pin center
- Coefficient of friction
- Circular thickness factor
- Single pitch deviation
- Pitch deviation
- Total cumulative pitch deviation
- Total profile deviation
- Runout
- Total helix deviation

A numerical subscript is used to distinguish “pinion” from “gear” (Example z1 and z2), “worm” from “worm wheel”, “drive gear” from “driven gear”, and so forth. (To find an example, see next page Fig. 2.1).

Table 1.5 indicates the Greek alphabet, the international phonetic alphabet.

Table 1.5 The Greek alphabet

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