Gears are identified by many types and there are many specific technical words to describe their definition. This section introduces those technical words along with commonly used gears and their features.

1.1 Types of Gears

The most common way to classify gears is by category type and by the orientation of axes.
Gears are classified into 3 categories; parallel axes gears, intersecting axes gears, and nonparallel and nonintersecting axes gears.
Spur gears and helical gears are parallel axes gears. Bevel gears are intersecting axes gears. Screw or crossed helical, worm gear and hypoid gears belong to the third category. Table 1.1 lists the gear types by axes orientation.

Table 1.1 Types of Gears and Their Categories

  • Categories of Gears
    Parallel AxesGears
  • Types of Gears
    Spur Gear
    Spur rack
    Internal gear
    Helical gear
    Helical rack
    Double helical gear
  • Efficiency (%)
    98.0 – 99.5

 

  • Categories of Gears
    Intersecting AxesGears
  • Types of Gears
    Straight bevel gear
    Spiral bevel gear
    Zerol bevel gear
  • Efficiency (%)
    98.0 – 99.0

 

  • Categories of Gears
    Nonparallel and Nonintersecting
  • Types of Gears
    Screw gear (Efficiency 70.0 – 95.0 %)
    Worm gear (Efficiency 30.0 – 90.0 %)

 

Also, included in table 1.1 is the theoretical efficiency range of various gear types. These figures do not include bearing and lubricant losses.

Since meshing of paired parallel axis gears or intersecting axis gears involves simple rolling movements, they produce relatively minimal slippage and their efficiency is high.
Nonparallel and nonintersecting gears, such as screw gears or worm gears, rotate with relative slippage and by power transmission, which tends to produce friction and makes the efficiency lower when compared to other types of gears.
Efficiency of gears is the value obtained when the gears are installed and working accurately. Particularly for bevel gears, it is assumed that the efficiency will decrease if improperly mounted from off-position on the cone-top.

(1) Parallel Axes Gears

1 Spur Gear
Fig.1.1 Spur Gear
Fig. 1.1 Spur Gear
This is a cylindrical shaped gear, in which the teeth are parallel to the axis. It is the most commonly used gear with a wide range of applications and is the easiest to manufacture.

2 Gear Rack
Fig.1.2 Spur Rack
Fig. 1.2 Gear Rack
This is a linear shaped gear which can mesh with a spur gear with any number of teeth. The gear rack is a portion of a spur gear with an infinite radius.

3 Internal Gear
Fig.1.3 Internal Gear and Spur Gear
Fig. 1.3 Internal Gear and Spur Gear
This is a cylindrical shaped gear, but with the teeth inside the circular ring. It can mesh with a spur gear. Internal gears are often used in planetary gear systems.

4 Helical Gear
Fig.1.4 Helical Gear
Fig. 1.4 Helical Gear
This is a cylindrical shaped gear with helicoid teeth. Helical gears can bear more load than spur gears, and work more quietly. They are widely used in industry. A disadvantage is the axial thrust force caused by the helix form.

5 Helical Rack
Fig.1.5 Helical Rack
Fig. 1.5 Helical Rack
This is a linear shaped gear that meshes with a helical gear. A Helical Rack can be regarded as a portion of a helical gear with infinite radius.

6 Double Helical Gear
Fig1.6 Double Helical Gear
Fig1.6 Double Helical Gear
A gear with both left-hand and right-hand helical teeth. The double helical form balances the inherent thrust forces.

(2) Intersecting Axes

1 Straight Bevel Gear
Fig.1.7 Straight Bevel Gear
Fig.1.7 Straight Bevel Gear
This is a gear in which the teeth have tapered conical elements that have the same direction as the pitch cone base line (generatrix). The straight bevel gear is both the simplest to produce and the most widely applied in the bevel gear family.

2 Spiral Bevel Gear
Fig.1.8 Spiral Bevel Gear
Fig.1.8 Spiral Bevel Gear
This is a bevel gear with a helical angle of spiral teeth. It is much more complex to manufacture, but offers higher strength and less noise.

3 Zerol Bevel Gear
Fig.1.9 Zerol Bevel Gear
Fig.1.9 Zerol Bevel Gear
This is a special type of spiral bevel gear, where the spiral angle is zero degree. It has the characteristics of both the straight and spiral bevel gears. The forces acting upon the tooth are the same as for a straight bevel gear.

(3) Nonparallel and Nonintersecting Axes Gears

1 Worm Gear Pair
Fig.1.10 Worm Gear pair
Fig.1.10 Worm Gear pair
Worm gear pair is the name for a meshed worm and worm wheel. An outstanding feature is that it offers a very large gear ratio in a single mesh. It also provides quiet and smooth action. However, transmission efficiency is poor.

2 Screw Gear (Crossed Helical Gear)
Fig.1.11 Screw Gear
Fig.1.11 Screw Gear
A pair of cylindrical gears used to drive non-parallel and nonintersecting shafts where the teeth of one or both members of the pair are of screw form. Screw gears are used in the combination of screw gear / screw gear, or screw gear / spur gear. Screw gears assure smooth, quiet operation. However, they are not suitable for transmission of high horsepower.

(4) Other Special Gears

1 Face Gear
Fig.1.12 Face Gear
Fig.1.12 Face Gear
A pseudo bevel gear that is limited to 90° intersecting axes. The face gear is a circular disc with a ring of teeth cut in its side face; hence the name Face Gear.

2 Enveloping Gear Pair
Fig.1.13 Enveloping Gear Pair
Fig.1.13 Enveloping Gear Pair
This worm set uses a special worm shape that partially envelops the worm gear as viewed in the direction of the worm gear axis. Its big advantage over the standard worm is much higher load capacity. However, the worm gear is very complicated to design and produce.

3 Hypoid Gear
Fig.1.14 Hypoid Gear
Fig.1.14 Hypoid Gear
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 The symbol of Centre distance
  • Reference pitch The symbol of Reference pitch
  • Transverse pitch The symbol of Transverse pitch
  • Normal pitch The symbol of Normal pitch
  • Axial pitch The symbol of Axial pitch
  • Base pitch The symbol of Base pitch
  • Transverse base pitch The symbol of Transverse base pitch
  • Normal base pitch The symbol of Normal base pitch
  • Tooth depth The symbol of Normal base pitch
  • Addendum The symbol of Addendum
  • Dedendum The symbol of Dedendum
  • Chordal height The symbol of Chordal height
  • Constant chord height The symbol of Constant chord height
  • Working depth The symbol of Working depth
  • Tooth thickness The symbol of Tooth thickness
  • Normal tooth thickness The symbol of Normal tooth thickness
  • Transverse tooth thickness The symbol of Transverse tooth thickness
  • Crest width The symbol of Crest width
  • Base thickness The symbol of Base thickness
  • Chordal tooth thickness The symbol of Chordal tooth thickness
  • Constant chord The symbol of Constant chord
  • Span measurement over k teeth The symbol of Span measurement over k teeth
  • Tooth space The symbol of Tooth space
  • Tip and root clearance The symbol of Tip and root clearance
  • Circumferential backlash The symbol of Circumferential backlash
  • Normal backlash The symbol of Normal backlash
  • Radial backlash The symbol of Radial backlash
  • Axial backlash (Radial play) NOTE 1 The symbol of Axial backlash (Radial play)
  • Angular backlash The symbol of Angular backlash
  • Facewidth The symbol of Facewidth
  • Effective facewidth The symbol of Effective facewidth
  • Lead The symbol of Lead
  • Length of path of contact The symbol of Length of path of contact
  • Length of approach path The symbol of Length of approach path
  • Length of recess path The symbol of Length of recess path
  • Overlap length The symbol of Overlap length
  • Reference diameter The symbol of Reference diameter
  • Pitch diameter The symbol of Pitch diameter
  • Tip diameter The symbol of Tip diameter
  • Base diameter The symbol of Base diameter
  • Root diameter The symbol of Root diameter
  • Center reference diameter The symbol of Center reference diameter
  • Inner tip diameter The symbol of Inner tip diameter
  • Reference radiusThe symbol of Reference radius
  • Pitch radius The symbol of Pitch radius
  • Tip radius The symbol of Tip radius
  • Base radius The symbol of Base radius
  • Root radius The symbol of Root radius
  • Radius of curvature of tooth profile The symbol of Radius of curvature of tooth profile
  • Cone distance The symbol of Cone distance
  • Back cone distance The symbol of 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 The symbol of Reference pressure angle
  • Working pressure angle The symbol of Working pressure angle
  • Cutter pressure angleThe symbol of Cutter pressure angle
  • Transverse pressure angle The symbol of Transverse pressure angle
  • Normal pressure angle The symbol of Normal pressure angle
  • Axial pressure angle The symbol of Axial pressure angle
  • Transverse working pressure angle The symbol of Transverse working pressure angle
  • Tip pressure angle The symbol of Tip pressure angle
  • Normal working pressure angle The symbol of Normal pressure angle
  • Reference cylinder helix angle The symbol of Reference cylinder helix angle
  • Pitch cylinder helix angle The symbol of Pitch cylinder helix angle
  • Mean spiral angle NOTE 2 The symbol of Mean spiral angle
  • Tip cylinder helix angle The symbol of Tip cylinder helix angle
  • Base cylinder helix angle The symbol of Base cylinder helix angle
  • Reference cylinder lead angle The symbol of Reference cylinder lead angle
  • Pitch cylinder lead angle The symbol of Pitch cylinder lead angle
  • Tip cylinder lead angle The symbol of Tip cylinder lead angle
  • Base cylinder lead angle The symbol of Base cylinder lead angle
  • Shaft angle The symbol of Shaft angle
  • Reference cone angle The symbol of Reference cone angle
  • Pitch angle NOTE 3 The symbol of Pitch angle
  • Tip angle NOTE 4 The symbol of Tip angle
  • Root angle NOTE 5 The symbol of Root angle
  • Addendum angle The symbol of Addendum angle
  • Dedendum angle The symbol of Dedendum angle
  • Transverse angle of transmission The symbol of Transverse angle of transmission
  • Overlap angle The symbol of Overlap angle
  • Total angle of transmission The symbol of Total angle of transmission
  • Tooth thickness half angle The symbol of Tooth thickness half angle
  • Tip tooth thickness half angle The symbol of Tip tooth thickness half angle
  • Spacewidth half angle The symbol of Spacewidth half angle
  • Angular pitch of crown gear The symbol of Angular pitch of crown gear
  • Involute function (Involute α The symbol of 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 Symbol of Number of teeth
  • Equivalent number of teeth Symbol of Equivalent number of teeth
  • Number of threads, or number of teeth in pinion Symbol of Number of threads, or number of teeth in pinion
  • Gear ratio Symbol of Gear ratio
  • Transmission ratio Symbol of Transmission ratio
  • Module Symbol of Module
  • Transverse module Symbol of Transverse module
  • Normal module Symbol of Normal module
  • Axial module Symbol of Axial module
  • Diametral pitch Symbol of Diametral pitch
  • Transverse contact ratio symbol of Transverse contact ratio
  • Overlap ratio symbol of Overlap ratio
  • Total contact ratio symbol of Total contact ratio
  • Angular speed symbol of Angular speed
  • Tangential speed symbol of Tangential speed
  • Rotational speed symbol of Rotational speed
  • Profile shift coefficient symbol of Profile shift coefficient
  • Normal profile shift coefficient symbol of Normal profile shift coefficient
  • Transverse profile shift coefficient symbol of Transverse profile shift coefficient
  • Center distance modification coefficient symbol of Center distance modification coefficient
  • Tangential force (Circumference) symbol of Tangential force (Circumference)
  • Axial force (Thrust) symbol of Axial force (Thrust)
  • Radial force symbol of Radial force
  • Pin diameter symbol of Pin diameter
  • Ideal pin diameter symbol of Ideal pin diameter
  • Measurement over rollers (pin) symbol of Measurement over rollers (pin)
  • Pressure angle at pin center symbol of Pressure angle at pin center
  • Coefficient of friction symbol of Coefficient of friction
  • Circular thickness factor symbol of Circular thickness factor
  • Single pitch deviation symbol of Single pitch deviation
  • Pitch deviation symbol of Pitch deviation
  • Total cumulative pitch deviation symbol of Total cumulative pitch deviation
  • Total profile deviation symbol of Total profile deviation
  • Runout symbol of Runout
  • Total helix deviation symbol of 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
Table 1.5 The Greek alphabet

Related links :
Gear Types and Characteristics – A page of The ABCS of Gears – B
Basic Gear Terminology and Calculation – A page of The ABCS of Gears – B
Types of Gears – A page of Introduction to Gears
Characteristics of Gears – A page of Introduction to Gears
Gear Terminology – A page of Introduction to Gears