1.00 The following terms are basic and will be referred to in subsequent sections.

1.01 Gears are machine elements that transmit motion by means successively engaging teeth. (Figure 1.1)

1.02 A gear is any machine part with gear teeth. Of two gears that run together, the one with the larger number of teeth is called the gear. (Figure 1.1)

1.03 A pinion is a gear with a small number of teeth. Of two gears that run together, the one with the smaller number of teeth is called the pinion.

1.04 A rack is a gear with teeth spaced along a straight line, and suitable for straight-line motion. (Figure 1.1)

1.05 A worm is a gear with one or more teeth in the form of screw threads. (figure 1.2)

1.06 A wormgear is the mate to a worm. A wormgear that is completely conjugate to its worm has line contact and is said to be single enveloping. It is usually cut by a tool that is geometrically similar to the worm. An involute spur gear or helical used with a cylindrical worm has only point contact.

 

1.07 A helical gear is cylindrical in form and has helical teeth. (Figure 1.3)

1.08 Parallel helical gears operate on parallel axes and where both are external, the helices are of opposite hand. (Figure 1.3)

  

1.09 Crossed Helical gears operate on crossed axes and may have teeth of the same or opposite hand. The term crossed helical gears has superseded the old term "Spiral Gears." (Figure 1.4)

1.10 Bevel Gears are conical in form and operate on intersecting axes which are usually at right angles. (Figure 1.5a)

1.11 Miter gears are mating bevel gears with equal numbers of teeth and with axes at right angles. (Figure 1.5b)

1.12 Straight bevel gears have straight tooth elements, which if extended, would pass through the point of intersection of their axes. (Figure 1.5a)

1.13 Angular bevel gears are bevel gears in which the axes are not at right angles. (Figure 1.5c)

 

1.14 An internal gear is one with the teeth formed on the inner surface of a cylinder or cone. An internal gear can be meshed only with an external pinion. (Figure 1.6)

1.15 An external gear is one with the teeth formed on the outer surface of a cylinder or cone. (Figure 1.6)

 

1.16 The axial plane of a pair of gears is the plane that contains the two axes. In a single gear, an axial plane may be any plane containing the axis and a given point. (Figure 1.7)

1.17 The pitch plane of pair of gears is the plane perpendicular to the axial plane and tangent to the pitch surfaces. A pitch plane in an individual gear may be any plane tangent to its pitch surface. The pitch plane of a rack or crown gear is the pitch surface. (Figure 1.7)

1.18 A transverse plane is a perpendicular to the axial plane and to the pitch plane. In gears with parallel axes, the transverse plane and plane of rotation coincide. (Figure 1.7)

1.19 A plane of rotation is any plane perpendicular to a gear axis. (Figure 1.7)

  

1.20 A normal plane is in general normal to a tooth surface at a pitch point, and perpendicular to the pitch plane. (Figure 1.8)

1.21 A tangent plane is tangent to the tooth surfaces at a point or line of contact. (Figure 1.8)

1.22 Circular pitch is the distance along the pitch circle or pitch line between corresponding profiles of adjacent teeth. (Figure 1.9)

 

1.23 Normal circular pitch is the circular pitch in the normal plane, and is also the length of the arc along the normal helix between helical teeth or threads. (Figure 1.10)

1.24 Axial pitch is linear pitch in an axial plane and in a pitch surface. In helical gears and worms, axial pitch has the same value at all diameters. In gearing of other types, axial pitch may be confined to the pitch surface and may be a circular measurement. (Figure 1.10)

The term axial pitch is preferred to the term linear pitch. The axial pitch of a helical worm and the circular pitch of its wormgear are the same.

1.25 Transverse circular pitch is the circular pitch in the transverse plane. (Figure 1.10)

 

1.26 Addendum is the height by which a tooth projects beyond the pitch circle or pitch line; also, the radial distance between the pitch circle and the addendum circle. (Figure 1.11)

1.27 Dedendum is the depth of a tooth space below the pitch circle or pitch line; also the radial distance between the pitch circle and the root circle. (Figure 1.11)

1.28 Clearance is the amount by which the addendum in a given gear exceeds the addendum of its mating gear. (Figure 1.11)

1.29 Working depth is the depth of engagement of two gears; that is, the sum of their addendums. (Figure 1.11)

1.30 Whole depth is the total depth of a tooth space, equal to addendum plus dedendum, also equal to working depth plus clearance. (Figure 1.11)

1.31 Pitch diameter is the diameter of the pitch circle.

1.32 Outside diameter is the diameter of the addendum outside circle.

1.33 Backlash is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the operating pitch circles. (Figure 1.12)