Gear, Types of Gears & Gear Terminology
What is a Spur Gear?
A spur gear is one of the simplest and most common types of cylindrical gears. Spur gears have straight teeth that run parallel to the shaft.
These gears are easy to manufacture and can be used in a variety of applications. These applications include speed increase or reduction, torque multiplication, and enhancing accuracy for positioning systems.
In this blog, we are going to define spur gear terminology and provide formulas for determining the values of these terms.
(1) Parallel Axes Gears
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 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
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
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
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
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
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
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
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
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
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
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
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
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.Spur Gears – Terms, Definitions, and Calculations
The following terms are related to spur gears:
Addendum: The height of the tooth above the pitch circle.
Backlash: The clearance between two mating teeth of separate gears.
Base circle: A theoretical circle used to generate the involute curve when creating tooth profiles.
Center distance: The distance between the center shafts of two gears.
Chordal addendum: The distance between a chord, passing through the points where the pitch circle crosses the tooth profile and the tooth top.
Chordal thickness: Tooth thickness measured along a chord passing through the points where the pitch circle crosses the tooth profile.
Circular pitch: Measurement of the pitch circle arc length from one point on a tooth to the same point on the adjacent tooth.
Circular thickness: The thickness of the tooth at the pitch circle.
Clearance: The space between one gears minor diameter and the mating gears major diameter.
Dedendum: Depth of the tooth between the pitch circle and the minor diameter.
Diametral pitch: The number of teeth per inch of pitch diameter.
Fillet: The small radius that connects the tooth profile to the root circle.
Module: Teeth per millimeter of pitch diameter.
Outside diameter: The major diameter of the gear.
Pinion: The smaller-sized gear in any meshed pair.
Pitch circle: The circle, the radius of which is equal to the distance from the center of the gear to the pitch point. This is where the gear’s speed is measured.
Pitch diameter: Diameter of the pitch circle.
Pitch point: The point of tangency of the pitch circles of a pair of mating gears.
Pressure angle: The angle between the line of action and a line perpendicular to the line of centers.
Root (or dedendum) circle: The minor diameter of the tooth.
Velocity ratio: Ratio of input gear revolutions to output gear revolutions within a specified amount of time.
Whole depth: The height of the tooth from major diameter to minor diameter of a gear.
Working Depth: The depth to which a tooth extends into the space between teeth on the mating gear.
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