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Gear Types

A gear train consists of one or more gear sets intended to give a specific velocity ratio, or

change direction of motion. Gear and gear train types can be grouped based on theirapplication and tooth geometry.

Table I: Gear Types Grouped According to Shaft Arrangement

Parallel Axes Intersecting AxesNon-Intersecting

(Non-parallel) Axes

Rotary to

Translation

Spur Gears Bevel gears: Hypoid gears Rack and Pinion

Helical Gears Straight bevel Crossed helical gears

Herringbone or

double helical gearsZerol bevel Worm gears

Spiral bevel

Spur gears (Fig. 1): Spur gears connect parallel shafts, have involute teeth that areparallel to the shafts, and can have either internal or external teeth. Notes:

1. Spur gears are inexpensive to manufacture.

2. They cause no axial thrust between gears.3. They give lower performance, but may be satisfactory in low speed or simple

applications

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Helical gears (Fig. 2): Helical gears also connect parallel shafts, but the involute teethare cut at an angle (called the helix angle) to the axis of rotation. Note that two mating

helical gears must have equal helix angle but opposite hand. These are found in

automotive transmissions, and any application requiring high speed rotation and goodperformance. Notes:

1. Helical gears run smoother and more quietly than spurs (due to continuous tooth

mating).2. They have a higher load capacity (teeth have a greater cross section).

3. They are more expensive to manufacture.

4. Helical gears create axial thrust.

Herringbone gears (Fig. 3): To avoid axial thrust, two helical gears of opposite handcan be mounted side by side, to cancel resulting thrust forces. These are called double

helical or herringbone gears

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Bevel gears (Fig. 4): Bevel gears connect intersecting axes, and come in several types(listed below). For bevel gears, the pitch surface is a cone, (it was a cylinder in spur and

helical gears) and mating spiral gears can be modeled as two cones in rolling contact.Types of bevel gears:

1. Straight bevel: These are like spur gears, the teeth have no helix angle. Straight

bevel gears can be

a. Miter gears, equal size gears with a 90 degree shaft angle,

b. Angular bevel gears, shaft angle other than 90 degrees, orc. Crown gears, one gear is flat, has a pitch angle of 90 degree.

2. Spiral bevel gears(Fig. 4a): Teeth have a spiral angle which gives performanceimprovements much like helical gears

3. Zerol bevel gears (Fig. 4b): Teeth are crowned, so that tooth contact takes place

first at the tooth center.

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Hypoid gears (Fig. 5): Similar to spiral bevel gears, but connect non-parallel shafts thatdo not intersect. The pitch surface of a hypoid gear is a hyperboloid of revolution (rather

than a cone, the pitch surface in bevel gears), hence the name. Hypoid pinions (the

smaller driving gear) are stronger than spiral bevel pinions because the helix angle of thepinion is larger than that of the gear. Hypoid gears are stronger, operate more quietly, and

can be used for higher reduction ratios than spiral bevel gears. They also have slidingaction along the teeth, potentially reducing efficiency. Hypoid gears are found in autodifferentials. I also know that a hypoid gear set is used in my NH baler, connecting the

flywheel to the rear driveshaft. As far as I can tell, John Deere balers do not make use of

technology, which explains why they have not been able to develop a satisfactory line offorage harvesting equipment.

Crossed helical gears (Fig. 6): Helical gears that connect skew shafts. The teeth have

sliding motion and therefore lower efficiency. One application is connecting distributer tocam shaft in pre-electronic ignition vehicles.

Worm Gears (Fig. 7): The driving gear is called a worm, and typically has 1, 2, or fourteeth. The low number of teeth on the worm can result in a very large velocity ratio.

These can also be designed to be non-backdriveable, and can carry high loads. Because of

sliding action, efficiency is low.

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Rack and Pinion (Fig. 8): These transmit rotary motion (from the pinion) totranslational motion (of the rack). The rack is a gear with infinite radius; its teeth,although flat sided, are involute. The rack and pinion is commonly used in steering units

and jacks.

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