Gear Types
External vs. internal gears
An external
gear is one with the teeth formed on the outer surface of a cylinder or
cone. Conversely, an internal gear is one with the teeth formed on the
inner surface of a cylinder or cone. For bevel gears, an internal gear is one
with the pitch angle exceeding 90 degrees. Internal gears do not cause
direction reversal.
Gear Design
Gear designs vary depending on the requirements for rotation speed, degree of
gear reduction and torque loading. Transmissions commonly use spur gears, while
hypoid gear designs are usually employed as the main gearing in differentials.
Common gear types include:
Spur
Spur (straight cut) gears are widely used in parallel shaft applications, such
as transmissions, due to their low cost and high efficiency. The design allows
for the entire gear tooth to make contact with the tooth face at the same
instant. As a result, this type of gearing tends to be subjected to high shock
loading and uneven motion. Design limitations include excessive noise and a
significant amount of backlash during high-speed operation.
Helical
Helical gears differ from spur gears in that their teeth are not parallel to the
shaft axis; they are cut in a helix or angle around the gear axis. During
rotation, parts of several teeth may be in mesh at the same time, which reduces
some of the loading characteristics of the standard spur gear. However, this
style of gearing can produce thrust forces parallel to the axis of the gear
shaft. To minimize the effects, two helical gears with teeth opposite each
other are utilized, which helps to cancel the thrust out during operation.
Double Helical (Herringbone)
Double helical gears, or herringbone gears, overcome the problem of
axial thrust presented by "single" helical gears, by having two sets
of teeth that are set in a V shape. A double helical gear can be thought of as
two mirrored helical gears joined together. This arrangement cancels out the
net axial thrust, since each half of the gear thrusts in the opposite
direction. However, double helical gears are more difficult to manufacture due
to their more complicated shape. Herringbone gears are capable of transmitting
large amounts of horsepower and are frequently used in power transmission
systems.
Bevel
Bevel gears (straight and spiral cut) transmit motion between shafts that are
at an angle to each other. Primarily found in various types of industrial
equipment as well as some automotive applications (differentials), they offer
efficient operation and are easy to manufacture. As with spur gears, they are
limited due to their noisy operation at high speeds, and are not the top choice
where load carrying capacity is a requirement. Bevel gears with equal numbers
of teeth and shaft axes at 90 degrees are called miter gears.
Hypoid
Hypoid gear sets are a form of bevel gears, but offer improved efficiency and
higher ratios over traditional straight bevel gears. Commonly found in axle differentials,
hypoid gears are used to transmit power from the driveline to the axle shafts.
Worm
Worm gear sets employ a specially-machined “worm” that conforms to the arc of
the driven gear. This type of design increases torque throughput,
improves accuracy and extends operating life. Primarily used to transmit power
through non-intersecting shafts, this style of gear is frequently found in gear
reduction boxes as they offer quiet operation and high ratios (as high as
100:1). Downfalls with this type of gear set are its efficiency, high price per
HP and low ratios (5:1 minimum).
Rack and pinion
A rack and pinion
is a type of linear actuator that comprises a pair of gears which convert
rotational motion into linear motion. A rack is a toothed bar or rod that can
be thought of as a sector gear with an infinitely large radius of curvature.
Torque can be converted to linear force by meshing a rack with a pinion: the
pinion turns; the rack moves in a straight line. Such a mechanism is used in
automobiles to convert the rotation of the steering wheel into the
left-to-right motion of the tie rod(s).
Planetary
Planetary gear sets, such as those found in automatic transmissions, provide
the different gear ratios needed to propel a vehicle in the desired direction
at the correct speed. Gear teeth remain in constant mesh, which allows for gear
changes to be made without engaging or disengaging the gears, as is required in
a manual transmission. Instead, clutches and bands are used to either hold or
release different members of the gear set to get the proper direction of
rotation and/or gear ratio.
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Rack and Pinion |
Planetary Gear |
Shifting of gears
In some machines
(e.g., automobiles) it is necessary to alter the gear ratio to suit the task, a
process known as gear shifting or changing gear. There are several ways of
shifting gears, for example:
- Manual transmission
- Automatic transmission
- Derailleur gears which are
actually sprockets in combination with a roller chain
- Hub gears (also called
epicyclic gearing or sun-and-planet gears)
There are several outcomes of gear shifting in motor vehicles. In the case of vehicle noise emissions, there are higher sound levels emitted when the vehicle is engaged in lower gears. The design life of the lower ratio gears is shorter, so cheaper gears may be used (i.e. spur for 1st and reverse) which tends to generate more noise due to smaller overlap ratio and a lower mesh stiffness etc. than the helical gears used for the high ratios.