1-1 Types of Gears

There are many types of gears such as spur gears, helical gears, bevel gears, worm gears, gear rack, etc. These can be broadly classified by looking at the positions of axes such as parallel shafts, intersecting shafts and non-intersecting shafts.

It is necessary to accurately understand the differences among gear types to accomplish necessary force transmission in mechanical designs. Even after choosing the general type, it is important to consider factors such as: dimensions (module, number of teeth, helix angle, face width, etc.), standard of precision grade (ISO, AGMA, DIN), need for teeth grinding and/or heat treating, allowable torque and efficiency, etc.

Besides this page, we present more thorough gear technical information under Gear Knowledge (separate PDF page). In addition to the list below, each section such as worm gear, rack and pinion, bevel gear, etc. has its own additional explanation regarding the respective gear type. If it is difficult to view PDF, please consult these sections.

It is best to start with the general knowledge of the types of gears as shown below. But in addition to these, there are other types such as face gear, herringbone gear (double helical gear), crown gear, hypoid gear, etc.

button of gear selector

Spur Gear
The most common and easy to produce parallel shaft cylindrical gears. Of a pair of gears, the larger one is called a gear and the smaller one a pinion.
spur gear

Helical Gear
Quiet and able to transmit larger torque than spur gears. Cylindrical gears with spiral shaped tooth trace.
helical gear

Gear Rack
Changes rotary motion to linear motion. A set consisting of rectangular or circular rod shaped gear with mating small gear.
gear rack

Bevel Gear
Cone shaped gears used in intersecting shaft applications. There are also bevel gears with spiral shaped tooth trace called spiral bevel gears.
bevel gear

Spiral Bevel Gear
Cone shaped gears used in intersecting shaft applications. There are also bevel gears with straight shaped tooth trace called straight bevel gears.
spiral bevel gear

Screw Gear
Used in offset shaft application. Shape wise, they are the same as helical gears.
screw gear

Miter Gear
Type of bevel gears in which the pair is made of same number of teeth and used where speed reduction or increase is not needed.
miter gear

Worm Gear
Used when a large speed reduction is needed. Worm and worm gear set. Normally, different materials are used for worm and worm gear.
worm gear

Internal gear
Gear teeth are cut on the inside surface of hollow cylindrical forms and used in planetary gear systems. The gear teeth are cut using gear shaper machines.
internal gear

overview of gears

(Important Gear Terminology and Gear Nomenclature in this picture)

  • Worm
  • Worm wheel
  • Internal gear
  • Gear coupling
  • Screw gear
  • Involute spline shafts and bushings
  • Miter gear
  • Spur gear
  • Helical gear
  • Ratchet
  • Pawl
  • Rack
  • Pinion
  • Straight bevel gear
  • Spiral bevel gear

There are three major categories of gears in accordance with the orientation of their axes

Configuration :

  1. Parallel Axes / Spur Gear, Helical Gear, Gear Rack, Internal Gear
  2. Intersecting Axes / Miter Gear, Straight Bevel Gear, Spiral Bevel Gear
  3. Nonparallel, Nonintersecting Axes / Screw Gear, Worm, Worm Gear (Worm Wheel)
  4. Others / Involute Spline Shaft and Bushing, Gear Coupling, Pawl and Ratchet

The difference between a gear and a sprocket

Simply said, a gear meshes with another gear while a sprocket meshes with a chain and is not a gear. Aside from a sprocket, an item that looks somewhat like a gear is a ratchet, but its motiion is limited to one direction.

Classification of types of gears from the point of positional relations of the attached shafts

  1. When the gears’ two shafts are parallel (parallel shafts)
    Spur gear, rack, internal gear and helical gear, etc.
    Generally they have a high transmission efficiency.
  2. When the gears’ two shafts intersect each other (intersecting shafts)
    Bevel gear is in this category.
    Generally they have a high transmission efficiency.
  3. When the gears’ two shafts are not parallel or intersect (offset shafts)
    Worm gear and screw gear belong in this group.
    Because of the sliding contact, the transmission efficiency is relatively low.

Precision class of gears

When a type of gears is grouped by accuracy, precision class is used. The precision class is specified by the standards set by ISO, DIN, JIS, AGMA, etc. For example, JIS specifies each precision class’ pitch error, tooth profile error, helix deviation, runout error, etc.

Existence of teeth grinding

Existence of teeth grinding greatly affects the performance of gears. Therefore, in considering types of gears, teeth grinding is an important elememt to consider. Grinding the teeth surface makes gears quieter, increases force transmission capacity and affects the precision class. On the other hand, the addition of teeth grinding process increases cost and is not suitable for all gears. To obtain high precision other than by grinding, there is a process called shaving using shaving cutters.

Kinds of tooth shape

To broadly classify types of gears by their tooth shape, there are involute tooth shape, cycloid tooth shape and trochoid tooth shape. Among these, involute tooth shape is most commonly used. They are easy to produce and has the characteristic of being able to correctly mesh even when the center distance is slightly off. Cycloid tooth shape is mostly used in clocks and trochoid tooth shape is mainly in pumps.

Creation of Gears

This article is reproduced with the permission.
Masao Kubota, Haguruma Nyumon, Tokyo : Ohmsha, Ltd., 1963.

Gears are wheels with teeth and are sometimes called toothed wheels.

Gears are mechanical components that transmit rotation and power from one shaft to another, if each shaft possesses appropriately shaped projections (teeth) equally spaced around its circumference such that as it rotates, the successive tooth goes into the space between the teeth of the other shaft. Thus, it is a machine component in which the rotary power is transmitted by the prime mover’s tooth surface pushing the tooth surface of the driven shaft. As an extreme case, when one side is a linear motion (this can be thought as rotational motion around an infinite point), it is called a rack.

There are many ways to transmit rotation and power from one shaft to another such as by rolling friction, wrapping transmission, etc. However, in spite of a simple structure and a relatively small size, gears have many advantages such as certainty of transmission, accurate angular speed ratio, long lasting and minimal loss of power.

From small clocks and precision measuring instruments (motion transmission applications) to large gears used in marine transmission systems (power transmission applications), gears are used widely and are ranked as one of the important mechanical components along with screws and bearings.

There are many types of gears. However, the simplest and most commonly used gears are the ones used to transmit specific speed ratio between two parallel shafts at a defined distance. In particular, gears with their teeth parallel to the shafts as shown in Figure 1.1 called spur gears are the most popular.

spur gears
[Figure 1.1  Spur Gears]

The simplest method to transmit specific angular speed ratio between two parallel shafts is a rolling friction drive. This is accomplished as shown in Figure 1.2, by having two cylinders, with diameters in inverse ratio to the speed ratio, in contact and rotating without slippage (if two shafts are counter rotating, contact is on the outside; and if rotating in the same direction, contact is on the inside). That is to say that the rotation is obtained from the friction force of the rolling contact. However, it is impossible to avoid some slippage and, as a result, reliable transmission cannot be hoped for. To get a larger power transmission requires heavier contact forces which in turn result in high bearing loads. For these reasons, this arrangement is not suitable for transmitting large amount of power. As a result, an idea to create suitable form of teeth equally spaced on the rolling surfaces of the cylinders in such a way that at least one pair or more of teeth are always in contact was invented. By pushing the teeth of the trailing shaft with the teeth of the driving shaft, the certainty of a strong transmission is assured. This is called a cylindrical gear and the reference cylinder on which the teeth are carved is the pitch cylinder. Spur gears are one type of cylindrical gears.

Pitch Cylinders
[Figure 1.2 Pitch Cylinders]

When two shafts intersect, the references for carving teeth are the cones in rolling contact. These are the bevel gears as shown in Figure 1.3 where the base cone on which teeth are carved is called the pitch cone. (Figure 1.4).

bevel gears
[Figure 1.3  Bevel Gears]

pitch cones
[Figure 1.4  Pitch Cones]

When the two shafts are not parallel and non-intersecting, there are no true rolling contacting curved surfaces. Based on the type of gears, teeth are created on a pair of reference contacting rotating surfaces. In all cases, it is necessary to set the tooth profile such that the relative motion of the contacting pitch surfaces matches the relative motion of the meshing of the teeth on the reference curved surfaces.

When gears are considered as rigid bodies, in order for two bodies to maintain a set angular speed ratio while in contact at teeth surfaces, without running into each other or separating, it is necessary for the common normal components of speed of the of the two gears at the contact point to be equal. In other words, at that instant, there is no relative motion of the gear surfaces in the direction of the common normal, and the relative motion exists only along the contact surface at the point of contact. This relative motion is nothing but the sliding of gear surfaces. The tooth surfaces, with the exception of special points, always involve the so-called sliding contact transmission.

In order for the tooth forms to satisfy the conditions as explained above, utilization of the enveloping surface can lead to the desired tooth form as a general method.

Now, specify one side of gear A’s surface as a curved surface FA, and give both gears a specified relative rotation. Then on the coordinate system attached to gear B, a group of successive positions of the gear surface FA is drawn. Now think of the envelope of this group of curves and use it as the tooth surface FB of gear B. Then from the theory of envelope surfaces, it is clear that the two gear surfaces are in constant line contact and the two gears will have the desired relative motion.

It is also possible to lead to tooth forms by the following method. Consider, in addition to a pair of gears A and B with specified relative motion, a third imaginary gear C in mesh where A and B are in mesh and give it an arbitrary tooth form surface FC (curved surface only without tooth body) and an appropriate relative motion.

Now, using the method as before, from the imagined meshing of gear A with the imaginary gear C, obtain the tooth form FA as the envelope of tooth form FC. Designate the contact line of tooth surfaces FA and FC as IAC. Similarly, obtain the contact line IBC and tooth surface FB from the imaginary meshing of gear B and the imaginary gear C. Thus, the tooth surfaces FA and FB are obtained by the mediation of FC. In this case, if the contact lines IAC and IBC match, gears A and B are in line contact, and if IAC and IBC intersect, gears A and B will have a point contact at that intersection.

That means, with this method, it is possible to lead to point contact tooth forms as well as line contact tooth forms.

However, there are limits to geometrically obtained tooth forms as explained above, especially when the tooth bodies of surfaces FA and FB invade each other, or when those areas cannot be used as tooth forms. This invasion of one tooth body into another is called interference of tooth profiles.

As clear from the above explanation, there are theoretically many ways to produce tooth forms which create specified relative motion. However, in reality, consideration for the gear mesh, tooth form strength and difficulties of tooth cutting will limit the usage of these kinds of tooth forms to just a few.

Related Links :
Gear Types and Characteristics – A page of The ABCS of Gears – B
Gear Types and Terminology – A page of Gear Technical Reference
齿轮类型 – 中文页
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