The most detailed explanation of gear classification: detailed explanation of 17 kinds of gears

Cylindrical gear

Meshing cylindrical gear diagram and photo
The axes of the meshing cylindrical gears are parallel

A cylindrical gear is a gear whose indexing surface is cylindrical. It is a part with teeth engraved on the outer circumference or inner side of a cylinder or cylinder.

A cylindrical gear is generally a part that transmits rotational motion between two parallel shafts.

1. Spur gear (straight gear)

steel spur gear
spur gear shaft

Spur gears are cylindrical gears whose tooth lines are parallel and straight on the shaft.

It is a ubiquitous gear; it can also be said to be representative of gear, also called straight gear.

It comes in various shapes, including ones integrated with the shaft.

2. Helical gear

helical gear

Helical gears are cylindrical gears with a helical tooth line.

It is generally believed that it looks like a part that skews the teeth of a linear gear. But to be precise, its definition should be a part with a helix carved on the outer circumference of a cylinder.

3. Herringbone gear (double helical gear)

A herringbone gear is a combination of left and right helical gears.

Looking at the gear from the front (end face), the teeth of the gear are in the shape of a mountain. However, it becomes the mountain’s reflection when viewed from the opposite side, also known as double helical gear.

herringbone gear

4. Gear rack

gear rack

A rack is a part engraved with teeth of the same shape at equal intervals on one side of a flat plate or a straight rod.

You can think of it as part of a cylindrical gear with an indexing cylinder of infinite radius. Its tooth line has straight teeth and helical teeth.

The pinion gear that meshes with it is an ordinary cylindrical gear.

5. Internal gear

An internal gear is a gear with teeth machined on the inside of a cylinder or a cone.

So, in contrast to this, ordinary gears are external gears. The meshing object of the internal gear must be the outer gear. If its counterpart is an internal gear, it cannot mesh anyway. Moreover, the internal gear is usually a bull gear, and the external gear meshing with it is a pinion gear.

Because of the parts engraved with teeth on the inside of the cone, its matching gear is also a bevel gear, and its two axes are not parallel.

internal gear

Bevel gear

Bevel gears are conical gears that transmit motion between two intersecting shafts. It is similar to cutting on the top of an umbrella and is a gear with teeth carved on the outside bevel. “Bevel ” means slant, bevel, and oblique angle.

Any angle between the two intersecting axes is acceptable, but generally, they intersect perpendicularly. Because from the perspective of mechanical structure and processing, the right angle is the best, and the accuracy is the easiest to satisfy. The formation of right angles is also not limited to just two 45° angles.

1. Straight bevel gear

straight bevel gear set

Straight bevel gears are the simplest type of bevel gears. The tooth line of the pitch cone is radially linear, and each tooth line passes through the tip of the pitch cone. Its tooth direction is along the direction of the conical generatrix and gradually shrinks in proportion from the large end to the small end of the gear truncated cone, finally, at the intersection point where the space intersects the axes of the two-phase meshing gears.

Just like the relationship between spur gears and helical gears, it can be considered that the opposite of straight bevel gears is helical bevel gears.

2. Helical bevel gear

helical bevel gear set model

In the development diagram of the pitch bevel surface of the helical bevel gear, the pitch bevel tooth line is tangent to a circle concentric with the axis of the pitch bevel, so it is also called the tangential bevel gear. Its advantages over straight bevel gears are the same as those of helical spur gears over spur gears: less shock load and smoother operation. However, due to the low processing productivity of this kind of gear, and the performance is not as good as the spiral bevel gear that can be mass-produced with a unique high-efficiency machine tool, it is gradually replaced by the latter and is rarely used.

3. Spiral bevel gear

spiral bevel gear set

A bevel gear is a part that cuts teeth on an indexing cone, so its tooth top and tooth root are both conical. Therefore, the tooth height of a general bevel gear decreases gradually towards the top of the cone. On the other hand, there are also contour bevel gears whose outer and inner tooth heights are the same. The advantage of bevel gears is that the teeth are solid and easy to measure.

4. Zerol bevel gear

zerol bevel gear model

Zerol spiral bevel gear refers to the spiral bevel gear whose midpoint helix angle is zero degrees. It has the same curved teeth as a straight bevel gear. This gear is processed on a machine tool that processes ordinary spiral bevel gears (that is, circular arc bevel gears) whose midpoint helix angle is not zero, so it can have teeth that are in local (point) contact, just like ordinary spiral bevel gears. On the surface, this is where the zerol spiral bevel gear is superior to the general straight bevel gear. Zerol spiral bevel gears generate the same bearing loads as spur bevel gears. Therefore, the two-gear transmission pairs are interchangeable when their bearings are the same.

5. Miter bevel gear

miter gear

Miter bevel gear refers to two paired bevel gears with the same number of teeth intersecting vertically. The miter is a verb, and the noun form is miter, which means to keep leaning. Miter bevel gear can also be called mitre gear or mitre bevel gear.

6. Angular bevel gears

angular bevel gear

The oblique bevel gear refers to a pair of bevel gears that transmit motion between two shafts that are not perpendicular to each other. The so-called “not perpendicular” means that both angles are more significant than a right angle and smaller than a right angle.

Here is a more confusing concept: compared to the single gear of the bevel gear, the definition of the bevel gear includes two gears that are paired with each other. Because if it is just a gear, the angle (perhaps oblique) between its axis and the mating gear cannot be known.

7. Crown gear

crown gear

The crown gear is a bevel gear with a flat indexing surface. The teeth of the crown gear are distributed on the end face and are generally used in mechanical transmission and reversing clutches, and are also widely used in toys. It is named for its shape like a crown. The teeth of the crown gear are distributed on the end face and are generally used in mechanical transmission and reversing clutches, and are also widely used in toys.

Its advantages are: small size, compact structure, ample transmission torque, as long as the axial positioning part is slightly designed, it can “slip” when overloaded, and it has a self-protection effect.

Crossed-axis helical gears

Cross-axis gear is a general term for gears transmitting motion between disjoint and non-parallel shafts. When the two shafts are parallel, it is a cylindrical gear; if the two shafts intersect, it is a bevel gear. If neither of the above two is true, it is a cross-axis gear.

Crossed-axis helical gears in gearbox

1. Cross-axis helical gear set

cross-axis helical gear set

A cross-axis helical gear pair is a spur gear pair that transmits motion between staggered axes. It can be thought of as twisting the two parallel shafts of a cylindrical gear into a gear that is no longer parallel. So, the single one on one side is the spur gear.

2. Hypoid bevel gear set

hypoid bevel gear

A hypoid gear pair is a conical-shaped gear pair that transmits motion between alternating shafts. In a sense, it is a bevel gear with two disjoint axes. In terms of shape, it is a conical gear.

The gear with a considerable stagger distance between the two axes of the hypoid gear becomes an off-axis spiral crown wheel (face gear).

3. Worm gear pair

The worm gear pair is the general term for the gear pair formed by combining the worm and the worm gear meshing with it.

It can be regarded as a small gear similar to a cross-axis helical gear pair, and one of them is twisted forcefully. Twist one side hard, and the pinion becomes like a screw thread. The other gears are in rolling contact while it is in sliding contact. Because it is a threaded part, it must rotate in a spiral.

For other gears, even if the driving gear and the driven gear are reversed, they can still rotate. That is to say, a gear pair can turn no matter which side is used as the driving gear. However, only this worm pair, only the worm can play an active role, and the worm wheel will not actively rotate. Their two axes are approximately perpendicular.

worm gear pair

4. Worm shaft

worm shaft

A worm is a helical gear with one or more teeth.

Worm originally meant bug; maybe the worm’s name comes from the shape of this bug. To engage more with the worm gear, sometimes the worm is made into a “drum-shaped worm” that matches the outer peripheral arc of the worm gear.

5. Worm gear

worm gear

A worm gear is a gear that meshes with a worm. But this description does not make it clear; it does not explain the principle of the gear; only by looking at the photos or the real thing can we understand it. Worm gears have completely different teeth than other gears.

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