MECHANICAL GEAR TYPE

 

Mechanical Engineering Gear 

Creation of Gears

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

What is a gear ?

A gear is a kind of machine element in which teeth are cut around cylindrical or cone shaped surfaces with equal spacing. If it is difficult to view PDF, please consult these sections.

Types of Gears

• Spur gear

• Helical gear

• Ratchet

• Pawl

• Rack

• Pinion

• Straight bevel gear

• Spiral bevel gear

• Worm

• Worm wheel

• Internal gear

• Gear coupling

• Screw gear

• Involute spline shafts and bushings

• Miter gear

There are many types of gears such as spur gears, helical gears, bevel gears, worm gears, gear rack, etc.

 Gears can be classified by shape as involute, cycloidal and Trophies gears. 

There are straight miter and spiral miter gears.

 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.

Spur Gear

Gears having cylindrical pitch surfaces are called cylindrical gears. Spur gears belong to the parallel shaft gear group and are cylindrical gears with a tooth line which is straight and parallel to the shaft. Spur gears are the most widely used gears that can achieve high accuracy with relatively easy production processes. They have the characteristic of having no load in the axial direction (thrust load). The larger of the meshing pair is called the gear and smaller is called the pinion.

Helical Gear

Helical gears are used with parallel shafts similar to spur gears and are cylindrical gears with winding tooth lines. They have better teeth meshing than spur gears and have superior quietness and can transmit higher loads, making them suitable for high speed applications. When using helical gears, they create thrust force in the axial direction, necessitating the use of thrust bearings. Helical gears come with right hand and left hand twist requiring opposite hand gears for a meshing pair.

Gear Rack

Same sized and shaped teeth cut at equal distances along a flat surface or a straight rod is called a gear rack. A gear rack is a cylindrical gear with the radius of the pitch cylinder being infinite. By meshing with a cylindrical gear pinion, it converts rotational motion into linear motion. Gear racks can be broadly divided into straight tooth racks and helical tooth racks, but both have straight tooth lines. By machining the ends of gear racks, it is possible to connect gear racks end to end.

 

Worm Gear

A worm gear is a gear consisting of a shaft with a spiral thread that engages with and drives a toothed wheel. Worm gears are an old style of gear, and a version of one of the six simple machines. Basically, a worm gear is a screw butted up against what looks like a standard spur gear with slightly angled and curved teeth.

It changes the rotational movement by 90 degrees, and the plane of movement also changes due to the position of the worm on the worm wheel (or simply "the wheel"). They are typically comprised of a steel worm and a brass wheel.

Bevel gear 

Bevel gears have a cone shaped appearance and are used to transmit force between two shafts which intersect at one point (intersecting shafts). A bevel gear has a cone as its pitch surface and its teeth are cut along the cone. Kinds of bevel gears include straight bevel gears, helical bevel gears, spiral bevel gears, miter gears, angular bevel gears, crown gears, zero bevel gears and hypoid gears.

Spiral bevel gear 

Spiral bevel gears are bevel gears with curved tooth lines. Due to higher tooth contact ratio, they are superior to straight bevel gears in efficiency, strength, vibration and noise. On the other hand, they are more difficult to produce. Also, because the teeth are curved, they cause thrust forces in the axial direction. Within the spiral bevel gears, the one with the zero twisting angle is called zero bevel gear.

Screw gear

Screw gears are a pair of same hand helical gears with the twist angle of 45° on non-parallel, non-intersecting shafts. Because the tooth contact is a point, their load carrying capacity is low and they are not suitable for large power transmission. Since power is transmitted by the sliding of the tooth surfaces, it is necessary to pay attention to lubrication when using screw gears. There are no restrictions as far as the combinations of number of teeth.

Internal gear 

Internal gears have teeth cut on the inside of cylinders or cones and are paired with external gears. The main use of internal gears are for planetary gear drives and gear type shaft couplings. There are limitations in the number of teeth differences between internal and external gears due to involute interference, trochoid interference and trimming problems. The rotational directions of the internal and external gears in mesh are the same while they are opposite when two external gears are in mesh.

Miter gear 

Miter gears are bevel gears with a speed ratio of 1. They are used to change the direction of power transmission without changing speed. There are straight miter and spiral miter gears. When using the spiral miter gears it becomes necessary to consider using thrust bearings since they produce thrust force in the axial direction. Besides the usual miter gears with 90° shaft angles, miter gears with any other shaft angles are called angular miter gears.

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

Configuration :

Parallel Axes / Spur Gear, Helical Gear, Gear Rack, Internal Gear

Intersecting Axes / Miter Gear, Straight Bevel Gear, Spiral Bevel Gear

Nonparallel, Non intersecting Axes / Screw Gear, Worm, Worm Gear (Worm Wheel)

Others / Involute Spline Shaft and Bushing, Gear Coupling, Pawl and Ratchet

its own additional explanation regarding the respective gear type. 

Helical gears come with right hand and left hand twist requiring opposite hand gears for a meshing pair.

It is necessary to accurately understand the differences among gear types to accomplish necessary force transmission in mechanical designs. 

The rotational directions of the internal and external gears in mesh are the same while they are opposite when two external gears are in mesh.

It is best to start with the general knowledge of the types of gears as shown below. 

In addition to the list below, each section such as worm gear, rack and pinion, bevel gear, etc.

 They have better teeth meshing than spur gears and have superior quietness and can transmit higher loads, making them suitable for high speed applications. 

The larger of the meshing pair is called the gear and smaller is called the pinion.

An overview of gears

• Spur gear

• Helical gear

• Ratchet

• Pawl

• Rack

• Pinion

• Straight bevel gear

• Spiral bevel gear

• Worm

• Worm wheel

• Internal gear

• Gear coupling

• Screw gear

• Involute spline shafts and bushings

• Miter gear

(Important Gear Terminology and Gear Nomenclature in this picture)