# How many types of gears are there

## gear

As gear is the name given to a wheel whose circumference is provided with small elevations and depressions, the so-called "teeth" and "tooth gaps".

The teeth have such a shape that they can roll off one another. In order to achieve smooth rotation of both gears, at least two teeth must always be in mesh. The curve of a tooth shape is called an involute. The form of power transmission is a form-fitting connection.

A distinction is made between different basic forms of gears: gears with involute gearing, headstock gearing or cycloid gearing, the most widespread is involute gearing.

### General

Gears are used in gearboxes. For this purpose, they are mounted on shafts or axles or attached in such a way that their teeth mesh with one another, so that the rotary motion of one gear can be transferred to the other. The direction of rotation is reversed, which can be a desired effect of this arrangement. If the wheels are of different sizes, the speed can be increased or decreased in accordance with the laws of physics (force times distance is constant), with the torque being decreased or increased. In this way, gears translate forces and speeds.

### Spur gears

Spur gears

The Spur gear or Cylinder gear is a simple wheel that has a cylindrical outer contour and is provided with teeth on its circumference. The axes of a spur gear and its mating gear (also a spur gear or a spur-toothed shaft) are parallel, a Spur gear.

The toothing can be straight, i. H. axially parallel or inclined (helical toothing). The size of the toothing is determined as a "module". The mating gear must have a toothing of the same module.

The spur gear is used to transmit torques. The torque can be absorbed "from the inside", via the seat of the wheel by means of the entrainment. This entrainment can be non-positive or positive.

The torque can be absorbed and passed on, for example by means of a further gear wheel on the circumference. In this case, the bore of the spur gear has a pure storage function. This use is occasionally found in racing engines or in idler gears.

### Bevel gears

Bevel gears

The axes are at an angle (usually 90 °) to each other, but must intersect. The basic shapes are cones, the tips of which coincide. One distinguishes straight toothed (Picture) and spiral-toothed Bevel gears.

### slug

Worm gear

A particularly frequently used form is slug and the Worm wheel, which together form the worm gear. The two axes are at an angle of 90 ° to each other. The worm is, as it were, a single-toothed gear, the tooth being wound helically around the cylinder, with one turn corresponding to one tooth. This means that one revolution of the worm axis corresponds to a partial revolution of 360 ° / x (x = number of teeth on the worm wheel). This also results in the transmission ratio of x: 1.

Worm gears are often designed to be self-locking, i.e. the drive for the transmission of a rotary movement can only be from the worm to the gear, but not vice versa. A torque that acts on the worm from the gear is blocked by frictional forces. In this form, worm gears are used, for example, on the hoisting rope of cranes, so that the load is held if the drive fails.

### Racks

Rack

Racks are straight bars. They enable a conversion of a rotating into a linear movement and vice versa. The freedom of movement is restricted, however, since the rod is limited in length and thus only allows an alternating movement in the two opposite directions along the rod. One application is the cog railway.

### Determining variables of straight spur gears

Gear pair in mesh, gear with 24 teeth, module = 2mm, pitch circle diameter d = 48mm, tip circle diameter dk = 52mm

Two diameters are important for determining a gear with straight flanks: the outside and the working diameter. The outside diameter determines the space required by the gear. The working diameter determines the distance between the gear axles. In the specialist literature, the outside diameter is called Tip circle-Diameter and the working diameter as Pitch circle-Diameter designated. The pitch circle is shown in technical drawings with a dash-dotted line.

The division p of the gear is the width of a tooth plus the width of a gap on the pitch circle diameter. The module m is the ratio of the division p to the number pi, m = p / π.

The diameter of the pitch circle results from the product of the module and the number of teeth z, d = m * z.

Gear and counter gear must always have the same module. The head height of the teeth is equal to the module, hk = m. The foot height is equal to the module plus play; 25% of the module clearance is usual, hf = 1.25 · m dk is equal to dk = m (z + 2). The root diameter df is df = m (z - 2.5).

The center distance a two spur gears 1 and 2 can be calculated with the following two formulas:

The module for spur gears is to be selected in accordance with DIN 780-1.