Spur gear

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Sketch of a single-stage spur gear
Section through a three-stage spur gear (helical toothed)

The spur gear is a type of gear that is characterized by spur gears on parallel axes.
The simplest design is the single-stage spur gear, which consists of two shafts , each with a gear . However, by adding additional gears and intermediate shafts, multi-stage gears can be formed.

Areas of application and advantages and disadvantages compared to other gear types

Spur gears are widely used; They are used, for example, in clockworks , motor vehicle gearboxes through to large industrial gearboxes .

Their advantages consist in the relatively simple design, since few moving parts are used and the externally toothed spur gears are easier to manufacture than z. B. ring gears in planetary gears or worm or bevel gears as well as robustness and high efficiency through direct, purely mechanical transmission.
The disadvantage is the small translation that can be achieved in one step; Usually a maximum gear ratio of about 6 can be achieved in practice with one step. A spur gear is larger and therefore heavier than a planetary gear with the same given transmission power ; compared to worm gears, spur gears are louder.

Straight, helical and herringbone teeth

Frequently, in spur gears obliquely inserted gears. The teeth do not run parallel to the gear axis, but at an angle. If a pair of teeth (of gear and mating gear) comes into contact, it does not carry directly over its entire width, as is the case with straight-toothed spur gears without profile correction. Instead, the loaded face width increases slowly as the wheels continue to turn, until the pair of teeth carries over their entire width, and only slowly decreases again when turning out of the contact zone. With helical gear pairs there are usually always two or more teeth in contact at the same time, with straight gear pairs usually only one to three teeth.
In the case of helical gears, there are fewer hard impacts during tooth meshing, which leads to less vibration excitations and quieter running. Furthermore, the tooth root and pit load capacity is slightly higher. You can often hear the difference in older cars. In the straight-toothed reverse gear, the transmission makes more distinct noises than in one of the helical-toothed forward gears. In addition to the straight toothing, this is also due to the fact that in reverse gear, a considerably poorer toothing quality is usually accepted and post-processing after hardening is usually dispensed with.
The disadvantage of the helical gearing is a slightly higher friction, which results in greater losses. In addition, there are axial forces that push the gears apart laterally and therefore require more complex storage.

In addition to the straight and helical gearing, there is also the herringbone gearing, in which two helical gears with different helical directions but with the same pitch angle are attached next to one another. This arrangement avoids the axial forces characteristic of helical gears, but mostly at the expense of more complex production.

Load capacity calculation

The most important and most expensive component of a spur gear is usually the toothing. The load capacity calculation of the gears, d. H. Proof of whether the gear can transmit a certain power is provided, for example, with the help of DIN 3990. This calculation rule is well secured and reliable thanks to extensive practical experience. The tooth flank pressure and the tooth root tension are essential parameters here. The simpler guideline VDI 2736 (sheet 2) can be used for thermoplastic gears . There are numerous advanced, advanced calculation methods, e.g. B. take into account the deformation of the housing, the shafts or the teeth and allow the highest possible performance, with a small design.

The choice of lubrication is also an important factor for the load-bearing capacity of the gearing . While simple spur gears are only lubricated by a lubricating oil, high-performance spur gears require highly developed lubricants that are injected directly into the toothing.

Individual evidence

  1. ^ Karl-Ludwig Haken: Fundamentals of automotive engineering

See also