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Internal vs External Spur Gears

spur gear

Spur gears are the most common kind of gears, and are mainly used to transmit power and/or motion between parallel shafts. They can also be used to reduce or increase speed, multiply torque, and enhance accuracy enhancement for positioning systems.

Spur gears have straight-edged teeth that are cut parallel to the axis of rotation. These gears are mounted on parallel shafts to ensure that the teeth mesh together correctly. Spur gears mate only one tooth at a time, and the contact occurs instantaneously over the full face width of the mating teeth. This leads to loud operations along with high stress on the mating teeth.

Spur gears are of two kinds – external and internal. As the name suggests, external gears have teeth cut on the outside surface of the mating cylindrical wheels, and the teeth point away from the center. The input and output shafts move in opposite directions.

The teeth of an internal spur gear point inwards towards the center, and are cut on the internal surface of one of the mating cylindrical wheels. Both the input and output shafts rotate in the same direction — this eliminates the need for an idler gear. Since the centers of the mating wheels can be closer than those of external gears, internal spur gears are preferred in applications where space is a constraint. (However, these gears require complicated housing and support, since the external gear is contained within the internal gear.) Internal spur gears also make for a stronger drive, since there is more area contact and less sliding action than with similar external gears. Internal spur gears are an integral part of planetary gear drives and gear couplings.

AmTech OEM manufactures high quality internal and external spur gears of various sizes and dimensions, in high quality stainless steel and aluminum alloys, with custom manufacturing options to meet OEM customer needs.

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