thumb|upright=1.1|Railroad car Train wheel|wheels affixed to a straight axle, limiting them to rotate in unison. This is called a wheelset. thumb|upright=1.1|A Denney axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle is supported. In the latter case, a bearing or bushing sits inside a central hole in the wheel to allow the wheel or gear
An axle is a central shaft that allows wheels or gears to rotate, and it can be designed either to spin together with the wheels or to stay fixed while the wheels rotate around it. On vehicles, axles are essential because they support the wheels and determine how they move—for example, on a train, wheels affixed to the same axle must rotate together, while on other vehicles, wheels may rotate independently around a fixed axle.
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thumb|upright=1.1|Railroad car Train wheel|wheels affixed to a straight axle, limiting them to rotate in unison. This is called a wheelset. thumb|upright=1.1|A Denney axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle is supported. In the latter case, a bearing or bushing sits inside a central hole in the wheel to allow the wheel or gear to rotate around the axle. Sometimes, especially on bicycles, the latter type of axle is referred to as a spindle.
==Terminology== On cars and trucks, several senses of the word axle occur in casual usage, referring to the shaft itself, its housing, or simply any transverse pair of wheels. Strictly speaking, a shaft that rotates with the wheel, being either bolted or splined in fixed relation to it, is called an axle or axle shaft. However, in looser usage, an entire assembly including the surrounding axle housing (typically a casting) is also called an axle.
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