Axle

Railroad car wheels affixed to a straight axle, limiting them to rotate in unison. This is called a wheelset.
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.[1] 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.

  1. ^ Mechanical Engineering design (9th ed.). McGraw Hill. 2010. p. 360. ISBN 978-0073529288.