Muley axle
A car axle without collars at the outer ends of the journals.
Muley axle: a journal without shoulders to hold wheels
A muley axle is a railroad car axle where the journals (the cylindrical bearing surfaces) have no integral collars or shoulders. On a standard axle, collars are machined as raised rings at the outer ends of each journal; these collars serve as mechanical stops that prevent the wheel assemblies from sliding inward toward the car body. A muley axle omits these collars entirely, relying instead on external keying or wedges to position the wheels.
The muley design emerged in 19th-century railroading as a cost-cutting measure. Eliminating the collar-turning operation reduced manufacturing time and scrap loss. Early freight cars, especially four-wheel trucks, often carried muley axles. The wheels were held in place using taper keys or shims driven into slots between the wheel hub and the axle shoulder, a method that worked adequately at the speeds and loads of that era.
A muley axle is weaker than a collared axle at the journal shoulder, where stress concentration occurs. The absence of a collar means the diameter transitions more abruptly from the reduced journal to the wheel-seating surface, creating a sharper stress riser. This weakness becomes significant under dynamic loading, particularly in heavier cars or at higher speeds.
Why muley axles fell out of favor
By the early 20th century, the railroad industry moved decisively toward collared axles. Heavier cars, faster trains, and more demanding service revealed the durability problem. The keying method proved unreliable; keys could loosen or shift, allowing wheels to creep inward. Modern freight cars and passenger stock use solid collars forged as part of the axle, providing both mechanical strength and secure wheel retention without maintenance concerns.
Today muley axles appear chiefly on heritage railway equipment, industrial switching gear, and very light narrow-gauge lines where loads remain minimal and speeds low. When encountered during car inspection or repairs, a muley axle signals equipment of substantial age and warrants close scrutiny of the keying system and journal wear, as the original hardware may have corroded or shifted over decades of service.