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Castle nuts are said to be so named because they are reminiscent of the battlements (parapets with openings) found atop castle towers. They are hex nuts with six large slots (60° apart) in a cylindrical top a little smaller in diameter than the width of the nut; slot depth and cylindrical top height are the same. Castle nuts are used with bolts and studs that have a hole drilled in their threaded section through which a cotter pin is inserted to prevent the nut from rotating. The nut is quite effectively retained until the pin is removed or shears. They are commonly used to secure the position of wheel bearings on spindles and are also called:
castellated nuts, finished hex castle nuts, slotted nuts, spindle nuts.
Castle nuts may be interchanged with slotted nuts, although castle nuts have a higher profile because of their cylindrical top. They are not used with lock washers.
Threads are standard right-hand and often Unified inch fine (UNF, Unified National Fine); Unified inch coarse series (UNC, Unified National Coarse) threads may be available.
Castle nut size refers to its nominal thread diameter. Typically, sizes range from about 1/4" to 1 1/2". Size is specified in inches, usually fractional rather than decimal.
Nut width and height vary with the size of the castle nut.
Grade 2, steel, is a common grade and material, and plain finish and zinc plated are typical finishes. (Plain finish is unplated so there is no surface treatment to inhibit rust.)
The nut is torqued to the desired value, then rotated clockwise or counterclockwise—up to 30°—to align the nearest slot with the through hole in the bolt or stud. The cotter pin, or possibly a lock wire or spring pin, is then inserted through opposing slots in the nut and the hole. NOTE: Always follow the manufacturer's instructions carefully to determine whether the nut should be tightened further or loosened to align the slots. For this reason, castle nuts are not used when a specific preload is required because it isn't possible to "fine tune" torque.
Although well-suited for certain applications, there are several disadvantages to consider: the through hole must be correctly located along the threaded shaft to allow insertion of the pin or wire after the nut has been properly tightened, an extra component is needed (pin or wire) and additional time is usually required to align and secure the nut.
Refer to ASME B18.2.2 for specifications relating to castle nuts.