fox
A wedge driven into the split end of a bolt to tighten it.
fox: a wedge that locks a split bolt tight
A fox is a tapered wedge inserted into the axial split of a bolt's shank to create a permanent mechanical lock. The bolt is deliberately slit lengthwise, usually for half its length or more, and as the fox is driven inward, it forces the split sides outward against a receiving hole or collar, creating friction and preventing rotation or axial movement. This method is older than modern lock washers or thread-locking compounds, and remains in use where reliability under vibration is critical and where re-tightening is not expected or desired.
The fox itself is typically a small wedge of hardened steel, roughly 10 to 25 millimeters long depending on bolt diameter, with one end tapered at a shallow angle (often 1:10 or shallower). It is driven flush with the split end using a hammer or press. The split bolt must be of sufficient wall thickness to accommodate the wedge without cracking; tensile strength alone is not enough. The receiving surface, whether a drilled hole in a shaft or a retaining collar, must be rigid enough to resist the outward pressure without deforming.
Variants and limitations
Variations exist in the number of splits (two opposite splits are most common, but three or four radial splits are used in some applications) and in the depth and angle of the taper. Some foxes are knurled or serrated to prevent slipping during insertion. The method works well in cast iron, forged steel, and rigid assemblies, but is less reliable in thin-walled tubing or soft materials where the split sides can yield. It offers no protection against corrosion or galling and is difficult to disassemble without damage to the fox and often the bolt itself.
The term 'fox' appears in older British and Continental mechanical engineering texts, though its precise origin is unclear. It has largely been superseded by modern fastening techniques in new designs, but remains common in legacy equipment, railway infrastructure, heavy machinery, and applications where the assembly must withstand sustained shock or vibration without loosening. It is rarely found in applications where disassembly and re-use are expected.