A Philidas nut  is a locknut with one or more slots cut laterally in the reduced-diameter circular top for less than half the diameter, the metal above the slot(s) being deformed downwards so that over the last one or two turns, the thread for half the diameter is "axially depitched" or displaced from its normal position. As the nut is threaded on, the displaced sections are elastically forced back axially to their original position, the load increasing the friction between the nut and the fastener, creating the locking action. These nuts retain their locking action at temperatures limited only by the base material, as no polymeric insert is used, and as the locking action is by elastic deformation, they can be re-used multiple times.
They may look similar, but differ substantially from the Split beam nut because the latter has a radial displacement of the deformed portion, while the Philidas nut uses axial deflection or depitching.
There are at least two basic types, the original "Industrial" type with two slots, one on each side of the diameter but axially aligned, and the "Turret" where both slots are aligned radially, one above the other. The Turret type appears to reduce the risk of failure of the locking action due to fracture between the base of both slots, which may be a limitation of the Industrial type.
Philidas, the original and current manufacturer, is a tradename for these nuts, which were patented in 1942.
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