the screw by shaft bearings). This is due to the need to avoid "wedging" of the thread flanks when subjected to radial loads, which would contribute substantially to friction forces and thread wear. However, there is a Centralizing Acme-thread standard (also specified in ASME/ANSI B1.5-1997) which caters to applications where the threads are not radially supported, where the roots and crests of opposing threads are designed to come into contact before the flanks do under radial loads. This adds the requirement that the sum of the allowances (clearances) and tolerances on the major diameters of nut and bolt be less than the sum of the allowances on the pitch diameters (PD). The drawback is that for a given amount of end play (axial clearance due solely to PD clearances), closer tolerances and a cleaner work environment are necessitated in the application of a Centralizing Acme thread. Compared to square threads, disadvantages of the Acme thread form are lower efficiency due to higher friction and some radial load on the nut (angular offset from square). When created before 1895, Acme screw threads were intended to replace square threads and a variety of threads of other forms used chiefly for the purpose of traversing on machines, tools, etc. Acme screw threads are now extensively used for a variety of purposes. Long-length Acme threads are used for controlled movements on machine tools, testing machines, jacks, aircraft flaps, and conveyors. Short-length threads are used on valve stems, hose connectors, bonnets on pressure cylinders, steering mechanisms, and camera lens movement. The thread form shown in the figure (Basic ACME thread profile) is called "basic". The actual thread heights on both the internal (nut) and external (bolt) threads differ from by allowances (or clearances): A minimum root-crest clearance of (diametral) between opposing threads with 10 tpi (threads-per-inch) or fewer, and for finer pitches. (This is also true for the minor diameters of the Centralizing Acme thread, though not its major diameters, where the allowance is made less than the PD allowance.) A PD allowance, which makes the PD smaller than "basic" in the case of the GP and external Centralizing Acme threads, but greater in the case of the internal Centralizing Acme thread. The net effect is that the minimum thread heights are greater than "basic" for internal and external GP threads and for external Centralizing threads, and the maximum height for internal Centralizing Acme threads