What is End Fixity?
Screw end fixity is the engineering term for screw end support. Fixity is an important element in screw and nut drive systems. The rigidity of the screw end support determines the screw drive system’s resistance to column buckling and limit of the speed of rotation to avoid natural frequency vibration. (See Column Loading and Critical Speed sections.)
Theoretically, there are only 3 types of screw end mountings:
Free is just that – no support of any kind. The illustrations in Table 34 demonstrate “fixed” and “supported” screw end fixities. A supported end will resist axial and radial loads but not moment (overturning) loads. A fixed end will support axial, radial and moment loads.
Different Types of End Fixity
Fixed ends offer the highest column load support and the highest resistance to vibration. A supported end and a free end should never be used. The relative rigidity and the factors for critical speed and column loading are listed in Table 40. These factors show the relative effect of end configuration on a screw system’s ability to support column loads and its vibratory limit of critical speed. For more detail on how these factors are used, see Column Loading, Critical Speed and the Useful Formulas sections.