Selecting for Axial Compliance
Longitudinal, or axial, shaft displacement can either be intentional or unintentional. The movements due to tolerances, settlement and thermal expansion or contraction cause the latter. While these movements maybe small, they can contribute to substantial thrust loads and result in bearing damage. In these cases a coupling with axial compliance capacity should be selected, predominantly bellows, sliding disc or even helical beam. Multi-stage bellows create the greatest amount of axial compliance while the single-stage disc or bellows provides the smallest amount of axial compliance. For intentional shaft displacement, such as push/pull systems or those with extensible drives where the distance between actuator and load is variable, a teleshaft coupling such as the HUCO-POL should be used. This embodies precision-drawn nesting tubes manufactured from square-section brass that can be cut to the appropriate length to provide a wide range of axial movement. This ability to customise the length of the coupling means they can be tailored to required length of stroke. In push/pull situations the coupling should be capable of resisting the corresponding forces. These values are listed under ‘end loading’ for mechanical couplings and ‘axial spring rate’ for flexural couplings. Another option is the Oldham de-mountable, three-part coupling. By mounting the hubs slightly out of full engagement, a limited amount of axial compliance is created.