ISO 14691:2008 specifies the requirements for couplings for the transmission of power between the rotating shafts of two machines for general-purpose applications in the petroleum, petrochemical and natural gas industries. Such applications typically require couplings to transmit power at speeds not exceeding 4 000 r/min, between machines in which the first lateral critical speed is above the running speed range (stiff-shaft machines). ISO 14691:2008 can, by agreement, be used for applications outside these limits.ISO 14691:2008 is applicable to couplings designed to accommodate parallel (or lateral) offset, angular misalignment and axial displacement of the shafts without imposing excessive mechanical loading on the coupled machines. Couplings covered by ISO 14691:2008 include gear (and other mechanical contact types), metallic flexible-element and various elastomeric types. Such couplings can be of all metal construction or can include components of non-metallic materials, such as composites. ISO 14691:2008 covers design, materials of construction, inspection and testing of couplings and methods of attachment of the coupling to the shafts (including tapered sleeve and other proprietary devices).ISO 14691:2008 does not apply to special types of couplings, such as clutch, hydraulic, eddy-current, rigid and radial-spline types.ISO 14691:2008 does not define criteria for the selection of coupling types for specific applications.