A Single-Flex CD Coupling can be used where parallel misalignment is minimal.  The Single-Flex CD Coupling can be ideal in these situations, such as many servomotor and automation equipment applications, where the shafts are typically well aligned.  With only one disc-pack, the Single-Flex CD Coupling offers a more compact size, higher torsional stiffness, lower inertia, yet it can still handle any inevitable small amounts of parallel misalignment that may exist.  Both the Single-Flex and Double-Flex CD Couplings handle up to 3° of angular misalignment.
A Double-Flex CD Coupling has two disc-packs, which allows it to handle a much greater amount of parallel misalignment.  Also, by using two open-arm disc-packs, the design of the Double-Flex CD Coupling greatly reduces the reaction loads on the connected equipment.
Other than the misalignment capacities, in some cases the DBSE (distance between shaft ends) will determine whether you should use the shorter Single-Flex or the longer Double-Flex Solution.  Floating Shaft CD Couplings should be considered in very large DBSE applications.