Electricpete,
You are exactly correct, but all couplings are not created equal when it comes to restoring forces (reactionary forces). A couplings ability to withstand misalignment is mainly predicated on the design and material selection of the flex element. so although the fatigue limit of the flex element will not be reached, that doesn't mean the adverse reactionary forces generated are acceptable. Gear couplings are particularly challenging because the more torque you drive through them, the less willing they are to misalign. This is due to the fact that torque is driven through the misalignment plane via friction. higher torque driven = high force required to overcome friction. Disc couplings are different, torque is driven through 1 plane, misalignment occurs through another. So no matter what torque you drive though a disc coupling the reactionary force are virtually unchanged. But the stress strain relationship in a disc coupling flex element changes as high torques are driven, so some disc coupling manufactures advertise high misalignment, but that is at high service factors (low torque) as you lower the service factor (drive more torque) the misalignment rating has to get de-rated. So no matter what any disc coupling manufacturer claims, all disc couplings at a given diameter have about the same amount of misalignment capability. The only way to increase misalignment in a disc coupling is to have more discs to lower the stress in the disc pack. Then you are working that stress strain relationship, trading stress for strain. However, that comes at a price, higher reactionary forces. More discs is a thicker flex element pack which leads to higher reactionary forces. So it is a balancing act.
Second point, axial misalignment to angular misalignment.
This is a little tricky, a coupling by design can only bend out of alignment so far until it hits a dead stop. So axial misalignment changes where that dead stop will occur. that can be a concern it is basically limiting the amount of movement that is available, while this isn't as critical in a gear coupling, in a disc coupling it can be very critical.
However, the more concerning aspect of of axial misalignment applies to 1 part flex elements (flex elements that function off a stress strain relationship) like discs or elastomers. Axial misalignment in these 1 part flex elements kind of works like a pre-load on a beam. For example, assume loading a beam cyclically with 1000 lb force generates a fatigue limit over time. Then, loading that same beam cyclically with a 1000 lb force AFTER a 250 lb constant force is applied, will generate a shorter fatigue limit then if the 250lb static force wasn't present. That 250 lb force is acting like axial misalignment. axial misalignment in 1 part flex elements is the pre-strain present in the system. The easiest safest way to account for it is resolve it back to angular misalignment per flex element.
When it comes to couplings we are always here to help.
