As evident in the photo at right, spacers for couplings can get quite large. The spacers in the shown photo are approximately 20 feet in length and mate up to two size 9 flex-rigid gear couplings for an application installation in Asia. (This photo was taken at Lovejoy's Downers Grove, Illinois facility with Lothar Gädtke of R+L Hydraulics from Werdohl, Germany serving as our guest human model. Thanks Lothar!)
While coupling spacers can get quite long, it is important to remember that they do create an additional overhung load, and the power transmission system's bearings must be able to support the weight. (Weight is a major reason why long spacers are sometimes designed out of lightweight composite materials.)
Longer spacers also cost more, given more material and manufacturing time is needed, with composites costing considerably more.
Due to weight and cost, best practices generally dictate that equipment being connected by a coupling be located as close to each other as possible (while still allowing adequate room for coupling installation and removal). However, there is one benefit of longer spacers that is often overlooked... and, that is misalignment handling capability.
Given two flexing planes and a set distance between them (as pictured in the 4 bolt disc coupling at left), it is the distance between the two flexing planes that largely determines how much parallel misalignment the coupling can handle. As the distance between the two flexing planes increases, so does the maximum amount of parallel misalignment that they system can handle.
Note: While couplings are designed to handle a certain amount of misalignment, and larger spacers can provide for increased maximum allowances, broadly speaking, the best solution for managing misalignment and maximizing system life is to actually eliminate or reduce misalignment in a system should be minimized. (Just because a coupling can take a certain amount of misalignment, doesn't mean you should accept that amount of misalignment in your system. This is somewhat analogous to "just because your car is designed to handle and survive potholes... doesn't mean you shouldn't try to avoid driving through a bunch of them".)