Thanks to Myles' idea, to machine the splined shaft to fit into a round coupling instead of the bass ackwards way I was looking at doing it, I have come up with a tentative solution using agricultural tractor PTO shafting and coupling. No need to click the links, they are just there for the curious. First one is a shaft splined at each end and second is an internally splined coupler.
http://www.surpluscenter.com/Power-Transmission/Shafting/Splined-Shafting/1-3-8-6T-SHAFTING-SPLINED-BOTH-ENDS-1-2937-6.axd
This 12" piece of 1-3/8" shaft has both ends splined and the middle portion round. I think I can just turn down the central part between the splined sections, one side 7/8" dia and the other side 1" dia, then cut the shaft in half to have my two adapters, each of which will couple to their respective round shafts and meet in a splined coupling fixed to one of the two adapters.
http://www.surpluscenter.com/Power-Transmission/Shaft-Couplers/1-Piece-Solid-Couplers/1-3-8-6T-SPLINED-COUPLING-167-02206-1-1562.axd
This coupling is 5" long so I can easily set up to have up to an inch of fore/aft play I think.
This is a standard agricultural PTO shaft and the minor diameter in the standards is listed at 29mm, or a bit over 1-1/8" so there is enough steel to work with, for a 1" round shaft termination. So this would only be a $70 + machining solution, the machining being simple, nothing more than turning half the central portion to 7/8" and half the central portion to 1" diameter. I will just use "Redneck milling", i.e. my angle grinder, to cut a keyway in the round ends of the two pieces.
In fact, actually since the central portion is 1-3/8" diameter, PLENTY of steel. Don't know why I was even concerned with minor diameter since I don't need to turn down the splined part. I could use a plain splined shaft for donor material, and save some money, because THAT would also have a 1-1/8"+ minor diameter.
There are actually three different standards for PTO coupling shafts. This is type 1. There is another standard, type 2, which uses 21 splines, same diameter. And still another with a much bigger diameter and I'm not going there at all. I suppose I could use the type 2, with 21 splines, but somehow I get the idea that for this application, 6 splines would be better. Any input? Those little splines on the type 2 look kinda delicate, to me. You know how us Rednecks like to hammer on stuff.
This isn't going to happen any time soon, so I have time to consider other options. The important thing is that the motor never under any circumstances come under any axial loading, even if there is play in the thrust bearing or the motor shaft is longitudinally stressed when bolting stuff together. For now, I am going back to the reduction gear that already worked okay for me. In fact I am fixing to get busy on the unconversion right now.
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