Thanks for the link to Lovejoy couplers. Yeah, I've been looking at couplers supplied by McMaster. I do a lot of business with them. The Oldham and Lovejoy couplers are variations on the plastic "spider" type of coupler: two metal or plastic pieces facing each other with fingers or teeth and a softer plastic cushion between them.
Pump is horizontal, not vertical. It's made by Gorman-Rupp and sold under their AMT badge or rebadged as Dayton and sold by Grainger/Zoro. It's also sold as Teel brand. As sold, the pump frame supports the close-coupled motor. See the picture.
I'm definitely sticking with this pump model. After 10 years and more than 6000 hrs run time, the pump that had the motor die had been faultless and performed exactly as expected. The backup pump (same model) has been faultless after a year of use. If I change the pump model, I'll have to redo the plumbing ... ugh, no, not happening! I can get a new backup motor/pump combo (same model) on sale now for about US$1000. In terms of money and time that would probably be more economical than putting a motor with a coupler on the pump that has the dead motor. The pump that had the motor die could be kept for parts. So, it's a learning project that might save time down the road because if the motor should need servicing or replacement, the pump won't have to be disconnected from the plumbing and disassembled.
Mike in California
Tautech,
I know you weren't saying I should switch to vertical. My response about not switching pump models was in response to PaulAm. The plumbing does have unions, but a change in geometry or inlet/outlet diameters, would necessitate some significant plumbing changes. But it's not happening anyway.
I believe I understood your post and appreciate the advice. The picture below shows the shaft, seal and impeller arrangement. What is called "adapter" in the diagram is actually part of the pump case, and the motor bolts to it. It's called adapter because the shafts are slightly different for the 1 Ph vs 3 Ph versions of the motor. A more expanded view of the adapter (labeled #2) is in the second picture.
Mike in California
Now I can see that a flexible coupling could allow a disastrous amount of wiggle because the far end of the shaft is not supported and the impellers might contact the case. And a rigid coupling would require some tricky alignment. Sounds like it's time to give up my idea of a separate motor and coupler. I'll either buy a replacement close-coupled motor or a motor/pump. Thanks for all the responses. I learned a lot.
Mike in California