coupler for AC servo

That depends on how accurately you can mount the servo. The big CNC machines I used to work on had rigid couplings with precisely located motors. For a retrofit you are probably looking at a coupling that will accommodate some misalignment. I'd look at the helical couplings or the flex disk couplings. I wouldn't even consider anything like the lovejoy or similar with any sort of insert.

Try Mcmaster.com, flexible shaft couplings, catalog page 1165.

I like Oldham couplings for this sort of thing. They're much cheaper than higher performance options like bellows couplings, less fragile and cheaper than helical beam couplings, and usually good enough in all but the most demanding motion control apps. McMaster has a limited selection; Lovejoy and Huco have much more. Lovejoy has some comparison info in their catalog.

Hmm. I like the lovejoy for retrofits. Easy to disassemble. The only possible solution on a couple locationsI've had. I suppose a purist would say there's an RCH of backlash in them. IMHO, a tinsy fraction of the total system backlash. I suppose if your building parts for NASA or Medtronic it might not be good enough.

Just my 2 cents

Karl

Those are not zero backlash couplers. McMaster has zero backlash flexible couplers specifically intended for servo use. They aren't particularly cheap, but if you only need one of them they should be acceptable.

The Oldham couplings from Huco and Lovejoy are zero backlash, as are those that McMaster stocks. There's a light interference fit between the aluminum hubs and the delrin discs. The disc is much stiffer torsionally than the elastomer spider in typical jaw couplings, which also suffer from backlash. Oldhams are stiffer and have lower inertia than a helical beam of similar torque rating. And if you bust one, replacement discs are a couple dollars -- cheap enough to keep a couple spares on hand.

In highly dynamic systems a disc or bellows coupling may be necessary, but I've used Oldhams in many less demaninding apps with no problems.

The link you provided does not indicate the Oldham style is zero backlash. The only three piece coupler on that page indicated as zero backlash is the curved jaw GS series.

You're right, it doesn't say so specifically, but the page also neglects to specify that the bellows coupling is zero backlash.

Here's Huco's description:

"Rotation is transmitted through a central disc that slides over the tenons on the hubs under controlled preload conditions to eliminate backlash."