Are you looking for a hysteresis coupling (slips when torque reaches a specific maximum), or a torque coupling (transmits torque, up to a maximum level)?

I've found magnetic couplings require a lot of communication for a given design to be successful.  I've experienced all sorts of problems & miscommunications so I'm not sure what the "usual problems and misunderstandings" are.

A short answer to your question about limiting torque to 140% of maximum: Yes, it certainly is possible.  A properly designed coupling can work to almost any torque that a customer is looking for.  If the coupling uses SmCo as a permanent magnet material than there is minimal chance that it will lose its magnetic properties and overheating will not be a concern.

The biggest challenge that I've experienced is convincing customers to pay for a proper design.  Most want a standard, off-the-shelf coupling that wasn't designed for over-torquing or temperature extremes.  If you are willing to invest in a custom design, the chances are that it will work well for you.

I´m not sure if I understood the difference between "slips when torque reaches a maximum" and "transmits torque, up to a maximum"....after reaching a maximum won't it start slipping...and so, aren't they the same?

Do you, with your experience in Magnetic coupling, think its applicable to conveyor belts, with usually has very different torques, as it can be fully loaded, or with only a small % of its nominal capacity? Is there any "in field setting for torque limit"....just like a hydraulic coupling has? As easy as filling it with oil?

At low torque levels this isn't a big problem.  You can used face couplings (two discs with magnets mounted, face to face).  Changing the air gap changes the torque limit.  These are used on bottle cap machines.
The higher torque coupling that use inner and outer magnet assemblies are not really adjustable.

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I apologize, I didn't explain the difference between those couplings very well.

I consider a regular torque coupling one which will transmit a specific amount of torque up to a certain speed.  A hysteresis coupling will transmit a constant amount of torque irregardless of speed.  Each have a different construction and it is important to understand the application to specify the correct one.

As EdStainless mentioned, in a face-to-face torque  coupling, the torque can easily be controlled by adjusting the air gap.

Coaxial torque couplings (inner & outer magnet assemblies) can also be adjusted but not as easily.  The adjustment involves controlling the amount of axial engagement between inner & outer assemblies.  It's been done, but not very often in my experience.  I wouldn't consider it "as easy as filling with oil."

The other issue with PM tourque couplings is that when they slip they pulse.  If you want the system smooth you need to go with hysterisis or hydrol.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm