Couplings and Brakes
Permanent magnetic couplings and brakes can be divided into three basic types:
» Synchronous couplings, which includes the disc couplings and concentric ring couplings
» Hysteresis clutches and brakes
» Eddy current clutches and brakes
 Fig.1 Design of a disc coupling |
 Fig.2 Design of a concentric ring coupling |
 Fig.3 Design of a hysteresis clutch or brake |
 Fig.4 Design of an eddy current coupling or brake |
For all types of coupling and brake the relevant efficiency equation is: P1 – Pv – P2 = 0
P1 is the influx power from the drive side.
P2is the transmitted power from the driven side and
Pvis the power loss which occurs through the transmission mechanism in coupling and brake.
In synchronous couplings Pv = 0, as the slip S = 0. AOn the drive and driven sides, permanent magnets are set opposite one another with an equal, even number of poles mirror balanced (disc couplings: fig. 1) or dynamicallybalanced (concentric ring couplings: fig. 2).
The magnetic requirements for synchronous couplings
» permeability µrev -> 1
» coercive field strength as great as possible
are best met by the ceramic materials barium or strontium ferrite as well as intermetallic combinations of rare earths and cobalt.
One of the halves of the synchronous coupling – suitable for the drive side for cooling reasons – is replaced in hysteresis clutches by a ring or disc made from a permanently magnetic material with relatively greater remanence and permeability and relatively smaller coercive field strength, so that this half of the coupling can be magnetically reversed from the other for greater resistance (fig. 3).
Finally, in eddy current clutches and brakes Pv > 0, as S > 0. One of the halves of the sychronous coupling – suitable for the drive side – is replaced here by an electrical conductor, iron backed, in ring or disc form (fig. 4).
General advantages of permanent magnetic couplings and brakes:
Free of wear, no contact, maintenance-free, low bearing- friction on concentric ring couplings, unlimited life under normal conditions of use.
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