The flux density pattern on the disc face of a magnetic clutch is analysed by conformal mapping of the coil-gap configuration. The suspension rheology of iron/silicone mixtures is used to derive the viscous torque transmitted by the clutch disc rotating in a non-Newtonian medium in a narrow enclosure. Since disc Reynolds numbers are small for normal operating slips of fluid clutches, only Couette-type laminar flow occurs without boundary layer in the clutch gap. This viscous torque cannot be predicted by theories based on the shear failure of dry masses. Experimental measurements on a disc-type clutch using torque, speed and temperature transducers show that the theory based on apparent viscosity of the fluid mixture under various shear rates, magnetic inductions and volume fractions is more pertinent to the fluid magnetic clutch. List of symbols b = modulus of elliptic integral B = flux density, T B o = flux density at centre of clutch gap, T C v = volume fraction C vm = maximum possible value of C v d = thickness of clutch disc, m E(4>, yfb) = elliptic integral of the second kind E, E = complete elliptical integral of the second kind with modulus b and (1 -b), respectively F = elliptic integral of the first kind H = magnetising force, A/m g = inner radius of excitation coil, m h = clearance between disc edge and housing, m K, K' = complete elliptic integral of the first kind with modulus b and (1 -b), respectively K a = coefficient denoting pseudoplastic behaviour / = semiaxial length of excitation coil, m r = radius vector r 0 = radius of clutch disc, m R = reluctance of clutch S = shear rate, s " 1 t = width of clutch gap, m T = torque, N m V o = MMF, ampere turns x =(((-b)(c -l) y\ a = apparent viscosity, Ns/m 2 rj^ = asymptotic viscosity, N s / m 2 VaB = apparent viscosity under magnetic induction, N s / m 2 0 = temperature, °C Hi = relative permeability of iron /z 0 = magnetic constant fi p = relative permeability of fluid mixture Me
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