a fluid shear coupling apparatus including a driving member comprising a rotor mounted to a shaft . a driven member including a bearing housing is mounted to the shaft . a complementary surface between the driven member and the driving member is provided between which a shear fluid provides variable coupling according to the proportion of fluid in the reservoir vis - a - vis the fluid shear chamber defined by the complementary surface . a temperature responsive valve is provided to modulate the shear fluid level . the valve may modulate an overlapping inlet having a constant area . an overlap inlet may be define by an arcuate slot or by a fixed size valve aperture . the valve may be pivotable on a valve axis which is eccentric to , and radially outward of , the common axis of the driving and driven members . temperature responsive rotation of the valve member may be provided by coupling links or by gearing mechanism .