In an example, a device includes a base unit, a cover unit, and a hinge assembly to couple the cover unit to the base unit The hinge assembly is moveable between a folded position and an unfolded position. The hinge assembly includes a channel, an annular holder disposed in the channel, a shaft engaged with the annular holder, and a hydraulic cylinder coupled to the shaft. The hydraulic cylinder is to move the shaft along a longitudinal axis of the shaft, based on a movement of the hinge assembly between the folded position and the unfolded position, to vary an operating torque of the hinge assembly.