A device generates electrical energy from mechanical motion in a downhole environment. The device includes a magnetostrictive element and an electrically conductive coil. The magnetostrictive element has a first end and a second end. The first and second ends are coupled between a rotor and a bearing. The magnetostrictive element is configured to experience axial strain in response to radial movement of at least one of the rotor or the bearing with reference to the other. The electrically conductive coil is disposed in proximity to the magnetostrictive element. The coil is configured to generate an electrical current in response to a change in flux density of the magnetostrictive element.