Imparting compressive hoop stress into a bonded magnetoelastic element by means of diameter reduction of the underlying shaft

Abstract

A method for producing a low-hysteresis magnetoelastic element comprising applying a magnetoelastic material onto a metal substrate. After the magnetoelastic element is securely bonded to the underlying shaft, a reduction in the outside diameter of the shaft draws the magnetoelastic element radially inward causing it to occupy a smaller volume. This process provides the magnetic anistropy required for the sensor to operate properly, and also results in a densification of the magnetoelastic element making it inherently stable. Several methods of reducing the outside diameter of the shaft to draw the magnetoelastic element radially inward have been devised. Prior to deposition of the magnetoelastic material, the outside diameter of a hollow shaft is elastically expanded by applying force to its inside diameter. This is accomplished by using an expanding insert, by pressurizing the cavity within the shaft, or by inducing a managed thermal expansion of the shaft prior to material deposition. Post-deposition operation can also be implemented to reduce the shaft diameter, such as drawing the shaft axially until it yields. Any process that results in a shaft reduction after the magnetoelastic active material is deposited on the shaft can be used in the present invention.