Solenoid drive device

A solenoid drive device including

• a cylindrical coil,
• a core placed radially inward of the cylindrical coil and having a bottomed cylindrical inner surface, and
• a plunger that can be displaced along an axial direction of the core and is subjected to biasing force in a direction in which the plunger is separated from a bottom surface portion of the core along the axial direction, and that is placed radially inward of the core,
• the solenoid drive device being configured so that, according to an amount of current that is applied to the cylindrical coil, the plunger can be displaced between a maximum stroke position as a position where a tip end surface of the plunger which faces the bottom surface portion is located farthest from the bottom surface portion due to the biasing force when no current is applied to the cylindrical coil and a minimum stroke position as a position where the tip end surface of the plunger is located closest to the bottom surface portion by current application to the cylindrical coil, wherein
• a biasing direction is a direction in which the plunger is biased by the biasing force along the axial direction, and an opposite biasing direction is a direction opposite to the biasing direction,
• a first imaginary boundary plane as an imaginary plane perpendicular to the axial direction is set in a boundary portion between a magnetic flux limiting portion and an increased diameter portion of the core so as to be located at the maximum stroke position in the axial direction or a position located on a side in the biasing direction of the maximum stroke position, and a second imaginary boundary plane as an imaginary plane perpendicular to the axial direction is set at the minimum stroke position in the axial direction or a position located on the side in the biasing direction of the minimum stroke position and on a side in the opposite biasing direction of the maximum stroke position,
• the magnetic flux limiting portion is located in a region on the side in the biasing direction of the first imaginary boundary plane, and is formed so as to have a smaller radial thickness than a region on the side in the opposite biasing direction of the first imaginary boundary plane,
• the increased diameter portion is formed so that its radial thickness is increased in a continuous or stepwise manner from the first imaginary boundary plane toward the side in the opposite biasing direction,
• an increased diameter reference line is a straight line connecting a first reference point and a second reference point, the first reference point being an intersection of the first imaginary boundary plane and a radial outer surface of the core in an axial section of the core taken along a plane including a central axis of the core, and the second reference point being an intersection of the second imaginary boundary plane and the radial outer surface of the core in the axial section, and
• the increased diameter portion has a recessed portion whose outer surface is recessed toward a radial inner surface with respect to the increased diameter reference line.