1. Field of the Invention
The present invention relates to a magnetic switch for a starter used in an internal combustion engine, and in particular, relates to an improvement in a structure of a magnetic switch for a starter including an electromagnet having a hollow cylindrical coil which controls the current supply between a battery and a starter motor.
2. Conventional Art
As an example of conventional structures of magnetic switches for starters, U.S. Pat. No. 5,256,992 discloses a magnetic switch in which a rectangular plate shaped movable contact is disposed on a movable control rod between an annular collar shaped head at the end thereof and a helical spring arranged around the rod, while interposing at the respective faces of the plate shaped movable contact a pair of an insulative washer and an insulative bush having a protrusion, and the other end of the helical spring is supported by a guide sleeve fixed to the other end of the rod.
Further, U.S. Pat. No. 5,757,256 discloses another example in which a rectangular plate shaped movable contact is disposed on a presumably insulative movable control rod between a constant spring and a metal washer fixed axially on the rod against a tapered shoulder thereof by a clawed metal washer at the axial end of the control rod, and the other end of the constant spring is supported by another shoulder formed integrally on the control rod.
Still further, JP-A-4-337223(1992), JP-A-07-253072(1995) and JP-A-11-30169(1999) discloses still other examples in which a rectangular plate shaped movable contact is disposed on an electrically conductive movable shaft between a movable contact holding member fixed at an end thereof and a contact pushing spring, while interposing at the respective faces of the plate shaped movable contact a pair of an insulative washer and an insulative bush having a protrusion for insulating the movable contact from the movable shaft, and the other end of the contact pushing spring is supported by an annular collar formed integrally on the movable shaft.
Further, in the above examples an inner stationary core forming a magnetic flux passage together with a casing thereof for a hollow cylindrical coil is usually formed through cold forging to have a cylindrical protrusion so as to secure the hollow cylindrical coil at the circumference thereof together with the casing and to permit axial movement of the movable rod or shaft of non-magnetic material, therefore, an elastic member such as a leaf spring is interposed between the side face of the casing and the hollow cylindrical coil so as to prevent vibration of the hollow cylindrical coil with respect to the casing.
With the structure including the fitting member secured to the shaft and/or shaft end caulking and for receiving an impact force of the plate shaped movable contact which is caused when recovering compressed distance of the contact pressing spring corresponding to over travelling of the rod immediately after being released from activation or deenergization of the magnetic switches for the starters as has been explained above, it was difficult to adjust the grip strength of the fitting member and the caulking strength at the shaft top end which can endure the repeating impact force from the plate shaped movable contact as well as it took time for the adjustment. On the other hand, the structure which receives the impact force with the collar formed integral with the shaft satisfies a required mechanical strength, however, since a spring stopper has to be provided at he remote side of the contact pressing spring from he movable contact along the shaft, while interposing intermediate members at respective faces of the movable contact plate, the total length of the shaft carrying the movable contact is prolonged as well as the diameter of the shaft receiving through hole in the inner stationary core cylindrical protrusion has to be enlarged which reduces the cross sectional area of the magnetic flux passage circuit in the stationary magnetic core, or otherwise enlarges the radial size of the stationary magnetic core. Moreover, in the conventional structure, in order to fixedly secure the hollow cylindrical coil in the casing together with the inner stationary core, such as a leaf spring has to be interposed between the hollow cylindrical coil and the side face of the casing, which causes an increase of parts number and assembly manhour to increase the manufacturing cost thereof.