An electromagnetic device for controlling the current supply to the electric starter motor of an internal combustion engine for motor vehicles

Abstract: The device comprises an electromagnet (3) including a solenoid (6) for controlling a movable contact (8) which is adapted to cooperate with a pair of fixed contacts (10, 10a) carried by a support (2) of electrically-insulating material. Each fixed contact comprises a screw-like, threaded metal member (10) clamped to the support (2) with a predetermined force by a respective nut (11). The threaded members (10) constituting the fixed contacts are made of a copper-based alloy including chromium and zirconium.

Description: The present invention relates to an electromagnetic device for controlling the current supply to the electric starter motor of an internal combustion engine for motor vehicles comprising an electromagnet including a solenoid for controlling a movable contact which is adapted to cooperate with a pair of fixed contacts carried by a support of electrically-insulating material, each fixed contact comprising a screw-like, threaded metal member clamped to the support with a predetermined force by a respective nut. An electromagnetic device of the type specified in the preamble of claim 1 is known from PCT patent application WO-A-87/02824, where the material of the fixed contacts is not specified. In applications in which the fixed contacts have to be clamped to the insulating support with a fairly high clamping torque (for example, 15 Nm or more) the fixed contacts typically have a two-component structure; a steel screw suitable for withstanding the clamping torques envisaged has a recess in its head, in which a pellet of electrolytic copper is fixed for cooperating with the movable contact of the electromagnet. The copper pellet may be fixed to the head of the steel screw by braze-welding or clamped thereto mechanically, for example by upsetting. Fixed contacts produced in this way are quite expensive. Moreover, the copper-steel connection involves a voltage drop and also problems of reliability. European patent application EP-A-0 172 912 relates to a vacuum breaker having a fixed contact and a movable contact, each constituted by an alloy pellet soldered to a respective end of a conducting rods and in which the material used for the pellets is a copper-based alloy including chromium, approximately 25% by weight, and zirconium, up to 5% by weight. The results of tests using different quantities of chromium and zirconium are reported and an insufficient weld-resisting ability and breakdown voltage was found for material using 0.3 % by weight of zirconium and less than 20% by weight of chromium. The material used for the contacts is different from the material used for the conductors and it causes problems of cost as in the case mentioned above. The object of the present invention is to provide an electromagnetic device of the type specified above, whose fixed contacts overcome the problems explained above. According to the invention, this object is achieved by means of a device of the type specified above whose main characteristic lies in the fact that the threaded members constituting the fixed contacts are made of copper-based alloy including chromium and zirconium, said alloy comprising: between 98.5%, and 99.67%, preferably 99.1%, by weight of copper, between 0.3% and 1.2%, preferably 0.8%, by weight of chromium, and between 0.03% and 0.3%, preferably about 0.1%, by weight of zirconium. The fixed contacts may be formed either by machining, that is, by turning, or by die-forming. Further characteristics and advantages of embodiments of the invention will become clear from the detailed description which follows with reference to the appended drawing provided purely by way of non-limiting example, which is a partially-sectioned view of an electromagnetic device for controlling the current supply to the electric starter motor of an internal combustion engine for motor vehicles. Figure 1 shows a device embodying the invention, generally indicated 1. It includes, in known manner, a support element 2 of electrically-insulating material which, in the embodiment illustrated, is substantially cup-shaped. An electromagnet, generally indicated 3, is fixed to this element. The electromagnet includes a casing containing a spool 5 carrying a control winding or solenoid 6. A movable core, generally indicated 7 is associated with the solenoid. The movable core has a head 7a which extends into the cavity in the support element 2 and carries a plate 8 of electrically-conductive material, preferably copper, which constitutes the movable contact. The end wall of the support element 2 has two holes 9 through which the shanks of two screws 10 extend. In the embodiment illustrated, the screws have hexagonal heads 10a which, in the assembled condition, face a side portion of the plate 8 constituting the movable contact. The screws 10 are clamped to the support 2 by respective nuts 11, with the interposition of washers 12. The screws 10 are formed in one piece from a copper-based alloy including small quantities of chromium and zirconium. In particular, the alloy includes: between 98.5% and 99.67%, preferably 99.1%, by weight of copper, between 0.3% and 1.2%, preferably 0.8%, of chromium, and between 0.03% and 0.3%, preferably about 0.1%, by weight of zirconium. For example, such an alloy is the alloy No. 2.12.33 conforming to DIN standard 17666. As stated above, the screws 10 may be formed either by machining (turning) or by die-forming. The fixed contacts thus produced have very good electrical conductivity, better than that of the prior-art contacts made of copper and steel, and can whithstand the highest clamping torques currently envisaged.

Claims: An electromagnetic device for controlling the current supply to the electric starter motor of an internal combustion engine for motor vehicles, comprising an electromagnet (3) including a solenoid (6) for controlling a movable contact (8) which is adapted to cooperate with a pair of fixed contacts (10, 10a) carried by a support (2) of electrically-insulating material, each fixed contact comprising a screw-like, threaded metal member (10) clamped to the support (2) with a predetermined force by a respective nut (11), characterised in that the threaded members (10) constituting the fixed contacts are made of copper-based alloy including chromium and zirconium, said alloy comprising: between 98.5%, and 99.67%, preferably 99.1%, by weight of copper, between 0.3% and 1.2%, preferably 0.8%, by weight of chromium, and between 0.03% and 0.3%, preferably about 0.1%, by weight of zirconium. A device according to Claim 1, characterised in that the fixed contacts are formed by machining. A device according to Claim 1, characterised in that the fixed contacts are formed by die-forming.

Inventors: BUSCHI LUCIANO, UGO BRUNO, BUSCHI, LUCIANO, UGO, BRUNO
Applicants: MAGNETI MARELLI SPA, INDUSTRIE MAGNETI MARELLI S.P.A.
Application year: 1991
Main classification: H01H1
IPC codes: H01H1, H01H51