The invention relates to a contact bolt of the type set forth in the preamble of claim 1.

A contact bolt of this type, which is known for example from DE-OS 3,632,469, must fulfill a triple function: firstly, it must ensure the current supply through the housing, secondl it must form in the interior of the housing a contact sur¬ face for the cooperation with a movable switch contact and thirdly at the outside of the housing it must serve as con- tacting and securing point for connection of the current connection cables.

Because of these three functions contact bolts are subjected to very high electrical, thermal and mechanical stresses. For example, in a starting operation very high currents occur, typically of 200 to 2000 amps. Failure of the con¬ tacting, for example by contact burn-up, renders the switch and the starter motor inoperative. Even worse is a welding or fusing of the contacts. In this case the starter motor is not switched off after the internal-combustion engine has started, the switching relay remaining in engagement, and this leads in extremely short time to complete destruct¬ ion of the starter motor.

To match the high currents correspondingly largely dimensioned and heavy cables must lead to the starter motor to keep the voltage drop and heat losses as low as possible. Due to the mechanical connection of these heavy cables, coupled with the vibrations of the engine, the contact bolt is also sub¬ jected to high mechanical stresses.

Hitherto, two types of contact bolts were usual.

In the first type the bolt consists of copper. This has the advantage that the voltage drop both in the bolt and in the switch contact is small and that due to the good thermal conductivity of copper the heat dissipation through the bolt out of the switch is very good.

Disadvantageous is firstly the high price of such a bolt, which must be turned from solid copper, and the low mechanical strength of the bolt, as a result of which the load of the cable can lead to fracture of the bolt. Also, copper does not have ideal properties for a switch contact surface.

Under high load or with high switching frequency the contact burn-up is relatively great and under unfavourable conditions in spite of the good heat dissipation out of the contact contact welding may occur.

In the second type the bolt consists of steel. It is dis¬ tinguished by very high mechanical strength so that the danger of breakage is reduced. Since steel has poor contact properties, at the switch contact point, i.e. in the head of the contact bolt, a contact bead of copper is inserted. In this type failure due to breakage of the contact bolt is practically completely eliminated. However, due to the high resistance of the steel bolt, the transition resist¬ ance from the contact bead to the bolt and the poor thermal conductivity of the steel bolt very high temperatures occur

in the region of the switch contact. As a result the danger of contact fusing or welding is very great. Admittedly, this danger can be reduced by replacing the contact bead of copper by one of a relatively weld-resistant contact ater- ial. The high temperatures however are not reduced by this step and as a result contact burn-up and the tendency to welding remain troublesome.

DE-PS 631,839, 898,498 and 926,745 each disclose contact pins for electrical contacts which consist of a sheath of base mat and a core arranged in a longitudinal bore of the sheath and consisting of a material of high electrical conductivity, for example copper or noble metal. The sheath serves to mechanically strengthen the contact pin and may consist for example of steel. The core may have a widening at the head of the contact pin and form the contact surface. Although this increases the strength with simultaneous good electrical conductivity of the contact pin, it does not however solve the problem of contact burn-off at the contact surface.

The invention is based on the problem of providing a contact bolt or pin of the aforementioned type which is simple and economical to make and which is distinguished by high contact loadability of the contact surface with simultaneous good thermal conductivity and high mechanical loadability.

The solution of the problem characterized in the claim has the advantage that the contact pin according to the in¬ vention consists of three different materialswhich with regard to the various functions of the contacting at the contact surface, the good electrical thermal conductivity and the good mechanical loadability can be specifically selected independently of each other and the core of material of good conductivity additionally performing the function of

a connecting element holding the parts together.

An embodiment of the invention will be explained in detail. According to the drawing the contact pin or bolt 3 consists of a threaded shank 4 and the for example hexagonal head 5 which are integrally formed and are made from steel or a similar material with high mechanical strength. Extending in a central bore of the contact bolt 3 is a core 6 of a material with high electrical and thermal conductivity, pre- ferably copper.

A contact bead 8 is placed on the head 5 of the steel bolt 3. The core 6 of copper or the like extends through bores of the steel bolt 3 and the contact bead 8 and after in- sertion is upset at both ends so that the steel bolt 3 and the contact bead 8 are thereby riveted together and mechanic¬ ally joined.

The upsetting operation leads to the copper core 6 completely filling the bore of the steel bolt and the contact bead 8. As a result an intimate mechanical and electrical contact of the copper core 6 with the steel bolt 3 and the contact bead 8 is established and consequently the thermal conduct¬ ivity of the contact bolt as a whole is improved and in particular excellent heat dissipation from the contact area is ensured.

The contact bead 8 consists. of a contact material which is resistant to burn-up and welding or fusion, in particular in the form of suitable metal alloys such as copper-nickel, copper-graphite, tungsten-molybdenum or tungsten-nickel alloys. The contact bead 8 can in particular be made by powder metallurgy techniques.

By corresponding configuration and shape of the contact and of the cross-sectional dimensioning the mechanical, electrica and thermal properties of the contact bolt can be varied within wide limits. In spite of its superiority to convent¬ ional contact bolts the bolt according to the invention can be economically made in mass production.