GB2102705A

GB2102705A - Electrical contact

Info

Publication number: GB2102705A
Application number: GB08109111A
Authority: GB
Inventors: John Kevin Neary
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-03-24
Filing date: 1981-03-24
Publication date: 1983-02-09

Abstract

The contact making face of the contact of a light current electromagnetic relay is provided with a pair of parallel ribs faced with or composed of precious metal. The ribs of a co-operating similar contact extend at right angles to those of the first rib. The precious metal (2) may be palladium silver and the body (1) of the contact copper nickel. <IMAGE>

Description

SPECIFICATION Electrical contact This invention relates to the provision of contacts on contact springs such as for instance those of the contact pile-up of a light current contact making electromagnetic relay.

It has been the practice to use bar contacts for this purpose in which the contacting surface of a contact is domed in one plane only, while that of its co-operating contact is similarly domed, but with its doming in the orthogonal plane.

For a typical pair of contacts mounted side by side near one end of the contact spring of a light current relay each contact may have a base measuring approximately 1 mm by 1 mm. This base is typically 0.4 mm high, is made of copper/nickel, and is coated with palladium silver contact metal to a depth of about 0.25 mm.

Under direct current testing conditions, using 50 volts to deliver a current of 1 amp into a resistive load, and performing a switching cycle of operations at the rate of 5 Hz it was found that craters tended to form in the contact metal over a relatively small proportion of the total surface of the contact face.

Typically the crater was found to have a diameter of between a third and a quarter of the width of the contact. Eventually such cratering is liable to penetrate the entire thickness of the precious contact metal and expose the underlying base metal, whereupon the risk of contact welding or of open contact conditions occuring is significantly increased.

The present invention is concerned with a way of making more efficient use of the precious contact metal. It also provides a contact design which enables the contact to be welded to the contact spring without having to pass the electric welding current through the operating surface of the precious contact metal and thus risking the contamination of this surface with weld electrode material.

According to the present invention there is provided an electrical contact the operating surface of which contact is provided with a pair of projecting parallel ribs faced with or composed of precious contact metal.

Typically such contacts are produced when they are required for welding to the contact springs by cropping off an appropriate length from a piece of stock.

Accordingly the invention also embraces a strip of stock from which such contacts can be cut.

There follows a description of the contacts of a contact pile up of a light current relay, the design of these contacts embodying the invention in a preferred form. The description refers to the accompanying drawings in which: Figure 1 depicts a piece of stock from which the contacts are cut, and Figure 2 depicts a contact in position on a contact spring and shows the relative orientation of its co-operating contact.

Contacts for a relay are cut from a strip of material depicted in Fig. 1. This strip consists of a main part 1 of base metal, typically copper nickel, from the top surface of which protrude two ribs each bearing a coating 2 of precious contact metal, typically palladium silver. Two smaller ribs 3 project from the bottom surface of the main part 1 of the strip to facilitate the welding of contacts cut from the strip to contact springs (not shown in Fig.

1).

The precious metal coatings 2 are rounded on top, and typically their maximum thickness is greater than their width, though this is not essential for all applications. The thickness of coating required will depend upon the application and upon the lifetime requirements, but will typically lie between 200 and 250 microns. The total thickness of the strip is typically about 700 microns and its width at the widest point about 1.1 mm.

Fig. 2 shows a contact cut from the strip of Fig. 1 and in position for welding on to a contact spring 4. Preferably the welding tool is introduced between the precious metal coatings 2 to the exposed region 5 of base metal so that the welding operation should leave the precious metal in its pristine condition.

Fig. 2 shows in broken outline the position of the co-operating contact. The two contacts are arranged with the ribs of one extending subtantially at right angles to those of the other so that when they mate they will touch at four places 6.

It may be that initially contact will not be made at each one of the four places because of small misalignments, but in the course of use this situation is liable to change because the first places to come into contact in a switching operation are liable to wear at a faster rate than those contacting later.

1. An electrical contact the operating surface of which contact is provided with a pair of projecting parallel ribs faced with or composed of precious contact metal.

2. A contact as claimed in claim 1 wherein the thickness of precious metal on the rib is greater than its width.

3. A contact as claimed in claim 1 or 2 wherein no precious metal is present in the region between its two ribs.

4. A contact as claimed in claim 1, 2 or 3 wherein on the face opposite its operating surface the contact is provided with a pair of welding projections.

5. A contact as claimed in any preceding claim wherein the precious contact metal is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Electrical contact This invention relates to the provision of contacts on contact springs such as for instance those of the contact pile-up of a light current contact making electromagnetic relay. It has been the practice to use bar contacts for this purpose in which the contacting surface of a contact is domed in one plane only, while that of its co-operating contact is similarly domed, but with its doming in the orthogonal plane. For a typical pair of contacts mounted side by side near one end of the contact spring of a light current relay each contact may have a base measuring approximately 1 mm by 1 mm. This base is typically 0.4 mm high, is made of copper/nickel, and is coated with palladium silver contact metal to a depth of about 0.25 mm. Under direct current testing conditions, using 50 volts to deliver a current of 1 amp into a resistive load, and performing a switching cycle of operations at the rate of 5 Hz it was found that craters tended to form in the contact metal over a relatively small proportion of the total surface of the contact face. Typically the crater was found to have a diameter of between a third and a quarter of the width of the contact. Eventually such cratering is liable to penetrate the entire thickness of the precious contact metal and expose the underlying base metal, whereupon the risk of contact welding or of open contact conditions occuring is significantly increased. The present invention is concerned with a way of making more efficient use of the precious contact metal. It also provides a contact design which enables the contact to be welded to the contact spring without having to pass the electric welding current through the operating surface of the precious contact metal and thus risking the contamination of this surface with weld electrode material. According to the present invention there is provided an electrical contact the operating surface of which contact is provided with a pair of projecting parallel ribs faced with or composed of precious contact metal. Typically such contacts are produced when they are required for welding to the contact springs by cropping off an appropriate length from a piece of stock. Accordingly the invention also embraces a strip of stock from which such contacts can be cut. There follows a description of the contacts of a contact pile up of a light current relay, the design of these contacts embodying the invention in a preferred form. The description refers to the accompanying drawings in which: Figure 1 depicts a piece of stock from which the contacts are cut, and Figure 2 depicts a contact in position on a contact spring and shows the relative orientation of its co-operating contact. Contacts for a relay are cut from a strip of material depicted in Fig. 1. This strip consists of a main part 1 of base metal, typically copper nickel, from the top surface of which protrude two ribs each bearing a coating 2 of precious contact metal, typically palladium silver. Two smaller ribs 3 project from the bottom surface of the main part 1 of the strip to facilitate the welding of contacts cut from the strip to contact springs (not shown in Fig. 1). The precious metal coatings 2 are rounded on top, and typically their maximum thickness is greater than their width, though this is not essential for all applications. The thickness of coating required will depend upon the application and upon the lifetime requirements, but will typically lie between 200 and 250 microns. The total thickness of the strip is typically about 700 microns and its width at the widest point about 1.1 mm. Fig. 2 shows a contact cut from the strip of Fig. 1 and in position for welding on to a contact spring 4. Preferably the welding tool is introduced between the precious metal coatings 2 to the exposed region 5 of base metal so that the welding operation should leave the precious metal in its pristine condition. Fig. 2 shows in broken outline the position of the co-operating contact. The two contacts are arranged with the ribs of one extending subtantially at right angles to those of the other so that when they mate they will touch at four places 6. It may be that initially contact will not be made at each one of the four places because of small misalignments, but in the course of use this situation is liable to change because the first places to come into contact in a switching operation are liable to wear at a faster rate than those contacting later. CLAIMS

5. A contact as claimed in any preceding claim wherein the precious contact metal is palladium silver.

6. A contact as claimed in any preceding claim wherein the material of the contact carrying the precious contact metal is copper nickel.

7. A contact substantially as hereinbefore described with reference to the accompanying drawings.

8. A contact spring carrying a contact or pair of contacts as claimed in any preceding claim.

9. A pair of co-operating contact springs as claimed in claim 8 wherein the ribs of the or each contact of one contact spring extend substantially at right angles to those of the or each contact of the other contact spring.

1 0. A light current electromagnetic relay provided with one or more pairs of co-operating contact springs as claimed in claim 9.

11. A strip of stock from which contacts as claimed in any claim of claims 1 to 7 can be cut which strip has the same cross-section as that of said contacts.