Because the selection and application of a contact material generally involves many different electrical, mechanical, metallurgical, and cost factors, each case should be evaluated individually. Usually, each type and model of an electrical device is unique.
Wide experience in the selection and fabrication of electrical contacts is invaluable when contacts are to be designed. As a regular part of its business process for decades, CMW assists customers with various types of electrical contact applications.
To Engineer an Electrical Contact
1.) Study the Problem
The complexity of contact phenomena is such that a final selection of a contact material and design cannot always be made from existing data. The choice of material and a design can best be made by balancing the physical and electrical properties of available contact materials against the electrical and mechanical characteristics of the apparatus.
The problems encountered when working on electrical contacts are basically four:
Electrical factors are directly related to the type and magnitude of the current, the voltage of the circuit (and also across the contacts), and electrical circuit parameters.
Mechanical considerations include contact force, spacing, bounce, opening and closing rates and wiping in the mechanism using the electrical contact, as well as the duration of open and closed periods.
Metallurgical studies are concerned with the chemical characteristics, together with the mechanical, electrical and thermal properties of the contact material.
Cost factors involve contact materials, sizes, tolerances, shapes and method of fabrication. The total cost of the contact and it's sub-assembly on the assembled apparatus is a prime consideration.
2.) Annex CMW Manufacturing and Engineering Experience
By consulting with the CMW representatives during the engineering and design stages of a contact application, you effectively add the full scope of CMW experience, manufacturing and engineering facilities to their own. Not only these extensive resources, but also the broad range of CMW experience in the fields of metallurgy, electro-mechanics and electro-chemistry is dedicated toward adding CMW's specialization in contact manufacture to the your facilities.
3.) Select a Sound, Low Cost Solution
CMW representatives help customers assemble all the facts concerning their application thereby minimizing the overall cost of engineering electrical contacts. Based on cumulative experience, CMW will help you identify a sound, low-cost solution. The extent and variety of available production equipment at CMW permits unrestricted selection of economical methods of fabrication of the actual contacts or contact assemblies required. Proven techniques and specialized methods cut manufacturing costs without sacrificing quality.
Some Design Considerations:
Electrical
Voltage
Circuit Voltage
Voltage at the contacts during circuit interruption
Current
Amperes
Direct Current
Alternating Current
Frequency
Type of Load
Inductive
Capacitive
Resistive
Motor load
Overload Requirements (if any)
Contact protection or arc suppression
Condenser circuit
Resistor circuit
Mechanical
Contact Force
Static force with contacts closed
Impact force at instant of closing
During operation (if contacts operate at high frequency)
Force available to open the contacts
Frequency of Operation
Number of makes and breaks per unit of time
Speed of Opening and Closing
Wipe or Slide between contacts on closure, or butt type closure
Chatter during opening, or bounce during closing
Contact gap when fully open
Method of operating contacts
Mechanical (by cam)
Electro-magnetic (by coil and armature)
Thermostatic (by bi-metal)
Manual (simple lever or push-button)
Environmental
Atmosphere in which contacts operate
Air
Other gases such as nitrogen
Presence of tarnishing gases
Salt air
Humidity
Dust
Particles from wear of adjacent parts
Pressure of atmosphere
Ambient temperature
Maximum
Minimum
Contacts in open, or sealed
Determine Service Requirements
Contact Resistance
Can affect both reliability and operating life. Specific resistance must permit carrying of required current without destructive heating. Interface resistance must not prevent surety of the circuit.
Contact Erosion
May result from burning under arcing, or mechanical wear. Erosion rate must be evaluated in choosing both contact material and design.
Material Transfer
A deposit of material on one contact from another, with corresponding peaks and craters. Excessive transfer may interfere with proper operation or calibration of many devices.
Contact Sticking (or welding)
Occurs when contacts fuse together due to heat from electrical arcing or excessive electrical overloads. Material selection, provision for heat dissipation, and opening forces must be adequate for the device.
Materials and Designs
A particular application generally requires one or more of the unique properties of a class of materials. Further analysis of a suitable material class often indicates that a single material will do the best job. Some basic material classes are:
CMW Elkonite® Silver Tungsten and Copper Tungsten materials
Silver Semi-Refractory Compositions
Silver Cadmium Oxides
Silver Tin Oxides
Silver Nickels
Silver Graphites
Silver and basically silver alloys
Fine Silver
Coin Silver
Noble Metals
Copper and copper base alloys
Tungsten and Molybdenum
CMW supplies Contact Materials in 10 Basic Designs
Rivet contacts
Solid button contacts
Composite button contacts
Screw contacts
Springs
Stampings
Discs, Washers, Rectangles
Special molded shapes
Wire, rod, bar, strip
Castings, forgings, extrusions
CMW is also proficient in manufacturing complete contact assemblies
