Wireforms for electrical switches and relays: contact pressure and fatigue

The wireform is the spring in the switching device

In a switch or a relay, the formed wire part usually does three jobs at once. It carries current to the contact, it stores the energy that produces contact force, and it allows a small sliding movement, called wipe, that breaks through surface film. The shape is simple, but the part sits between electrical performance and mechanical life.

That combination is why a switch wireform cannot be designed as a pure spring or as a pure conductor. A material chosen for conductivity may have poor fatigue strength. A shape chosen for force may be too stiff to assemble. The design has to satisfy the contact, the mechanism and the production line at the same time.

Contact force is a window

Every switching device has a minimum contact force below which contact resistance rises, chatter appears and the contact welds or erodes faster. It also has a maximum force above which the actuator struggles, the wipe becomes excessive and the contact face wears. The design target is a window between those limits, with tolerance on both sides.

The window has to hold over the full range of wire diameter, temper, plating thickness and installed height. A spring that sits in the middle at nominal conditions can fall outside the window when the wire runs to the top of its diameter tolerance and the plating adds thickness. That is why the drawing should state the load at a defined installed height, with the height measured from a real datum rather than a nominal stack.

Stress at the root: a quick check

A cantilever contact spring is a beam fixed at one end and loaded at the other. The bending stress at the root is proportional to the material modulus, the thickness and the deflection, and inversely proportional to the square of the active length. For a rectangular section the usual form is stress equals three times modulus times thickness times deflection, divided by two times the square of the length.

Take an illustrative phosphor bronze spring with a modulus of about 110 GPa, a thickness of 0.4 mm, an active length of 12 mm and a deflection of 0.6 mm. The root stress works out at roughly 275 MPa. That is a useful number because it can be compared with the fatigue strength of the material at the intended stress ratio. If the result is too high, the options are thinner wire, a longer active length or less deflection. Increasing the contact force is not one of them.

The calculation is a screening tool, not a validation. It ignores the radius at the root, the shape of the contact crown and the way the wire is clamped. A finite element model or a physical fatigue test is the right next step for a part that will cycle millions of times.

Fatigue is driven by the stress range

A switch that cycles once a minute for ten years sees more than five million operations. For illustration, a relay in a control panel may see far fewer, and a domestic switch may see more. The fatigue life depends on the alternating stress range, the mean stress and the surface condition of the wire, not just the peak stress at assembly.

Formed wire has two features that concentrate stress: the bend radius and the clamp point. A tight bend on the outside of the spring puts the highest tensile stress exactly where the surface is most likely to have a drawing mark. A sharp clamp edge creates a dent that behaves like a crack starter. Both are design issues, and both are cheaper to fix on the drawing than in a failed batch.

• Keep the inside bend radius at or above one wire diameter for spring temper material
• Avoid a clamp on a bend; clamp on a straight leg
• Specify the stress relief or shot peening only if the process route supports it
• Check the surface for drawing marks, seams and plating pits
• Test at the stress ratio the application will actually see

Wipe and overtravel are not the same thing

Wipe is the small sliding motion at the contact face as the spring deflects. It breaks oxide and contamination and helps the contact settle. Overtravel is the extra travel after the contacts touch, which stores force and absorbs tolerance. A design can have plenty of overtravel and no wipe if the contact face moves purely perpendicular to the mating surface.

Both need to be visible on the drawing. Wipe is often defined as a tangential movement over the contact stroke, and overtravel as a minimum travel after first touch. If they are left to the assembly, the switch may pass a force test and still fail a contact resistance test after a few thousand operations.

Materials and plating for the contact spring

Phosphor bronze is the common choice for a contact spring that needs moderate conductivity and good fatigue behaviour at a sensible cost. Beryllium copper offers higher strength and conductivity but costs more and brings handling requirements. Stainless steel is used where corrosion resistance matters more than conductivity, often with a separate contact rivet or pad. Nickel silver sits between the copper alloys and stainless on conductivity.

Plating changes both contact resistance and fatigue. A thin tin layer is cheap and solderable but can fret and oxidise. Nickel under gold is a common combination for low level signals. Silver gives low resistance at higher currents but tarnishes and can migrate. Whatever the finish, the plating thickness has to be stated, and the bend radius has to be large enough that the coating does not crack at the outside of the bend.

Validation and the drawing calls that support it

A switch wireform should be validated with three checks: load at the installed height, contact resistance after a defined number of operations, and a fatigue or life test at the application stroke. Add a visual check for plating damage at the bends and a dimensional check on the features that set the stroke.

On the drawing, state the material grade and temper, the wire diameter with tolerance, the load at the installed height, the active length, the bend radii that matter, the plating thickness and the contact zone. Mark the surfaces that must not be plated or must stay bare. Then agree the inspection method, because a load measured on a compression platen is not the same as a load measured in the housing.