Contact force and bounce: the two spring calls that decide reliability

Why force is the first variable

Two contacts touch at a small number of spots, and the real contact area grows with force. More area means lower constriction resistance, lower local temperature and a better chance of breaking through an oxide film. Force is the variable that ties the mechanical design to the electrical result.

Force also sets the ability to resist welding. A contact that closes with enough force has a larger real area, which spreads the current and reduces the local heating that forms a molten bridge. The same force gives the mechanism a better chance of breaking a weak weld when it tries to open.

The catch is that force is not free. A stiffer spring needs more actuator energy, puts more stress on the hinge and can increase the impact velocity at make. That is why force and bounce have to be designed together rather than in sequence.

The force stack

A contact mechanism has several force values that are often confused. The initial contact force is measured at the instant the contacts first touch. The final contact force is measured at the rest position, after the mechanism has travelled through its overtravel. The difference comes from the spring rate and the overtravel distance.

Overtravel is the travel available after first touch. It does two jobs: it builds the final force, and it provides the reserve that absorbs contact wear and mounting tolerance. A mechanism with no overtravel loses force as soon as the contacts erode or the stack settles.

Contact follow is the related measure of how much the spring deflects after touch. It is usually expressed as a distance, and it converts to force through the spring rate. If the drawing gives only the final force, the supplier cannot set the mechanism without knowing the follow.

How much force is enough

There is no universal number. A signal relay may work with a few millinewtons, a power relay with a fraction of a newton, and a contactor with several newtons or more. The right value depends on the contact material, the load, the environment and the film that forms on the face.

The practical method is a force-resistance curve. Measure contact resistance at several forces with the real material and the real surface finish, and find the point where the curve flattens. Below that point, a small change in force causes a large change in resistance. Above it, extra force buys little electrical benefit.

Then add margin for life. Set the minimum force above the knee of the curve by an amount that covers wear, relaxation and the worst-case film. If the spring cannot reach that force within the space, the contact material or the face form may need to change.

What bounce does

Bounce is the rebound of the contact after first touch. Instead of closing once, the contacts make and break several times in a few milliseconds before settling. Each break draws an arc at a current that may be close to the full load current, and each make re-establishes the spot.

The electrical effect is a burst of noise and a series of erosion events. In a logic circuit, the noise can be read as multiple pulses. In a power circuit, the arcs erode the face and can weld it. The mechanical effect is impact fatigue at the contact and the spring.

Bounce is usually measured as the time from first contact to the last break, or as the total duration of the chatter. A clean contact may settle in well under a millisecond. A poorly damped mechanism can chatter for several milliseconds. Both figures are illustrative, because the acceptable value depends on the circuit.

What causes bounce

The energy at impact is the main cause. A heavier armature, a higher closing velocity or a stiffer spring stores more kinetic energy, and that energy has to go somewhere when the contacts stop. Some of it is absorbed by the contact and the spring, and some of it rebounds.

The stiffness of the contact support matters. A thin blade flexes on impact and returns, which adds a secondary oscillation. A rigid support with a damping element absorbs the energy instead. The mounting of the fixed contact is as important as the moving armature.

The drive also contributes. A solenoid that pulls in too fast hits the pole face hard and transfers the shock to the contacts. A mechanism with a pre-travel stop, a damped armature or a two-stage pull-in can reduce the impact velocity without reducing the final force.

Reducing bounce without losing force

Start with the impact velocity. A slower final approach reduces the kinetic energy that has to be absorbed, and it can be achieved with a damper, a lost-motion link or a modified solenoid drive. The final force can stay the same because it is set by the overtravel and the spring rate, not by the velocity.

Then look at the mass. A lighter armature and a lighter moving contact reduce the impact energy at the same velocity. The limit is stiffness: a lighter part may flex more, which can add its own bounce. A modal check or a simple tap test can show whether the moving parts have a resonance near the drive frequency.

Damping is the third lever. Elastomer dampers, friction dampers and hydraulic dampers all absorb energy, but they add cost and temperature sensitivity. A well-designed elastomer stop can be enough for a small relay. A contactor may need a hydraulic or friction damper.

• Reduce the final approach velocity before increasing spring force.
• Reduce moving mass where stiffness allows.
• Add a damping stop at the armature or the contact support.
• Use a pre-travel stop to separate the drive impact from the contact impact.
• Check the natural frequency of the contact blade.
• Measure bounce at the worst-case voltage and temperature.

Spring design and material

The spring has to supply the force at end of life without taking a set. Phosphor bronze, beryllium copper and stainless steel are common contact spring materials. Each has a different conductivity, relaxation behaviour and cost. Beryllium copper gives good strength and conductivity, but its dust and machining controls need attention.

Stress relaxation is the slow loss of force under load at temperature. It is the reason a spring that measures correctly at build can fail the end-of-life force check. Specify the material, the temper and the stress level, and ask for relaxation data at the operating temperature.

The spring rate links force, follow and natural frequency. A stiffer spring gives more force for the same follow, but it also raises the natural frequency and can increase the impact force. A softer, longer spring gives more follow and lower stress, but it needs more space. The choice is a balance, not a single optimum.

Testing force and bounce

Measure force with a calibrated load cell and a micrometer stage. Record the force at first touch and at the rest position, and convert the difference to the spring rate. Do this at the start and end of the life test, and at the temperature extremes if the application requires it.

Measure bounce with a high-speed acquisition. A simple circuit with a wetting voltage and a series resistor lets you see the make and break events as voltage steps. A data logger with a fast sample rate is enough; an oscilloscope is not required. Record the number of bounces and the total duration.

Keep the measurements together. A force reading without the bounce data can lead to a stiffer spring that fixes resistance and makes erosion worse. The pair is the evidence that the mechanism is balanced.