FAQ Ring-shaped burn on spring-loaded contacts

Why does the moving contact pin show a ring-shaped burn on its side (the sliding surface)?

In short: A circumferential, ring-shaped burn on the side of the moving pin does not form at the contact tip, but at the sliding contact between the pin and its barrel or guide. If the head moves while current is flowing – typically due to vibration or repeated compression – this circumferential contact zone wears. It is a combination of fretting corrosion and micro-arcing, not an overload caused by excessive current.

Overheating and loss of spring force in spring-loaded contacts

The typical damage pattern

The burn runs as a circumferential, ring-shaped dark band around the side of the moving pin – exactly where it slides inside the barrel or guide. Alongside this, the contact resistance rises, and fine dark debris or particles are often visible. The marks cannot be wiped or polished off.

The cause: movement under current at the sliding contact

In a spring-loaded contact the current flows across the contact zone between the moving pin and the barrel that guides it. If the pin is moved while current is flowing – due to vibration, shock or repeated compression – a continuous relative movement under load occurs at this circumferential contact zone. That is precisely what causes the wear. Because the pin touches the barrel all the way around, the damage spreads over the whole circumference – hence the ring-shaped appearance.

Two mechanisms act together

Fretting corrosion: the small, repeated sliding movements abrade the protective precious-metal plating and expose the less noble base material. This oxidises, and the resulting hard oxide particles act like an abrasive and increase the contact resistance.

Micro-arcing: if the pin lifts off even minimally due to the movement while current is flowing, tiny arcs ignite at the separation point. They melt and erode the surface locally – all the way around the contact circumference.

Key point: It is neither current alone nor movement alone that causes the burn – but both at the same time: relative movement at the contact while current is flowing.

Why can't the burn be polished off?

Because it is not a loose deposit but altered base material: an abraded and oxidised or locally re-melted surface. It no longer conducts cleanly through metal-to-metal contact – hence the increased contact resistance.

How to avoid the burn

Minimise relative movement under current: mechanical fixing or damping and avoidance of resonances, so that the contact does not move while current is flowing.
Sufficient contact / spring force: a firmly seated pin prevents brief lift-off ("chatter") and thus the micro-arcs.
Choose a contact suited to vibration and current: e.g. biased designs with a defined, stable current path, or designs with a robust/separate current path rather than only through the sliding contact.
Distribute the current or add margin: split the load across several contacts or use a higher-rated contact.
Suitable plating: harder or thicker surfaces improve resistance to sliding wear.

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