Contact pressure

Contact pressure, that is, the force with which the contact points press against each other, not only determines the magnitude of the contact resistance but also directly affects the service life of the contacts. The operation of electrical switches cannot do without contacts, which are responsible for transmitting current and ensuring the smooth opening and closing of the circuit. However, the pressure between these contacts also affects their performance.

Definition and Function of Contact PressureContact Pressure. Contact pressure refers to the force per unit area applied between two contact points, usually measured in Newtons per square millimeter (N/mm²). In design, it is provided by springs, screws, or mechanical structures. Why is contact pressure needed? Because without pressure, the contacts cannot overcome surface roughness and oxide layers to form a stable contact. Imagine two pieces of metal lightly touching; their contact area might be very small, resulting in high resistance. When pressure is applied, the metals undergo microscopic deformation, increasing the number of contact points and making the contact tighter. Contact pressure is not the greater the better; it has an appropriate range. Too little, and the contact is unstable; too much, and it causes excessive wear. The most direct effect of contact pressure on contacts is contact resistance, which differs from the bulk resistance of the conductor itself and is caused by incomplete contact at the surface of the contacts. On the surface, the two contacts may appear to fit perfectly, but microscopically, only a few protruding points actually make contact. The total area of these points is much smaller than the size of the contacts, usually only one-thousandth or less of the total area. Current can only flow through these points, leading to increased resistance.

The Effect of Contact Pressure on Contact ResistanceContact pressure is the most direct way to reduce contact resistance. The principle lies in the fact that when two surfaces are pressed together, the contact points undergo elastic or plastic deformation, increasing the actual contact area. At low pressure, the contacts touch only at a few points, and the resistance may be as high as several milliohms. As the pressure increases, both the number and size of the contact points grow, causing a sharp drop in resistance. This is because the pressure breaks the surface oxide film, exposing the pure metal, which ensures that electrons can flow more easily. However, pressure is not infinitely beneficial. After reaching a certain threshold, further increasing pressure brings only minimal improvement to resistance. This is because the contact points are already near their maximum, and additional pressure only results in unnecessary stress.

The Effect of Contact Pressure on Contact LifeContact life refers to the number of times or duration that a contact can reliably operate, usually counted in millions of cycles. It is affected by wear, erosion, and fatigue, with contact pressure being a key controlling factor. Proper pressure extends life: it ensures stable contact and reduces contact bounce and arcing. Arcing is a killer for contacts, as it can melt surfaces, create pits, and increase resistance. When pressure is moderate, contacts close quickly, minimizing arc. However, excessive pressure shortens life, mainly due to mechanical wear. Under high pressure, each closure involves friction, scraping away surface material. Over time, contacts thin, resistance rises, and eventual failure occurs. This is more evident in sliding contacts (such as brushes), where high pressure causes rapid wear. Low pressure also brings many problems: unstable contact easily affected by vibration, resulting in poor contact. This produces intermittent arcs, accelerating oxidation and corrosion, sharply reducing life. In humid environments, low pressure allows moisture to seep in, forming a corrosion layer and further worsening the situation.

The Wisdom Behind PressureEven slight contact pressure controls the destiny of contact resistance and lifespan. Applying the right amount of pressure ensures efficient conductivity and long-lasting durability. Though microscopic, contacts have a grand impact—from household appliances to aerospace engineering, all rely on their stability.