Friction locking

In friction locking, a force holds rather than a shape. In the narrow sense this is friction: a contact force presses two faces together, and the friction force F = μ · N prevents sliding. In the wider sense suction cups and magnets belong here too – their holding force acts perpendicular to the face, but across it they also carry by friction only.

The load stays for exactly as long as the holding force exceeds everything trying to release it. That distinguishes friction locking from positive locking, where geometry blocks the movement.

The friction coefficient μ is not a material constant. It depends on the pairing, the roughness, on oil, water, dust and temperature, and can differ considerably between a clean sample and a production part. Because holding force is proportional to it, every change comes through in full.

Sizing therefore uses the least favourable value occurring in operation, determined on the real part – not taken from a table for ideal surfaces.

The holding force required does not follow from weight alone. Lifting, braking and slewing produce accelerations; tilting changes the direction of the weight force relative to the gripping face; wear of pads and sealing lips reduces the force over time. The safety factor brings these influences together.

Its size depends on the gripping principle, the movement and the relevant standard. For vacuum, for example, 2 is customary for lifting with a horizontal suction face and 4 for tilting.

A grip that holds by force almost always needs energy – compressed air, vacuum or electricity. If it fails, the holding force must be maintained until the load has been set down.

Friction locking is quick, needs no particular geometry on the part and copes with variants. It is the method of choice for smooth panels, sheet metal, containers and anything without a rim or bore. As soon as the load is rotated, tilted or joined against resistance, it is not enough on its own: a positive-locking safeguard is then added.