# Static and kinetic friction – what exactly is the difference?

When setting up or maintaining a conveyor belt, there are often issues due to friction. Here’s a refresher on the difference between static and kinetic friction, and the influence they have on proper belt performance.

#### The difference between sticking and sliding

The friction between two bodies is never exactly the same when at rest and when moving. The force required to start a body sliding is greater than the force needed to keep it sliding. As a result, we differentiate between static friction and kinetic friction (also called sliding friction).

#### Static friction

Static friction occurs when two bodies are not moving relative to each other (like a conveyor belt on the driving pulley surface).

The coefficient of static friction is usually denoted as µs (sometimes just µ or µo or µA):

µs   = static friction [-]

FF  = frictional [N]

FN  = normal force [N]

#### Kinetic friction

Kinetic friction occurs when two bodies are moving relative to each other (like a conveyor belt on a slider bed). An object begins to move if the tractive force (FA) is greater than the frictional force (FF).

The coefficient of kinetic friction is usually denoted as μk (sometimes ):

μk   =  kinetic friction [-]

FA  =  tractive force [N]

FN  =  normal force [N]

#### Try this experiment

Pull an object slowly using a spring scale and read off the tractive force (FA):

The tractive force will first increase without the object moving. Suddenly, the object will start to move, and the tractive force will decrease.

• The maximum tractive force shortly before the object starts to move represents the static friction (µs).
• The medium value of the tractive force when the object is moving represents the kinetic friction (μk).

#### Why static friction is significant for power transmission

Static friction has a decisive function in non-positive drives. This kind of transmission takes place in two- and multi-pulley drives with flat belts, V-belts and Poly-V-belts, as well as on the driving pulley of conveyor belt installations and live roller conveyors.

The coefficient of static friction is needed for the Euler-Eytelwein formula.

The coefficient of friction is very sensitive, with more uncertainty associated with it than all the other factors that impact a belt’s ability to transmit power. Sometimes it can be strongly reduced by barely discernible, outside influences, such as dirt or oil mist. This may cause the belt to completely slip on the pulleys, so that it no longer transmits the required power. In such cases, both the belt and the pulleys must be cleaned.

A belt will show completely different coefficients of friction against the same pulley depending on whether the belt surface is smooth like a mirror, or textured like fabric. The mirror-like surface will adhere much more to the pulley, but will lose virtually all adhesion as soon as any humidity or dust gets between it and the pulley. The fabric-like texture adheres somewhat less, but is also much less affected by humidity and dust.

The same requirement, namely that the surface must be neither too smooth nor too rough, also applies to the pulleys. This is why Habasit specifies the roughness of the pulley surface for power transmission pulleys (max.: CLA = 1.6 ㎛ or Ra = AA 63 μin ).

Very often, the surface of the driving pulley in conveyor belt installations is covered with adhesive material (like rubber) in order to increase the coefficient of static friction between the pulley and the belt, thus increasing the load capacity of the conveyor or reducing the shaft load.

#### The significance of kinetic friction in conveyor belt installations

Kinetic friction is decisive for achieving the required force to pull the conveyor belt over a slider bed. The coefficient of kinetic friction is therefore indispensable for the calculation of conveyor belts running on slider beds.

Oil or sticky stuff, or sometimes just water, between the conveyor belt and the slider bed may produce adhesion of the belt (sometimes called the suction effect), thus increasing the friction and consequently the energy consumption. In the worst cases, this can stop the conveyor belt completely.