# Sliding Friction Force

In a sliding joint, contact force always acts perpendicular to the surface in contact. This contact force is commonly referred to as a normal force because it acts perpendicular to the surfaces in contact.

When friction cannot be neglected in machine analysis, an additional force, friction force (Ff) is observed. Friction always try to oppose the motion of the body. Therefore, a friction force acts on a sliding link, perpendicular to the normal force, and in a direction opposite to the motion (velocity) of the body.

For a stationary object, friction works to prevent motion until the maximum attainable friction has been reached. This maximum value is a function of a coefficient of friction (μ). The coefficient of friction is a property that is determined experimentally and is dependent on the type of material and surface conditions of the contacting links.

Average values of friction coefficients for commonly used materials in dry and lubricated surfaces are given in the below table.

The magnitude of the sliding friction force (Ff) that acts on sliding components is calculated as

Ff = μN

As mentioned, for moving objects, the friction force acts opposite to the direction of the relative sliding motion.

Approximate Coefficients of Sliding Friction
Dry Lubricated
Hard steel On hard steel 0.45 0.08
On Babbitt 0.35 0.15
Mild steel On mild steel 0.60 0.12
On bronze 0.34 0.17
On brass 0.44 ___
0.23 0.13
On aluminium 0.50 ___
On laminated plastic 0.35 0.05
On Teflon ___ 0.04
Cast iron On cast iron 0.15 0.07
On bronze 0.22 0.07
On brass 0.30 ___
On copper 0.29 ___
On zinc 0.21 ___
Aluminium On aluminium 1.40 ___