Shear forces

Shear forces are tangential forces acting parallel to a surface or boundary layer. They cause relative movement between material layers, particles or component surfaces. In powder technology, shear forces arise in particular during the mixing, conveying, dosing, agglomeration or comminution of bulk materials.

The effect of shear forces is often described in terms of shear stress. This is defined as force per unit area and can in many cases be approximated by

τ = μ ⋅ γ′

where τ represents the shear stress, μ the effective viscosity and γ the shear rate. In the case of bulk materials, the relationship between shear stress and normal stress is also considered via the material’s internal friction angle.

Shear forces have a significant influence on particle structure and product properties. They can lead to deagglomeration, particle comminution, dust formation, segregation or surface activation. Shear forces are often the determining factor in the selection of process machinery.