What is the primary size-reduction mechanism in toothed roll crushers when handling friable or soft rock and coal?

Introduction / Context:
Toothed roll crushers incorporate intermeshing teeth that grip and pull material into the crushing zone. They are widely used for coal preparation and soft ores, achieving controlled top size with minimal fines.

Given Data / Assumptions:

• Material is friable/soft.
• Device has toothed rolls rotating toward each other.

Concept / Approach:
As particles are nipped, the teeth apply compressive forces while the relative motion introduces shear. The combined compression–shear (tearing) mode fractures the material along planes of weakness.

Step-by-Step Solution:
Identify action of teeth: grip, compress, and shear.Differentiate from smooth rolls (mainly compression) and impact mills (impact/attrition).Select “Tearing (shear) and compression.”

Verification / Alternative check:
Operation manuals specify shear-compression as the main breakage mode for toothed rolls.

Why Other Options Are Wrong:
Impact/attrition: characteristic of hammer/pin/ball mills.Cutting only: teeth do not slice like knives; they compress and tear.

Common Pitfalls:
Assuming impact dominates because of rotation; intermeshing geometry favors compression–shear.

Final Answer:
Tearing (shear) and compression