In smooth steel crushing rolls handling ordinary rock, what is the typical angle of nip (in degrees)?\nChoose the value most commonly cited for practical design.

Introduction / Context:
The angle of nip in roll crushers is a foundational concept in comminution. It is the maximum angle between the two roll faces at which a particle can be nipped (gripped) for crushing without slipping. Properly estimating this angle ensures efficient crushing and prevents excessive recirculation.

Given Data / Assumptions:

• Material: ordinary rock with typical frictional characteristics against steel.
• Rolls: smooth steel.
• We seek the commonly used design angle.

Concept / Approach:
The angle of nip depends on the coefficient of friction between particle and roll surface and the particle size relative to roll diameter. For smooth rolls and common rock, handbooks report a practical design range around 15°–20°, with 16° often quoted as the representative value for calculations.

Step-by-Step Solution:

Verification / Alternative check:
Design examples for roll crushers frequently adopt 16° to ensure conservative sizing and reliable nipping for a wide range of feed particles.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing corrugated/roughened rolls (higher nip angles) with smooth rolls; ignoring material friction; or assuming angle increases with feed size regardless of surface condition.

Final Answer:
16°