Question

Which is the neutral point in rolling?

• A. Friction changes direction
• B. Thickness is minimum
• C. Temperature is highest
• D. Strain is zero

Hint:

At the neutral point:
- Relative velocity \( = 0 \).
- Friction changes direction.
- Maximum pressure occurs here along the contact arc.
This peak pressure is often referred to as the "friction hill."

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question asks for the definition or defining physical characteristic of the "neutral point" (also known as the no-slip point) in a flat metal rolling process.
Rolling is a metal forming process where slab or sheet thickness is reduced as it passes between two counter-rotating rolls.

Step 2: Key Formula or Approach:
The metal velocity changes continuously as it passes through the roll gap.
By mass conservation, because the cross-sectional area decreases, the velocity of the metal sheet must increase from entry (\( v_0 \)) to exit (\( v_f \)):
\[ v_0 \lt v_r \lt v_f \]
where:
\( v_0 \) is the entry velocity of the metal.
\( v_r \) is the constant peripheral linear velocity of the rolls.
\( v_f \) is the exit velocity of the metal.

Step 3: Detailed Explanation:

• Friction and Relative Velocity:
-

Before the Neutral Point (Entry Zone): The rolls move faster than the sheet (\( v_r \gt v_{\text{metal}} \)).
Relative motion drag acts to pull the sheet into the roll gap.
Frictional forces act in the direction of rolling.
-

After the Neutral Point (Exit Zone): The deformed sheet moves faster than the rolls (\( v_{\text{metal}} \gt v_r \)).
The metal is being extruded forward relative to the rolls.
Frictional forces act opposite to the direction of rolling, trying to resist this relative forward sliding.
-

At the Neutral Point: The velocity of the sheet matches the roll surface velocity exactly (\( v_{\text{metal}} = v_r \)).
Because there is zero relative velocity, the frictional force drops to zero and changes its direction across this boundary.

• Other Options: Thickness is minimum at the exit plane of the rolls, not at the neutral point.
The strain is maximum at exit, not zero.


Step 4: Final Answer:
The neutral point is mathematically and physically defined as the plane where relative velocity is zero and the friction force changes its direction.
Therefore, the correct choice is option (A).