Question

In a thresher, the cylinder separation efficiency can be improved by increasing

• A. cylinder diameter
• B. cylinder speed
• C. cylinder–concave clearance
• D. feed rate

Hint:

For better separation, increase cylinder speed (within crop-specific limits) and keep concave clearance and feed rate near their recommended values to avoid grain damage and overloading.

Answer 1

The Correct Option is B

Step 1: Mechanism of separation.
Separation (detaching kernels from heads/pods) occurs via impact, rubbing, and shearing between cylinder bars/spikes and the crop against the concave. The specific kinetic energy imparted scales with the square of tip speed: \[ v_{\text{tip}} = \pi D N, E_k \propto v_{\text{tip}}^2, \] where \(D\) is cylinder diameter and \(N\) is rotational speed (rev/s).

Step 2: Influence of parameters.
- Cylinder speed \(N\!\uparrow\) \(\Rightarrow v_{\text{tip}}\!\uparrow\) \(\Rightarrow\) stronger impact and better rubbing → higher separation efficiency (until an optimum to avoid grain damage). - Cylinder diameter \(D\) alone, at fixed \(N\), slightly increases \(v_{\text{tip}}\) but is not the standard operational lever; design change rather than an in-field adjustment. - Cylinder–concave clearance \(\uparrow\) reduces contact intensity and retention → poorer separation. - Feed rate \(\uparrow\) risks overloading, increasing mat thickness and reducing exposure to bars → poorer separation.

Step 3: Practical rule.
Operators tune \(N\) upward from a nominal setting to improve separation until losses are minimized without cracking grain—speed is the primary controllable that directly raises separation.

Final Answer:
\[ \boxed{\text{B — cylinder speed}} \]