Question

The condition for a screw jack to be self-locking is that:

• A. Its efficiency should be the maximum possible
• B. Its efficiency should be the minimum possible
• C. It should not unwind to lower the load if left to itself
• D. Its efficiency should be more than 50%

Hint:

For self-locking in screw jacks: 
- Friction angle>Helix angle. 
- Efficiency must be less than 50%. 
- Overhauling occurs if efficiency exceeds 50%.

Answer 1

The Correct Option is C

Step 1: Understanding the self-locking condition. A screw jack is considered self-locking if, when the external effort is removed, the load does not descend on its own. This occurs when the friction angle is greater than the helix angle. 
Step 2: Efficiency and self-locking. For a screw jack to be self-locking: 
- The efficiency must be less than 50%. 
- If the efficiency were greater than 50%, the screw would become overhauling, meaning the load could start descending without external force. 
Step 3: Choosing the correct answer. 
- (a) Incorrect – Maximum efficiency would lead to overhauling. 
- (b) Incorrect – Minimum efficiency does not define self-locking. 
- (c) Correct – A self-locking screw jack should not unwind on its own. 
- (d) Incorrect – Efficiency must be less than 50% for self-locking. 

Thus, the correct answer is (c) It should not unwind to lower the load if left to itself.