Question

A self-locking screw is one which

• A. Has locking arrangement
• B. Has a hole drilled through for inserting locking pin
• C. Has coefficient of friction equal to or greater than the tangent of the load angle
• D. Has fine pitch screw threads

Hint:

Efficiency of a self-locking screw is always less than 50%. If efficiency exceeds 50%, the screw is "overhauling" and will unwind under its own load.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to identify the correct technical condition that makes a power screw "self-locking".


Step 3: Detailed Explanation:
A self-locking screw is defined mechanically as a screw that will not lower or unwind under the action of the axial load alone. It requires an external applied torque to lower the load.
The condition for self-locking depends on the relationship between the friction angle (\(\phi\)) and the helix angle or load angle (\(\alpha\)). For a screw to be self-locking, the friction holding it in place must be greater than or equal to the force component tending to unwind it down the thread incline.
Mathematically, this geometric condition is expressed as: \[ \phi \geq \alpha \] Taking the tangent of both sides: \[ \tan(\phi) \geq \tan(\alpha) \] By definition, the coefficient of static friction, \(\mu\), is equal to the tangent of the friction angle (\(\mu = \tan(\phi)\)).
Therefore, the condition for self-locking becomes: \[ \mu \geq \tan(\alpha) \] This translates precisely to the statement that a screw is self-locking if its coefficient of friction is equal to or greater than the tangent of the load angle.


Step 4: Final Answer:
A self-locking screw has a coefficient of friction equal to or greater than the tangent of the load angle.

Answer 2

To answer this, it helps to check what "self-locking" technically requires in a power screw, and then test each of the four options against that requirement.

1. Has locking arrangement: This describes an external mechanical add-on, like a separate latch or clamp, a different concept altogether from the screw's own thread geometry naturally resisting motion. Self-locking in the engineering sense is a property of the thread and friction, not of an added mechanical part.
2. Has a hole drilled through for inserting locking pin: This describes a physical modification used in some fasteners to physically prevent rotation. It has nothing to do with the friction-based self-locking property of a power screw thread.
3. Has coefficient of friction equal to or greater than the tangent of the load angle: This describes exactly the friction condition that keeps a screw from unwinding under load alone: modeling the thread as an inclined plane, the load's tendency to slide back down (proportional to the tangent of the helix/load angle) must be matched or exceeded by the friction available (proportional to the coefficient of friction). This is the actual technical definition of self-locking.
4. Has fine pitch screw threads: A fine pitch reduces the helix angle, which does make self-locking easier to achieve, but pitch alone is not the defining condition; a coarse-pitch screw with high enough friction can still be self-locking, and a fine-pitch screw with very low friction may not be.

Only the friction-versus-load-angle condition captures the actual physics of why a screw resists unwinding under load; the other options describe unrelated hardware features or a related-but-not-defining geometric factor.

Therefore, the correct answer is Has coefficient of friction equal to or greater than the tangent of the load angle.