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.