Self Locking in Screws
Mufaddal Rasheed 
A brief article on the concept of Self locking in Power screws, what is conceptual basis and how to design for self locking ?
Power screws are very commonly used to transfer motion and a very important feature in applications like automation , machine tools is the self locking feature of the power screw.
The ability of the screw to maintain its position at a required point instead of self loosening .
Lets understand the analysis for this feature in design of power screws.
This Analysis is for Square threaded Power screws
Start by understanding the Principle of the screw.
Screws are akin to inclined plans wrapped around a cylinder
When the thread is unwound they are nothing but inclined planes.
- Moving the Nut up is like moving a weight on an inclined plane
- Force required to push the body up is analogous to Force which is generated by the input torque on screw.
- Amount of movement per revolution is the Pitch.
Free body diagram of an inclined plane :
Free body diagram of the power screw with a load F resting on the nut :
If we unwrap one complete thread, and lay it flat and then analyze the forces at contact.
From the triangle :
Raising Load and Torque:
Lowering Load and Torque:
It is seen that when the friction is low (between screw and nut) then the load will lower itself.
That is
In order for the screw to lock it self in position and not loosen on its own without external force.
When there is a positive torque obtained here , the lead screw is said to be self locking
Condition for self locking :
Or
Whenever coefficient of friction is higher than tangent of lead angle of screw the screw will be self locking.
Categories: : Mechanism and Machine design