friction of screw and nut

FRICTION OF SCREW AND NUT

Fig.6.10 shows a square threaded screw A free to rotate in a fixed nut B. A screw is defined as a cylinder with a uniform helical ridge projecting from its surface. A nut is defined as a body with an internal surface made to fit closely the outer surface of a screw. These two form a screw pair.

1. Two Forms of Screw and Nut

1. In screw pairs, the nut forms a fixed part and the screw on rotating advances or recedes in the direction of its axis. Example: Screw jack.

2. In other case, the screw forms a turning part and nut a sliding part. As the screw rotates, the nut moves along the axis of the screw. Example: Lead screw of lathe.

2. Types of Threads

1. According to the shape of the threads

(a) Square threads, and

(b) V-threads.

2. According to the surface of the threads

(a) External threads: When threads are cut on the outer surface of a solid rod, then they are known as external threads.

(b) Internal threads: When threads are cut on the internal surface of the hollow rod, then they are known as internal threads.

3. According to the number of threads

(a) Single threaded screw: If only one thread is cut in one lead distance of screw, it is known as single threaded screw.

(b) Multi threaded screw: If more than one thread is cut in one lead distance of a screw, it is known as multi-threaded screw.

4. According to the direction of movement of threads

(a) Right handed screws

(b) Left handed screws

As they move from or towards the observer looking at one end when they are rotated in clockwise direction to a fixed nut, we may get right handed or left handed screws.

3. Terminology Used for the Study of Screw

1. Helix: It is the curve traced by a particle while moving along a screw thread.

2. Pitch: It is the axial distance from a point on a screw threads to a corresponding point on the next thread.

3. Lead: It is the distance, a screw thread advances axially in one turn.

4. Depth of thread (crest or root of a thread): It is the distance between the top and bottom surfaces of a thread.

5. Nominal diameter of thread: The outside diameter, also called as major diameter, of the screw thread is known as the nominal diameter of the screw thread.

6. Core diameter of thread: The root diameter, also called as minor diameter, of the screw thread is known as the core diameter of the screw thread.

7. Single threaded screw: If the lead of screw is equal to its pitch, it is known as single threaded screw.

8. Multi-threaded screw: If more than one thread is cut in one lead screw of a screw, it is known as multi-threaded screw.

⸫ Lead = Pitch × Number of threads

9. Slope of thread: It is the inclination of the thread with the horizontal.

where 

p = Pitch of the thread,

α = Helix angle of the thread,

d = Mean diameter of the thread, and

n = Number of threads in one lead.

Note 

Relationship between nominal diameter (d0), core diameter (dc), mean diameter (d), and