Screw and screw threads

Screws form an important part of any machinery. Screws are present almost everywhere, ranging from a wristwatch to an automobile. In this post, the manufacturing of a screw thread, and the types  of screw thread will be discussed.

First of all one needs to know the term 'Helix'.
A helix is a 3D structure which is obtained when a particle rotates with a constant angular velocity along a fixed axis along with a linear motion in a direction parallel to the axis with a constant linear velocity.
The screw thread also forms a helix.

Helix

Now knowing what a helix is, when we have a path carved on a cylindrical metal piece in the form of a helix, we get a screw.

Now let us see various terms related to a screw.

• While holding a screw horizontally, a wave kind of pattern can be seen made by the threads. The upper extreme position is called 'Crest' and the lowermost position is called 'Trough' (or 'Root'). 
• The diameter of the screw including the threads is called 'Major Diameter' and the diameter of the cylinder formed by not considering the threads is called 'Minor Diameter'.
• The distance between two consecutive crests ( which is same as the distance between two consecutive troughs) is called the 'Pitch'.
• The angle formed between the threads is called 'Thread Angle'.
• The mean of Major Diameter and Minor Diameter is called 'Pitch Circle Diameter'.

 

Now to discuss the classification:

1. Based on orientation of threads, we have right hand threads and left hand threads.
When the screw is held vertically, if the threads incline from right at top to left at bottom, we have right-hand threads.
Similarly, if the threads incline from left at top to right at bottom, we have left-hand threads.

2.  Based on the start of threads:
[Note: The linear distance vertically upwards or downwards the screw travels during one complete rotation in known as the 'Lead' of the Screw.]
If a single thread is present, it is known as a single start screw. If two threads start simultaneously we have a double start screw. Similarly we can have a triple start screw and so on.

The main point is that for a fixed pitch, as the number of starts increases, the lead of the screw increases i.e. one revolution the lead of a double start screw will be more than single start screw.

So, for one complete revolution, a triple start screw will travel more linear distance than a double start screw, which in turn would be more than a single start screw.

 3. Based on the shape of the threads:

The controlling factor is the thread angle.

Square threads give  higher friction than v-threads and also the load bearing surface is higher.

But they are difficult to manufacture and the efficiency is low.

V-threads are easy to manufacture, are highly efficient but provide lower friction and lesser bearing surface. Both square and v-threads are used for power transmissions.

Buttress threads are ideally used for power transmission in only one direction, and are easy to manufacture, provide high friction, high bearing surface along with good efficiency.

The methods of manufacturing screw threads are:

• Die and tapping
• Threading on Lathe
• Milling 
• Rolling, Forming

Rolling, forming, etc. are used for mass production of screws and bolts.Whereas lathe may be used where a part of the work-piece needs to be threaded, and not for small diameter screws.

To watch manufacturing of threads on a bolt, click here

To watch threading operation on a Lathe machine, click here