Single and multi-start threads
Shery George | 9 September 2021
What does “thread” actually mean in relation to a lead screw? And what is the difference between single and multi-start threads?
Lead screws are machine elements that convert a rotary movement into a translatory one. Lead screws consist of a cylindrical round rod on which a simple transmission thread is attached. The transmission threads can have, for example, the trapezoidal thread form or the dryspin® geometry designed by igus.
In this blog we will deal with the topic of the threadability or single and multi-start threads. What do these terms actually stand for and what is their significance in my application?
This can be easily seen by means of the following graph:
L = Linear travel in mm per one screw revolution , P = Spacing from one thread to the next
Explanation of single and multi-start threads
Single start lead screws (1st graph from left) consist, as the name suggests, of only one single thread (single start). This means that the distance between the flanks, marked by pitch, is also the linear travel (lead) in millimetres per revolution of the lead screw.
For the second lead screw (2nd graph from left) a further thread has been added. This means that with one revolution of the lead screw the two-fold stroke is achieved (l = 2 x p). The start of the thread is offset by 180°, i.e. opposite each other. These are the multi-start threads.
The third lead screw in this example shows a 4-start lead screw. This means we have 4 thread starts (orange, green, purple and light blue). For the linear travel the same calculation applies here as for the other examples. My linear travel (lead) is therefore four times the distance between the flanks. The start of the thread is at a position of 90° to each other.
Can I recognise a multi-start thread visually and without marked lines? YES! This is done by viewing the front side of the lead screws:
First graph from the left: a start of thread can be seen from above. The thread flank distance (pitch) is equal to the lead length of the thread (lead).
Remaining graphs: looking at the front surface of the lead screw, you can see two or more starts of thread symmetrical to each other. The number of starts of thread defines the number of threads. (Two starts of thread = two-start lead screw)
What does the thread designation TR20x8P4 actually tell me and how do I calculate the number of threads of this type?
Example of trapezoidal thread:
Trapezoidal thread: TR20x8P4
Thread diameter: 20mm
No. of starts: 8/4=2 (L) / P = No. of starts)
► TR20x8P4 = 2-start thread
Example of high helix thread:
High helix thread: DS10x12
Nominal thread diameter: 10mm
No. of starts: 12/3=4 (L/P) = No. of starts)
►DS10x12 = 4-start thread
Single-start trapezoidal lead screw units are self-locking in most cases, depending on the coefficient of friction. This effect can only be weakened at a very low coefficient of friction. This means that the flank angle and the sliding friction prevent the nut or lead screw from moving without outside forces being applied. As soon as the static friction is exceeded, the components are no longer self-locking. Multistart trapezoidal screw drives have a “residual self-locking” feature; high helix screw drives have no self-locking feature.
My advantage when using multi-start threads:
When multi-start lead screws are used, the effective support surface increases under the condition of a constant core diameter.
► Low surface pressure = longer service life
Furthermore, the clearance between the tooth flanks is reduced, which tends to make the entire system more precise. Multi-start threads are often used for high helix threads.