12
$\begingroup$

When looking at thread descriptions, one of the basic properties is always the number of thread starts.

wikipedia thread starts

As far as I could tell, all of the major standard bolt threads are single-start. This includes:

  • Unified Standard (UNC, etc.)
  • National Pipe Thread (NPT, NPS)
  • British Standard

I only found one standard thread that can also come in a multiple-starts: ACME.

What are the reasons why single-start threads are so common and multiple-start threads are rare? I am specifically interested in bolts and other fasteners.

Improve this question
$\endgroup$
3
  • $\begingroup$ I once found a null-start threaded bolt; thread creating loops instead of helix... Took me a good while to spot just why I couldn't screw it in. I wonder just how did it happen... $\endgroup$
    – SF.
    Commented Mar 9, 2015 at 15:43
  • $\begingroup$ Complete guess with no research to back it up: would a double thread give a possibility of cross-threading a bolt, which is undesirable, hence making single threads more attractive? $\endgroup$
    – AndyT
    Commented Sep 1, 2015 at 13:28
  • $\begingroup$ Thank you but tomorrow I start my new journey to cut double start thread stainless 304 for valve stem which has 4tpi. $\endgroup$
    – Brhian'z Salundaga
    Commented Nov 21, 2020 at 17:45

5 Answers 5

Reset to default
11
$\begingroup$

As Dave Tweed points out, the ratio of torque to tension is lower the lower the lead angle is. Since the important measure of bolt tightness is generally the tension in the bolt, we want to achieve that minimum pretension with the least effort possible. Assuming we have to maintain a certain shear area of the thread (so the the threads are stronger than the bolt when fully engaged) having two starts means we double the lead angle and greatly increase the amount of force required in the wrench to tighten the fastener appropriately. On its own though, this isn't the end of the world in practical applications because a big enough torque arm, (or a shear wrench) makes this just a matter of using a bigger motor.

The bigger problem is that we want bolt threads to be self-locking. That is, we wouldn't want the pretension in the bolt to cause it to loosen. Imagine if the bolt had 10 starts, and therefore a very steep helix for the threads - no matter how hard we tighten the bolt, the pretension will immediately loosen the bolt when we let go of the wrench. This is because the lead angle lets so much force transfer into the rotation of the bolt (or nut) that it can overcome the friction between the internal and external threads. This would make the bolt not very effective without an external locking device. By contrast, standard single start fasteners are often pretensioned (or simply made snug tight) and trusted to self-lock based on their shallow lead angle. In situations with high vibration or thermal cycles, additional locking elements may be used, but they aren't normally required if the bolt is properly tightened.

This is why single start threads are typically used for fasteners (things that aren't supposed to move) but multiple-start threads aren't uncommon for leadscrews (which are supposed to move freely, or have an external brake.)

Screws which form their own mating thread in another material are often two-start, as they have an additional resistance to unscrewing from the compression and roughness of the substrate they displaced around the threads. This is the case with some sheet metal screws, and also most wood screws.

Improve this answer
$\endgroup$
9
$\begingroup$

For any given size of fastener and given thread pitch, a single-start thread gives you the greatest mechanical advantage in terms of the torque required to achieve a given tension.

Aside from acme threads that are often used on leadscrews for mechanical motion (e.g., CNC machinery), the only other place I have seen multiple-start screws is on self-tapping screws used to fasten plastic cases together. In such applications, the speed of assembly is more important than the tension, which is limited by the plastic anyway.

Improve this answer
$\endgroup$
3
  • 1
    $\begingroup$ A single thread is the "standard" it's also much easier to manufacture. $\endgroup$
    – George Herold
    Commented Mar 9, 2015 at 15:22
  • 1
    $\begingroup$ Most commercially available fasteners are roll-threaded, which is no harder for two starts than one. Cut threads are harder to fabricate with two starts, but only a little. $\endgroup$
    – Ethan48
    Commented Mar 9, 2015 at 18:46
  • 1
    $\begingroup$ I suppose forming internal threads using conventional tapping would be significantly harder as more material has to be removed at once, leading to a higher force of machining and therefore more broken taps. $\endgroup$
    – Ethan48
    Commented Mar 9, 2015 at 18:53
1
$\begingroup$

In Addition to the other answers, manufacturing of single-start threads is more simple thus easier to control from a quality perspective. Also, Multi-start threads are typically used for motion control not fastening(unless special cases as described above for specialty plastic fasteners and such)

Improve this answer
$\endgroup$
0
$\begingroup$

Single start threads are commonly use since it has large mechanical advantage & also it has self locking property because in multistart thread,this property is lost

Improve this answer
$\endgroup$
1
  • $\begingroup$ Emmi: Welcome to Engineering SE. Your answer would be better if you could provide a link, or more, which could verify your answer. $\endgroup$
    – Fred
    Commented Sep 1, 2015 at 6:32
0
$\begingroup$

Advantages of double lead threading: Starting ease and eliminating cross-threading (i.e. happy customers).

Common usages:

  • Any bottle cap or jaw lid
  • Catsup bottles
  • Tooth paste tubes
  • Split bolts (Kearneys)
  • Flash light end caps
  • Any assembly not requiring high tension
Improve this answer
$\endgroup$
1
  • 2
    $\begingroup$ How does this address the OP's question regarding why most threads are single start? $\endgroup$
    – user16
    Commented Sep 10, 2015 at 16:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.