What is the contact area between threads and how does it change?

Question

1. "https://www.youtube.com/watch?v=3l6EcBg9EPA" [@24:30 - 25:10] He says something about the load of a threaded interface being on only one small part of the spiral. To me this seems a bit odd right?

2. How much contact area is there really between a screw and a nut? "Where is the contact point between screw spindle teeth and nut teeth?" - Here one answer suggests that it's dependant on the thread type and tolerances which makes sense, but I couldn't find any reference values in my machining books, besides this link which briefly mentions a contact area of 30-35% without further explaining it "https://fastfixtechnology.com/rail/the-benefits-of-lockbolts/"

3. Can you even talk about "contact area" or is it like with the teeth of gears more like a contact line?

4. Isn't the contact area almost 100% along the whole thread because of the elongation of the screw when torqued down correctly?

5. Why doesn't this value apear when calculating screws, as I would imagine it to be important when considering surface pressure between the two threaded parts(e.g. what material to use)?

I guess it's important to mention that I was thinking about machined threads not self tapping screws or something like that.

Answer 1

When you see the distribution of the load in a thread, it usually involves only the first few threads.

That is easier to understand when you consider that the thread is under load. The load is gradually transferred to the plate through the bolts. The fact that the core of the bolt transfers that load, creates a small elongation/compression (enough to develop pressure onto the thread).

As the load is gradually transferred to the plate, the remaining threads are less and less loaded. Eventually there is no pressure on the threads, because - like you pointed out - machined threads are expected to fit without interference when they are unloaded.

That is why the contact area is defined in the first few threads, and also, when you are doing calculation on the thread (when you are using a hard bold with a softer material), you tend to calculate that most of the load will be on just on thread so that the the contact area is even less.

• Regarding the 5th question: depending on the calculation that you are using the contact area comes into play. However, most of the time, this is not required (i.e. in most cases performing that calculation will yield a result that is not a constrained). The reason is that bolts have been developing and improving for many years. As a result, the final form of bolts nowadays has a ratio of thread size to thread core, which (most of the time) makes sure that the bolt does not cut across the core cross-section. (Probably the worst scenario is to have a bolt break inside the thread). And although threads are not the top priority, the ratio of strength is very close to one.

Keeping in mind that bolts are usually designed with a safety factor, and that a portion of the core carries the total of the load, while a single thread carries about 35 to 50% of the load at any time, you can understand that the failure of threads is not usual (unless there are material of uneven hardness involved).