Thread depth

How much thread depth do I need?

How does thread depth influence the strengt of the connection?

When mating two parts with a screw, more often than not you will want to tap the threads in one of the mating parts. Tapping a thread in one of the parts instead of making a though hole and using a nut is preferable for many reasons. Sometimes it’s the only option. Thread depth is one of the consideration when designing a part.

When tapping a thread into the part you want to make sure that you have the enough thread length so that the screw is securely tighten. Required length of the thread is directly correlated with the standard pitch of the screw. This in turn depends on the size of the screw/hole.

Thread depth obviously needs to be larger than the depth of the bottom of the screw. Also the hole drilled for tapping the thread needs to be larger then the thread itself.

Thread depth, strength of connection

That’s all pretty obvious, but what are some standard dimensions that will get rid of the hassle of thinking about it?

First let’s consider the threaded connection a bit closer. In this picture you can see the interface that a screw makes with at threaded hole in a close up view. In this example the thread depth is not limited, it runs all the way though.

Threaded connection

If we imagine that a load (a force) is applied to pull the screw out of the threaded hole then we can conclude that this interaction between the outer thread dimension of the screw and the inner thread dimension of the hole is holding the screw put. We can simplify this even more with this example of a pulling a pin with a plug through a hole.

Peg connection

Now if we increase the force at some point the connection is going to brake right? It can break on the joint or somewhere else in the material. From this thought experiment we can derive that factors that have influence in the strength of this connection are the surface of the interface or the dimensions A and B and the strengths of the material of the pieces 1 and 2.

These are exactly the parameters that are included in the standard for the optimal thread depth and therefore all other dimensions that we have talked in the beginning. The parameters are the strength of the screw (Property class), the strength of the material with a threaded hole and the interface surface that depends on dimension of the screw and the type and pitch of the thread.