The flank diameter is the effective diameter of a thread. It lies between the outside diameter and the center diameter and describes the theoretical diameter at which the thread flanks of internal and external threads are in a defined geometric relationship to one another. It is one of the most important reference parameters for fit, force transmission, and the function of threaded connections and screw drives.
The flank diameter is decisive for:
- the load-carrying capacity of the thread,
- force transmission via the thread flanks,
- the calculation of friction, efficiency, and torque,
- the fit between the nut and the screw.
Since the load in threads is primarily transmitted through the flanks, the flank diameter is often more critical to mechanical function than the outside and pitch diameters. It therefore plays a central role in the design, tolerancing, and evaluation of threaded connections.
For an external thread, such as that on a spindle, the following applies:
- The outside diameter is the largest diameter at the tips of the threads.
- The core diameter is the smallest diameter at the root of the threads.
- The flank diameter lies between these two and is also referred to as the effective diameter.
The flank diameter is used, among other things, for:
- Tolerance and fit specifications,
- calculations of surface pressure,
- the design of ball screws and trapezoidal screws,
- strength and wear analyses.
Thus, the flank diameter is the key reference value for the geometric and functional evaluation of a thread.