The outer screw diameter is the largest diameter of a lead screw, i.e., the diameter across the thread crests (also known as the major diameter). It is a central dimensional parameter in the design of screw drives and influences both the mechanical strength and the load capacity of the screw.
The larger the outer diameter, the more stable the screw is against bending or buckling, especially with long travel distances. A larger diameter generally also allows for the transmission of higher axial forces. Furthermore, the outer diameter determines how much space the screw requires, thereby influencing the installation space.
The outer diameter of the screw can be determined by various methods, depending on the type of thread (metric, trapezoidal, etc.). For metric threads, the outer diameter is often specified directly, while for trapezoidal threads, additional calculations may be required.
The outer screw diameter should not be confused with the core diameter (the smallest diameter at the thread root) or the pitch diameter (the mean diameter of the thread profile). All three dimensions are relevant, but the outer diameter is the visibly largest and often the first value specified in thread designations.
Practical examples
For an M10 metric thread, the outer diameter is 10 mm, meaning that the screw has this diameter at the thread crests.
