Glossary term
Screw Thread
A helical ridge used to fasten components or convert rotation into linear motion.
Definition
deviceA screw thread is a helical ridge on a cylindrical or conical surface used for fastening, adjustment, sealing, or converting rotation into linear motion.
Screw threads are load-transmitting geometries. Their performance depends on thread form, pitch, diameter, engagement length, material, surface finish, lubrication, preload, tolerance class, manufacturing method, corrosion exposure, and whether the thread is used as a fastener or as a motion screw.
A screw thread converts relative rotation into axial motion through an inclined helical surface. In fasteners, that geometry creates clamping force when torque is applied. In lead screws and ball screws, it converts rotation into controlled linear displacement. In pipe and sealing applications, it may also provide alignment and pressure containment.
Important geometric terms include major diameter, minor diameter, pitch diameter, pitch, lead, flank angle, root radius, crest, thread depth, and engagement length. Metric ISO, Unified, Acme, trapezoidal, buttress, pipe, and special thread forms exist because load direction, manufacturability, self-locking, sealing, and wear requirements differ.
Engineering design
For bolted joints, the thread is only one part of the load path. The design must account for clamp load, proof strength, tightening scatter, friction under the head and in the thread, embedment, relaxation, vibration loosening, fatigue, joint stiffness, and whether the fastener is reused. Thread roots are stress concentrations, so fatigue failures often initiate at the first engaged thread where load transfer is highest.
For motion screws, the design focus shifts to efficiency, backlash, wear, buckling, lubrication, critical speed, bearing support, and drive torque. A coarse thread moves faster per revolution but reduces mechanical advantage and self-locking tendency. A fine thread gives finer adjustment and higher tensile stress area but may be more vulnerable to galling or damage.
Manufacturing and inspection
Threads can be cut, rolled, ground, molded, printed, tapped, or formed. Rolled threads often improve fatigue resistance through favorable grain flow and compressive residual stress. Inspection may use go/no-go gauges, pitch-diameter measurement, optical measurement, or coordinate measurement depending on tolerance and function.
Common mistakes
A common mistake is specifying only nominal diameter and pitch while ignoring tolerance class, engagement length, material pairing, coating thickness, lubrication, and assembly torque method. Another is assuming applied torque directly equals preload; friction variation can dominate the result. A strong thread review states thread standard, tolerance class, strength grade, engagement length, preload target, tightening method, lubrication, corrosion protection, and fatigue or stripping margin.