Mechanisms: The Screw Thread

They hold together everything from the most delicate watch to the largest bridge. The world is literally kept from coming apart by screws and bolts, and yet we don’t often give a thought to these mechanisms. Part of that is probably because we’ve gotten so good at making them that they’re seen as cheap commodities, but the physics and engineering behind the screw thread is interesting stuff.

We all likely remember an early science lesson wherein the basic building blocks of all mechanisms laid out. The simple machines are mechanisms that use an applied force to do work, such as the inclined plane, the lever, and the pulley. For instance, an inclined plane, in the form of a splitting wedge, directs the force of blows against its flat face into a chunk of wood, forcing the wood apart.

Screw threads are another simple machine, and can be thought of as a long, gently sloped inclined plane wrapped around a cylinder. Cut a long right triangle out of paper, wrap it around a pencil starting at the big end, and the hypotenuse forms a helical ramp that looks just like a thread. Of course, for a screw thread to do any work, it has to project out more than the thickness of a piece of paper, and the shape of the projection determines the mechanical properties of the screw.

The most common thread profile is the simple V thread, with opposing faces of the thread forming an angle of 60°. This thread angle is a compromise that balances the efficiency, wear rate, strength, and most importantly, the friction of the threads. Along with the pitch, or number of threads per unit length, thread friction contributes to the self-locking, or “non-overhauling” property of most screw threads. Self-locking threads easily convert a rotational force to an axial force, but not the other way around. It’s easy to see this property in action — a nut spins easily on a screw with finger pressure, but push the nut as hard as you can along the long axis of the screw and the screw will not start spinning. Self-locking keeps tools like screw jacks from unwinding under load.

Screw thread profiles can be modified from the standard V profile for better performance under different loads. The Acme profile, a trapezoidal shape with faces forming an included angle of 29° and with large flat crests and roots, is particularly suited to high-load applications like vises and clamps. It’s also found in lead screws such as those used in linear actuators found in everything from DVD players to CNC machines. The Acme profile is also found in the lead screws of most metal lathes because it works well with split nuts. Split nuts are just what they sound like — an internally threaded element that has been split lengthwise and can be opened and closed around a lead screw. This is used to advantage in thread cutting operations, which the split nut closes around the lead screw at the start of thread cutting and opens at the end of the cut, starting and stopping the movement of the cutting tool against the workpiece in repeatable locations.

For precision threads and low-volume production, cut threads are common. Cut threads are produced in either a lathe or a CNC machine by removing material to form the thread profile, either with a thread cutting tool translated lengthwise against a rotating workpiece, or with a tap (for internal threads) or a die (for external threads).

Cutting threads is time-consuming, though, so for mass production, external threads are generally formed with a rolling operation. A forged blank is clamped tightly between a pair of grooved dies, one of which is stationary. The opposing die moves perpendicular to the long axis on the blank, pressing the thread profile into it. Since no material is removed from the blank and because the material is work hardened, rolled threads can be stronger than cut threads. The rolling process is also automation-friendly, and rolling machines can produce thousands of parts per minute.

The internal threads of nuts don’t lend themselves to rolling, so most mass-produced nuts are formed with a hot forging process. Hot slugs of metal are struck by dies that form the head profile and punch the center hole. Later, the cooled blanks are sent to a tapping machine which cuts the internal threads using a tap.

There’s much more to screw threads than this quick tour, of course. The simplicity of screw threads and the ubiquity of threaded fasteners belie the physics behind these mechanisms, but understanding the basics is a great place to start.