Screw thread

A screw thread is a ridge following a helix around an external or internal surface. Cylindrical threads repeat around a cylinder; tapered forms can instead follow a conical spiral. The thread's shape and dimensions determine how it mates with another threaded component.^[ref]

A simple geometric model represents one reference helix on a cylindrical thread bywhere $R$ is the chosen radius, $\theta$ is an angle measured in radians, and $L$ is the screw lead. Increasing $\theta$ by $2\pi$ returns to the same angular position while increasing the axial coordinate by $L$. This curve describes a reference path, rather than the full three-dimensional ridge profile.

A thread start is a distinct continuous helical ridge. With $n$ equally spaced starts and thread pitch $p$, the advance of one start per turn is $L=np$. The same axial spacing can therefore coexist with different advances per turn.^[ref]

When a matching nut is guided against rotation, relative rotation can produce axial movement in a lead screw. Clearance can produce backlash, while deformation of loaded components contributes mechanical compliance. Thus ideal thread geometry predicts a motion ratio, while the actual load path also depends on contact and deformation.

A thread start is one continuous helical ridge of a screw thread. A single-start thread has one such ridge; a multiple-start thread has several interleaved ridges. The number of starts controls how screw lead and thread pitch differ:where $n$ is the number of starts, $p$ the axial spacing between adjacent thread crests and $L$ the advance of one ridge in a complete turn.^[ref]

An ideal cylindrical construction explains the relation. At reference radius $R$, represent the $j$th start by the helixAt one fixed angular position, successive complete turns of the $j$th start occur at axial offsets $kL+jp$. When $L=np$, these offsets are $p(nk+j)$. As $k$ ranges over the integers and $j$ over the $n$ starts, they give every integer multiple of $p$. Adjacent crests can therefore be spaced by $p$ even though following one continuous ridge for a turn advances by $np$.

For example, a three-start thread with $p=2\mathrm{mm}$ has $L=6\mathrm{mm}$. At a chosen angular position, crests at axial coordinates $0,2,4,6\mathrm{mm}$ do not identify four starts: the crests at 0 and $6\mathrm{mm}$ can be successive turns of the same start. Following the three ridges gives advances $0\to 6$, $2\to 8$ and $4\to 10\mathrm{mm}$ over one turn. The unwrapped geometry in screw thread displays this construction.

With the screw restrained axially and its matching nut guided against rotation, a quarter-turn in this example advances the nut by $L/4=1.5\mathrm{mm}$. A single-start thread with the same pitch advances only $0.5\mathrm{mm}$. Counting starts is therefore essential when predicting a lead screw's motion from its pitch.

Thread pitch, usually $p$, is the axial distance between corresponding points on adjacent turns of a screw thread. It is measured parallel to the thread axis, rather than along the sloping helix. For a thread with several thread starts, this spacing differs from the screw lead, which measures advance per complete relative turn.^[ref]

The distinction between points and intervals is useful in a measurement. Suppose ten successive ridge crests along an axial line span $13.5\mathrm{mm}$ from the first crest to the tenth. They delimit nine pitch intervals, soDividing by ten would give $1.35\mathrm{mm}$ and would count the endpoints incorrectly. Measuring several intervals also distributes a fixed endpoint-position error over the number of intervals, provided the thread spacing is sufficiently uniform for that average to be meaningful.

A specification in threads per inch expresses the reciprocal spacing. For $q$ threads per inch,Thus $20$ threads per inch correspond to $p=1.27\mathrm{mm}$, using the exact inch-to-millimetre conversion.

For a two-start example with $p=1.5\mathrm{mm}$, the screw lead is $L=2p=3\mathrm{mm}$. A quarter-turn then advances a suitably constrained nut by $0.75\mathrm{mm}$ in the ideal geometry. A single-start thread with the same pitch advances only $0.375\mathrm{mm}$ in a quarter-turn. Equal pitch therefore does not establish equal motion per revolution.

In screw thread geometry, lead $L$ is the axial advance in one complete relative revolution of the screw and its matching nut. The term screw lead distinguishes this geometric quantity from the chemical element. For $n$ thread starts with axial thread pitch $p$,A single-start thread has $L=p$, whereas a three-start thread with $p=2\mathrm{mm}$ has $L=6\mathrm{mm}$.^[ref]

Assume the screw is restrained axially and the nut is guided against rotation. If the relative angular displacement is $\theta$, consistent signs giveDifferentiating gives $v=L\omega /(2\pi )$, where $\omega$ is relative angular velocity. Equivalently, rotation at $f$ revolutions per second gives axial speed $v=Lf$. With $L=6\mathrm{mm}$ per revolution and $f=5$ revolutions per second, the ideal speed is $30\mathrm{mm}{\mathrm{s}}^{-1}$; a $90^{\circ }$ turn advances $1.5\mathrm{mm}$.

Unwrapping a cylindrical reference surface of radius $R$ gives a right triangle with circumferential run $2\pi R$ and rise $L$. Its helix inclination $\alpha$, measured relative to a plane perpendicular to the axis, therefore obeysThis is a geometric statement about the chosen reference radius. It does not by itself determine the drive torque, because contact geometry and friction also matter.

A commanded turn is a prediction from geometry, rather than a guarantee of measured travel. Backlash can delay reversal motion, and mechanical compliance changes displacement under load. These effects remain relevant even when the nominal screw lead is known exactly.

A nut is a component with an internal screw thread that engages a matching external thread. In a lead screw mechanism, it transfers load through thread contact and can provide the translating output. Either the screw or the nut can be the rotating member, provided the other required motions are constrained.^[ref]

The useful motion depends on relative rotation. Consider an ideal assembly with screw lead $L=2\mathrm{mm}$ per relative revolution. Restraining the nut against rotation while turning an axially restrained screw through a quarter-turn produces $0.5\mathrm{mm}$ of nut travel. If both rotate together through the same angle without relative rotation, that common rotation produces no axial advance through the thread geometry.

An ideal work calculation connects the motion conversion to load transfer. For steady slow motion without friction or changes in stored energy,With $L=0.002\mathrm{m}$ and $\tau =0.10\mathrm{N}\mathrm{m}$, the ideal axial force is approximately $314\mathrm{N}$. Real contact friction and other loads enter the drive-torque balance; the ideal calculation is not a torque specification for a manufactured assembly.

A matching thread must establish a suitable contact path, rather than simply fit through the nut's opening. Clearance can permit backlash when the loaded contact surfaces change on reversal. After contact is established, further displacement under changing force includes mechanical compliance. Counting rotations alone therefore does not measure the load-dependent position of the nut.