Screw thread


A screw thread is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. A screw thread is the essential feature of the screw as a simple machine and also as a threaded fastener.
The mechanical advantage of a screw thread depends on its lead, which is the linear distance the screw travels in one revolution. In most applications, the lead of a screw thread is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied, as long as no external rotational force is present. This characteristic is essential to the vast majority of its uses. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight elastic deformation.

Applications

Screw threads have several applications:
  • Fastening:
  • * Fasteners such as wood screws, plastic screws, machine screws, nuts, and bolts.
  • * Connecting threaded pipes and hoses to each other and to caps and fixtures.
  • Gear reduction via worm drives
  • Moving objects linearly by converting rotary motion to linear motion, as in the leadscrew of a jack.
  • Measuring by correlating linear motion to rotary motion, as in a micrometer.
  • Both moving objects linearly and simultaneously measuring the movement, combining the two aforementioned functions, as in a leadscrew of a lathe.
In all of these applications, the screw thread has two main functions:
  • It converts rotary motion into linear motion.
  • It prevents linear motion without the corresponding rotation.

    Design

Gender

Every matched pair of threads, external and internal, can be described as male and female. Generally speaking, the threads on an external surface are considered male, while the ones on an internal surface are considered female. For example, a screw has male threads, while its matching hole has female threads. This property is called gender. Assembling a male-threaded fastener to a female-threaded one is called mating.

Handedness

The helix of a thread can twist in two possible directions, which is known as handedness. Most threads are oriented so that the threaded item, when seen from a point of view on the axis through the center of the helix, moves away from the viewer when it is turned in a clockwise direction, and moves towards the viewer when it is turned counterclockwise. This is known as a right-handed thread, because it follows the right-hand grip rule. Threads oriented in the opposite direction are known as left-handed.
By common convention, right-handedness is the default handedness for screw threads. Therefore, most threaded parts and fasteners have right-handed threads. Left-handed thread applications include:
  • Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to applied torque or to fretting induced precession. Examples include:
  • * The left foot pedal on a bicycle
  • * The left grinding wheel on a bench grinder
  • * The axle nuts, or less commonly, lug nuts on the left side of some automobiles
  • * The securing nut on some circular saw blades – the large torque at startup should tend to tighten the nut
  • * The spindle on brushcutter and line trimmer heads, so that the torque tends to tighten rather than loosen the connection
  • * The hand-tightened nut holding the fan blade to the motor spindle in many designs of oscillating table fans and floor standing fans
  • In combination with right-hand threads in turnbuckles and clamping studs
  • In some gas supply connections to prevent dangerous misconnections, for example:
  • * In gas welding the flammable gas supply uses left-handed threads, while the oxygen supply if there is one has a conventional thread
  • * The POL valve for LPG cylinders
  • In a situation where neither threaded pipe end can be rotated to tighten or loosen the joint. In such a case, the coupling will have one right-handed and one left-handed thread.
  • In some instances, for example early ballpoint pens, to provide a "secret" method of disassembly
  • In artillery projectiles, anything that screws into the projectile must be given consideration as to what will happen when the projectile is fired, e.g., anything that screws into the base from the bottom of the projectile must be left hand threaded
  • In mechanisms to give a more intuitive action as:
  • * The leadscrew of the cross slide of a lathe to cause the cross slide to move away from the operator when the leadscrew is turned clockwise
  • * The depth of cut screw of a "Bailey" type metal plane for the blade to move in the direction of a regulating right hand finger
  • Some Edison base lamps and fittings have a left-hand thread to deter theft, because they cannot be used in other light fixtures

    Form

The cross-sectional shape of a thread is often called its form or threadform. It may be square, triangular, trapezoidal, or other shapes. The terms form and threadform sometimes refer to all design aspects taken together, but commonly refer to the standardized geometry used by the screw. Major categories of threads include machine threads, material threads, and power threads.
Most triangular threadforms are based on an isosceles triangle. These are usually called V-threads or vee-threads because of the shape of the letter V. For 60° V-threads, the isosceles triangle is, more specifically, equilateral. For buttress threads, the triangle is scalene.
The theoretical triangle is usually truncated to varying degrees. A V-thread in which there is no truncation is called a sharp V-thread. Truncation occurs for practical reasons—the thread-cutting or thread-forming tool cannot practically have a perfectly sharp point, and truncation is desirable anyway, because otherwise:
  • The cutting or forming tool's edge will break too easily;
  • The part or fastener's thread crests will have burrs upon cutting, and will be too susceptible to additional future burring resulting from dents ;
  • The roots and crests of mating male and female threads need clearance to ensure that the sloped sides of the V meet properly despite error in pitch diameter and dirt and nick-induced burrs.
  • The point of the threadform adds little strength to the thread.
In ball screws, the male-female pairs have bearing balls in between. Roller screws use conventional thread forms and threaded rollers instead of balls.

Angle

The included angle characteristic of the cross-sectional shape is often called the thread angle. For most V-threads, this is standardized as 60 degrees, but any angle can be used.
The cross section to measure this angle lies on a plane which includes the axis of the cylinder or cone on which the thread is produced.

Lead, pitch, and starts

Lead and pitch are closely related concepts. They can be confused because they are the same for most screws. Lead is the distance along the screw's axis that is covered by one complete rotation of the screw thread. Pitch is the distance from the crest of one thread to the next one at the same point.
Because the vast majority of screw threadforms are single-start threadforms, their lead and pitch are the same. Single-start means that there is only one "ridge" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn, it has advanced axially by the width of one ridge. "Double-start" means that there are two "ridges" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn, it has advanced axially by the width of two ridges. Another way to express this is that lead and pitch are parametrically related, and the parameter that relates them, the number of starts, very often has a value of 1, in which case their relationship becomes equality. In general, lead is equal to pitch times the number of starts.
Whereas metric threads are usually defined by their pitch, that is, how much distance per thread, inch-based standards usually use the reverse logic, that is, how many threads occur per a given distance. Thus, inch-based threads are defined in terms of threads per inch. Pitch and TPI describe the same underlying physical property—merely in different terms. When the inch is used as the unit of measurement for pitch, TPI is the reciprocal of pitch and vice versa. For example, a -20 thread has 20 TPI, which means that its pitch is inch.
As the distance from the crest of one thread to the next, pitch can be compared to the wavelength of a wave. Another wave analogy is that pitch and TPI are inverses of each other in a similar way that period and frequency are inverses of each other.

Coarse versus fine

Coarse threads are those with larger pitch, and fine threads are those with smaller pitch. Coarse threads have a larger threadform relative to screw diameter, where fine threads have a smaller threadform relative to screw diameter. This distinction is analogous to that between coarse teeth and fine teeth on a saw or file, or between coarse grit and fine grit on sandpaper.
The common V-thread standards include a coarse pitch and a fine pitch for each major diameter. For example, -13 belongs to the UNC series and -20 belongs to the UNF series. Similarly, M10 as per ISO 261 has a coarse thread version at 1.5 mm pitch and a fine thread version at 1.25 mm pitch.
The term coarse here does not mean lower quality, nor does the term fine imply higher quality. The terms when used in reference to screw thread pitch have nothing to do with the tolerances used or the amount of craftsmanship, quality, or cost. They simply refer to the size of the threads relative to the screw diameter.
Coarse threads are more resistant to stripping and cross threading because they have greater flank engagement. Coarse threads install much faster as they require fewer turns per unit length. Finer threads are stronger as they have a larger stress area for the same diameter thread. Fine threads are less likely to vibrate loose as they have a smaller helix angle and allow finer adjustment. Finer threads develop greater preload with less tightening torque.