A screw thread, often shortened to 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 (and simultaneously amplifying it), 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 (whether in nut or substrate) has female threads. This property is called gender. Assembling a male-threaded fastener to a female-threaded one is called mating.
Handedness
Right- and left-handed screw threads
The right-hand rule of screw threads
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 (RH) thread, because it follows the right-hand grip rule. Threads oriented in the opposite direction are known as left-handed (LH).
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 hand pedal on a bicycle.
- The left-hand 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
- 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 (e.g. in traditional heating pipes running through several rooms in a building). 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” (or “Stanley-Bailey”) type metal plane (tool) for the blade to move in the direction of a regulating right hand finger.
- Some Edison base lamps and fittings (such as those formerly used on the New York City Subway or the pre-World War I Sprague-Thomson rolling stock of the Paris Metro) have a left-hand thread to deter theft, since they cannot be used in other light fixtures.
Form
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 (whether in nut or substrate) has female threads. This property is called gender. Assembling a male-threaded fastener to a female-threaded one is called mating.
Handedness
Right- and left-handed screw threads
The right-hand rule of screw threads
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 (RH) thread, because it follows the right-hand grip rule. Threads oriented in the opposite direction are known as left-handed (LH).
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 hand pedal on a bicycle.
- The left-hand 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
- 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 (e.g. in traditional heating pipes running through several rooms in a building). 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” (or “Stanley-Bailey”) type metal plane (tool) for the blade to move in the direction of a regulating right hand finger.
- Some Edison base lamps and fittings (such as those formerly used on the New York City Subway or the pre-World War I Sprague-Thomson rolling stock of the Paris Metro) have a left-hand thread to deter theft, since they cannot be used in other light fixtures.