Screw

A screw is essentially an inclined plane wrapped around a cylinder. Each turn of the screw advances it forward or backward by a small distance, multiplying a small turning force into a much larger pushing or pulling force. The wood screws holding furniture together, the metal bolts in machines, the jar lids on your pickles, the drills used in mining and the Archimedes screw that lifted ancient water are all examples. Although the screw was the last of the six simple machines to be identified, it is now one of the most widely used mechanical devices in the world.

• What it isInclined plane wrapped on a cylinderSpiral thread
• Multiplies forceTurning becomes pushingA tiny turn = strong push
• PitchDistance per turnFiner thread = bigger advantage
• Common usesWood screws, bolts, jar lidsPlus drills and clamps
• Ancient inventorArchimedes3rd century BC
• In your kitchenJar lids, taps, jarsSpiral threads everywhere

The big idea

Imagine you have a small inclined plane: a long, thin right-angled triangle. Now wrap it around a pencil. The angled side becomes a spiral ridge running around the pencil. That spiral is a screw thread.

Turning the screw makes the thread move along its surroundings (or, if the screw stays still, makes the thread inside an object move along the screw). Either way, one full turn moves the screw by exactly one thread-width forward or backward.

Mechanical advantage

Two things decide how much a screw multiplies force:

• Pitch: the distance the screw advances per full turn. A fine thread (small pitch) gives more mechanical advantage but needs more turns. A coarse thread (large pitch) is faster but needs more force per turn.
• Lever arm: the radius at which you apply the turning force (the length of the screwdriver handle, spanner or knob). The longer the arm, the more force you can apply.

This is why a long screwdriver with a wide handle makes screwing in wood screws so much easier than trying to twist them with your fingers.

Fact The screw is the last of the six simple machines to have been formally identified. Although ancient peoples used spiral-threaded objects for thousands of years (such as Archimedes screw for lifting water around 250 BC), it was not until the Renaissance that screws became widely used in everyday tools. Mass production of metal screws started in the 1700s with new machinery.

Types of screws

• Wood screws: have sharp threads and tapered tips that bite into wood.
• Machine screws (bolts): have straight, regular threads designed to fit into matching threaded holes or nuts.
• Self-tapping screws: have sharp tips that cut their own thread into materials like sheet metal.
• Drywall screws: long, thin and sharp; specially designed for plasterboard.
• Sheet metal screws: very sharp tips for piercing thin metal.
• Lag screws (coach screws): very large heavy-duty screws for structural carpentry.
• Set screws: small headless screws that fix one object to another (such as locking a wheel onto an axle).

Screws around the house

Have a look around: screws are absolutely everywhere.

• Door hinges: each hinge is held in place with several wood screws.
• Light bulb fittings: most bulbs screw into their sockets.
• Jar lids: a single thread spiral seals the jar tightly.
• Plastic bottle tops: same principle on a smaller scale.
• Taps: turning a tap opens or closes a valve via a screw mechanism.
• Plumbing: pipe joints are often sealed with threaded fittings.
• Furniture assembly: most flat-pack furniture is held together with screws.
• Eyeglasses: tiny screws hold the arms to the frames.
• Cars and bikes: hundreds of screws, bolts and threaded fittings hold them together.
• Computers: laptops, desktops, phones all contain dozens of small screws.

Did you know? The Phillips head screw (with the cross-shaped indentation) was invented in the 1930s by an American businessman called Henry F. Phillips. The cross shape was designed to "cam out" if you applied too much force, allowing the screwdriver to slip rather than break the screw or the work piece. This made it ideal for assembly lines using powered screwdrivers. Today there are dozens of different drive types: slot, Phillips, Pozidriv, Torx, Robertson, hex and many specialised security types.

Beyond fasteners

Screws are not just for holding things together. They also:

• Lift: car jacks use screw threads to lift cars when changing tyres. A tiny turning effort lifts hundreds of kilograms.
• Squeeze: vices, clamps and presses squeeze with screw-driven force.
• Drill: a drill bit is essentially a long, sharp screw that pulls itself into the material as it spins.
• Convey: an auger (a giant screw turning inside a tube) moves grain, snow, mince or other loose materials along its length.
• Lift water: the ancient Archimedes screw still works today. A giant screw turning inside a slightly tilted cylinder lifts water from a low source to a higher channel.
• Measure: a micrometer uses a precision screw thread to measure tiny distances to within a thousandth of a millimetre.
• Adjust precisely: telescope mounts, microscope stages and many other precision instruments use screws for fine adjustment.

Tightening and loosening

Almost all screws are made with right-handed threads: turn clockwise (rightward, when looking at the head) to tighten, anti-clockwise to loosen. A common memory aid is "righty tighty, lefty loosey".

A few special cases use left-handed threads:

• Pedals on a bicycle: the left pedal has a left-handed thread, so it tightens itself as you pedal.
• Gas cylinder valves: certain gases (like acetylene and other fuels) use left-handed threads, so the cylinders cannot be accidentally connected to wrong equipment.
• Some old saw blades and grinder wheels.

Try this Take a screw (any size) and look at it through a magnifying glass. Trace the spiral thread from the tip to the head with your eye. That single spiral is the wrapped-around inclined plane. Now slowly turn the screw and watch it slowly creep forward (or backward) by exactly one threads worth each turn. The same principle scales up to enormous industrial screws and down to the microscopic threads on tiny eyeglass screws.

Deeper dive: the screw that lifts the rivers of London

The Archimedes screw is one of the oldest known mechanical inventions still in everyday use, over 2,200 years after it was first described. The Greek mathematician Archimedes wrote about a long screw turning inside a cylindrical tube, used to lift water uphill: ideal for irrigation, mining and ship pumping. The Romans copied it widely. Medieval European farmers used it for centuries. Today, you might be surprised to learn that giant Archimedes screws are still pumping water around modern cities.

The Thames Water network in London, for example, uses dozens of Archimedes screws to lift sewage and storm water through the citys treatment works. Each screw is several metres in diameter and weighs many tonnes. The screws are durable, can handle messy water with debris in it (which would clog more delicate pumps), and are very efficient. Some of the screws at Mogden sewage treatment works in west London are over 3 metres across, lifting tens of thousands of litres per second.

The same screws are now also used as generators, doing the job in reverse. Water flowing downhill turns the screw, and the screw turns a generator producing electricity. Reverse Archimedes screws are used at small hydroelectric stations on many UK rivers, including ones in Devon, Yorkshire and even within central London on the Thames itself. They produce moderate amounts of electricity (enough for tens or hundreds of homes per installation) while being friendly to fish (which can swim through the slow-moving screws unharmed).

So the next time you walk past a small hydroelectric installation on a UK river, or see workers cleaning out a Thames sewage treatment plant, you might be looking at a 2,200-year-old invention still working perfectly. Few simple machines have had such a long career.

For more, see inclined plane and the six simple machines.