STEM Gear Tech

Gears play a central role in the history of mankind. Simple gears were used to make fire, more complex ones were needed to build structures such as Stonehenge (ca. 3,500 BC) or the seven wonders of the ancient world, including the pyramids of Giza (ca. 2,500 BC), which can still be admired today.

Gears were crucial for the development of human civilization: they made it possible to construct large buildings, draw water or lift and transport heavy objects. Later, they were used to generate energy (pedal wheels, water wheels, windmills, steam engines), pump water (Marly machine) or drive vehicles. Gears also played an important role in measuring time: it was only with pendulum clocks and precision mechanical gears that it was possible to construct clocks with which time could be determined more precisely than by determining the position of the sun.

Today, gears are found in almost every electrical appliance, usually invisible to the user. Washing machines, dishwashers, vacuum cleaners, sewing machines, bicycles, elevators, hair dryers, blenders, coffee machines, garage doors, wall clocks, scales - none of these technical achievements of everyday life would be possible without gears. Even the simplest tools such as corkscrews or bottle openers are gears. Without gears, we would probably still be living in caves. And possibly not have survived the dinosaurs.

Let's illustrate this using what is probably the simplest transmission of all, the lever.

A lever consists of a rigid body (e.g. a beam) that is pivoted at one point. Imagine a seesaw on a playground - that's exactly what a lever is. The two parts of the seesaw beam that protrude to the left and right of the bearing are called the lever arm. One of the lever arms is the drive, the other is the output. If you sit on one side of the rocker, your lever arm moves downwards and the other upwards - a rocker (a lever) therefore changes the direction of movement. It also changes the path of the movement, because your movement of one lever arm is transferred to the end of the other lever arm. And the lever can also change the speed of the movement: If your lever arm is longer than the lever arm of the output, then you will cover a longer distance than the end of the other lever arm when rocking - but in the same amount of time. So the movement of the output slows down.

Something fascinating happens: The force that you exert on the drive lever arm of the seesaw with your weight is transferred to the output lever arm. If this is shorter than the drive lever arm, the force increases! You have probably observed this before: If a larger child is sitting on one side of the seesaw than on the other, the seesaw can be balanced by the larger child shortening its lever arm by sliding forwards (towards the pivot joint). So a gearbox can do something else: it can amplify force.

Simple gears such as the lever or pulley have been known to man for thousands of years and were mainly used to lift loads (e.g. in buildings or for loading and unloading ships and carts). 

The oldest known records in which gears were systematically studied date back to the Greeks. As far as we know, the "law of the lever" was first described by Archimedes of Syracuse (approx. 287-212 BC). He was so enthusiastic about increasing the force of a lever that he even exclaimed: "Give me a fixed point in space and I will unhinge the world."

The Roman architect and master builder Marcus Vitruvius Pollio (Vitruvius, ca. 75-15 BC) wrote the first ever work on architecture with his "Ten Books on Architecture" (still extant today). He dedicated volume 10 to "Mechanical Engineering" and described in detail the machines and gears known at the time. These included the pulley block, the corrugated wheel, the treadwheel, the trispastos (a simple crane), Greek water clocks and the Archimedean screw (a "screw" for pumping water). The works of the Greek Heron of Alexandria (probably a contemporary of Vitruvius) also contain "automata" with gears, e.g. a wind-driven organ or the first cogwheels. The Romans also constructed war machines (throwing machines and slingshots) in which gears were used.

A few centuries later, gears played an important role in energy generation. Water power and wind power were converted into rotary motion by water and wind wheels. This was sufficient for mills; for stone saws, however, the rotary motion had to be converted into a reciprocating motion of the saw. The first evidence for the existence of such slider-crank gears is provided by the "drawing" of a stone saw with a water wheel on a tombstone from the 3rd century AD.

Positive and non-positive gears

There is another important characteristic of gearboxes according to which we can differentiate between the gearboxes presented. Gear, worm and chain gearboxes are described as positive-locking: The gear elements (teeth, worm, chain links) engage firmly with each other. The drive and output are thus firmly connected to each other: As soon as an input occurs, the output of the gearbox also moves.

However, there are also gearboxes in which the gearbox elements do not interlock via their shape, but are "loosely" connected to each other. These include belt transmissions, for example: the belt is only prevented from "slipping through" by the frictional forces acting on it. These gears are called friction-locked. Unlike form-fit gears, the input and output are not firmly connected: If, for example, the resistance force at the output becomes too great, the belt slips. This protects the drive (e.g. a motor) from damage - it simply continues to run. The point at which the resistance forces of the output exceed the frictional forces of the connection can even be calculated.