Observations of the movements of the planets inspired the development of mechanical clocks. In 1900, Greek divers discovered a highly complex mechanism (the “Antikythera mechanism”) in the remains of a sunken Greek ship from the first century B.C. consisting of over 40 toothed gears that was reconstructed as an astronomical clock. However, the knowledge required to build it was lost once again. Only around 1,500 years later, in the second half of the 14th century, were the first clock towers created with complex gears for astronomical displays (such as the phases of the moon).
The first systematisation of gear elements comes from the Swede Christopher Polhem (1661-1751), who founded the first engineering school in 1697 and developed a “mechanical alphabet” with models of elementary gears. Around a hundred years later, Franz Reuleaux (1829-1905) developed a gearing system that became established as a standard in mechanical engineering.We can categorise gears into different types, for instance based on the change in motion they generate, based on the components they contain (rollers, toothed gears, cranks), or based on the way in which they transmit force. Many gears cause multiple changes in motion, and can be implemented with different components; therefore, category classifications are rarely totally unambiguous.
A crankshaft gear also converts a rotational movement into a horizontal movement. It does so continuously, however, in a back and forth motion. It is not limited. However, the gear ratio is not even, unlike the other gears we have already learned about: while the crank rotates, the speed of the output axle changes. The gear is not self-locking like the worm gear, but if the input is a back and forth movement, the gear does have a “dead point” at each end of the back and forth motion. If it stops exactly there, the movement is arrested and cannot be continued.
Even a simple pulley with just one sling doubles the length of the tow rope pulled in to complete the stroke, cutting the force required to do so in half. A person who can lift a maximum of 50 kg can use such a pulley to lift up to a 100 kg load with the same tensile force. The force amplification can be increased by adding additional rope slings: The tensile force required to complete the linear work FZ drops by n rope paths (= pulleys) to an n-th of the weight force FL of the load.
Pulleys also have a positive side effect: They stabilise the tow rope by making it more difficult for the rope to twist up: They make it possible to pull an object up very straight. The more rope slings there are, the more resistant the pulley is to torsion.
Ultimately, this reduces the load on the tow rope, since only a fraction of the weight force of the lifted object acts on each individual strand of the rope. A pulley can therefore be used to lift even very heavy objects with a relatively thin rope.