A simple machine is a mechanical device that changes the direction or magnitude of a force.[1] In general, they can be defined as the simplest mechanisms that use mechanical advantage (also called leverage) to multiply force.[2] Usually the term refers to the six classical simple machines that were defined by Renaissance scientists:[3][4][5]
A simple machine uses a single applied force to do work against a single load force. Ignoring friction losses, the work done on the load is equal to the work done by the applied force. The machine can increase the amount of the output force, at the cost of a proportional decrease in the distance moved by the load. The ratio of the output to the applied force is called the mechanical advantage.
Classical Simple Machines
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Simple machines can be regarded as the elementary "building blocks" of which all more complicated machines (sometimes called "compound machines"[6][7]) are composed.[2][8] For example, wheels, levers, and pulleys are all used in the mechanism of a bicycle.[9][10] The mechanical advantage of a compound machine is just the product of the mechanical advantages of the simple machines of which it is composed.
Although they continue to be of great importance in mechanics and applied science, modern mechanics has moved beyond the view of the simple machines as the ultimate building blocks of which all machines are composed, which arose in the Renaissance as a neoclassical amplification of ancient Greek texts. The great variety and sophistication of modern machine linkages, which arose during the Industrial Revolution, is inadequately described by these six simple categories. Various post-Renaissance authors have compiled expanded lists of "simple machines", often using terms like basic machines,[9] compound machines,[6] or machine elements to distinguish them from the classical simple machines above. By the late 1800s, Franz Reuleaux[11] had identified hundreds of machine elements, calling them simple machines.[12] Modern machine theory analyzes machines as kinematic chains composed of elementary linkages called kinematic pairs.
During the Renaissance the dynamics of the mechanical powers, as the simple machines were called, began to be studied from the standpoint of how far they could lift a load, in addition to the force they could apply, leading eventually to the new concept of mechanical work. In 1586 Flemish engineer Simon Stevin derived the mechanical advantage of the inclined plane, and it was included with the other simple machines. The complete dynamic theory of simple machines was worked out by Italian scientist Galileo Galilei in 1600 in Le Meccaniche (On Mechanics), in which he showed the underlying mathematical similarity of the machines as force amplifiers.[17][18] He was the first to explain that simple machines do not create energy, only transform it.[17]
If a simple machine does not dissipate energy through friction, wear or deformation, then energy is conserved and it is called an ideal simple machine. In this case, the power into the machine equals the power out, and the mechanical advantage can be calculated from its geometric dimensions.
Although each machine works differently mechanically, the way they function is similar mathematically.[20] In each machine, a force F in \displaystyle F_\textin is applied to the device at one point, and it does work moving a load F out \displaystyle F_\textout at another point.[21] Although some machines only change the direction of the force, such as a stationary pulley, most machines multiply the magnitude of the force by a factor, the mechanical advantage
Simple machines do not contain a source of energy,[22] so they cannot do more work than they receive from the input force.[21] A simple machine with no friction or elasticity is called an ideal machine.[23][24][25] Due to conservation of energy, in an ideal simple machine, the power output (rate of energy output) at any time P out \displaystyle P_\textout is equal to the power input P in \displaystyle P_\textin
So in non-ideal machines, the mechanical advantage is always less than the velocity ratio by the product with the efficiency η \displaystyle \eta . So a machine that includes friction will not be able to move as large a load as a corresponding ideal machine using the same input force.
A compound machine is a machine formed from a set of simple machines connected in series with the output force of one providing the input force to the next. For example, a bench vise consists of a lever (the vise's handle) in series with a screw, and a simple gear train consists of a number of gears (wheels and axles) connected in series.
In many simple machines, if the load force F out \displaystyle F_\textrm out on the machine is high enough in relation to the input force F in \displaystyle F_\textrm in , the machine will move backwards, with the load force doing work on the input force.[29] So these machines can be used in either direction, with the driving force applied to either input point. For example, if the load force on a lever is high enough, the lever will move backwards, moving the input arm backwards against the input force. These are called reversible, non-locking or overhauling machines, and the backward motion is called overhauling.
However, in some machines, if the frictional forces are high enough, no amount of load force can move it backwards, even if the input force is zero. This is called a self-locking, nonreversible, or non-overhauling machine.[29] These machines can only be set in motion by a force at the input, and when the input force is removed will remain motionless, "locked" by friction at whatever position they were left.
Simple machines are elementary examples of kinematic chains that are used to model mechanical systems ranging from the steam engine to robot manipulators. The bearings that form the fulcrum of a lever and that allow the wheel and axle and pulleys to rotate are examples of a kinematic pair called a hinged joint. Similarly, the flat surface of an inclined plane and wedge are examples of the kinematic pair called a sliding joint. The screw is usually identified as its own kinematic pair called a helical joint.
The identification of simple machines arises from a desire for a systematic method to invent new machines. Therefore, an important concern is how simple machines are combined to make more complex machines. One approach is to attach simple machines in series to obtain compound machines.
However, a more successful strategy was identified by Franz Reuleaux, who collected and studied over 800 elementary machines. He realized that a lever, pulley, and wheel and axle are in essence the same device: a body rotating about a hinge. Similarly, an inclined plane, wedge, and screw are a block sliding on a flat surface.[32]
Throughout history, humans have developed several simple machines to make work easier. The most notable of these are known as the "six simple machines": the wheel and axle, the lever, the inclined plane, the pulley, the screw, and the wedge, although the latter three are actually just extensions or combinations of the first three, according to Encyclopedia Britannica (opens in new tab).
Simple machines are devices with no, or very few, moving parts that make work easier. Many of today's complex tools are just combinations or more complicated forms of the six simple machines, according to the University of Colorado at Boulder (opens in new tab). For instance, we might attach a long handle to a shaft to make a windlass, or use a block and tackle to pull a load up a ramp. While these machines may seem simple, they continue to provide us with the means to do many things that we could never do without them.
The other five machines all help humans increase and/or redirect the force applied to an object. In their book "Moving Big Things (opens in new tab)," Janet L. Kolodner and her co-authors write, "Machines provide mechanical advantage to assist in moving objects. Mechanical advantage is the trade-off between force and distance." In the following discussion of the simple machines that increase the force applied to their input, we will neglect the force of friction, because in most of these cases, the frictional force is very small compared to the input and output forces involved.
When a force is applied over a distance, it produces work. Mathematically, this is expressed as W = F D. For example, to lift an object, we must do work to overcome the force due to gravity and move the object upward. To lift an object that is twice as heavy, it takes twice as much work to lift it the same distance. It also takes twice as much work to lift the same object twice as far, according to Auburn University (opens in new tab). As indicated by the math, the main benefit of machines is that they allow us to do the same amount of work by applying a smaller amount of force over a greater distance.
As simple as pulleys are, they are still finding use in the most advanced new machines. For example, the Hangprinter, a 3D printer that can build furniture-sized objects, employs a system of wires and computer-controlled pulleys anchored to the walls, floor, and ceiling.
The simplest machines in general terms can be defined, or specified as the simplest of devices or mechanisms that apply the law of leverage (mechanical advantage) to convert or multiply the input force, to change the direction or magnitude of the output force.
Usually the term refers to the six classical simple machines which were defined by Renaissance scientists. But more often than not in recent times, machines made up from combinations of the six basic simple machines are also included. That's progress for ya!
In the 20th century, a realization that at least one machine, the hydraulic press, had been left out of the simple category and arguments that some of the six classical devices can be considered as modifications of others, has led some modern sources to avoid specifying any list of simple devices as "basic". 2ff7e9595c
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