Note: the following has been abstracted from the Grolier Encyclopedia.

Machine

Machines can be divided into four broad groups: mechanical, electrical, fluid power, and prime movers. Prime movers convert natural forms of energy, such as those found in streams, wind, and potentially in fuels, into mechanical energy. Common examples of prime movers include the waterwheel, windmill INTERNAL-COMBUSTION ENGINE, and NUCLEAR REACTOR. Fluid-power machines convert mechanical energy to flow energy, as in hydraulic pumps; or convert flow to mechanical energy, as in fluid motors, hydraulic cylinders, and turbines. Similarly, electric machines convert mechanical to electrical energy in such devices as generators and alternators, or convert electrical to mechanical energy in electric motors and loudspeakers. Mechanical machines, the focus of this article, modify mechanical energy to convert limited input forces and motions into those required to perform specific work. The manner in which a machine modifies and transmits motion is called a mechanism. Although the variety of mechanisms is unlimited, motion is mechanically transmitted in only three basic ways: by a linkage; by direct contact between surfaces such as gear teeth or a cam and follower; or by a wrapping connector such as a belt, rope, or chain.

Work

Work is defined as force multiplied by the distance over which the force acts. Work is measured in foot-pounds. (A foot-pound equals the work done by a force of one pound acting through a distance of one foot.) For example, if a work task involves lifting a 300-pound refrigerator two and one-half feet into a truck, then 750 foot-pounds of work is required. Since very few humans can lift 300 pounds directly, a device must be employed to modify the required effort into something manageable. One common device is an inclined plane--in this case, a loading ramp that slopes from the ground to the truck. If the ramp were 10 feet long and friction forces were negligibly small, then 75 pounds of force would be required to roll the refrigerator up the ramp. The total work is still 750 foot-pounds (10 feet multiplied by 75 pounds), but the effort has been modified so that the maximum force required is only 75 pounds.

A device that decreases the necessary applied force while increasing the distance over which the smaller force acts is called a force multiplier. Machines can also multiply speed and distance. A broom is an example of a speed and distance multiplier because it converts input force and distance at the handle into a lower force and larger distance at the sweeping end. Since the larger distance at the sweeping end is covered in the same time as the input distance, then the speed is also increased. In addition to modifying forces and distances, machines can also change the direction of motion.

Efficiency and Mechanical Advantage

Efficiency and mechanical advantage are used to gauge the performance of mechanical machines. Efficiency is defined as the useful mechanical output work, expressed as a percentage of the input work. Efficiency is always less than 100 percent because of friction between moving parts. If someone actually wheeled the refrigerator of the initial example up the ramp, they might discover that it required 84 pounds. The nine-pound difference is the force required to overcome the resistance of the wheels and bearings. Under these conditions the machine would have an efficiency of 89 percent. If they slid the refrigerator up the ramp without wheels, the required force could be 215 pounds or more, which corresponds to an efficiency of less than 35 percent. Ideal mechanical advantage neglects friction and is equal to the distance the input force travels divided by the useful distance the load travels. For force-multiplying machines, the input distance is greater than the load distance and the ideal mechanical advantage is greater than 1. In the loading ramp example, the ideal mechanical advantage is 4, since the input distance is 10 feet (the length of the ramp) and the useful load distance is two and one-half feet (the vertical distance the load travels). An inclined plane is always a force-multiplying machine. For speed-multiplying machines the input distance is less than the load distance and the ideal mechanical advantage is less than 1. Machines that simply change the direction of motion have an ideal mechanical advantage equal to 1. The actual mechanical advantage includes friction and is equal to the actual output force divided by the actual input force. The actual mechanical advantage in the loading ramp example is about 3.6 with wheels and about 1.4 without wheels.

Simple Machines

Simple machines are the most basic devices that can modify the effort required to accomplish work. The simplest are the inclined plane and LEVER. More complicated variations consist of the PULLEY, wheel-and-axle, wedge, and SCREW.

Inclined planes and their principle of operation can be found in a broad range of applications such as roads, loading ramps, and escalators. They are also the basis of two other simple machines, the screw and the wedge. A wedge is often the shape of two inclined planes placed back to back, while a screw is the shape of an inclined plane spiraled around a cylinder.

Levers are rigid bars that rotate about a pivot called a fulcrum. Levers are divided into three classes, depending on the relative positions of the handle, fulcrum, and load. The ideal mechanical advantage of a lever is equal to the distance between the handle and the fulcrum divided by the distance between the load and the fulcrum. Second- class levers are force multipliers: a good example is a wheelbarrow, where the wheel acts as the fulcrum. To illustrate first- and third-class levers, consider a broom that is gripped with one hand on top and the other hand in the middle. If the sweeping motion is obtained by holding the hand on top still while moving the other hand, it is a third-class lever. All third-class levers are speed multipliers. If the hand in the middle is now held still and the hand on top moved to sweep, then it is a first-class lever. First-class levers can be either force or speed multipliers and also change the direction of motion. If the hand that grips the middle is moved toward the top, then it is a speed multiplier. If the hand is moved past the middle toward the sweeping end, then it is a force multiplier. If the hand remains precisely in the middle, then only the direction of motion is changed.

The principle of the lever is found in a wide variety of machines including the pulley and the wheel-and-axle. A pulley is basically a circular lever that consists of a grooved wheel rotating about a center hub. Pulleys are classified either as fixed or movable, depending on whether the hub remains at a fixed position or moves. A fixed pulley only changes the direction of effort, since it is a first-class lever with the fulcrum located at its center. A movable pulley is a second-class lever with an ideal mechanical advantage of 2. Several fixed and movable pulleys can be used together in a lifting device called a block and tackle, if larger mechanical advantages are desired.

A wheel-and-axle is also a circular first-class lever but differs from a pulley in that the wheel is fixed to the axle. Examples are screwdrivers, chain sprockets, and gears. The wheel-and-axle can be either a force or speed multiplier. A screwdriver functions as a force multiplier because the handle diameter (machine input) is much larger than the width of the blade (machine output). A bicycle functions as a speed multiplier because the pedal sprocket (input) is smaller than the rear wheel (output). The ideal mechanical advantage of a wheel-and-axle is the diameter at the input divided by the diameter at the output.

Gears

Gears are toothed wheels that transmit force and motion from one rotating, geared shaft to another. Teeth can be formed on the outside as for external gears, or on the inside, as for internal gears. Depending on the relative sizes (or number of teeth) of the meshing gears, a variety of speed or force multiplications can be obtained. It is also possible to simply change the direction of motion.

For gearing it is more common to speak of TORQUE than force. Torque is equal to force multiplied by the distance from the point of force application to the center of rotation. A small gear driving a large gear multiplies torque, while a large gear driving a small gear multiplies speed. The ideal mechanical advantage of two gears (or two chain sprockets wrapped by a chain) is equal to the number of teeth on the driven gear divided by the number of teeth on the driving gear. Two external gears in mesh rotate in opposite directions, while an external and internal gear in mesh rotate in the same direction. Gears can also convert rotary motion to linear motion by meshing with a toothed bar called a rack. When large mechanical advantages are desired, many gears are meshed successively in a gear train.

With the exception of some lever applications all of the devices discussed thus far have a constant mechanical advantage over their input cycle and are called linear machines, or linear mechanisms. The input cycle consists of a repetitive input such as a gear rotation or screw turn. There are also many machines, such as cams and linkages, where the mechanical advantage changes over the input cycle. These are called nonlinear mechanisms.

Cams and Linkages

Cams generally rotate on shafting and are shaped to impart a specified motion to a contacting follower. The follower has a flat face or roller and reciprocates, or oscillates, when in contact with the cam surface. A common application is in actuating the valves of an internal combustion engine, where cams control when and how the valves open. As the cam shaft rotates, the lobed portion of the cam pushes down and opens the valve. When the valve is fully open, the follower stops moving and remains idle, or dwells, momentarily. The cam then rotates further and the valve closes. For one typical revolution of the cam the follower accelerates, decelerates, dwells, accelerates, and then decelerates again. The cam surface is shaped to provide the best follower positions, velocities, accelerations, and dwells for a specific application.

Linkages are a broad class of mechanisms. Two of the most important are the four-bar linkage and the slider crank. A four-bar linkage has four links that are connected by pin joints. One of the links is stationary and is generally formed from the machine housing. The other links consist of the input, output, and the coupler, which connects the input and output links. Depending on the relative lengths of the four links, the input-output of a four-bar linkage can be configured as a rocker-rocker, crank-rocker, crank-crank, and a rocker-crank. A crank has rotary motion, while a rocker oscillates back and forth. A common example of the use of the crank-rocker configuration is in a washing machine. During the wash cycle the input motor has constant rotary motion, while the agitator oscillates back and forth to wash the clothes. The four-bar linkage can multiply speed or force of both rotary and oscillatory motion.

The slider crank is commonly used to convert linear, reciprocating motion to rotary motion in internal-combustion engines. The slider crank has four links, similar to the four-bar linkage, except that one of the links is constrained by a sliding joint rather than by a pin joint. In internal-combustion engines the piston slides up and down along the cylinder wall, which serves as the fixed ground link. The piston is the input link and is pushed by exploding combustion gases. The crankshaft is the output link that converts the linear motion of the piston to the rotary motion that will turn the wheels. The coupler link that connects the piston to the crankshaft is called the connecting rod.

The slider crank and four-bar linkage are planar mechanisms, because their motions can be viewed in a single plane such as the paper on which they are sketched. Spatial mechanisms--such as the action of a joint in a robot--have more complex motions in three-dimensional space.

The process of inventing, selecting, and proportioning machines so they provide desired motions is called kinematic synthesis. Thousands of different mechanisms exist in millions of machines, and thousands more are yet to be invented. The variety of mechanisms as well as the way they are utilized to make complex machinery is limited only by the imagination of engineers and inventors.