Mechanical energy transmission

Summary: Lifting devices such as levers are among the simplest devices for mechanical energy transmission and have been in use since prehistoric times. During the Industrial Revolution many more types of mechanical energy transmission devices were developed.

Probably the simplest form of mechanical energy transmission is the lever. Although the lever has existed since the first unknown human used a tree branch as a crowbar, Archimedes has been identified as the first known scientist to describe the lever mathematically sometime before his death in 212 b.c.e. Three main types of levers exist, and all have four components. The first component is the fulcrum, the pivot point on which the lever moves. The second component is the lever itself, which is usually a stiff rod or structure. The third component is the load the lever is trying to work against, and the forth component is the applied force to move the load. Class one levers work by pivoting on the fulcrum, with the load on one side of the fulcrum and the applied force on the other. If the lever is longer on the side of applied force, the load can be lifted with an applied force less than if the force needed to raise the load if the lever did not exist. The distance the applied force moves will be greater than the distance the load moves. Conversely, if the lever is shorter on the side of applied force, then the applied force to move the load must be greater than if the lever did not exist, and the load will move a greater distance than the applied force. A simple crowbar is an example of a class one lever.

A class two lever has the load between the fulcrum and the applied force. A standard wheelbarrow is an example of a class two lever. A class three lever is similar to a class two lever; it has the applied force between the load and the fulcrum. Most mobile cranes are class three levers. A deep-sea fisherman using a fishing rod attached to a belt on his waist turns the fishing rod into a class three lever when he is pulling upward on the middle of the fishing rod to lift a large fish as the load.

Gearing and gears constitute another common and often used means of transmitting mechanical energy transmission. Early gears existed in simple machines, such as potter’s wheels, at the dawn of civilization at least 4,000 years ago in the Middle East. Both early Greeks and Romans used gears made of metal, wood, and stone in primitive machines. Systems of chain-driven gears were depicted by Leonardo da Vinci in the 15th century, and chain-driven energy transmission is common today in bicycles and many relatively low-speed machines.

Many types of gearing exist, and most gears are used to change torque and speed from a primary drive to a secondary drive. The simplest type of gear is the spur gear. Spur gears use teeth to transmit power, and the teeth are perpendicular to the face of the gear. When two gears mesh, the smaller of the two is commonly referred to as the pinion. Helical gears represent a slight improvement on spur gears; the main difference is that the gear teeth are not perpendicular to the face of the gear but at an angle. The angle makes the tooth have greater surface area for transmitting force, which is the advantage that helical gears have over spur gears. Gears of this type can have a hunting tooth design, which means that one gear has an odd number of teeth and its meshing gear has an even number of teeth. In this arrangement, the teeth do not contact the same corresponding tooth with every revolution, and thus wear on the gear teeth is reduced. Herringbone gears are an improvement over helical gears; the herringbone design comprises two helical gear teeth with opposite angles on the same gear. This type of gear can resist axial forces, an advantage over helical gears. When forces need to be transmitted by gears over right angles, bevel gears are used. The simplest form of bevel gearing is a cog-style gear, and early examples used wooden pegs. Worm gears are 90-degree transmission gears, like bevel gears, but instead of a bevel, a fluted worm is used for one of the gears. These gears are often used for fine-motion applications.

Many other types of gearing exist, including rack-and-pinion gears, but all gears transmit energy through teeth and rotating motion. Sprocket gearing can allow gears to be belt- or chain-driven instead of requiring direct contact between teeth. The belt or chain thus can be the weak point and can break if the machine driving, or driven by, the belt locks up or malfunctions. Belts exist in many forms and can be smooth or have teeth or ribs to assist in the transmission of force. In the early days of belt transmission, most belts were made of leather or cotton fibers, but most modern belts are constructed of synthetic elastomers such as Viton. Most 21st-century energy transmission chains are link chains. High-strength links are made and can be added in series to form a chain of any length desired. Chains can be self-lubricating or can require external lubricators, but all chains need some form of lubrication to keep friction low enough to make the chains practical for energy transmission.

Couplings are also used to transmit energy. Couplings exist in two main types: flexible and rigid. Rigid couplings are usually solid metal and joined together by friction fit or bolting. Flexible couplings are usually more tolerant of misalignment and perform better than rigid couplings at higher shaft speeds. Flexible couplings take many forms; some of the main types are gear couplings, fluid couplings, wire grid couplings, disk pack couplings, and magnetic couplings. Gear couplings use gear teeth to transmit the force, and slight misalignment between the gear teeth of the two machines is tolerated. Wire grid couplings use wire springs to transmit the force between the driver machine and the driven machine. The flexibility of the wire grid also allows for some misalignment of the coupling and still lets the force and energy be transmitted. Disk pack couplings are very similar to wire grid couplings but use large metal disks or washers instead of wire as the springs to transmit the force. Even magnetic force can be used to transfer power between two machines; the magnetic coupling uses magnets that do not touch each other to transmit the force. Hydraulic couplings work the same way as magnetic couplings, but fluid friction transmits power instead of magnetic force.

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