10 Power Transmission Devices used in Mechanical Engineering
Introduction to Power Transmission Elements
There are many ways to store or generate power, but all are useless without a transmission to transmit the power from a source to the place where it is needed in a form in which it can be used. Furthermore, the basic physics of power generation and transmission indicate that high speed and low torque are more efficient to create than low speed and high torque. The former is generated by high voltage and low current which is more economical than low voltage and high current.Once again, a transmission is often required to transform power into a useful form. But what combination of elements provides the best price/performance? For example, how should high speed low torque rotary motion from a motor be transformed into low speed, high force linear motion?
Pulleys are one of the oldest and most ubiquitous power transmission elements, but they require careful design. If a belt or cable runs around a fixed shaft, friction between the belt and the shaft can cause the efficiency to be low, and the cable can rapidly wear. A pulley reduces these effects with rolling contact between the cable and the machine, but it must be of sufficient size, typically 20 times the cable diameter, to prevent fatiguing of the cable strands.
A winch is a device used to control the tension and position of a cable by winding it onto a drum. Winches are one of the principle elements of cranes, as they provide the lifting force for the hook and often the boom.
Winches are also commonly used to provide a pulling force for equipment such as tow trucks. Until the introduction of hydraulics, winches were also one of the principle means of actuation for construction equipment. Robots, particularly those used in competitions, may use a winch to help pull themselves up a steep slope, or to deploy a wall.
3. Belts & Cables
Belts and cables are very common power transmission elements because their elastic nature enables them to pass over round objects (pulleys) typically with a high degree of efficiency. The term power transmission literally means the power output from the device equals the product of the efficiency and the power input to the device. Therefore, analyzing any power transmission system really is as simple as keeping track of the products of speeds and torques (or forces) and efficiencies.
Belts and cables are laterally compliant, which makes them forgiving of misalignment, so they are commonly used applications from automobiles to office equipment. In all cases, the key functional requirements of a belt or cable are to transfer tensile loads and to pass over a pulley. The life of a belt or cable is a function of its pre-tension, the diameter of the smallest pulley, and the load it is expected to carry. The key efficiency issue is to minimize contact at different diameters to prevent differential slip between the belt and the pulley.
The belt cross section must be chosen to withstand the sum of the stresses from these three sources while maintaining a desired level of stiffness.
A cable is a flexible tensile element whose elements are stranded together such that all of the strands share the load. When the cable passes over a pulley, the strands locally slip over each other so the bending stress in any single strand is far lower than if a solid bar of the same diameter were bent over the pulley. However, slip dissipates energy, so cable drives are used primarily in low speed applications. The load that cables can carry is a function of the pre-tension, the coefficient of friction between the cable and the pulley, and the wrap angle. Hence the pulley acts like a rotating capstan.
A belt can have many forms, but the three principle types are: flat, toothed, and vee belts. Flat belts transfer a load by being pre-tensioned, and as with cables, the pulleys act like capstans.
Gear are used to transmit motion from one shaft to another shaft or between a shaft or slide. This is accomplished by successively engaging teeth.There are many types of gears such as spur gears, helical gears, bevel gears, worm gears, gear rack, etc.
5. Chains and Sprockets
Chains are often used to transmit very large forces and torques relative to their size. There are many different types of chains, and one of the most common types is roller chain which uses metal links connected together by pins and spaced apart by bushings. Sprockets, toothed wheels which are a special form of gear, positively mechanically engage the chain so there can be no slip. Accordingly, chains are extremely versatile. They can be used to transmit power between two rotating shafts, and they can also be used to convert rotary to linear motion or to enable linear to linear motion.
Sprockets are readily available in many different sizes. A chain breaker tool is used to press out pins from bushings so the chain can be made the proper length and then rejoined using master links. Various special types of links can also be obtained so other elements can be more readily attached to the chain. Cutting teeth can also be attached as in a chain saw. All are readily available from catalogs.
The wheel and axle is one of six simple machines identified by Renaissance scientists drawing from Greek texts on technology. The wheel and axle consists of a wheel attached to a smaller axle so that these two parts rotate together in which a force is transferred from one to the other. A hinge or bearing supports the axle, allowing rotation. It can amplify force; a small force applied to the periphery of the large wheel can move a larger load attached to the axle.
7. Cams and Followers
A cam is a machine element whose specially shaped lobes are followed by a cam follower, which causes the cam profile to be imparted on another object. A common application is in an internal combustion engine where the cam is driven by a chain or belt connected to the crankshaft. The cam lobes are thus synchronized with the crankshaft rotation to open and close intake and exhaust valves as required. The shape of the cam lobe determines not only when a valve is opened, but how fast, and how long it stays open, which is called the dwell time. In a modern overhead cam engine, the cam pushes on a cam follower, which pushes on the valve stem. The valve is held in a normally closed position by valve springs, which, through the chain of elements cause the cam follower to remain in contact with the cam and keep the valve normally closed.
Shafts may seem like relatively benign machine elements, but they are often subject to large cyclic bending and torsional loads. There are also many different components which might be attached to them in a myriad of different ways. In addition, to reduce bearing friction losses, it is desirable to minimize shaft diameter. All these facts combine to make shaft design one of the more challenging aspects of machine design. The first step in shaft design is to assess the bending and torsional loads on the shaft. The bending loads can be determined from a free-body diagram of the proposed system, taking care to note where the shaft is supported in bearings and weather the support points act as simply supported constraints, or moment supporting constraints.
Couplings are required between rotary and linear actuators and driven components because the actuators are intended to move in one degree of freedom (linear or rotary) but they can never be perfectly aligned. As a component moves, it will not always be aligned with the actuator. The product of the net difference in motion with the stiffness of the connection between the two systems yields the misalignment force on the actuator. If a rigid coupling is used, the forces can be extremely high, and something, usually the bearings, will have to give and they will soon fail. Furthermore, a significant portion of the system’s power can be expended in the process. When a coupling is ideal, and only restrains the intended motion with negligible effect (stiffness) in all other axes, it is said to be a non-influencing coupling
10.Clutches & Differentials
Clutches and differentials are very important transmission components whose operation is normally transparent to the user until they are needed. A clutch normally behaves as a rigid element until a certain torque is exceeded, and then it slips.
A standard or “open” differential equalizes the torque on two output shafts while constraining the sum of their rotary displacements to be equal to the input rotary displacement.
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