40墨 DESIGN OF MACHINERY CHAPTER 2 Linkages have the disadvantage of relatively large size compared to the output dis- placement of the working portion;thus they can be somewhat difficult to package.Cams tend to be compact in size compared to the follower displacement.Linkages are rela- tively difficult to synthesize,and cams are relatively easy to design (as long as a com- puter is available).But linkages are much easier and cheaper to manufacture to high precision than cams.Dwells are easy to get with cams,and difficult with linkages.Link- ages can survive very hostile environments,with poor lubrication,whereas cams cannot. unless sealed from environmental contaminants.Linkages have better high-speed dy- namic behavior than cams,are less sensitive to manufacturing errors,and can handle very high loads,but cams can match specified motions better. So the answer is far from clear-cut.It is another design trade-off situation in which you must weigh all the factors and make the best compromise.Because of the potential advantages of the pure linkage it is important to consider a linkage design before choos- ing a potentially easier design task but an ultimately more expensive solution. 2.16 MOTORS AND DRIVERS Unless manually operated,a mechanism will require some type of driver device to pro- vide the input motion and energy.There are many possibilities.If the design requires a continuous rotary input motion,such as for a Grashof linkage,a slider-crank,or a cam-follower,then a motor or engine*is the logical choice.Motors come in a wide va- riety of types.The most common energy source for a motor is electricity,but compressed air and pressurized hydraulic fluid are also used to power air and hydraulic motors. Gasoline or diesel engines are another possibility.If the input motion is translation,as is common in earth-moving equipment,then a hydraulic or pneumatic cylinder is usual- ly needed. Electric Motors *The terms motor and Electric motors are classified both by their function or application and by their electrical configuration.Some functional classifications (described below)are gearmotors,ser- engine are often used interchangeably,but they vomotors,and stepping motors.Many different electrical configurations as shown in do not mean the same Figure 2-31 are also available,independent of their functional classifications.The main thing.Their difference is electrical configuration division is between AC and DC motors,though one type,the largely semantic,but the universal motor is designed to run on either AC or DC. "purist"reserves the term motor for electrical, AC and DC refer to alternating current and direct current respectively.AC is typ- hydraulic and pneumatic ically supplied by the power companies and,in the U.S.,will be alternating sinusoidally motors and the term engine at60 hertz (Hz),at about±120，±240，or±480 volts(V)peak.Many other countries for thermodynamic devices supply AC at 50 Hz.Single-phase AC provides a single sinusoid varying with time,and such as stcam engines and 3-phase AC provides three sinusoids at 120 phase angles.DC current is constant with internal combustion time,supplied from generators or battery sources and is most often used in vehicles,such engines.Thus,your automobile is powered by as ships,automobiles,aircraft,etc.Batteries are made in multiples of 1.5 V,with 6,12, an engine,but its and 24 V being the most common.Electric motors are also classed by their rated power windshield wipers and as shown in Table 2-5.Both AC and DC motors are designed to provide continuous ro- window lifts are run by tary output.While they can be stalled momentarily against a load,they can not tolerate motors. a full-current,zero-velocity stall for more than a few minutes without overheating.* The terms motor and engine are often used interchangeably, but they do not mean the same thing. Their difference is largely semantic, but the "purist" reserves the term motor for electrical, hydraulic and pneumatic motors and the term engine for thermodynamic devices such as steam engines and internal combustion engines. Thus, your automobile is powered by an engine, but its windshield wipers and window lifts are run by motors. Linkages have the disadvantage of relatively large size compared to the output dis￾placement of the working portion; thus they can be somewhat difficult to package. Cams tend to be compact in size compared to the follower displacement. Linkages are rela￾tively difficult to synthesize, and cams are relatively easy to design (as long as a com￾puter is available). But linkages are much easier and cheaper to manufacture to high precision than cams. Dwells are easy to get with cams, and difficult with linkages. Link￾ages can survive very hostile environments, with poor lubrication, whereas cams cannot, unless sealed from environmental contaminants. Linkages have better high-speed dy￾namic behavior than cams, are less sensitive to manufacturing errors, and can handle very high loads, but cams can match specified motions better. So the answer is far from clear-cut. It is another design trade-off situation in which you must weigh all the factors and make the best compromise. Because of the potential advantages of the pure linkage it is important to consider a linkage design before choos￾ing a potentially easier design task but an ultimately more expensive solution. 2.16 MOTORS AND DRIVERS Unless manually operated, a mechanism will require some type of driver device to pro￾vide the input motion and energy. There are many possibilities. If the design requires a continuous rotary input motion, such as for a Grashof linkage, a slider-crank, or a cam-follower, then a motor or engine* is the logical choice. Motors come in a wide va￾riety of types. The most common energy source for a motor is electricity, but compressed air and pressurized hydraulic fluid are also used to power air and hydraulic motors. Gasoline or diesel engines are another possibility. If the input motion is translation, as is common in earth-moving equipment, then a hydraulic or pneumatic cylinder is usual￾ly needed. Electric Motors Electric motors are classified both by their function or application and by their electrical configuration. Some functional classifications (described below) are gearmotors, ser￾vomotors, and stepping motors. Many different electrical configurations as shown in Figure 2-31 are also available, independent of their functional classifications. The main electrical configuration division is between AC and DC motors, though one type, the universal motor is designed to run on either AC or DC. AC and DC refer to alternating current and direct current respectively. AC is typ￾ically supplied by the power companies and, in the U. S., will be alternating sinusoidally at 60 hertz (Hz), at about ±120, ±240, or ±480 volts (V) peak. Many other countries supply AC at 50 Hz. Single-phase AC provides a single sinusoid varying with time, and 3-phase AC provides three sinusoids at 1200 phase angles. DC current is constant with time, supplied from generators or battery sources and is most often used in vehicles, such as ships, automobiles, aircraft, etc. Batteries are made in multiples of 1.5 V, with 6, 12, and 24 V being the most common. Electric motors are also classed by their rated power as shown in Table 2-5. Both AC and DC motors are designed to provide continuous ro￾tary output. While they can be stalled momentarily against a load, they can not tolerate a full-current, zero-velocity stall for more than a few minutes without overheating