【20160315】Mechanism Terminology" q3 R+ w+ f) I+ j2 J
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As stated, mechanisms consist of connected parts with the objective of transferring motion and force from a power source to an output. A linkage is a mechanism where rigid parts are connected together to form a chain. One part is designated the framebecause it serves as the frame of reference for the motion of all other parts. The frame is typically a part that exhibits no motion. A popular elliptical trainer exercise machine is shown in Figure 1.3. In this machine, two planar linkages are configured to operate out-of-phase to simulate walking motion, including the movement of arms. Since the base sits on the ground and remains stationary during operation, the base is considered the frame.0 N( g" ?1 o7 v% h
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Links are the individual parts of the mechanism. They are considered rigid bodies and are connected with other links to transmit motion and forces. Theoretically, a true rigid body does not change shape during motion. Although a true rigid body does not exist, mechanism links are designed to minimally deform and are considered rigid. The footrests and arm handles on the exercise machine comprise different links and, along with connecting links, are interconnected to produce constrained motion. 2 _. ~9 ^ h" K8 I" O
Elastic parts, such as springs, are not rigid and, therefore, are not considered links. They have no effect on the kinematics of a mechanism and are usually ignored during kinematic analysis. They do supply forces and must be included during the dynamic force portion of analysis. . V- F, B3 O' {! w- P1 q9 E
A joint is a movableconnection between links and allows relative motion between the links. The two primaryjoints, also called full joints, arethe revolute and sliding joints. The revolutejoint is also called a pin or hinge joint. Itallows pure rotation between the two links that it connects. The sliding joint is also called a pistonor prismaticjoint. It allows linear sliding between the links that it connects. Figure 1.4 illustrates these two primary joints.
A cam joint is shownin Figure 1.5a. It allows for both rotation and sliding between the two links that it connects. Because of the complex motion permitted, the cam connection is called a higher-order joint, also called half joint. A gear connection also allows rotation and sliding between two gears as their teeth mesh. This arrangement is shown in Figure 1.5b. The gear connection is also a higher-order joint.
A simple link is arigid body that contains only two joints, which connect it to other links. Figure 1.6a illustrates a simple link. A crank is a simple link that is able to complete a full rotation about a fixed center. A rocker is a simple link that oscillates through an angle, reversing its direction at certain intervals.
A complex link is arigid body that contains more than two joints. Figure 1.6b illustrates a complex link. A rocker arm is a complex link, containing three joints, that ispivoted near its center. A bellcrank is similar to a rocker arm, but is bent in the center. The complex link shown in Figure 1.6b is a bellcrank. $ d: ?; r1 ^ B. B* F& k6 Z
A point of interestis a point on a link where the motion is of special interest. The end of the windshield wiper, previously discussed, would be considered a point of interest. Once kinematic analysis is performed, the displacement, velocity, and accelerations of that point are determined. ; i. w' B: Y5 _9 M3 v- N+ L
The last general component of a mechanism is the actuator. An actuator is the component that drives the mechanism. Common actuators include motors (electric and hydraulic), engines, cylinders (hydraulic and pneumatic), ball-screw motors, and solenoids. Manually operated machines utilize human motion, such as turning a crank, as the actuator. Actuators will be discussed further in Section 1.7.
/ E# _1 @4 j- L/ v. v+ MLinkages can be either open or closed chains. Eachlink in a closed-loop kinematic chain is connected to two or more other links. The lift in Figure 1.2 and the elliptical trainer of Figure 1.3 are closed-loop chains. An open-loop chain will have at least one link that is connected to only one other link. Common open-loop linkages are robotic arms as shown in Figure 1.7 and other “reaching” machines such as backhoes and cranes.
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再次感谢鹰大的支持！本来总觉得自己每天在这冒泡大家会觉得水，毕竟每天浮上来，内容又是极其基础的对大家没什么参考的东西……各位的鼓励是对我更好的鞭策！我得更努力学习了呐，争取早日达到与各位更深入探讨的境界！
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楼主: 黑森林的鹿
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发表于 2016-3-15 07:53:31
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