What are Linkage Mechanisms（连杆机构）?
Have you ever wondered what kind of mechanism causes the windshield wiper（刮雨器） on the front widow of cars to oscillate（摆动）( Figure 5-1a)? The mechanism, shown in Figure 5-1b, transforms（转换） the rotary motion of the motor into an oscillating motion of the windshield wiper.
Figure5-1 Windshield wiper
Let''s make a simple mechanism with similar behavior（行为）. Take some cardboard（纸板） and make four strips as shown in Figure 5-2a.
Take 4 pins and assemble（装配） them as shown in Figure 5-2b.
Now, hold the 6in. strip so it can''t move and turn the 3in. strip. You will see that the 4in. strip oscillates.
Figure 5-2 Do-it-yourself four bar linkage mechanism
The four bar linkage is the simplest and oftentimes, the most useful mechanism. As we mentioned before, a mechanism composed of rigid bodies and lower pairs is called a linkage. In planar mechanisms（平面机构）, there are only two kinds of lower pairs --- revolute pairs（转动副） and sliding pairs（移动副）.
The simplest closed-loop linkage is the four bar linkage which has four members, three moving links, one fixed link and four pin joints. A linkage that has at least one fixed link is a mechanism.
This mechanism has three moving links. Two of the links are pinned to the frame which is not shown in figure 5-3.
How many DOF（自由度） does this mechanism have? If we want it to have just one, we can impose one constraint on the linkage and it will have a definite（确定的） motion. The four bar linkage is the simplest and the most useful mechanism.
Reminder（提示）: A mechanism is composed of rigid bodies and lower pairs called linkages. In planar mechanisms there are only two kinds of lower pairs: turning pairs and prismatic pairs（移动副）.
Functions of Linkages
The function of a link mechanism is to produce rotating, oscillating, or reciprocating（往复的） motion from the rotation of a crank or vice versa. Stated more specifically linkages may be used to convert:
1. Continuous rotation into continuous rotation, with a constant or variable angular velocity ratio.
2. Continuous rotation into oscillation or reciprocation (or the reverse), with a constant or variable velocity ratio.
3. Oscillation into oscillation, or reciprocation into reciprocation, with a constant or variable velocity ratio.
Linkages have many different functions, which can be classified according to the primary goal of the mechanism:
·Function generation（函数生成）: the relative motion between the links connected to the frame,
·Path generation（轨迹重现）: the path of a tracer point, or
·Motion generation（运动生成）: the motion of the coupler link.
Four Link Mechanisms
One of the simplest examples of a constrained linkage is the four-link mechanism. A variety of useful mechanisms can be formed from a four-link mechanism through slight variations（变化）, such as changing the character of the pairs, proportions（比例） of links, etc. Furthermore, many complex link mech
anisms are combinations of two or more such mechanisms. The majority of four-link mechanisms fall into one of the following two classes:
1.
the four-bar linkage mechanism, and
2.the slider-crank mechanism.
Examples
Parallelogram Mechanism（平行四边形机构）
In a parallelogram four-bar linkage, the orientation（方位） of the coupler does not change during the motion. The figure 5-4 illustrates（举例） a loader（载重机）. Obviously, the behavior of maintaining parallelism is important in a loader. The bucket（铲斗） should not rotate as it is raised and lowered.
Slider-Crank Mechanism(曲柄滑块机构)
The four-bar mechanism has some special configurations(配置) created by making one or more links infinite（无限的） in length. The slider-crank (or crank and slider) mechanism shown in figure 5-5 is a four-bar linkage with the slider replacing an infinitely long output link.
This configuration translates a rotational motion into a translational（平移） one. Most mechanisms are driven by motors, and slider-cranks are often used to transform rotary motion into linear motion（直线运动）.
Crank and Piston（活塞）
You can also use the slider as the input link and the crank as the output link. In this case, the mechanism transfers translational motion into rotary motion. The pistons and crank in an internal combustion engine（内燃机） are an example of this type of mechanism (Figure 5-6).
You might wonder why there is another slider and a link on the left. This mechanism has two dead points（死点）. The slider and link on the left helps the mechanism to overcome these dead points.
Block Feeder
One interesting application of slider-crank is the block feeder (Figure 5-7).
