tailieunhanh - Mechanisms and Mechanical Devices Sourcebook P13

KEY EQUATIONS AND CHARTS FOR DESIGNING MECHANISMS FOUR-BAR LINKAGES AND TYPICAL INDUSTRIAL APPLICATIONS All mechanisms can be broken down into equivalent four-bar linkages. They can be considered to be the basic mechanism and are useful in many mechanical operations. FOUR-BAR LINKAGES—Two cranks, a connecting rod and a line between the fixed centers of the cranks make up the basic four-bar linkage. Cranks can rotate if A is smaller than B or C or D. Link motion can be predicted. | CHAPTER 13 KEY EQUATIONS AND CHARTS FOR DESIGNING MECHANISMS FOUR-BAR LINKAGES AND TYPICAL INDUSTRIAL APPLICATIONS All mechanisms can be broken down into equivalent four-bar linkages. They can be considered to be the basic mechanism and are useful in many mechanical operations. FOUR-BAR LINKAGES Two cranks a connecting rod and a line between the fixed centers of the cranks make up the basic four-bar linkage. Cranks can rotate if A is smaller than B or C or D. Link motion can be predicted. PARALLEL CRANK FOUR-BAR Both cranks of the parallel crank four-bar linkage always turn at the same angular speed but they have two positions where the crank cannot be effective. NON-PARALLEL EQUAL CRANK The centrodes are formed as gears for passing dead center and they can replace ellipticals. CRANK AND ROCKER the following relations must hold for its operation A B C D A D B C A C - B D and C - A B D. DOUBLE PARALLEL CRANK This mechanism avoids a dead center position by having two sets of cranks at 90 advancement. The connecting rods are always parallel. SLOW MOTION LINK As crank A is rotated upward it imparts motion to crank B. When A reaches its dead center position the angular velocity of crank B decreases to zero. FOUR-BAR LINK WITH SLIDING MEMBER One crank is replaced by a circular slot with an effective crank distance of B. PARALLEL CRANK Steam control linkage assures equal valve openings. TRAPAZOIDAL LINKAGE This linkage is not used for complete rotation but can be used for special control. The inside moves through a larger angle than the outside with normals intersecting on the extension of a rear axle in a car. DOUBLE PARALLEL CRANK MECHANISM This mechanism forms the basis for the universal drafting machine. ISOSCELES DRAG LINKS This lazy-tong device is made of several isosceles links it is used as a movable lamp support. WATT S STRAIGHT-LINE MECHANISM Point T describes a line perpendicular to the parallel position of the cranks. 430 STRAIGHT SLIDING LINK .