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Four-Bar Mechanisms
When one of the members of a constrained linkage is fixed, the linkage becomes a mechanism capable of performing a useful mechanical function in a machine. On pin-connected linkages the input (driver) and output (follower) links are usually pivotally connected to the fixed link; the connecting links (couplers) are usually neither inputs nor outputs. Since any of the links can be fixed, if the links are of different lengths, four mechanisms, each with a different input-output relationship, can be obtained with a four-bar linkage. These four mechanisms are said to be inversions of the basic linkage. Slider-Crank Inversions
When one of the pin connections in a four-bar linkage is replaced by a sliding joint, a number of useful mechanisms can be obtained from the resulting in Fig. 1 (top) the connection between links 1 and 4 is a sliding joint that permits block 4 to slide in the slot in link 1. It would make no difference, kinematically, if link 1 were sliding in a hole or slot in link 4.
If link 1 in Fig. 1 (top) is fixed, the resulting slider-crank mechanism is shown in Fig. 1 (center). This is the mechanism of a reciprocating engine. The block4 represents the piston; link 1, shown shaded, is the block that contains the crankshaft bearing at A and the cylinder; link 2 is the crankshaft and link 3 the connecting rod. The crankpin bearing is at B, the wrist pin bearing at C. The stroke of the piston in twice AB, the throw of the crank. The slider-crank mechanism provides means for converting the translator motion of the pistons in a reciprocating engine into rotary motion of the crankshaft, or the rotary motion of the crankshaft in a pump into a translator motion of the pistons. In Fig. 1 (center), when B is in position B', the connecting rod would interfere with the crank if both were in the same plane. This problem is solved in engines and pumps by offsetting the crankpin bearing from the crankshaft bearing. By using an eccentric-and-rod mechanism in place of a crank, no offsetting is necessary and very small throws can be obtained.
In Fig.1 (bottom) the crankpin bearing at B has become a large circular disk pivoted at A with an eccentricity or throw AB. The connecting rod has become the eccentric rod with a strap that encircles and slides on the eccentric. The mechanisms in the center and bottom drawings of Fig. 1 are kinematically equivalent.
By fixing links 2, 3, and 4 instead of link 1, three other inversions of the linkage in Fig. 1 (top) are obtained.
Slider-crank mechanism in motion vibration and balance
Quality in all , the centroid of the acceleration component or components of the mechanical angular acceleration , there is a force of inertia . For example: When the downward stroke crank press , slide a " quick drop " on the transmission system to generate shock , vibration, will reduce the life of the transmission parts. Machinery at high speed during this period with the organization and running of the strong inertia of change will cause additional deputy in the dynamic pressure during exercise . This will not only increase the stress of friction and deputy campaign components , leading to increased wear and reduce efficiency, but also affect the strength member. And because the inertial forces with machine operation and for cyclical changes , but also make the basis of generating machinery and forced vibration , resulting in decline in the quality and reliability of mechanical work , increased fatigue damage internal parts and materials by vibration and noise pollution. Thus , variation of the inertia force of Mechanical high-speed operation , the use of balanced design and balance test method to balance the inertial force to eliminate or mitigate the adverse effects of inertia force is to reduce mechanical vibration and improve mechanical performance, improve the mechanical work quality , one of the important measures to prolong machine life , reduce noise pollution.