Lecture 14 Crank - Offset Slider Mechanism and its Inversions
(2) Crank - Offset Slider Mechanism :
Consider a 4 revolute pair (4R) planar mechanism where the fourth kinematic pair goes to infinity along a vertical direction. Consequently, that fourth kinematic pair is converted to a prismatic pair and what one get is a 3R - 1P [three revolute pair (3R) one prismatic pair (1P)] mechanism. Such a 3R -1P mechanism is known as a Crank - (Offset) Slider Mechanism that is used to convert uniform rotation of link 2 (Crank) into rectilinear to and fro translation / reciprocation of link 4 (Offset Slider).
(2) For zero offset ,
(2a) Link 1 (Frame = Fixed Link) has a revolute pair (R) with link 2 (Crank) at one end and a prismatic pair (P) with link 4 (Slider) at the other end.
(2b) Link 3 ( Coupler = Motion Transfer Link) has a revolute pair (R) with link 2 (Crank) at one end and another revolute pair (R) with link 4 (Slider).
Example of Crank - Slider Mechanism with offset zero is Reciptocating Air Compressor Mechanism and Pump Mechanism wherein Crank [Link - 2] is the Input Link.
Inversions of Crank - Offset Slider Mechanism
1st Inversion of Crank - Offset Slider Mechanism with offset zero : Reciptocating Air Compressor Mechanism and Pump Mechanism
Link 1 = Frame (Fixed Link)
Link 2 = Driving Crank
Link 3 = Coupler Link (Connecting Rod)
Link 4 = Slider (Piston)
Note: Flywheel is needed in machines that are built on this kind of mechanism so as to ensure rotation of crank (link - 2) at constant velocity.
2nd Inversion of Crank - Offset Slider Mechanism with offset zero : Quick Return (Motion) Mechanism (used in Slotting machine and Shaper)
Shortest link 2 (Crank) is fixed OR
Shortest link [ Crank - Originally second link ] is Frame [ Revised 1st Link].
Second link = Driving Crank = Originally [ link 3 - coupler ]
Third link = Coupler = Originally [link 4 - Slider]
Fourth link = Driven Slotted Bar = Originally [link 1 -frame]
Slider as Coupler OR Motion transfer link ( Revised link - 3) slides within the slot of Revised link - 4 ( Driven Slotted Bar).
3rd Inversion of Crank - Offset Slider Mechanism with offset zero : Oscillating Cylinder Mechanism
Coupler link 3 is fixed OR
Coupler link 3 is Frame [ Revised 1st Link]
Second link = Oscillating (Rocking) Cylinder = Originally [ link 4 - Slider]
Third link = Piston with Piston Rod as Coupler link= Originally [link 1 - frame]
Fourth link = Driven Crank = Originally [link 2 - Crank]
Note: Oscillating Cylinder Mechanism is used for example in a bicycle foot operated air pump. Here Revised link 3 ( Piston attached to piston rod) is given an input motion (oscillatory motion). This input link [piston - Revised link - 3] slides / reciprocates within Oscillating (rocking) cylinder [Revised 2nd link] that has the same angular velocity as piston [Revised link - 3]
4th Inversion of Crank - Offset Slider Mechanism with offset zero : Hand pump mechanism
Slider ( Originally 4th link ) is fixed OR
Slider link - 4 is Frame [Revised 1st Link] = water pipe
Second link = Piston rod attached with piston = Originally [link 1 - frame]
Third link = Coupler= Originally [link 2 - Crank]
Fourth link = Driven Rocker = Originally [ link 3 - Coupler]
Note: Third link [ Revised] , namely, Coupler is given the input motion such that Second link [ Revised], namely, piston moves vertically up and down within the Revised (fixed) 1st link, namely, grounded water pipe.