For procedures, see your mechanical laboratory manual.

Title: Slider Crank Mechanism


Aim

To find expression for the displacement, velocity, and acceleration of a slider-crank mechanism.

Theory





A crank is a device through which rotary motion and torque can be applied to a shaft. The simplest device is a crank handle. When several cranks are incorporated into a shaft, it is called a crankshaft. The most common application of the crankshaft is in the motor car engine. A crankshaft, connecting rod and piston, is one example of a crank and slider mechanism.

The base of the dynamic mechanism operation of the engine is a slider-crank mechanism, which consists of the crankshaft, connecting rod, and piston. Combustion pressure is transferred from the piston (the part merely has reciprocating motion) to the connecting rod (the part has both linear and rotation motion) and finally to the crankshaft (the part has merely rotation motion).

A slider-crank linkage is a four-link mechanism with three revolute joints and one prismatic, or sliding, joint. The rotation of the crank drives the linear movement of the slider, or the expansion of gases against a sliding piston in a cylinder can drive the rotation of the crank. It is also an arrangement of mechanical parts designed to convert straight-line motion to rotary motion, as in a reciprocating piston engine, or to convert rotary motion to straight-line motion, as in a reciprocating piston pump.

By rotating the crank, the slider is forced to move backward and forwards. This motion is called reciprocating motion. Alternately, if the slider produces the input motion (as in the case of a piston) the crank is forced to rotate. The distance moved by the slider is dependent upon the length of the crank. As the crank rotates through 180 degrees, the slider moves a distance equal to twice the length of the crank.

Full rotation of the crank is possible if the eccentricity, c, is less than the difference between the connecting rod and the crank lengths and the crank length is less than the connecting rod length (e.g. c<(a3-a2)  and a3>a2)



There are two types of slider-cranks: in-line and offset.
  1. In-line: An in-line slider crank has its slider positioned so the line of travel of the hinged joint of the slider passes through the base joint of the crank. This creates asymmetric slider movement back and forth as the crank rotates.
  2. Offset: If the line of travel of the hinged joint of the slider does not pass through the base pivot of the crank, the slider movement is not symmetric. It moves faster in one direction than the other. This is called a quick-return mechanism.

Applications

Some applications of a slider-crank mechanism are 
  1. Reciprocating engine
  2. Rotary engine
  3. Oscillating cylinder engine
  4. Hand Pump
  5. Scotch Yoke
  6. Oldham's coupling
  7. Elliptical Trammel
The slider-crank mechanism is an inversion of the 4 bar chain mechanism, in which, one turning pair is replaced by sliding pair. Now inversion is the process of obtaining as many mechanisms as possible by fixing different links successively.

First inversion
This inversion is obtained when link 1 (ground body) is fixed. Application- Reciprocating engine, Reciprocating compressor, etc.

Second inversion
This inversion is obtained when link 2 (crank) is fixed. Application- Whitworth quick return mechanism, Rotary engine, etc.

Third inversion
This inversion is obtained when link 3 (connecting rod) is fixed. Application- Slotted crank mechanism, Oscillatory engine, etc.

Fourth inversion
This inversion is obtained when link 4 (slider) is fixed. Application- Hand pump, pendulum pump or Bull engine, etc.

References

  1. Slider-crank linkage, wikipedia.org.
  2. Crank and Slider. WJEC.
  3. Slider – Crank Mechanism for Demonstration and Experimentation MQP by Eric Brigham, Chris Destefano, and Zachary Killoy.
  4. Application Value of Slider-Crank Mechanism in Pick-and-Place Operation of Delta Robot by QIN Zhe, LIU Xiao-chu, ZHAO Zhuan, XIAO Jin-rui1, Guangzhou University, School of Mechanical and Electric Engineering.
  5. Slider-crank mechanism, Britannica.
  6. Slider Crank Mechanisms, Middle East Technical University METU
  7. Full-Length Research Paper - Kinematics and kinetic analysis of the slider-crank mechanism in otto linear four-cylinder Z24 engine by Mohammad Ranjbarkohan, Mansour Rasekh, Abdol Hamid Hoseini, Kamran Kheiralipour, and Mohammad Reza Asadi.