Crank and Connecting Rod
To obtain the displacement diagram for the piston of a crank and connecting rod assembly Theory
The slider crank chain is one of the two basic mechanisms which from the basic for many more complicated motions. (The others are the four Bar Chain, Scoth Yoke or Chebyshev linkage of which over 800 forms are known). It transforms linear motion to circular motion or vice versa. A crank is an arm attached at right angles to a rotating shaft by which reciprocating motion is imparted to or received from the shaft. It is used to convert circular motion into reciprocating motion, or vice-versa. The arm may be a bent portion of the shaft, or a separate arm attached to it. Attached to the end of the crank by a pivot is a rod, usually called a connecting rod. The end of the rod attached to the crank moves in a circular motion, while the other end is usually constrained to move in a linear sliding motion. In a reciprocating piston engine, the connecting rod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts linear motion into rotating motion. Connecting rods may also convert rotating motion into linear motion. Historically, before the development of engines, they were first used in this way.
In this laboratory we will investigate the kinematics of some simple mechanisms used to convert rotary motion into oscillating linear motion and vice-versa. The ﬁrst of these is the slider-crank - a mechanism widely used in engines to convert the linear thrust of the pistons into useful rotary motion. In this lab we will measure the acceleration of the piston of a lawn mower engine at various speeds. The results exemplify a simple relation between speed and acceleration for kinematically restricted motions, which will discover. An adjustable slider-crank apparatus and a computer simulation will show you some eﬀects of changing the proportions of the slider-crank mechanism on piston velocity and acceleration. Other linkages and cam mechanisms may also be used for linear-rotary motion conversion and some of these will be included in the lab
The distance between the piston and the centre of the crank is controlled by the triangle formed by the crank, the connecting rod and the line from the piston to the centre of the crank, as shown in [ Figure 1 ]. Since the lengths of the crank and connecting rod are constant, and the crank angle is known the triangle OAP is completely defined. From this geometry, the distance s is given by :
The rightmost position of P occurs when the crank and connecting rod are in line along the axis at P’ and the distance from O to P’ is l + r. Since the distance measured in the experiment uses this position as the reference location, the distance measured is given by:
This means that x is a function of the crank angle and that the relationship is not linear.
Figure 1 - Geometry of Crank and Connecting Rod Mechanism
1.)All of equipments for experiment of slider crank are set in good condition.
2.)Before taking readings,we turned the crank slowly and watched the movement of the piston to make sure it moves in the correct direction
3.) The angle of the circle, is twisted at 5degrees and a resulting distance that the piston moves, q is measured. The position of sliding block/slider, x is calculated
4.) The procedures number 3 and number 4 are repeated with an increasing angle of 5 degrees until the angle of circle reaches 360°
5.) The graph of the position of slider, against angles of circle, is plotted.
Crank and connecting rod assembly
From the experiment we can conclude that the motion of the piston will gradually approach simple harmonic motion in increasing value of connecting rod and crank ratio. Even though that is the case in this experiment we did not really get the graph...
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