Straight line generators, Design of Crank-rocker Mechanisms

Straight line generators, Design of Crank-rocker Mechanisms:

·        Straight Line Motion Mechanisms:

The easiest way to generate a straight line motion is by using a sliding pair but in precision machines sliding pairs are not preferred because of wear and tear. Hence in such cases different methods are used to generate straight line motion mechanisms:

1. Exact straight line motion mechanism.

a. Peaucellier mechanism, b. Hart mechanism, c. Scott Russell mechanism

2. Approximate straight line motion mechanisms

a. Watt mechanism, b. Grasshopper’s mechanism, c. Robert’s mechanism,

d. Tchebicheff’s mechanism

·        a. Peaucillier mechanism :

The pin Q is constrained to move long the circumference of a circle by means of the link OQ. The link OQ and the fixed link are equal in length. The pins P and Q are on opposite corners of a four bar chain which has all four links QC, CP, PB and BQ of equal length to the fixed pin A. i.e., link AB = link AC. The product AQ x AP remain constant as the link OQ rotates may be proved as follows: Join BC to bisect PQ at F; then, from the right angled triangles AFB, BFP, we have AB=AF+FB and BP=BF+FP. Subtracting, AB-BP= AF-FP=(AF–FP)(AF+FP) = AQ x AP .

Since AB and BP are links of a constant length, the product AQ x AP is constant. Therefore the point P traces out a straight path normal to AR.

·        b. Robert’s mechanism:

This is also a four bar chain. The link PQ and RS are of equal length and the tracing pint ‘O’ is rigidly attached to the link QR on a line which bisects QR at right angles. The best position for O may be found by making use of the instantaneous centre of QR. The path of O is clearly approximately horizontal in the Robert’s mechanism.

a. Peaucillier mechanism                

b. Hart mechanism