STRIGHT LINE MOTION MECHANISMS

STRIGHT LINE MOTION MECHANISMS

Straight line mechanisms :-
The mechanism which is used for producing straight line motion is called straight line motion mechanism. These may be two types (1) exact straight line mechanism, (2) approximate straight line mechanism.

(1) exact straight line mechanism:-
(a) using only turning pairs
(i)peaucellier mechanism
(ii) hart’s mechanism
(b) using one sliding pair
(i) scott- russel mechanism.
(2) approximate straight line mechanism :-
(a) watt’s straight line motion mechanism
(b) modified scott- Russel mechanism
(c) grasshopper mechanism
(d) tchebicheff’s mechanism
(e) robert’s mechanism.

Pantograph:-

it is a mechanism or a kinematic arrangement having lower pairs used to enlarge or reduce movements. The pantographs are used to reproduce drawings to different scales.
These graphs may be used to reduce the type and cut moulds. The link ‘BCE’ is parallel to link AD. And AB is parallel to link DC. A line is drawn joing ‘O’ to ‘E’. it cuts ‘DC’ .
The bairs BA and BC are extended to O and E respectively, such that
= AD/BE
Thus, foe all relative positions of the bars, the triangles OAD and OBE are similar and the points O, D and E are in one straight line. It may be proved that point E traces out the same path as described by point D.
From similar triangles OAD and OBE, we find that Let point O be fixed and the points D and E move to some new positions  and  . then
OD/OE=
A little consideration will show that the straight line D′ is parallel to the straight line E
Hence, if O is fixed to the frame of a machine by means of a turning pair and D is attached to a point in the machine which has rectilinear motion relative to the flame, then E will also trace out a straight line path.
A pantograph is mostly used for the reproduction of plane areas and figures such as maps, plans etc.., on enlarged or reduced scales.

GRASS HOPER STRAIGHT LINE MECHANISM :-

This mechanism is farther improvement or modification of seott russel mechanism with the difference that the point ‘P’ does not slide along a straight line, but moves in a circular arc with centre O.

Watt’s mechanism:-

It is a crossed four bar chain mechanism and was used by watt for his early steam engines to guide the piston rod in a cylinder to have an approximate straight line motion.


Robert mechanism :-

This mechanism also consists of four bar chain. The links OA and BQ are equal in lengths and are hinge O and Q respectively.
Bar is one rigidly such that MR biset line AB and is at right angles to each. Best point ‘P’ may be found out by instantaneous centre method.
Produce OA and QB to interest at O31 then O31 is the instantaneous centre of links. From O31 drop a vertical line to interest MR at point ‘R’.