Kinematics - Synthesis of Mechanisms: Methods and Techniques

The motion to be generated by machines are generally irregular, any motion except uniform rotation about a fixed axis and uniform translation. The machine designer's task is to design such mechanisms which can generate these required irregular motions. The simplest way to design such mechanisms is by clever combination and assembly of cams and/or linkages. Thus, basically there are two types of motion generators:

1. Cams
2. Linkages

Each of the cam and linkage has their own advantages and disadvantages. Cams are easy to design but cams are difficult and expensive to manufacture. Cams generally have continuous point or line contact and thus wear out early. On the other hand, linkages are difficult to design but linkages are less expensive and easy to manufacture and also linkage mechanisms are more reliable.

Manufacturing of machine parts is more expensive process than designing or synthesis, so to save on costs one can put more effort in synthesis of mechanisms. From this view point linkages have an advantage over cams to be preferred for machine design.

The most widely used mechanisms are four link mechanisms. The four link mechanisms are simplest mechanisms capable of performing most desired functions. The design techniques used for four link mechanisms can be extended to be used for design of five and six link mechanisms. In this article series emphasis will be on four link mechanism synthesis.

The members in linkage mechanisms are connected through joints having surface contacts. Surface contact in joints provide good lubrication and wear resistance. Revolute joints and prismatic joints are the only two kinematic joints available to be used in linkage mechanisms. A four link mechanism has four joints and only two types of joints can be used, it makes only four possibilities for types of four link mechanisms with joints having surface contact.

1. The Four Bar Linkage Mechanism: This mechanism has all the four joints as revolute joints. The inversions of four bar linkage mechanisms are also four bar linkage mechanisms.
2. The Slider Crank Mechanism: This mechanism is used when either linear input is provided or a linear output is required. The slider crank mechanism is generally used to obtain linear oscillatory motion from rotary motion and vice versa. The inversions of slider crank mechanism also come under this same classification.
3. The Elliptical Trammel Linkage: This mechanism has two revolute joints and two prismatic joints on same links. The name Elliptical Trammel is given to this mechanism as the path of all the points on the coupler are ellipses.
4. The Rapson Slide Linkage: This mechanism also has two revolute joints and two prismatic joints but they are not on same links, each link has one revolute joint and one prismatic joint. Its inversions are also Rapson slide linkages.