2. Roller in a Cam Mechanism The purpose of the roller is to reduce the sliding friction between the surfaces and from the viewpoint of kinematics it can be eliminated. The DOF should be determined from an equivalent chain in which roller follower is replaced by a knife
edge follower. 3. Mechanisms with Critical Form Permanent Critical Form: Through out the whole phase of
the motion e.g. Parallelogram Mechanisms The DOF doesnt, in general depend on link dimensions.
However mechanisms with critical form are exceptions. Parallelogram mechanisms may make motion whereas GDOF eqn gives: F= 3(6-8-1)+8=-1. 3. Mechanisms with Critical Form (contd)
Instantaneous Critical Form: Movability is within an infinitesimal displacement, due to the dimensions of the links. (Used for moving loads within a small distance.)
4. Constrained-Unconstrained Mechanisms
Constrained Mechanisms: 1) Mechanisms in which F=1. 2) Mechanisms in which F1. No of inputs (no of independendent parameters)=F
e.g. Four bar mechanism with no of input=F=1, Five bar mechanism with no of input=F=2 .
Constrained-Unconstrained Mechanisms (contd) Unconstrained Mechanisms: F>1 and no of inputs
by the forces and dynamic characteristics of the system(e.g. Differential of the car). Used for load limiting or for the change of output due to change in load characteristic. F=2 system. However, due to the spring between 3-4 , if the
compressive force on 3 doesnt exceed a certain predetermined amount, there will be no relative motion between 3 and 4. In this case mechanism will act as a four bar mechanism.
Whenever this force is exceeded, link 5 will remain stationary and there will be relative motion between 3 and 4.
Fixing of different links in a mechanism
b) Oldham Coupling or equal to
1st kinematic transformation of Watts chain
2nd kinematic transformation of Watts chain A mechanism link can be of any shape
Classification of Mechanisms Reuleaux classification:
1. Screw mechanisms
2. Wheel mechanisms (gears or roller mechanisms) 3. Cam mechanisms 4. Crank mechanisms (also called linkages or link mechanisms)
5. Belt mechanisms 6. Ratchet and lock mechanisms (including Geneva drives)
Other Classifications According to; 1. Degree-of-freedom of the space 2. Degree-of-freedom
3. Number of links 4. Type of joints of the mechanism.
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