MEE 3025 MECHANISMS WEEK 3

INSTANTANEOUS EQUIVALENCE

INSTANTANEOUS EQUIVALNCE

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 doesn’t, in general depend on link dimensions. However mechanisms with critical form are exceptions. Parallelogram mechanisms may make motion whereas GDOF eq’n gives: F= 3(6-8-1)+8=-1.

3. Mechanisms with Critical Form (cont’d) 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 1) Mechanisms in which F=1. 2) Mechanisms in which F1. 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 (cont’d) F>1 and no of inputs<F. Motion is constrained 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 doesn’t 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 Watt’s chain 2nd kinematic transformation of Watt’s 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.