Motion & Machines: Line of Transmission

The line of Transmission:

The line along which the motion and the force is transmitted from driver to the driven is known as the line of transmission. Every machine is designed to perform a specific work and accordingly all the components, sub-assemblies and other elements are designed to carry out specific function and in a specific manner. 
You will appreciate, every machine is made up of a lot of components, as such, if you were to design a machine you would proceeds component by component and then take up the mating pair of components and so on. When we talk about mating components, the line of transmission comes into play. Since the line of transmission plays such an important role in functioning of any machine, it becomes a useful tool, for us, in determining ratio of linear or angular velocities of the driver and the driven and also the ratio of the forces or torque acting between the driver and the driven. The approach to analysing the transmission of motion and the forces will depend upon the type of connector. In the previous post we had categorized the connectors into four types. We will now proceed to analyze these connectors one by one.

Intermediate direct connectors:

Assuming that the friction is negligible, we go ahead with the analysis of the direct contact pairs. When forces and the motion is transmitted from the driver to the driven, when they are in direct contact with each other, the line of transmission is normal i.e. perpendicular to the surface of contact, as shown in the Fig. posted below;

The present discussion will be limited to only to compact mechanisms and also that only three links i.e. 1, 2 and 3 are required to transmit the force or motion from driver to the driven. It may be observed from the Fig. that there are only three links in case of the rigid connectors shown. 

As is apparent, from the arrangement shown above, the movement of the driver is in one direction only and the movement is a positive movement. One more point to be noted here is that the nature of the pair shown is a sliding pair. There can be rolling pair as well. There are some more considerations in the direct contact connectors which will be taken up next. The friction transmission has to be looked at too.