Knuckle joints are actually rod-couplings and are used to connect the members in trusses and other structures. it is a structural member and not a power transmitting member.
knuckle joints are used in various area but newly a wind shield wipers for automotive vehicles are being introduced.
The automotive engine piston is connected to the small end of connecting rod by means of piston pin. This is one major application of knuckle joint. There are numerous other areas where we can see knuckle joint being used. In fact in our body also, many joints are knuckle joints.
@mechky That is just a pin-joint and not exactly a knuckle joint. even the knuckle joint actually is an example of the pin joint. pin joints form a turning pair out of the two machine members which they join. this is the real knuckle joint: http://upload.wikimedia.org/wikipedia/commons/5/53/Knuckle_joint.jpg
The image you posted shows the one we normally study in our engineering drawing or machine design books. But actual applications of knuckle joint vary in form. A knuckle joint is one which consists of two forked ends with concentric holes and a rod end in the form of eye. A knuckle pin passes through forked ends and eye. In the example given by me, the piston acts as the forked ends and connecting rod small end acts as rod end with eye.
