1. Gears A gear is a rotating machine part, having cut like teeth, which fit together with another toothed part to transmit rotation. Geared devices can change the speed, torque, and direction of a power source. Gears almost always produce a change in torque, creating a mechanical benefit, through their gear ratio, and so may be carefully thought about a simple machine. The teeth on the two fitting together gears all have the same shape. Two or more fitting together gears, working in a repetition, are called a gear train or a transmission. A gear can fit together with a linear toothed part, called a rack, producing translation instead of rotation. Gears can be used in watches and clocks they are normally circle shaped.

2. Cams What is a cam and what does it do? As the camshaft spins, the lobes open and close the intake and exhaust valves in time with the motion of the piston. It turns out that there is a direct relationship between the shape of the cam lobes and the way the engine performs in different speed ranges. What is the function of a cam? A common example is the camshaft of an automobile, which takes the rotary motion of the engine and translates it into the reciprocating motion necessary to operate the intake and exhaust valves of the cylinders.

3. Linkages A mechanical linkage is an assembly of bodies connected to manage forces and movement. The movement of a body, or link, is studied using geometry so the link is considered to be rigid.[1] The connections between links are modeled as providing ideal movement, pure rotation or sliding for example, and are called joints. A linkage modeled as a network of rigid links and ideal joints is called a kinematic chain. Linkages may be constructed from open chains, closed chains, or a combination of open and closed chains. Each link in a chain is connected by a joint to one or more other links. Thus, a kinematic chain can be modeled as a graph in which the links are paths and the joints are vertices, which is called a linkage graph.

4. Ratios The Gear Ratio is simply the number of teeth on the driver gear compared with the number of teeth on the driven gear. It is the term used with gearing to mean Velocity Ratio. A gear will always rotate in an opposite direction to the gear it meshes Note: If both gears have the same number of teeth, they will rotate at the same speed, however if they have different numbers of teeth then the gear with fewer teeth will rotate more quickly - i.e. the Velocity Ratio (or ‘Gear Ratio’) of a pair of meshing gears is given as the Number of Teeth of the Driver Gear divided by the Number of Teeth of the Driven Gear. The Mechanical Advantage (or ‘Torque Ratio’) similarly is given as No. of Teeth on Driven divided by No. of Teeth on Driver. https://www.youtube.com/watch?v=GQGr-IN6rHU Here is a quick video: