Mechanical Systems: Motion, Linkages and Cams
Four Types of Motion
Mechanical systems transmit and transform movement. Before designing a mechanism you must identify what type of motion is needed at the output. There are four types that engineers work with every day.
Rotary motion is spinning around a fixed axis — a motor shaft, a gear, a wheel. Linear motion is movement in a straight line, such as the travel of a drill bit moving into a workpiece. Reciprocating motion is back-and-forth movement along a straight line — a piston, a saw blade or a needle in an industrial sewing machine. Oscillating motion is a swinging or rocking motion around a fixed point, like a pendulum or a windscreen wiper arm.
In the examination you must use the correct technical terms — answers like “goes round and round” or “goes sideways” are not credited. A motor produces rotary motion; a piston produces reciprocating motion.
The four types of motion are rotary (spinning around an axis), linear (straight line), reciprocating (back-and-forth straight line) and oscillating (swinging around a fixed point). Most mechanisms convert one type of motion into another.
Linkages and Conversion of Motion
A linkage is a set of rigid bars joined by pivots that transmits or changes the direction of movement. Engineers use linkages when they need to transmit force around corners, reverse direction, or change the shape of a motion path.
The simplest linkage is a reverse linkage (bell crank). Pushing one arm down causes the other arm to push outward — the motion direction changes by 90°. Bicycle brake levers use this principle: squeezing the handle pulls a cable that pushes brake pads against the wheel rim. A parallel linkage keeps two bars moving in the same direction — used in toy train pantographs and car suspension systems.
The crank and slider mechanism is the fundamental converter of rotary motion into reciprocating motion. A rotating crank pin is connected to a slider (piston) by a connecting rod. As the crank rotates, the slider moves back and forth. Every internal combustion engine uses this principle — the rotating crankshaft drives the pistons up and down.
The reverse is also possible: a reciprocating steam engine drives a piston back and forth, which turns a crankshaft via a connecting rod to produce rotary motion at the wheels. Engineers describe this as converting reciprocating to rotary motion.
Cams and Followers
A cam is a specially shaped piece that rotates and pushes a component called a follower up and down as it turns. The key principle: the position of the follower at any instant depends entirely on the radius of the cam at the point of contact. Where the cam radius is large, the follower is pushed up. Where the radius is small, the follower drops down. This is the core cam concept — you must be able to explain it, not just name it.
The follower’s position at any moment is determined by the cam profile — specifically the radius of the cam at the point of contact. A large radius pushes the follower high; a small radius lets it drop. The cam profile is therefore what engineers adjust to control the follower’s movement pattern.
Four cam profiles appear in engineering exam questions:
- Pear cam — egg-shaped profile. The follower rises slowly to a maximum height and then drops quickly. Used in car engine valve timing, where a valve must open slowly and snap shut.
- Eccentric (circular) cam — a circular disc with the pivot offset from the centre. As it rotates the follower rises and falls smoothly in a simple harmonic pattern. Used in pumps and simple oscillating machines.
- Snail (drop) cam — the follower rises very gradually around most of the rotation, then drops almost vertically as it reaches the “snail’s tail” drop. Used in clock escapements and stamping machines where a sudden downward force is needed.
- Heart cam — heart-shaped profile that converts the continuous rotation into a back-and-forth motion of equal rate in both directions. Used in mechanisms where a constant-rate reciprocation is needed.
Choosing the Right Mechanism
When an engineer needs to convert motion, they select the mechanism based on the motion required and the forces involved. To convert rotary to reciprocating: use a cam or a crank and slider. To change the direction of a force: use a linkage (bell crank, reverse linkage). To transmit rotary motion between shafts that are far apart or at an angle: use gears, chains or belts (covered in Lesson 3).
In an aero-engine control unit, precision cams govern the opening and closing of fuel valves, and their profiles are machined to very tight tolerances because a small timing error changes engine performance. The cam profile is machined to tolerances of ±0.005 mm because any deviation changes the valve timing and affects thrust output. This real-world context shows why understanding cam profiles matters beyond the classroom.