Parallel, reverse, and bell crank linkages differ mainly in the direction in which they transmit motion. A parallel linkage produces output in the same direction as the input, a reverse linkage produces output in the opposite direction, and a bell crank linkage redirects motion through an angle, commonly .
A linkage is a mechanical system of rigid members connected by pivots. It transmits force and motion from an input link to an output link, while fixed pivots connect parts of the mechanism to the frame.
| Linkage | Motion and arrangement | Typical application |
|---|---|---|
| Parallel linkage | The input and output links move in the same direction. Opposite links form a parallelogram, helping corresponding members remain parallel during movement. | Drawing boards, folding mechanisms, or mechanisms that must maintain orientation |
| Reverse linkage | The output moves in the opposite direction to the input. A pivoted member transfers an upward input into a downward output, or vice versa. | Control mechanisms and systems requiring opposing movement |
| Bell crank linkage | An angled, pivoted link changes the direction of motion, usually through . The arms may have different lengths, so the mechanism can also alter force and displacement. | Bicycle brakes, aircraft controls, and mechanisms operating around corners |
For example, pushing one arm of a bell crank downward can move its other arm sideways. If the input arm is longer than the output arm, the output force increases, but the output displacement decreases, reflecting the principle of mechanical advantage.
A common misconception is that all three linkages convert one type of motion into another. They primarily transmit or redirect motion; the key distinction is the relationship between the input and output directions.
In an IB Design Technology A3.3 response, identify the input, output, fixed pivots, and direction of movement. For an “explain” question, state how the linkage arrangement causes the observed output rather than merely naming the mechanism.
