Mechanisms, structures and control systems
What this domain tests
This domain tests force reasoning in structures, motion and ratio calculations in mechanisms, and the difference between open- and closed-loop control.
Mechanisms and structures
- Levers: class 1 has the fulcrum between effort and load; class 2 has the load between fulcrum and effort; class 3 has the effort between fulcrum and load. Mechanical advantage = load/effort = effort arm/load arm.
- Gears and pulleys: velocity ratio = teeth on driven / teeth on driver, or number of rope sections supporting the load; an idler changes direction but not ratio.
- Cams and linkages: convert rotary to reciprocating motion; follower displacement depends on the cam profile.
- Structures: members in tension (ties) or compression (struts); triangulation gives rigidity; bending is reduced by depth, flanges or bracing.
Worked example
A class 1 lever has the effort 250 mm from the fulcrum and the load 50 mm from it. Ideal MA = 250/50 = 5, so a 250 N load needs 50 N effort. In a test the actual effort is 60 N, so actual MA = 250/60 = 4.17 and efficiency = 4.17/5 × 100 = 83%. The loss is friction at the pivot and the mass of the lever. To improve it, fit a bearing at the pivot and reduce the lever’s own mass.
Control
An open-loop system, such as a timed sprinkler, has no feedback; a closed-loop system compares sensor output with a setpoint and corrects the actuator. The blocks are input, process, output, with feedback.
Traps
- Confusing MA and VR.
- Including an idler in the gear ratio.
- Claiming a gear train increases power; it changes torque and speed, with losses.
- Calling a system closed-loop when it only has a timer.