FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesHard
An overhaul technician inspects a crankshaft and finds pendulum-type counterweights attached to the crankshaft cheeks with pins riding in oversized holes. What is the primary function of these components?
- ATo balance the static weight of the crankshaft during grinding
- BTo absorb torsional vibrations occurring at specific engine speeds
- CTo reduce crankshaft journal wear by distributing oil evenly
- DTo increase crankshaft rigidity and prevent bending under load
Show answer & explanationAnswer & explanation
Correct answer: B. To absorb torsional vibrations occurring at specific engine speeds
Dynamic (pendulum-type) counterweights are tuned dampers that oscillate on their mounting pins to absorb torsional vibration at specific frequencies produced by the power impulses of the engine, preventing crankshaft fatigue failure.
Why the other options are wrong
- A. Static balancing is done separately and does not involve pendulum action.
- C. Oil distribution is handled by drilled passages, not counterweights.
- D. Rigidity is a function of crankshaft design/material, not the dampers.
Dynamic Counterweight (Pendulum Damper)
A crankshaft-mounted mass that swings on oversized pin holes to absorb torsional vibrations occurring at specific engine operating frequencies.
- Tuned to a particular vibration frequency, not all frequencies
- Prevents crankshaft fatigue cracking from torsional resonance
- Common on large radial and horizontally opposed engines
Memory trick: Swinging weights soak up the shaky twist