FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsMedium
A direct-cranking electric starter on a reciprocating engine uses a Bendix drive to engage the engine ring gear. What principle allows the drive pinion to move forward and mesh with the ring gear when the starter motor is energized?
- AThe pinion is held in mesh by spring pressure until the starter switch is released
- BInertia of the pinion causes it to screw forward along helical splines on the rapidly rotating armature shaft
- CAn electric solenoid physically shifts the pinion into mesh independent of shaft rotation
- DEngine oil pressure forces the pinion outward along the starter shaft
Show answer & explanationAnswer & explanation
Correct answer: B. Inertia of the pinion causes it to screw forward along helical splines on the rapidly rotating armature shaft
In a Bendix (inertia) drive, the pinion is mounted on a helically splined sleeve. When the starter motor spins up quickly, the pinion's inertia causes it to lag momentarily and screw forward along the splines into mesh with the ring gear. Once the engine starts and overspeeds the pinion, it automatically screws back out of mesh.
Why the other options are wrong
- A. Bendix drives are inertia-actuated, not spring-held into mesh.
- C. Solenoid-shifted engagement describes a different (positive-engagement) starter drive, not the Bendix type.
- D. Oil pressure is not used to actuate a Bendix drive.
Bendix Drive (Inertia Drive)
A starter drive mechanism that uses the inertia of a pinion gear on helical splines to automatically engage and disengage the engine ring gear.
- Pinion lags behind the rapidly accelerating shaft due to inertia, screwing into mesh.
- Automatically disengages when the engine overspeeds the starter after starting.
- Simple, self-contained mechanism requiring no separate solenoid to shift the pinion.
Memory trick: Bendix lags behind and screws itself in — inertia does the work.