Definition
The cheek is the section of a crankshaft that connects a main journal to a crankpin. It is the structural arm that offsets the crankpin from the crankshaft's axis of rotation, allowing the connecting rod to convert the piston's up-and-down motion into rotary motion. Cheeks also carry counterweights or have counterweight mass built into them to balance the forces created by the moving pistons and rods.
Plain English
The cheek is the flat arm-shaped piece of the crankshaft that links the main shaft to the offset pin where the connecting rod attaches. It is what makes the crankshaft 'crank-shaped' instead of straight.
Context Anchor
Seen in aircraft engine maintenance when studying crankshaft construction, inspection, and damage.
Derivation
The word 'cheek' has long been used in mechanical English to describe the side piece or flat face of a tool or part — the same way the cheek of a vise or the cheek of a pulley refers to its side wall. On a crankshaft, the cheek is literally the flat side piece that joins the rotating shaft to the offset pin.
Why Pilots Care
Cheeks are highly stressed parts of the crankshaft. Cracks or damage in this area can lead to crankshaft failure, which is why crankshaft inspections during overhaul focus closely on the cheek and the radius where it meets the journal and crankpin.
Analogy
Think of a bicycle pedal crank: the straight arm between the pedal axle and the main shaft is the cheek. It is what offsets the pedal so your foot can push it in a circle.
Intuition Check
Cheek does not mean part of a person’s face here. In a crankshaft, it means a solid side web or arm that connects parts of the shaft.
Example Sentence 1
During overhaul, the technician inspected the crankshaft cheeks for cracks, especially at the fillet radius where the cheek meets the crankpin.
Example Sentence 2
Counterweights bolt directly to the crankshaft cheeks to balance engine vibration.