lock up

Lock up occurs when a component that should rotate or slide freely becomes immobilized by friction, mechanical binding, or debris. In machinery, it means the part stops abruptly rather than decelerating gradually. This is distinct from normal stopping because the mechanical energy cannot dissipate through the intended pathway, often creating shock loads and potential damage.

The most common causes are inadequate lubrication, contamination, misalignment, or thermal growth. A spindle can lock up when oil viscosity drops below working specification and boundary contact occurs between journal and bearing. A sliding carriage locks up when swarf or foreign material wedges between the moving surface and its guide. Thermal lock up happens when components expand unevenly due to heat, tightening tolerances until friction becomes uncontrollable.

Recognition and consequences

Lock up typically announces itself as a sudden stall, often accompanied by a grinding or scraping sound, increased motor current draw, or a smell of overheating. The seized component may remain stuck until it cools or until manual intervention frees it. Repeated lock up events can damage bearings, seals, and guideways. In hydraulic systems, lock up can cause pressure spikes that rupture hoses or burst components if relief valves fail to respond quickly enough.

Prevention requires vigilance on lubrication schedules, cleanliness during assembly and operation, and routine inspection for early signs of drag or unusual resistance. Operators should monitor for gradual increases in running resistance, which often precedes full lock up. Proper design margins, correct assembly procedures, and protection from contamination are the shop floor's best defenses against this failure mode.