luff

To luff a crane jib is to rotate it upward or downward about its horizontal axis, changing the angle between the jib and the horizontal plane. On a mobile or tower crane, this motion allows the operator to position the jib so that a suspended load hangs level rather than tilted, and to adjust the hook height and reach simultaneously. The luffing mechanism uses hydraulic cylinders or a rope-and-pulley system to move the jib tip up or down while the boom length remains fixed.

Most commonly, luffing corrects for load swing or positioning errors during the lift. If a load is picked up unevenly or swings during slew, the operator luffs the jib to bring it parallel to the ground and ensure the load sits properly on its destination. On a tower crane with a horizontal jib, luffing also changes the effective radius: raising the jib tip increases the distance from the mast, reducing capacity but extending reach; lowering it does the opposite. This trade-off between load capacity and outreach is why luffing is essential to crane planning.

Luffing versus slewing and hoisting

Luffing is one of three primary motions on a crane. Hoisting raises or lowers the load on the hook cable; slewing rotates the boom or jib side to side around the mast; luffing tilts the jib fore and aft. Modern cranes often perform all three simultaneously under load. Confusion arises because some operators and older equipment use "luff" loosely to mean any upward movement of the load, but technically it refers only to the jib angle, not the hook position.

The term originates from sailing. A sail "luffs" when it flutters and loses tension by turning toward the wind. In crane work, the visual similarity between a slack sail and a tilted jib probably suggested the name in early 20th-century British and European practice. The word remains standard in European crane terminology; North American operators sometimes use "boom angle" or simply "up" and "down" instead.

Luffing speed is typically 5 to 15 degrees per second on mobile cranes and slower on tower cranes, depending on load, weather, and jib length. In high wind, operators limit luffing to prevent swinging. Hydraulic pressure loss, cylinder drift, or rope stretch can cause the jib to creep after luffing stops, so regular inspection of luffing actuators is essential for safety and load stability.