rocking tree
The axle from which the lay of a loom is suspended.
rocking tree: the loom's swing axle for the lay
The rocking tree is a horizontal shaft, typically made of cast iron or forged steel, from which the lay of a power loom is suspended by means of connecting rods or straps. It pivots at both ends in fixed bearings mounted on the loom frame, allowing the entire lay assembly to rock back and forth as the loom operates. The lay itself, which carries the reed and moves the warp threads apart to create the shed, depends entirely on this axle for its motion.
The rocking tree sits at the heart of the loom's timing mechanism. As the main shaft rotates, cams or eccentrics driven by it push downward on a connecting rod attached to the rocking tree, causing it to oscillate. This oscillation converts rotational motion into the reciprocating motion needed to work the lay. The rate and amplitude of rock are fixed by the profile of the cam and the mechanical linkage, making the rocking tree essential to controlling the beat of the loom and the fabric's consistency.
Stress and maintenance
The rocking tree endures repeated bending and shear stress with every pick, especially at high speeds. Modern looms often run at 200 to 400 picks per minute, meaning the rocking tree experiences millions of cycles over weeks of continuous operation. Wear in the bearing housings causes the axis of oscillation to shift, which throws off the lay's motion and creates uneven reed contact. Cracks can form at stress points where the connecting rod is attached, particularly if the loom is running out of timing or if the cam profile has been worn smooth.
The name reflects its function: it rocks, and it is a tree, meaning a main shaft or structural timber from which other parts hang or branch. In older narrow looms, the rocking tree was sometimes made from seasoned hardwood; modern industrial looms use metal exclusively for the rigidity and fatigue resistance required at speed. The term is standard in the British and European loom trades; North American mills often call it the lay shaft or lay beam, though those terms can also refer to the complete assembly rather than just the pivot axle.