Pipe coupling RK38 48 mm, matte black

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Inside the connector sits a molded in core of zinc plated steel with an internal M8 thread. The steel core makes the joint stiffer than a connector made entirely of plastic, and the thread lets you bolt on swivel casters, adjustable feet or the whole structure without drilling into the tubes. This is the version you choose when the frame is to stand on wheels or when the joint carries more than the weight of the tubes themselves.

With RK38 you build tube structures together without welding. The connector is a three way connector with a steel core and an internal thread for square tubes in nylon PA6, made for square tube 25x25 mm with a wall thickness of 1.5 mm; you cut the tubes to length, put the connector in and tap the parts together with a rubber mallet. No welding certificate, no surface finishing afterwards, no special tools.

PA6 is a tough engineering plastic that damps knocks and vibration better than an all metal joint. The surface is matte black and colored all the way through, so scratches and marks from assembly are barely visible. The material handles oils, greases and normal cleaning agents, and for natural reasons it never rusts; in damp premises and wash bays that is a heavier argument than you first think. Unlike welded joints there are no sharp weld spatter or burrs to hurt yourself on here, and the rounded shape of the connector is kind to both hands and goods passing close by.

This connector has a total length of 48 mm. Check both the outside dimension and the wall thickness of the tube before you order; the press fit relies on the inside dimension of the tube, and a tube with a different wall thickness gives either play or a connector that cannot be driven fully home. Three connections cover the corner where two frame tubes meet a leg, and the thread in the bottom takes a wheel or an adjustable foot.