By Josh Cosford, Contributing Editor
There are two major players in the hydraulic tubing game, and depending on who you ask, there might be two and a half, but I’ll get to that later. If you’re like me, you’ve heard of both seamless and welded tubes yet had no idea what made them different. Hopefully, this article can shed some light on those differences to clarify things.
The primary difference is in how they are manufactured. Neither method is easy, as both require heavy-duty machinery installed inside an expansive manufacturing facility. One method is more expensive but yields a stronger end product, while the other is more economical with reduced performance, at least as far as a fluid medium for hydraulics is concerned.
Seamless hydraulic tube is the stronger of the two, which is suitable for high-pressure hydraulic applications. It’s manufactured by forcing heated steel billets through an extruder, which is a series of drives, dies and a mandrel. The short, thick billet is formed into a longer, reduced-diameter tube, which then proceeds to downstream processes such as heat treating, quenching, straightening and resizing.
Seamless tube (SAE J524 and ASTM A519) is most frequently used in fluid power applications, especially those with high-pressure requirements. Its superior strength, smooth internal surface, and uniform shape are perfect for reducing flow-related pressure drop, while its ductility makes it easy to bend and flare.
Welded tube is still used as hydraulic tubing, and despite its lower pressure capacity and more economical production method, it’s still produced by large, impressive machinery. The factory starts by sourcing coiled steel of appropriate width for the circumference required for the finished tube diameter. Large tube manufacturers, especially, have the luxury of their own coil processing equipment to create the desired widths for the end customer’s tube size.
The coil is fed from an uncoiler directly into the tube mill, where a series of dies progressively shape the steel into a tubular shape (which doesn’t necessarily have to be circular). Once the final shape is achieved, it passes through the welder, where the ends are joined, sealing the tube. Similar to seamless tube, the welded tube also visits the heat treatment and straightening stations, and in many cases, you can’t tell the difference between each tube type by looking at the outside.
One of the benefits of welded tube is the possible length that can be manufactured. Depending on the thickness of the steel coil, mills can manufacture coils in excess of a mile. It’s unlikely anyone could ever use a single tube this long, but should you require something longer than what can be made with a seamless extruder, you know where to look.
A hybrid tube type is both welded and drawn. Using electric resistance welding before being drawn over a mandrel to improve accuracy while removing the interior weld bead, this tube is suitable for moderate-pressure hydraulic applications.
If you ever have the opportunity to visit a tube mill, I highly recommend you attend. Although we often work in short lengths, you’d be shocked by the scope of work involved in making these fluid power commodities.
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