A metal hose is a flexible metal line element. There are two basic types of metal hose that differ in their design and application: stripwound hoses and corrugated hoses. Stripwound hoses have a high mechanical strength (e.g. tensile strength and tear strength). Corrugated hoses can withstand high pressure and provide maximum leak tightness on account of their material. Corrugated hoses also exhibit corrosion resistance and pressure tightness under the most extreme conditions, such as in aggressive seawater or at extreme temperatures such as found in space or when transporting cooled liquid gas. They are particularly well suited for conveying hot and cold substances. With a history of more than one hundred years, metal hoses have given rise to other flexible line elements, including metal expansion joints, metal bellows and semi-flexible and flexible metal pipes. In Germany alone, there are about 3500 patents relating to metal hoses.
The origins
The first metal hose was technically a stripwound hose. It was invented in 1885 by the jewellery manufacturer Heinrich Witzenmann (1829–1906) of Pforzheim, Germany, together with the French engineer Eugène Levavassèur. The hose was modelled after the goose throat necklace, a piece of jewellery that consisted of interlacing metal strips. The original design of the hose was based on a helically coiled metal strip with an S-shaped profile. The profile interlocked along the windings of the helical coil. Due to a cavity between the interlocking profiles, this did not create a tight fit. The cavity was sealed by means of a rubber thread.
The result was a permanently flexible, leak-tight steel body of any length and diameter with a high mechanical strength. In France it was patented on 4 August 1885 with the patent number 170 479, and in Germany on 27 August 1885 with the German Reichspatent No. 34 871. From 1886 to 1905, Heinrich Witzenmann continued to develop numerous noteworthy profiles for hose production which are still of technical significance today. In 1894, he registered a patent for the double metal hose consisting of two coaxial metal hoses twisted in opposite directions. Further modifications of the original form focused on the use of different hose materials and different substances for the thread seal, including rubber, textile threads, asbestos and wire. An important variant of the metal hose can be attributed to the inventor Siegfried Frank of Frankfurt, Germany. In 1894, he patented the method of rolling a helical corrugation into a smooth rigid pipe. Witzenmann had already made experiments in this direction several years earlier, but did not continue his efforts to create a patentable result. It wasn't until the 1920 and 1930s that the hotel administrator Albert Dreyer of Lucerne, Switzerland, succeeded in creating a satisfactory annular corrugation for the manufacture of metal corrugated hoses.
Continued development Emil Witzenmann, son of Heinrich Witzenmann, developed a form of the metal hose in 1909 that eliminated the need for any kind of sealing thread, be it of rubber, textile fibre or asbestos. In this type of hose, the strip edges do not interlock but abut each other and are seamlessly welded together. In 1920, Emil Witzenmann invented the metal expansion joint. This invention was based on the double-walled, welded, corrugated metal hose (with a wound protection sheath) with radial flexibility. In 1929, it became possible for the first time to produce metal bellows. These were also developed by Albert Dreyer of Lucerne, but independently of Witzenmann. Metal bellows are created by rolling annular corrugations into a smooth extruded or welded pipe. In 1946, Dreyer developed a multi-walled joint that was designed to accommodate axial movements as well: the axial expansion joint.
The stripwound hose Stripwound hoses consist of spirals that are loosely interlocked. This causes them to be highly flexible. These hoses come in two basic variants – either with an engaged profile or with an interlocked profile such as the Agraff profile. Both variants offer high flexibility due to the profile structure. However, this results in their not being fully leak-tight. For this reason, they are often used as insulating or protective hoses around an inner tube.
Structure and function Stripwound hoses are created by helically winding cold-rolled, profiled metal strip onto a mandrel where the helical coils are interconnected but remain movable due to the type of profiling. This principle of a movable connection between the profiled coils leads to the high flexibility and movability of metal stripwound hoses. Most strips are made of galvanized steel, stainless steel or brass, which can optionally be chromium- or nickel-plated.
Properties of stripwound hoses Stripwound hoses exhibit enormous tensile and transversal pressure resistance, a high torsional strength and excellent chemical and thermal stability. Due to their structure, they are not 100% leak-tight.
Types of stripwound hose The metal hose properties are determined by several factors: the profile shape, the strip dimensions, the material and, if applicable, the type of seal. Stripwound hoses are available with round and polygonal cross-sections. Automotive engineering most often uses metallically sealed stripwound hoses. The introduction of a cotton, rubber or ceramic sealing thread into a specially profiled chamber during the winding process leads to greater tightness. For maximum tightness, the stripwound hoses can also be sheathed in PVC or silicone. The profile shapes range from simple engaged profiles to highly secure Agraff profiles.
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