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.

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.

Application areas for stripwound hoses

Stripwound hoses are frequently used as flexible temperature-resistant and ageing-resistant elements in exhaust equipment, especially in trucks and special vehicles such as tractors. They are also used as protective hoses for light conductors and electrical lines in fibre optics, or in measuring and control equipment. As miniature hoses with diameters ranging from 2.0–0.3 mm, they are also employed in medical technology, such as for endoscopy.

In addition, stripwound hoses are used for extracting and conveying substances such as smoke, shavings, granulate, etc. They are also suitable as protective hoses for corrugated lines to prevent over-extension and to act as a liner (guide hose inside a corrugated hose) to optimise flow conditions.

Stripwound metal hoses also include "bendable arms", or "swan necks". These consist of a round wire coil over which a triangular wire is wound. They can be bent in any direction and remain stationary in any position. These are used for the flexible supports of lamps, magnifying glasses and microphones, for example.

Structure and function

Corrugated hoses are used as economical, flexible connecting elements that permit movement, thermal expansion and vibrations, and that can be used as filling hoses. The starting material is a seamless or longitudinally welded, thin-walled tube into which corrugations are introduced by mechanical or hydraulic means using special tools. Corrugated hoses are absolutely leak-tight and are used to convey liquids or gases under pressure or as vacuum lines. They are also referred to as pressure hoses. Their special design achieves both flexibility and pressure resistance.

Types of corrugated hose

There are two basic variants of corrugated hoses that differ in their type of corrugation: annular corrugation and helical corrugation. In hoses with helical corrugation, usually a right-handed coil with a constant pitch that runs along the whole length of the hose. The annular corrugation, on the other hand, consists of a large number of equally-spaced parallel corrugations whose main plane is perpendicular to the hose axis. Hoses with annular corrugation have decided advantages over those with helical corrugation:

• When installed properly, they are free of damaging torsional strain during pressure surges.
• Because of the shape of their profile, they connect smoothly to connection fittings.

This increases process reliability during conduit assembly and use. For this reason, annular corrugated hoses are far more common, with only a few exceptions.

Manufacturing corrugated hoses

The first step in creating a corrugated hose is to shape the starting metal strip from the coil into a smooth, longitudinally welded tube. The strip is continuously welded using the highly precise shielding gas welding method. Then the tube is corrugated by one of the following procedures:

• The hydraulic corrugation method expands the tube from the inside to the outside. This method is used to create annular corrugated hoses.
• The mechanical corrugation method, on the other hand, is used to produce both annular and helical corrugated hoses. Usually, multiple profiled pressure rollers are positioned around the tube with an offset that enables them to roll the desired corrugation profile into the tube from the outside to the inside. Both corrugation methods cause material hardening and thus increase the pressure and fatigue resistance of the corrugated hoses.

In addition, corrugated hoses can be manufactured by a special method that is closely related to the manufacture of stripwound hoses. In this procedure, the starting strip is given a corrugated profile in a longitudinal direction. This profile strip is then wound helically and the overlapping coils are tightly welded along the helical seam. After corrugation, the hose may be equipped with a braided sheath (see below). In this case, the hose then passes through a braiding machine that has circumferential wire coil holders, or so-called bobbins.

The wire bundles are wrapped helically around the hose while also being alternately layered one over the other. This creates a tubular braid with the characteristic crisscross pattern. After the fittings are mounted, the hose line is complete. Production-related testing is an integral part of the manufacturing process. It encompasses incoming tests of the starting material as well as dimensional, leakage and pressure testing of the finished conduit.

Flexibility

The flexibility of the hose is achieved by means of the elastic behaviour of the corrugation profile. When the hose is bent, the outer corrugations separate while the inner corrugations are squeezed together. The flexibility, bending behaviour and pressure stability of corrugated hoses depend on the selected profile shape. While flexibility increases with an increase in profile height and a decrease in corrugation spacing, pressure resistance decreases. The frequently required semi-flexible bending behaviour is achieved by flat profiles. Depending on the use of the hose, special application-specific profile shapes can be implemented

miniature metal hoses for medical or optoelectronical applications

Pressure resistance and flexibility can also be altered by varying the wall thickness. A reduction in the wall thickness increases the bending capacity but reduces the pressure resistance of the hose.

Functional principle of the metal braid

Wire braiding functions on the lazy tongs principle. When axial tension is applied to the hose, the braid reaches its extension limit. This means that the wires lie tightly spaced with the smallest crossing angle, creating a hose braiding of the smallest possible diameter and the largest possible length. When the hose is axially compressed, the crossing angle and diameter increase to maximum values.