Everything a metal hose or a bellows does can be described as one of five movements. A metal hose is excellent at two of them, good at a third, poor at the fourth, and the fifth will destroy it.
| Movement | What it is | Metal hose | Bellows expansion joint |
|---|---|---|---|
| Axial | Compression or extension along the centreline | Limited — a hose is not an axial compensator | The primary duty of an axial joint |
| Lateral / offset | One end displaced sideways, ends staying parallel | Excellent — the natural duty of a hose | Lateral and universal configurations |
| Angular | Rotation about a point, changing the angle between ends | Good | Hinged and gimbal configurations |
| Vibration | Low amplitude, high frequency | Excellent — the classic pump connection duty | Yes, with care on resonance |
| Torsion | Twisting about the hose axis | No | No |
Axial movement in a hose — read this carefully
A braided hose resists axial extension by design; that is what the braid is for. A hose asked to absorb pipe growth by stretching will load the braid and the fitting welds directly, and it will fail at the collar. Where the line grows axially, the correct component is an axial expansion joint, not a longer hose.
A hose absorbs axial movement properly only when it is installed as a loop, so that pipe growth becomes a change in the loop geometry rather than a stretch along the hose axis. That is a lateral movement in disguise, and it works well.
Installation geometries that work
| Geometry | Movement absorbed | Notes |
|---|---|---|
| Straight, slack | Small lateral, vibration | Simplest; never install taut |
| 90° bend | Lateral in one plane | Common on pump connections |
| U-loop / travelling loop | Large lateral, repeated | The standard for high-cycle duty |
| Vertical loop | Movement in the vertical plane | Support the loop; do not let it hang on the fittings |
| Two hoses at 90° | Movement in two planes | Where a single hose would be twisted |
The length each geometry requires is given by the formulas on bend radius and live length.
Vibration
A braided hose is an excellent vibration decoupler, which is why pump and compressor connections are its highest-volume application. Two cautions. First, a PTFE-lined assembly is not a good vibration decoupler — the liner will not tolerate the repeated flexing. Second, high-frequency, low-amplitude vibration accumulates cycles very quickly: a hose flexing at 25 Hz sees more than two million cycles in a day, so the dynamic bend radius and any abrasion interlayer become critical rather than optional.
