Two numbers govern whether a moving hose reaches its design life: the radius it is allowed to bend to, and the flexible length available to do the bending. Get either wrong and the assembly fails at the fitting.
Live length is not overall length
Overall length is measured face to face, including the fittings. Live length is the flexible portion between the fitting terminations — the part that actually moves. Every formula below uses live length. When you specify a length on an enquiry, tell us which one you mean, because the difference on a short assembly with heavy flanges is substantial.
Static and dynamic bend radius
We publish both, because they answer different questions.
| Static bend radius | Flexing bend radius (Type-1 / Type-2) | |
|---|---|---|
| Definition | A permanently formed installation bend, made once | The radius to design to for repeated movement — two figures, ask us which duty applies |
| Use | Installed, clamped, left alone | Repeated movement, vibration, loops |
| Relationship | Baseline | Type-2 runs 2.9× the static figure at DN65, rising to 5.6× at DN6 |
Calculating the length you need
Most metal hose failures are length failures. Too short and it over-bends at the fittings. Forced to take axial movement and it destroys its own braid. Two rules cover nearly everything.
Rule 1 — movement in one plane only
A braided hose absorbs movement across its axis, not along it. Never apply axial compression, extension or torsion. If the line moves axially, use an expansion joint or install the hose as a loop.
Rule 2 — lateral offset, both ends fixed
Lv = live length, the flexible length between braid ferrules R = bend radius — flexing column for repeated movement, static for a one-time offset T = total lateral travel (mm)
Installed projected length: Lp = √( Lv² − T² )
Worked example. DN25 single-braid hose,
repeated lateral movement of 25 mm. Flexing radius from the table is 325 mm.
Lv = √(6 × 325 × 25 + 25²) = √49,375 ≈ 222 mm.
Specify a live length of at least 225 mm, plus the fitting lengths.
Rule 3 — 90° change of direction
DN50 as a static 90° bend: Lv ≥ 1.57 × 160 = 251 mm, so specify 255 mm live. The same bend as flexing duty uses R = 490 mm, giving Lv ≥ 770 mm. That is the difference the duty makes.
Rule 4 — vibration duty
Install the hose across the main vibration direction, use the flexing radius, and keep it slack at the operating position. Treat the vibration amplitude as T in Rule 2. Clamp the rigid pipework only — never the hose or the braid.
Rule 5 — keep fittings out of the bend
Leave a straight tangent of at least two times the hose outside diameter after each fitting before the bend starts. Never let a bend begin at the braid ferrule.
Rules that apply to every geometry
- Never install taut. A hose under tension has no slack to absorb anything and loads the braid and welds directly.
- Never bend at the fitting. Rule 5 above gives the tangent length. The reason is that the braid ferrule is the stiffest point on the assembly, so a bend that starts there puts all the strain into one or two convolutions instead of spreading it along the hose.
- Never let the hose take torsion. See movement types.
- Support long assemblies. Self-weight plus the weight of the medium will pull an unsupported horizontal run below its minimum radius.
