When the pressure exceeds what braid will carry, or - more often - when the circuit cycles hard enough that impulse fatigue, not burst pressure, governs the hose's life.

The pressure case is straightforward: above the R2 range, spiral is the only option. The impulse case is the one that gets missed. A circuit that pressurises and relaxes rapidly and repeatedly - a press, a mobile machine working a tight duty cycle, anything with fast-acting valves - wears out a braided hose by rubbing the braid wires against each other at every crossing, even at pressures the braid handles comfortably.

Spiral layers do not cross, so there is nothing to rub. Impulse ratings for spiral hose are typically several times those for braid.

If hoses in a given position keep failing well inside their rated pressure, the answer is usually to move to spiral rather than to a higher braid rating.

Principally the number of spiral layers and the pressure class, with R12 at the lower end and R13 and R15 in the high-pressure range used on heavy plant.

R12 uses four spiral layers and is the usual entry point into spiral construction, chosen for its impulse performance as much as for its pressure. R13 uses six layers for substantially higher pressure. R15 also uses six but carries a higher rating again, and is specified where system pressures are at the top of the hydraulic range.

An important practical difference is that R13 and R15 hold a broadly constant working pressure across their bore range, where braided classes lose pressure capacity as bore increases. That makes them predictable to specify on large-bore high-pressure lines.

Each step up adds stiffness, weight, bend radius and cost, so select the lowest class that satisfies both the pressure and the impulse requirement.

Because bending a spiral hose too tightly does structural damage that is invisible from the outside, and spiral hose has a much larger minimum radius than braid.

Inside a tight bend the wire layers are compressed and can buckle; on the outside they are stretched. Either distorts the geometry the hose depends on to share load evenly between layers, and the rubber between the layers is squeezed out of shape. The hose then fails well below its rating, usually at the bend, and often some time later.

A six-spiral hose can need a bend radius several times that of an equivalent braided hose, which regularly surprises people replacing braid with spiral in an existing installation.

Measure the available routing before ordering. Where the radius is not achievable, the answers are elbow fittings at the ends, a different route, or a shorter hose with an adapter - never forcing the bend.

With slack, without twist, and clamped so the hose cannot rub - the same principles as braided hose but with less tolerance for error.

Leave slack in every run. Hose shortens as it pressurises, so a hose installed taut pulls on its fittings each time the circuit builds pressure. Never twist a hose during installation: even a few degrees of twist substantially reduces life, because it unbalances the spiral layers. The lay line printed along the hose exists to make twist visible - it should run straight.

Where hoses run parallel or cross, clamp them so they cannot rub against each other or against the machine. Abrasion through the cover is one of the most common ways an expensive spiral hose is lost.

Avoid routing near hot surfaces, and use sleeving where contact with an edge is unavoidable.

Yes, generally - spiral hose needs fittings and crimp specifications designed for spiral construction, and they are not interchangeable with braid fittings.

The reason is how the crimp grips. A braid fitting is designed to bite into an interlaced structure; a spiral fitting must grip several discrete parallel layers with rubber between them, at much higher pressures. The insert profile, shell length and crimp diameter all differ.

As with braid, the assembly is rated as a system. Use the hose manufacturer's own fittings and their published crimp specification, with the correct dies on a calibrated machine.

Given the pressures involved, this matters more here than anywhere else in the hose range. A six-spiral assembly failing at the fitting releases fluid at pressures that will inject through skin and cause serious injury. Have assemblies made professionally and keep a record of what was specified.