The number of wire braids, and therefore the pressure the hose will carry.

R1 has a single braid of high-tensile steel wire over the inner tube. R2 has two braids, one laid over the other. For the same bore, the second braid roughly doubles the working pressure, which is why R2 dominates on modern equipment where system pressures have risen.

The second braid costs something. R2 is stiffer, has a larger minimum bend radius, weighs more and costs more per metre. On a low-pressure leg where R1 is comfortably adequate, fitting R2 makes the hose harder to route without making the circuit safer.

Both share the same basic geometry, the same fittings and the same fluid compatibility, so moving between them is straightforward. Work from the circuit's actual working pressure, including any pressure spikes, and check it against the rating for the specific bore - pressure ratings fall as bore rises within each class.

Because the braid wires move against each other every time the hose pressurises.

A braid is an interlaced structure - each wire passes over and under its neighbours. When pressure rises the hose tries to expand, the braid tightens, and the wires shift slightly at every crossing point. When pressure drops they shift back. Over hundreds of thousands of cycles that microscopic rubbing wears the wires where they cross, and the hose eventually fails by fatigue at a pressure it once carried easily.

This is why a hose can pass a static burst test and still fail in service. Impulse rating, not burst pressure, is what predicts life in a cycling circuit.

Spiral hose avoids the problem by laying wires in parallel layers that do not cross, so there is no rubbing. Where a circuit cycles hard - press applications, mobile plant working a duty cycle - that difference is the reason to move up to spiral even if braid would carry the pressure.

From the flow rate and the acceptable fluid velocity, not from the size of the port at each end.

For a given flow, a smaller bore means higher velocity. Excessive velocity causes turbulence, pressure loss and heating, and on a suction or return line it can cause cavitation. Too large a bore wastes money, adds weight and makes routing harder. Suppliers publish velocity guidance separately for pressure, return and suction lines, because the acceptable figures differ substantially between them.

A common and expensive mistake is sizing the hose to match the port thread. Ports are sized by the component manufacturer for their own reasons, and an adapter to a correctly sized hose is cheaper than a circuit that runs hot.

Work out the flow, pick the bore from the velocity table for that line type, then select fittings to suit both the hose and the port.

Yes - a hose assembly is rated as a complete system, and mixing components voids that rating.

The crimped connection between a fitting and a hose depends on precise geometry: the fitting's shell diameter, the insert profile, the crimp diameter and the die used. Manufacturers publish those figures for their own hose and fittings tested together. Another maker's fitting may crimp onto the hose and look entirely correct while gripping the reinforcement differently, and the failure appears later as the fitting blowing off under pressure.

This is a safety matter rather than a commercial one. A hydraulic hose failing at the fitting releases fluid at pressures that can inject through skin.

Use one manufacturer's hose, fittings and crimp specification throughout an assembly, and have assemblies made by someone with the correct dies and a calibrated crimper. Record what was fitted, so a replacement can match it.

Condition and service history rather than a fixed calendar interval, though many operators set one as a backstop.

Replace on sight if the cover is cut or abraded through to the wire, if wire is visible or rusting, if the hose is kinked, crushed, blistered or stiff, if there is any leak or weeping at the fitting, or if the fitting is corroded, slipping or damaged. A hose that has been pulled taut, twisted on installation or routed over a sharp edge should be treated as suspect regardless of appearance.

Beyond that, hose ages. The rubber inner tube and cover harden over time even when the hose is not cycling, so long-installed hoses in hot locations are worth replacing on schedule.

Never run a hand along a pressurised hose to look for a leak. A pinhole leak is an injection hazard and is often invisible - use a piece of card and depressurise before inspecting closely.