Pressure in a hydraulic system never simply disappears. Fluid moving through a line has to overcome resistance, and the pressure spent doing so does not vanish — it turns into heat. That is why higher pressure loss means faster oil temperature rise, and oil temperature is one of the most sensitive variables governing both system and hose life.Losses come in two kinds. Friction (along-the-line) losses arise from internal fluid friction and friction against the wall in straight runs.
Local losses occur wherever velocity changes abruptly in magnitude or direction — bends, couplings, diameter changes, valve ports. Both are described by the same core relationship: loss is proportional to the square of flow velocity and to the resistance coefficient.
That statement is stronger than it looks. Because flow equals velocity times cross-sectional area, at constant flow:
Double the velocity (as happens when the bore is undersized) and loss rises to roughly four times;
Friction loss is inversely proportional to the square of the cross-sectional area — i.e. to the fourth power of diameter — and with the diameter term in the denominator of the friction factor expression, the net effect is often approximated as inversely proportional to the fifth power of diameter.
A frequently cited consequence: halving the bore increases friction loss at the same flow by a factor of about 32; conversely, moving from 8 mm to 10 mm on a return line drops friction loss to roughly 0.17 of the original — about one-sixth. That is the arithmetic behind the old trade saying: one size up, half the pressure drop.
The problem is that almost nobody does this calculation at selection time. In many projects line sizes are still set by rule-of-thumb flow velocity, and pressure loss is measured later, during commissioning or even after delivery. Yet recommended velocity bands are well established: roughly 3–7 m/s for pressure lines, 2–4.5 m/s for return lines, 1–1.8 m/s for suction lines, with a common caution to stay below 8 m/s. Above that, losses and noise climb sharply and the suction side risks cavitation. Given flow and target velocity, bore follows directly: ID (mm) ≈ 4.61 × √(flow in L/min ÷ velocity in m/s).
What does this have to do with hose? More than it appears — and it forms a self-reinforcing loop.
Link one: undersized bore → higher velocity → more loss and heat. A hose specified one size down is simply a restriction in the circuit, and the pressure it drops becomes heat.
Link two: higher oil temperature → faster ageing of both fluid and hose. Heat accelerates oil oxidation and additive depletion, and it accelerates ageing of hose tube and cover compounds; several component and hose manufacturers list "rapid ageing of seals and hoses due to high heat" among the consequences of undersized line. In other words, a wrong bore does not merely waste a little energy — it quietly consumes the hose's own service life.
Link three: rising temperature → more leakage and lower efficiency. Hotter oil thins out, leakage increases, efficiency falls, and heat generation rises again. It is a positive feedback loop.
Link four: on electrified machines, every loss comes straight out of the battery. An electric excavator or aerial platform has no engine waste heat to spare and no coarse "fuel burn" ledger to hide behind. Pumping losses convert directly into range and work done per charge. Line sizing — long treated as a detail — becomes, for the first time, a variable that affects product competitiveness.
Once that is understood, the actions are refreshingly simple.
One: size the bore by flow, not by pressure. Pressure class determines whether a hose bursts; bore determines how hard the fluid has to work. They are separate questions. Two: go one size up at the margin. Larger bore costs more, weighs more, raises bend radius and oil volume — these must be weighed against gains in efficiency, temperature and life — but the best return is usually on the low-pressure side, in return and suction lines. Three: remove unnecessary bends and diameter changes, preferring swept bends over right-angle fittings; local losses at bends and couplings are routinely underestimated.
Four: consider tube surface condition and hose length. Smoother bore and shorter runs mean lower friction loss.For manufacturers this implies a change of role. The hose supplier is no longer just the party that answers "how much pressure will it take," but the one who points out, at selection time, that a given bore will generate heat at the customer's flow rate. The advice costs nothing to give and materially changes both machine energy consumption and hose service life — and under electrification, its weight will only grow.
Related: choosing the right bore size