Transmission lines: tubes, hoses & fittings

The plumbing that carries the oil. What to use where, how a hose is built, and why every bend and fitting quietly costs you pressure.

Source: Rabie, Fluid Power Engineering, Ch. 3.

Before you start

What you need first

The basic circuit — pump, valves, actuator and the lines between them (Topic 5).

What you'll be able to do

Choose rigid tube vs flexible hose.
Name the layers of a hose and what fittings do.
Quote the recommended line speeds and why they differ.

Start here · the choice

Rigid tube or flexible hose?

	Rigid tube (steel)	Flexible hose
Use for	Fixed runs (pump → manifold, along a frame)	Moving / vibrating parts (a boom cylinder, a swivel)
Good	Cheap, neat, low loss, no ageing	Absorbs movement & vibration, easy routing
Weak	Cannot move; needs careful bending/clamping	Ages, can burst, costlier, slightly more loss

Rule of thumb: tube where it doesn't move, hose where it does. Use the shortest, straightest route either way.

How a hose is built

A hydraulic hose is three layers working together:

Inner tube — oil-resistant rubber that carries the oil.
Reinforcement — steel-wire braid or spiral that takes the pressure (more layers → higher rating).
Outer cover — tough rubber that protects against abrasion and weather.

Hose cross-section: oil bore inside the inner tube, the braid takes the pressure, the cover protects it.

Every fitting quietly costs pressure

Lines are joined and turned with elbows, tees, crosses and couplings. Each one disturbs the smooth flow — the oil must turn, split or squeeze — and that disturbance costs a little pressure.

These are called minor (local) losses. One fitting is small, but a circuit full of bends and tees adds up — we put numbers on it in Topic 18. The design lesson is simple: fewer bends, gentler bends, shorter runs.

Recommended line speeds

Oil speed in a line is a balance: too fast wastes pressure and risks noise/cavitation; too slow needs bulky, costly pipe. Practice settles on these bands:

Line	Typical speed	Why
Pressure (pump → actuator)	2 – 6 m/s	Higher is OK; loss rises with speed
Suction (tank → pump)	0.6 – 1.6 m/s	Kept slow to avoid pump-inlet cavitation (Topic 12)
Return / low-pressure	0.6 – 1.6 m/s	Low-pressure line — Rabie groups it in the suction band

The low-pressure lines (suction & return) run far slower than the pressure line, and the suction especially is kept slow on purpose — low speed keeps the inlet pressure up and the pump out of cavitation. We turn these speeds into pipe diameters next.

✏️ Try it yourself — no numbers needed

For each, pick rigid tube or flexible hose and say why: (a) the line from a fixed pump to a fixed valve manifold; (b) the line to a cylinder on a digger's moving boom.

(a) Rigid tube — nothing moves, so a neat, cheap, low-loss steel tube is ideal. (b) Flexible hose — the boom moves and vibrates, so the line must flex; a rigid tube would fatigue and crack.

Recap — the whole topic on one screen

Idea	What you own now
Tube vs hose	Tube where fixed; hose where it moves
Hose layers	Inner tube · steel braid (pressure) · cover
Fittings	Each adds a minor loss → fewer/gentler/shorter
Line speeds	Pressure 2–6 · suction & return 0.6–1.6 m/s

Next topic

Sizing the line

Now turn a chosen speed into a pipe diameter: pick the speed from the table, and the flow sets the bore. Next we do exactly that with \(v = 4Q/\pi D^2\).

→ Sizing the line