Most hydraulic heat is made on purpose, by the way the system is designed. A fixed pump pushing a small job through a valve can turn three quarters of its input power into heat. A load-sensing pump doing the same job wastes a fraction of that. This lesson puts the three common designs side by side on one job. Move the sliders and watch the bars on the right.
In simple terms
Every hydraulic job needs two things: enough pressure to move the load, and enough flow to move it at the right speed. The job in the animation needs 120 bar and 20 litres a minute. Anything the pump makes beyond that is turned into heat.
Fixed pump and relief valve. The pump always delivers its full 60 litres a minute. The job only takes 20, so the other 40 must go somewhere: over the relief valve, at full relief pressure. And because the pump is at relief pressure, the control valve has to throttle the oil down from 210 bar to the 120 the load needs. Both losses are heat. Only about a fifth of the power does useful work.
Pressure-compensated pump. This pump turns itself down to deliver only the flow the system takes, so nothing goes over the relief. But it still holds its full set pressure, 210 bar, all the time, and the valve still has to burn the difference down to 120 bar. Better, but still wasteful when the load needs much less than full pressure.
Load-sensing pump. A small pipe, the load-sense line, carries the load’s pressure back to the pump. The pump makes that pressure plus a small margin, usually about 20 bar, and only the flow the valve lets through. For our job: 140 bar and 20 litres a minute. The only loss is the 20 bar margin across the valve.
When the load-sense line is blocked, the pump never hears what the load needs. It sits at its low standby pressure and the machine “goes weak”, even though the pump itself is fine.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free account.
For engineers
Power in a hydraulic line is p × Q / 600 (kW, with bar and L/min). Each design’s losses follow directly:
| Design | Pump pressure | Pump flow | Loss in the valve | Loss over the relief |
|---|---|---|---|---|
| Fixed pump + relief | p(relief) | Q(max) | (p(relief) − p(load)) × Q / 600 | p(relief) × (Q(max) − Q) / 600 |
| Pressure-compensated | p(comp) | Q | (p(comp) − p(load)) × Q / 600 | none |
| Load-sensing | p(load) + Δp(margin) | Q | Δp(margin) × Q / 600 | none |
For the default job (120 bar, 20 L/min, a 60 L/min pump and a 210 bar setting): the useful power is 4.0 kW. The fixed pump draws 21.0 kW and wastes 17.0 kW. The compensated pump draws 7.0 kW and wastes 3.0 kW. The load-sensing pump draws 4.7 kW and wastes 0.7 kW.
What the margin does. In a load-sensing system the control valve’s spool is a variable orifice. Flow through an orifice depends on its opening and the square root of the pressure across it. The LS margin is that pressure, held constant by the pump. So:
- With a constant margin, the flow depends only on how far the operator opens the spool. That is why LS machines feel proportional and predictable whatever the load.
- Raising the margin makes the machine faster at the same lever position, but every extra bar of margin is wasted across the valve. 30 bar instead of 20 adds 50% to the throttling loss.
- Lowering the margin saves a little energy but leaves too little pressure to overcome line losses at full flow, so the machine is sluggish, and with very low settings the pump control can hunt.
Pressure-compensated pumps still have a place. When a load needs close to full pressure most of the time (a press, a clamping circuit, an accumulator charging circuit), the compensator’s loss is small and the simpler control is easier to maintain.
Deep dive
Multiple functions. On a crane or excavator several functions share one pump. The LS signals from every section go through a chain of shuttle valves, and the pump follows the highest one. Lighter functions are then fed through pressure compensators in their valve sections, which drop the extra pressure. Those compensators make heat, so a machine that often runs a heavy and a light function together is less efficient than the single-function figures suggest. Flow-sharing (post-compensated) valves divide the flow in proportion when the operator asks for more than the pump can give, so no function stops dead.
Standby pressure. With no function operated, an LS pump drops to standby: the margin pressure with almost no flow, often 20 to 30 bar. The LS line is vented to tank through the valve so the pump can de-stroke. If standby pressure is high or the pump won’t come off stroke, check for a stuck shuttle, a leaking LS relief or a sticking pump control spool.
Faults we chase on LS systems:
- Blocked or kinked LS line, or a blocked LS orifice: the machine has no power on one or all functions, but the pump makes standby pressure normally.
- LS margin drift: the margin spring setting has been altered, often by someone trying to “get more speed”. The machine runs hot.
- Leaking LS relief or shuttle: maximum pressure is lower than it should be; heavy lifts fail.
- Pump control spool contamination: the pump hunts or responds slowly. Particles of a few microns can stop these small spools, so oil cleanliness matters (lesson 15).
Measuring. Fit gauges to the pump outlet and the LS port at the pump. With one function moving slowly against a load, the difference is the working margin. Compare it with the setting on the pump’s data plate or manual. Then check maximum pressure against a stalled function.
Why it matters in the UAE. At 45 °C ambient a cooler has much less temperature difference to work with. A system wasting 17 kW needs a large cooler running hard all summer. The same machine on load sensing may need a fraction of the cooling. Converting older fixed-pump power packs to variable pumps is often the most effective way to fix a chronic overheating problem (see hydraulic overheating in UAE heat).
Key numbers
| What | Typical value |
|---|---|
| LS margin (Δp) | 15 to 30 bar, often 20 |
| Standby pressure | about 20 to 30 bar |
| Loss from the margin | Δp × Q / 600 (20 bar at 100 L/min = 3.3 kW) |
| Fixed-pump waste at part load | often more than 50% of input |
| Pump response time, LS control | about 50 to 150 ms |
Work out your own figures with the pump flow, power and torque calculator.
