Load sensing explained: one job, three pumps, and where the power goes

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.