Hydraulic Load Sensing Control and How it Can Be VERY Inefficient

Typical load-sensing circuit.

If you know something about load sensing hydraulic systems, the above headline appears to be a contradiction. After all, load sensing circuits match pump flow to actuator demand. And this is supposed to increase the hydraulic system’s efficiency. Not reduce it.

HOWEVER, under certain conditions a load sensing hydraulic system can be VERY inefficient. Let me give you a real-life example. The manufacturer of an orchard mister decided to modernize the machine’s hydraulic circuit. The original circuit featured two fixed displacement hydraulic pumps. One pump driving the mister fan. And the other pump driving the chemical pump.

In the upgraded circuit, these two fixed displacement pumps were replaced with a more efficient, variable-displacement pump with load-sensing control. So the load-sensing pump powered both the mister fan and the chemical pump.

But when a prototype of the new, more modern design was put into service, the hydraulic system quickly overheated. The size of the heat exchanger wasn’t changed. So if anything, the engineering department expected the hydraulic system to run cooler due to the efficiency gains from switching to a variable-displacement, load-sensing pump.

So what went wrong?

Well, when redesigning this circuit, the manufacturer failed to take into account the large difference in operating pressure between two functions that operate simultaneously and continuously. The chemical pump drive motor operated at 40 bar (580 psi). But the mister fan drive motor required 200 bar (2900 psi).

And because a closed-center load sensing pump always gets its load pressure signal from the function operating at the highest pressure, the entire system in this case must operate at 200 bar, plus the pump’s flow regulator differential pressure, which is around 20 bar (290 psi).

So the overheating was being caused by 60 liters/minute (16 GPM) going to the chemical pump drive motor, dropping roughly 180 bar (2600 psi) across this function’s flow control. Remember, pressure drop without useful work creates heat. Which means, as built, this was a VERY inefficient load-sensing system.

As you’re probably aware, load sensing systems are widely used on both industrial and mobile hydraulic machines. And as the above case study shows, they’re nowhere near as well understood as they should be.

Which is why I’ve put together an e-Training program which explains everything you’ll ever need to know about load sensing hydraulic systems, including: fixed pump load sensing, open center load sensing (a.k.a. tank-line load sensing or negative flow control), closed center load sensing, Komatsu’s OCLS system, hybrid load sensing, load-signal bleed-down, load sensing directional control valves, LS inlet sections, differential pressure limiters, pre and post compensation, anti-saturation valves, load-signal amplifier valves and much more.

This training is delivered entirely ONLINE. It combines text, diagrams and simulation videos, which are accessed via a dedicated, password-protected web site. This means it’s accessible instantly, and available 24/7 via your smartphone, tablet, notebook or desktop computer.

There is a small investment required to access this training. But if in the unlikely event you judge the knowledge it contains is not worth the modest flag-fare involved, just say the word and I’ll cheerfully buy it back off you. No questions asked.

As you can see, if you’d like to know a LOT more about load sensing systems than the average bear, you’ve got nothing to lose and a big chunk of hydraulics expertise to gain. So to make sure you’re fully equipped to tackle problems like the one described above, get in on it here today.

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