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Heat generation in Hydraulic System

Heat generation in Hydraulic System

Heat generation in Hydraulic System

We just built some hydraulic application which uses variable pump, fixed disp. motor, hydraulic brake, directional control valve etc.

In this application, there are two subsystem, one system to  brake the wheel, and another is to turn the wheel.

Both application are not work simultaneuosly, depend on the requirement.
The problem is, when we switch the application to braking mode, the system is very easy to get hot. But, if we decrease displacement pump size, our hydraulic motor cannot turn the wheel.

We already put the hydraulic cooler, but the problem still exist. We don't want to change hydraulic cooler to bigger size, because we have very limited space. Hydraulic temp. inside the tank can reach 60 deg. C when the system on braking mode.

Please help me on this problem.


RE: Heat generation in Hydraulic System

Hello RK...

          What is the wheel turning subsystem doing when the braking system is operating?

          The most common causes of heat generation in systems with variable pumps is the system relief valve being set lower than the pressure compensator on the pump.

          Basically, the full pump flow is being pushed over the relief valve. For reference, a relief valve set to 200 BAR will heat the oil by 11.4 deg.C.

          The oil gets very hot very quickly, a 10KW motor will put 10KW of heat into the oil. You would need a very big cooler to keep the temperature down.

          You should check all of the relief valves around the complete system to make sure they are set correctly.

          Having said all of that, 60 deg.C is not hot, not for a hydraulic system anyway.

          Do you know what your optimum temperature is?

          Can you give us some more details of how the systems actually function?

          This is typical of engineering, you ask 1 question, you get another 5 back....

Let us know how you get on...



RE: Heat generation in Hydraulic System

DO you mean "Varible Flow Pump" OR Presure Compensated Pump?

Bud Trinkel CFPE
fluidpower1 @ hotmail.com

RE: Heat generation in Hydraulic System

I will answer one by one,
1. when the brake operate, there is other system/application which pull some wire/cable from wheel of system. This braking system objective is to give some constant tension, which is set by hydraulic brake pressure. This constant tension made by some capstan mechanism.
The hydraulic motor itself is in idle condition, no pressure applied to its. This hyd. motor used when we want to pull some wire from other drum to the wheel of system.

2. The operating pressure is set at 1000 psi. It is set by pressure relief on pump and system relief valve. The optimum temperature is designed at 40 deg. C.

3.Correct Budt, the pump is pressure compensated pump. When i said to change displacement is to change the maximum displacement of pump.


RE: Heat generation in Hydraulic System


You cannot have the pump compensator and the system relief set to the same pressure (1000 PSI).

If they are too close together there is a chance that the system relief will open before the compensator can reduce the pumps displacement.

The system pressure will be regulated to 1000 PSI but the pump will be at full displacement pushing all of the oil through the relief valve.

If you are familiar with the pumps controls, you should try to reduce the compensator setting below the relief valve setting.

If the maximum working pressure is 1000 PSI then that is what the compensator should be set to. The relief valve should be set slightly higher, just to protect the system from pressure spikes.

The only other reason I can think of for the rapid heat generation is size of the control valves in relation to the pump displacement. If the valves are too small, the system can be inefficient and become hot.

Can you tell us the pumps diplacement and also the details of the control valves?




RE: Heat generation in Hydraulic System


Setting pressure for system relief valve is 1500 psi.

For the pump, we used Rexroth PV7-1X/25-30RE01MC0-16, max.displ. pump is 30 cm3/min; hyd motor Rexroth MP 80/610-F-202, displ. 80 cm3/min; Directional control valve, Atos DHZO-A-0530S5/PA-NPT/7, max. flow 70 l/min.


RE: Heat generation in Hydraulic System


Suggest you look at this site...


With the pump displacement, I assume you mean 30cc/rev rather than per min.

Again assuming a 4 pole motor (1500 RPM), the flow rate is approx 45 litres per minute.

At this flow rate, according to Atos, there is 25 BAR pressure drop accross the directional control valve.

The theoretical max flow rate for the directional valve may well be 70 LPM, but it is a proportional valve and they run with quite large pressure drops. Large pressure drops generate heat.

Another problem you may have is with the pump. It is a pressure compensated vane pump and they are know to run hot when they are in the compensated position.

I know from personal experience that variable vane pumps put lots of heat directly into the oil reservoir.

Your options are limited. You can increase the size of directional valve from CETOP 3(NG6) to CETOP 5(NG10). This will reduce the pressure drop, but it will also reduce the performance of the valve. Or you can replace the pump, either with a piston pump or with a vane pump with a 2 stage compensator. A two stage compensator will allow you to run the pump at a lower pressure until the high pressure is required. Alternativley you can resize the cooler.

Can I ask why the system cannot run at 60 deg's?

Keeping the temperature at 40 deg's is asking a lot.

Even the most efficient hydraulic systems, by there very nature, generate some heat. Unless provision is made to remove it, either through a cooler or dissipation through the reservoir walls, they all get 'hot'.

Let us know what you think...



RE: Heat generation in Hydraulic System


As hydromech states, usually in hydraulic systems heat is generated by excess flow over relief and/ or high leakage.

What is the directional valve rated flow? dow you have a pressure drop curve for this?

One other thing to check is the fluid viscosity at 40 deg. check this against the Rexroth chart for oil grade at optimum operating temp.


RE: Heat generation in Hydraulic System

The hydraulic characteristics shown on the Atos data sheet for this valve state the maximum flow rate for a 70 BAR delta P as 45 LPM.

The link on my previous reply will take you to the data sheet.

With a 70 BAR pressure drop all of the oil that goes into the directional control will come out 4 deg's (C) warmer.

At 45 LPM, a 70 BAR pressure drop will add 20 deg's of heat to 300 litres of oil in 33 minutes.

That's about 5.25 KW of energy. To neutralise that heat,a cooler of the same capacity will be required.


RE: Heat generation in Hydraulic System

Being a Sundstrand guy from way back, I have to ask why this application wasn't addressed with a closed loop system. Overall, a lot simpler using the pump/motor combination and PL adjustment to control tension. (?)

RE: Heat generation in Hydraulic System

In a braking application, the work done against the brake turns to heat directly.  You have to dissipate that heat somewhere.

One possibility would be to keep the pump in the system and reduce its displacement so that it runs faster than the synchrounous speed of the electric motor.  The motor will then act as an induction generator, feeding some of the braking energy back into the electric power supply.

Works unless this is the only (or biggest) load on an isolated generator.

If this system will operate continuously you need to calculate the steady-state temperature rise.  Do you have a fan on the oil cooler?

One good way to get rid of braking energy is the system used in one type of exercise bicycle: drive a fan.  The work done to accellerate the air stream provides some of the braking and the air can help carry awat the heat dissipated in the rest of the system.

Consider whether there might be a better way to maintain the needed tension.  Is the tension level actually achieved exactly what you want or is it conservatively high to allow for dynamic effects?  If the latter is true, active controls may allow lower tension setpoints and reduced braking energy transfer.

The energy converted to heat in your brake is being put into the system by the prime mover of whatever is pulling the cable off your drum.  Cost to operate the system and the heat you have to get rid of are directly related.

RE: Heat generation in Hydraulic System

Appreciate your valuable help on my problem.

We select variable vane pump with pressure compensated, because between subsystem/aplication has different requirement on flow rate and pressure.
At that time, we think once the pressure reach the setting, the flowrate will be zero. To minimize heat generated by pump, we built some hole through the reservoir. Besides we used small hydraulic air cooler.

Why we cannot accept 60 deg C hydraulic temp, it's mainly because seal on our brake cannot withstand until 60 deg. We had experience that brake is leak when the temp. of oil reach 60 deg. C.

May be have to resize again the directional valve with the bigger one to have smaller heat generation, as hydromech said.

Thanks again guys.

RE: Heat generation in Hydraulic System

Excuse me if this was covered above, but...when in the braking mode my understanding is that the pump goes to near zero flow. If that is so how is the oil getting circulated through the cooler? I would use the pump to continuously run oil through the cooler while in braking mode.

RE: Heat generation in Hydraulic System

Hi jbthiel,
Correct, the pump goes to near zero flow. But, it's depend on pressure inside system.

In braking mode, the application has some mechanism to decrease or increase operating pressure depend on required braking torque.

So, oil will always circulate through oil cooler.


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