It seems like your hydraulic system is running rather warm, but you’re not sure if you need to add cooling capacity. From experience, you know that the piping on most hydraulic systems feels warm, or even hot to the touch, but how hot is TOO HOT?

How hot is too hot?

The main determining factor is the fluid, once again. Same as when we looked at the need for an oil heater, we need to look at the viscosity curve of the fluid. Below, we use the example of a VG32 fluid.

Maximum oil temperature at suction port of the pump for optimum efficiency and longevity1

Here we see that for a Rexroth piston pump, the optimum viscosity range is between 16 and 36 mm2/s. This is the optimum viscosity for pump efficiency and longevity. That means we get our biggest return on investment (ROI), if we operate within this range. This is the temperature of the fluid required at the suction port of the pump. Usually, this is similar to the temperature of the oil in the tank.

Maximum Oil Temperature

From the above graph, we can see that the maximum oil temperature we want is 58°C. We definitely want a cooler to maintain the oil below 58°C. In fact, I recommend to keep the fluid temperature at, or below 50°C. Why? Because oil leaks past the pistons of the pump. Leakage oil, just like oil going over a relief valve, heats up. The oil at the leakage port of a pump can be 20°C warmer than at the suction port.

Hot Oil Reduces Oil Life & Component Life

We also know that the hotter the oil, the lower is the life expectancy of the oil. According to Ing. R. van den Brink2, for every 10°C over 70°C, the service life of the fluid is cut in half. Therefore, if we keep the oil in the tank at 50°C, then we can be reasonably sure that we’re not going to exceed 70°C in the system. Thus, we’re maximizing our ROI on the oil, as well as the pump.

We also need to consider that leakage oil lubricates the bearings in the pump. The hotter the oil, the less lubrication these bearings get. Of course, this reduces the life of the pump.

Target Oil Temperature

I recommend to aim for an oil temperature that is right in the middle of that optimum viscosity range. That would be 25 mm/s2, which equates to a temperature of 47°C in our example. See below:

Ideal operating temperature for a VG32 mineral oil

At this temperature, we have a bit of room for fluctuations in ambient temperature, duty cycle, etc. Also, we can stay below that 70°C threshold in pump case, or even when oil goes over a relief valve.

Conclusion

Keeping the oil at the target temperature maximizes oil life and component life. Therefore, it maximizes the ROI of the hydraulic system and is worth some attention. If you cannot maintain the target temperature of the oil, then additional cooling capacity is in order to improve your ROI. Another option is to increase the viscosity grade of the oil, but keep in mind that operating over 70°C significantly reduces the life of your oil. Above that, you also need to consider the seals being used. Can they handle the heat?

Before Adding Cooling Capacity

Be sure to check your system first. Check if it’s working as designed. I talk about this in “How to improve the life of your existing system”. Ensure the existing cooler is clean and working correctly.

Your Turn

Do you have an interesting experience with oil temperature and coolers? Please share below. Also, for any questions or comments, please leave them in the comment box below.

References:

  1. Graph taken from Bosch Rexroth data sheet RE-A 92711/04.2017
  2. Author of Filtration Technology, first edition: 2020 by MK Publishing, Ede, The Netherlands