Look for Hydraulic Oil that Allows Your Machine to Start Up at -20°C

As described on Day 1, we’ll use Rexroth’s data sheet RDE90245 to find an oil that has proven itself through rigorous testing. For this example, we’ll use the Shell oils, as shown below:

Fluid Categories

There are two fluid categories shown in the table above: HLP & HVLP. These are hydraulic oils based on mineral oil and classified according to DIN standard 51524. The equivalent ISO classification is provided in brackets. Here’s a very simple explanation of what these categories mean.

ISO 6743-4: 2015	DIN 51524
HH	H	Basic mineral oil. No corrosion inhibitors. For light duty systems2
HL	HL	With corrosion inhibitors and antioxidants for systems under heavier thermal load2
HM	HLP	Like HL but with extra anti-wear additives for systems under heavier mechanical load2
HV	HVLP	Like HLP but with viscosity enhancing additives, making viscosity less sensitive to temperature2

Let’s start with Shell Tellus S2MX, since it seems to be the more standard type of fluid. In the technical data sheet, we find the following viscosity data points:

When we plot these points onto the viscosity graph, we see that this is very similar to the generic VG32 curve, as shown below. Based on this graph taken from Bosch Rexroth data sheet re-a92714_2015-09, the minimum start-up temperature is -17°C and we can start running the machine at full speed at -2°C. That certainly doesn’t meet our requirement of -20°C start-up temperature.

HVLP Fluid for Wider Operating Range

Now let’s look at Shell Tellus S4VE data. This is the HVLP fluid with viscosity enhancing additives.

Shell has given us the kinematic viscosity in centistokes at 40° and 100°C. Instead of the normal data point at 0°C, they’ve given us this viscosity at -25°C as 1,570 centipoise. We know that cP = cSt x specific gravity. Since they’ve given us the density of 827 kg/m³, the specific gravity is 0.827. Then we have a viscosity of 1,570/0.827 = 1,898 cSt. These points have been plotted in the curve below.

We can see that the minimum start-up temperatures of this HVLP fluid are about 5°C lower compared to the HLP fluid. This does allow us to start up the machine at -20°C, but we need to run the machine slowly until the oil temperature reaches -6°C. If we can trust the operators to do this, then we can use this oil. The good part about using this oil is that it also has a fairly high maximum temperature. So we probably won’t have to change the oil in the summer. So you can see that these HVLP fluids do provide a wider operating range. However, if we can’t trust the operator to run the machine slowly until the oil warms up to -6°C, then we need to look for another oil.

Lower Viscosity Required – Start with a VG22 HLP Fluid

If you need a lower starting temperature, then you’ll need to go with a VG22 fluid, or even VG15. Here we have the graph of the Shell Tellus S2 MX 22 (an HLP fluid). It shows that the machine can be started at -25°C and can run full speed at -10°C. Again, if we can’t trust the operator, then we need to look for lower viscosity.

Next we’ll try an HVLP fluid S2 VX 22

As you can see below, this allows us to start the machine at -28°C and start running at full speed when the tank temperature is at -13°C. If we need to run at full speed at -20°C, then we need to check a VG15 oil.

Colder Still? Try Shell Tellus S2 VX 15

With this one, we can start the machine at -37°C and run full speed at -22°C. Finally, this oil meets the requirement, allowing us to operate the machine at full speed with oil at -20°C. However, now we really have to start watching the operating temperature. The optimum temperature range for this fluid is from 17° to 38°C. Also, when the ambient temperatures warm up, we’ll have to change the fluid to a higher viscosity grade.

Conclusion

If you want to make sure that your hydraulic pump doesn’t experience damage, or catastrophic failure at cold temperatures, you need to choose the correct viscosity grade of fluid. The above fluids are just examples. As you can see in Rexroth data sheet RDE90245, there are many options. However, as you can see in this data sheet, none of the fluid manufacturers has had their VG22, or VG15 fluids tested yet. So you’ll have to go on the oil manufacturer’s word and reputation.

Your Turn

If you have any questions, or comments, please leave them in the comment box below. If you’d like some assistance with selecting a fluid for your application, that’s a service I can provide for you.

References:

1. Bosch Rexroth data sheet RDE90245 dated 2021-11-08
2. Technical Instruction – Filtration Technology, Ing. R. van den Brink, MK Publishing, First Edition: 2020
3. Shell website for technical data sheets