Hydraulic oil viscosity vs. temperature

                  Generally, the higher the oil temperature, the lower the viscosity. What is important – the viscosity characteristics in relation to the temperature change is very nonlinear

All oils, including hydraulic oils, have variable viscosities at different temperatures.

Therefore, it is important to design the filtration system to consider the operating conditions of the device, and for temperature ranges below and above the assumed thresholds, apply solutions to prevent system damage.

The following example of the popular hydraulic oil (available on the Polish market) shows a change in oil viscosity at the changing temperature of this oil.

At a temperature of 40 ° C, the oil of 32 has a viscosity of 32 cSt, respectively, the oil class 46 has a viscosity of 46 cSt at 40 ° C.


The kinematic viscosity of the example oil Shell Shell Tell S4 ME 46 is as follows:

Temperature Kinematic viscosity
0°C 450
40°C 46
100°C 7,7


As can be clearly seen from the characteristics presented, the viscosity span between 40 ° C and 100 ° C is much less than the span between 0 ° C and 40 ° C.

Lowering the temperature to -10 ° C causes the viscosity to exceed 1000cSt.


The temperature of 40 ° C is considered as the limit temperature necessary for the correct operation of the hydraulic system.



What does this mean in practice?

The filtration system can not work at full power until the minimum set operating temperature is reached.

At low temperatures of hydraulic oil, reduce the oil flow through the filter to prevent it from rupturing!

It has been accepted among filter manufacturers that the base conversion value is 30cSt or 32cSt. Initial calculations and calculations of flows and pressure drops are made for such values.



For a constant flow of hydraulic fluid through the filter, the pressure drop across the filter will be proportional to the change in viscosity of the selected hydraulic fluid.

ΔP = Q * K * (cSt_akt / cSt_base)


We accept the assumptions:

ΔP – calculated pressure drop on the filter

K – resistance coefficient, constant for the selected filter

Q – flow, we set a constant value

cSt_base – 32 cSt.


For our example oil we get patterns:

The temperature pattern
0 ° C ΔP = Q * K * (450/32)
40 ° C ΔP = Q * K * (46/32)

As you can see, lowering the oil temperature from 40 ° C to 0 ° C (eg trying to start a cold machine) means that when you try to maintain a constant flow / oil flow, the pressure on the filter would increase nearly 10 times!

Workaround methods.

Hydraulic oil heating before starting work      —->  To reduce the viscosity of the hydraulic oil, heat it. In mobile work machines, a                                                                                                              typical procedure coincides with starting an internal combustion engine that                                                                                                                 heats up the hydraulic oil at the same time. On stationary  machines, heaters                                                                                                                can be used.


By-pass in the hydraulic system     ——>  One of the methods of warming the oil is bypassing the hydraulic system, after the oil pump. At low oil temperatures the  by-pass is open and the oil circulates in a short loop, heating from heat sources, in particular due to friction on the wall. Of course, at this time the machine does not work, that is, it does not use a hydraulic system. After exceeding  the set by-pass temperature of the hydraulic system it closes and the oil begins to circulate in the full hydraulic system, supplying all the servo-motors.


By-pass on the filter     —–>         The standard method of circumventing the problem, in particular, when we expect relatively quickly to start work by the hydraulic  system, is to apply by-pass on each filter. By-pass on the filter always opens when either due to too high viscosity or eg due to clogging / filtering the filter the oil can not pass through the filter insert with the speed set by the pump.

The method of operation of such a by-pass is simple. By-pass is a spring set to the right pressure. For suction filters it is -0.2 ÷ -0.3 bar, for linear filters depending on the pressure in the system it is 2-6 bar, for return filters 1.25 ÷ 1.75 bar.

After exceeding the set by-pass differential pressure on the filter cartridge, the oil bypasses the filter insert.

In this case, the oil is not filtered, but under certain conditions, this functionality of the system is perceived as a better evil.