When temperature changes affect the viscosity of the hydraulic fluid, this effect usually appears dramatically in the time response of the hydraulic system. By using the Lee Visco Jet, which has a considerable amount of viscosity compensation built into the design, you will find that the time responses on hydraulic systems will remain more constant even though there are wide changes in the viscosity of the hydraulic fluids.
The curve below shows the viscosity compensation of the Lee Visco Jet while flowing on MIL-H-5606 hydraulic oil. The viscosity was changed by controlling the oil temperature. Test runs were conducted with both the standard Lee Jet and a sharp edge orifice for a reference. There was no appreciable difference between the standard Lee Jet and the sharp edge orifice. However, there was a dramatic difference in the flows obtained with the Lee Visco Jet. The pressure drop was held constant throughout the runs with a ΔP of 50 psi.
This compensation is obtained by two independent effects which tend to make the flow increase as the viscosity increases. The first effect is that of the back pressure on the spin slots. As previously mentioned, this varies as the square of the spin velocity; when the viscosity increase slows down the spin velocity, the back pressure tends to decrease, thereby permitting a higher flow through the spin slot into the spin chamber. The other effect occurs in the deceleration chamber. If the liquid is spinning at a high speed when it enters the deceleration chamber, energy is absorbed to bring this liquid to rest and subsequently to accelerate it out in the opposite direction. This shows up as a pressure drop. If, due to the increase in viscosity, the liquid is not spinning as fast when it enters the deceleration chamber, it then follows that it can be discharged with less pressure drop.
If a relatively flat flow vs. viscosity curve were desired, it can readily be obtained in the low viscosity ranges, that is up to 23 centistokes. Referring to the two curves below, one can see that if a standard Lee Jet and a Visco Jet were flowing in parallel, the two slopes would be cancelled out through this viscosity range and the total output would be reasonably constant for 1 centistoke through 23 centistokes. Again referring to the MIL-H-5606 oil, this would be from +72° to over 350°F.