There are always some particles of dirt or metal or lint in any hydraulic system even after it has been cleaned and filtered. Sometimes the hydraulic equipment actually generates these particles due to moving parts and there are always many more small particles than there are large ones. The size and number of these particles limit the orifice size which can be used for fluid restrictors. If a small orifice clogs, it usually means failure of a complete system.
The use of the Lee Visco Jet enables you to use much larger passages and still obtain the same amount of restriction which your system demands. Since the minimum passageways in the Lee Visco Jet are larger than for an ordinary orifice, it also means that the hole size in protecting screens may be made proportionately larger. With both larger passageways and larger screen hole sizes, your system will tolerate a much higher level of contamination. Any particle which will pass through the screens, which we use to protect our Visco Jets, will also pass right through the Visco Jet without causing any blockage or build-up of fluid restriction. Now you can use the fluid resistance values you need without fear of system failure due to contamination.
Lohm Versus Hole Size
An example of this is shown by a Visco Jet having 0.015 inch diameter holes and flowing at the same Lohm rate (32,600 Lohms) as the Standard Lee Jet having 0.0048 inch hole size. Thus the same Lohm rate can be obtained with 3.1 times the hole diameter and over 9 times the hole area!
If you need a high Lohm rate Jet, but are concerned with the possibility of clogging, even when protected by safety screens, you can get the same resistance to flow (Lohms) by using a Visco Jet with holes FIVE TIMES AS LARGE!
The unique Lee Visco Jet principle can provide an increase in flow passage as great as 25 times that of a corresponding (same Lohm rate) single stage restrictor, with a corresponding reduction in velocity. Since erosion is a function of the square of the velocity of the liquid medium, erosion can be reduced to 1/625 of that of the simple restrictor. Actual life tests have indicated that under certain conditions, a Lee Visco Jet can be expected to function without an appreciable change in Lohm rating for up to 20 years of continuous operation.
Liquid flowing through any orifice will cavitate whenever its velocity causes the pressure in the throat of the orifice to drop below the vapor pressure of the flowing liquid. Even though there may be a high supply pressure and a high back pressure on an orifice, if the velocity is high enough there will be a subsequent lowering of the pressure in the throat of the orifice and the possibility of cavitation.
The effects of cavitation are unstable flow and erosion – both of which are undesirable. To prevent cavitation, sufficiently high throat pressure must be maintained, either by:
The Lee Company manufactures a wide variety of fluid restrictors constructed with orifices in series which greatly reduce the velocity and thereby permit the use of much lower back pressures.
Cascade Restrictor Orifice: