Hydraulic Oil – Fluid Types ,Viscosity Selection and effect on System Performance in UAE

HYDRAULIC OIL  &  System Performance

Hydraulic Oil is medium through which power is transmitted in hydraulic machinery .Hydraulic Oil is used in equipments like excavators , cranes, forklifts ,lifts ,industrial machinery , power steering systems ,transmissions . Hydraulic Fluid has great impact on system performance. ISO International Organization for Standardization ( ISO )has set standards to distinguish between hydraulic oil based on properties like Viscosity , Operating Temperature Range .

Pump Efficiency is also dependent on Hydraulic Oil . Flow losses in pump depends on factors like viscosity of the fluid , pressure difference

VISCOSITY – TEMPERATURE CHART FOR HYDRAULIC OIL

Viscosity and its relation with temperature is of great importance .Viscosity changes with temperature shall be controlled . In the Viscosity- Temperature Chart  . Kinematic Viscosities at 40 C and 100C is measured using Ubbelohde Viscometer  which is widely used in other International Standards like ISO3104 , ISO3105 , ASTM D446 ,BS188,IP-71.

Type of Hydraulic Fluids

– Mineral Oil Based Hydraulic Fluids( HH / HL/HM Fluids )

-Fire Resistant Fluids

-Environmental Acceptable Hydraulic Fluid

Comparison between HH/ HL/ HM Hydraulic Fluids

Type HH hydraulic Fluids are made from refined mineral oils with no additives . They are used in simple operations like Manual pumps , jack systems  and low pressure applications . They are now not much used because of inability to operate at higher temperature and provides limited lubrication only. Type HL fluids are most commonly used as they have additives for antioxidants , rust inhibitors for providing extra  protection and is mainly used in piston pump applications . HM Fluid is a type of HL Fluid with  anti-wear properties  due to additional phosphorus , zinc and sulphur .

Viscosity Index

Higher the VI of oils , more stable the fluid is . If VI of fluid is very low , the viscosity of fluid becomes very high at low temperatures . During high working temperatures, if VI is low, the fluid becomes thin and wear occurs especially in pumps and motors. By adding VI improvers the anti-wear properties of the fluid can be improved. Standard to use test method is ASTM D2270 . Only  limitation of this methd  is that it can be used to calculate the viscosity index of the fluids where the kinematic viscosity is above 2 cSt at 100°C.

Bulk modulus (β) It is defined as the substance resistance to uniform compression or in simple terms it is also understood as  change in volume by change in pressure. The hydraulic fluids require low compressibility that means high bulk modulus which is useful for high pressure transmission and low power losses. The fluid with low bulk modulus acts as damping material for hydraulic system, but for poor bulk modulus the line sizes should be increased to compensate for the lower stiffness.[8] The SI unit of bulk modulus is Pascal (Pa) = N/m2

Effect of viscosity on System performance

System performance is measured in terms of efficiency namely – Mechanical efficiency and Volumetric efficiency . Performance of system comprising pumps and motors is best if fluid is selected such that viscosity of fluid is  enough to balance mechanical efficiency with volumetric efficiency.Mechanical efficiency relates to frictional losses .Volumetric efficiency relates to flow losses in the system. Both of these depend on the viscosity of the fluid. As shown in the below figure viscosity of fluid is directly proportional to volumetric efficiency and inversely proportional to mechanical efficiency of the system. So fluid should be selected satisfying both these efficiencies for the maximum overall efficiency of the system.

TOW Method ( Temperature Operating Window Method ) for viscosity selection

The most common method for selecting hydraulic fluid is Temperature Operating Window or TOW method. Based on experimental results majority of the pumps and motors provided satisfactory performance with a minimum of 13 cSt under operating conditions and maximum of 860 cSt during start –up of the system. The below fig is based on calculated temperature for the mid range ISO VG (between 13 cSt to 860 cSt) at 100 VI.