Hydraulic system uses different oil types for performing functions like lubrication, heat transfer, power transmission, and sealing. For achieving a properly working hydraulic system, the manufacturer or designer must need to consider various hydraulic fluid properties like viscosity, compressibility, demulsibility, fire resistance, etc.. Among all these, hydraulic oil viscosity is necessary and most considered property that will reduce machine wear and improves accuracy.
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We can discuss more on hydraulic oil viscosity here.
The viscosity of a fluid is the internal resistance to flow. Hydraulic oil viscosity is the key property that affects the performance of every hydraulic system. The viscosity of oil determines which oil to be selected for what application. The higher viscosity of fluids will result in increased friction, pressure drop, and heat generation. At the same time, lower viscosity will create internal leakages during higher temperature. So, it is important to choose a fluid having adequate viscosity. Otherwise, it will inversely affect the lifetime of that hydraulic system.
Hydraulic oil viscosity determines the selected fluids film strength and thickness in machines. A fluid with low viscosity will flow faster and that with higher viscosity will flow slowly. Hydraulic oil viscosity can be categorized in dynamic (absolute) viscosity or as kinematic viscosity. Dynamic viscosity is measured using onsite viscometers and kinematic viscosity is that measured and reported by oil analysis laboratories.
Also, Read: Understanding the Symptoms of Low Oil Pressure
What is hydraulic viscosity index?
It is the change in hydraulic oil viscosity with changes in temperature. Viscosity index is primarily classified into low(up to 35), medium(35-80), high(80-110) and very high(above 110). Low viscosity index indicates high viscosity changes with change in temperature and high viscosity index is just the reverse. In precise, we can say that viscosity will increase when cold and decreases when heat.
For viscosity index calculation, you can use the formula:
Here ‘L’ is the oil with viscosity index zero at 40°C, ‘H’ is the oil with a viscosity index 100 at 40°C, and ‘U’ is the kinematics oil viscosity at 40°C.
Applying the viscosity index formula helps you to choose the right hydraulic fluid for your application. The highly desirable hydraulic oil must have a viscosity index range more than 95. If a low viscosity index fluid is selected for an application requiring low-temperature, the fluid will freeze and resist free movement of fluid through the system.
The viscosity index of synthetic oils will be higher than that of mineral oil. Normally used mineral oils will have a viscosity index around 100.
Also, Read: Water in Hydraulic Oil
Importance of Hydraulic Oil Viscosity
Hydraulics have indoor and outdoor applications that work in extreme winter cold temperature to desert hot temperature. As we mentioned above, the temperature will affect the hydraulic oil viscosity. At lower temperature, the fluid will solidify and make it difficult to pump or flow. At higher temperature, the fluid will become thinner and reduces the lubricating power of fluids.
Low hydraulic oil viscosity creates a risk of internal leakages and will cause lower volumetric efficiency of pumps and motors. Likewise, higher viscosity of fluid will reduce mechanical efficiency and causes issues like high energy consumption, heat generation, cavitation, poor air release, and inadequate lubrication.
Too low or high hydraulic fluid viscosity can damage the hydraulic system components like pumps, valves, filters, etc.. High viscosity will resist the flow of fluid from the reservoir to the pump. Also, it will be difficult for the valves and filters to pass fluid through it. Similarly, low hydraulic oil viscosity will damage the sealing and cause wear to the metal parts due to direct metal-to-metal contact.
While selecting an oil for your hydraulic system, the primary factor you need to consider must be hydraulic oil viscosity. A fluid with adequate viscosity will provide maximum lifetime, performance and reduces maintenance.
Continuous operation of a hydraulic system for a longer period will reduce the hydraulic oil viscosity. To overcome this issue, viscosity index improvers are added to the fluid. VI improvers are polymeric additives that minimize the dependence between viscosity and temperature. During low temperature, the polymeric molecules contract and in high temperature, the molecules relaxes. Proper VI improvers will provide better fuel efficiency and durability to the system.