What is a Hydraulic System?
Hydraulic systems can be found today in a wide variety of applications, from small assembly processes to integrated steel and paper mill applications. Hydraulics enable the operator to accomplish significant work (lifting heavy loads, turning a shaft, drilling precision holes, etc.) with a minimum investment in mechanical linkage through the application of Pascal’s law, which states:

“Pressure applied to a confined fluid at any point is transmitted undiminished throughout the fluid in all directions and acts upon every part of the confining vessel at right angles to its interior surfaces and equally upon equal areas (Figure 1).”

Figure 1 - Pascal's Law

By applying Pascal’s law and Brahma’s application of it, it is evident that an input force of 100 pounds on 10 square inches will develop a pressure of 10 pounds per square inch throughout the confined vessel. This pressure will support a 1000-pound weight if the area of the weight is 100 square inches.

The principle of Pascal’s law is realized in a hydraulic system by the hydraulic fluid that is used to transmit the energy from one point to another. Because hydraulic fluid is nearly incompressible, it is able to transmit power instantaneously.

Hydraulic System Components
The major components that make up a hydraulic system are the reservoir, pump, valve(s) and actuator(s) (motor, cylinder, etc.).

Reservoir
The purpose of the hydraulic reservoir is to hold a volume of fluid, transfer heat from the system, allow solid contaminants to settle and facilitate the release of air and moisture from the fluid.

Pump
The hydraulic pump transmits mechanical energy into hydraulic energy. This is done by the movement of fluid which is the transmission medium. There are several types of hydraulic pumps including gear, vane and piston. All of these pumps have different subtypes intended for specific applications such as a bent-axis piston pump or a variable displacement vane pump. All hydraulic pumps work on the same principle, which is to displace fluid volume against a resistant load or pressure.

Valves
Hydraulic valves are used in a system to start, stop and direct fluid flow. Hydraulic valves are made up of poppets or spools and can be actuated by means of pneumatic, hydraulic, electrical, manual or mechanical means.

Actuators
Hydraulic actuators are the end result of Pascal’s law. This is where the hydraulic energy is converted back to mechanical energy. This can be done through use of a hydraulic cylinder which converts hydraulic energy into linear motion and work, or a hydraulic motor which converts hydraulic energy into rotary motion and work. As with hydraulic pumps, hydraulic cylinders and hydraulic motors have several different subtypes, each intended for specific design applications.

Key Lubricated Hydraulic Components
There are several components in a hydraulic system, that due to cost of repair or criticality of mission, are considered vital components. Pumps and valves are considered key components. Several different configurations for pumps must be treated individually from a lubrication perspective, including:

Vane Pumps
There are many variations of vane pumps available between manufacturers. They all work on similar design principles. A slotted rotor is coupled to the drive shaft and turns inside of a cam ring that is offset or eccentric to the drive shaft. Vanes are inserted into the rotor slots and follow the inner surface of the cam ring as the rotor turns.

The vanes and the inner surface of the cam rings are always in contact and are subject to high amounts of wear. As the two surfaces wear, the vanes come further out of their slot. Vane pumps deliver a steady flow at a high cost. Vane pumps operate at a normal viscosity range between 14 and 160 cSt at operating temperature. Vane pumps may not be suitable in critical high-pressure hydraulic systems where contamination and fluid quality are difficult to control. The performance of the fluid’s antiwear additive is generally very important with vane pumps.

Piston Pumps
As with all hydraulic pumps, piston pumps are available in fixed and variable displacement designs. Piston pumps are generally the most versatile and rugged pump type and offer a range of options for any type of system. Piston pumps can operate at pressures beyond 6000 psi, are highly efficient and produce comparatively little noise. Many designs of piston pumps also tend to resist wear better than other pump types. Piston pumps operate at a normal fluid viscosity range of 10 to 160 cSt.

Gear Pumps
There are two common types of gear pumps, internal and external. Each type has a variety of subtypes, but all of them develop flow by carrying fluid between the teeth of a meshing gear set. While generally less efficient than vane and piston pumps, gear pumps are often more tolerant of fluid contamination.

1. Internal gear pumps produce pressures up to 3000 to 3500 psi. These types of pumps offer a wide viscosity range up to 2200 cSt, depending on flow rate and are generally quiet. Internal gear pumps also have a high efficiency even at low fluid viscosity.

2. External gear pumps are common and can handle pressures up to 3000 to 3500 psi. These gear pumps offer an inexpensive, mid-pressure, mid-volume, fixed displacement delivery to a system. Viscosity ranges for these types of pumps are limited to less than 300 cSt.

Hydraulic Fluids
Today’s hydraulic fluids serve multiple purposes. The major function of a hydraulic fluid is to provide energy transmission through the system which enables work and motion to be accomplished. Hydraulic fluids are also responsible for lubrication, heat transfer and contamination control. When selecting a lubricant, consider the viscosity, seal compatibility, basestock and the additive package. Three common varieties of hydraulic fluids found on the market today are petroleum-based, water-based and synthetics.

1. Petroleum-based or mineral-based fluids are the most widely used fluids today. The properties of a mineral-based fluid depend on the additives used, the quality of the original crude oil and the refining process. Additives in a mineral-based fluid offer a range of specific performance characteristics. Common hydraulic fluid additives include rust and oxidation inhibitors (R&O), anticorrosion agents, demulsifiers, antiwear (AW) and extreme pressure (EP) agents, VI improvers and defoamants. Mineral-based fluids offer a low-cost, high quality, readily available selection.

2. Water-based fluids are used for fire-resistance due to their high-water content. They are available as oil-in-water emulsions, water-in-oil (invert) emulsions and water glycol blends. Water-based fluids can provide suitable lubrication characteristics but need to be monitored closely to avoid problems. Because water-based fluids are used in applications when fire resistance is needed, these systems and the atmosphere around the systems can be hot. Elevated temperatures cause the water in the fluids to evaporate, which causes the viscosity to rise. Occasionally, distilled water will have to be added to the system to correct the balance of the fluid. Whenever these fluids are used, several system components must be checked for compatibility, including pumps, filters, plumbing, fittings and seal materials. Water-based fluids can be more expensive than conventional petroleum-based fluids and have other disadvantages (for example, lower wear resistance) that must be weighed against the advantage of fire-resistance.

3. Synthetic fluids are man-made lubricants and many offer excellent lubrication characteristics in high-pressure and high- temperature systems. Some of the advantages of synthetic fluids may include fire-resistance (phosphate esters), lower friction, natural detergency (organic esters and ester-enhanced synthesized hydrocarbon fluids) and thermal stability. The disadvantage to these types of fluids is that they are usually more expensive than conventional fluids, they may be slightly toxic and require special disposal, and they are often not compatible with standard seal materials.

Fluid Properties
When choosing a hydraulic fluid, consider the following characteristics: viscosity, viscosity index, oxidation stability and wear resistance. These characteristics will determine how your fluid operates within your system. Fluid property testing is done in accordance with either American Society of Testing and Materials (ASTM) or other recognized standards organizations.

1. Viscosity (ASTM D445-97) is the measure of a fluid’s resistance to flow and shear. A fluid of higher viscosity will flow with higher resistance compared to a fluid with a low viscosity. Excessively high viscosity can contribute to high fluid temperature and greater energy consumption. Viscosity that is too high or too low can damage a system, and consequently, is the key factor when considering a hydraulic fluid.

2. Viscosity Index (ASTM D2270) is how the viscosity of a fluid changes with a change in temperature. A high VI fluid will maintain its viscosity over a broader temperature range than a low VI fluid of the same weight. High VI fluids are used where temperature extremes are expected. This is particularly important for hydraulic systems that operate outdoors.

3. Oxidation Stability (ASTM D2272 and others) is the fluid’s resistance to heat-induced degradation caused by a chemical reaction with oxygen. Oxidation greatly reduces the life of a fluid, leaving by-products such as sludge and varnish. Varnish interferes with valve functioning and can restrict flow passageways.

4. Wear Resistance (ASTM D2266 and others) is the lubricant’s ability to reduce the wear rate in frictional boundary contacts. This is achieved when the fluid forms a protective film on metal surfaces to prevent abrasion, scuffing and contact fatigue on component surfaces.

Ten Steps to Check Optimum Viscosity Range
When selecting lubricants, ensure that the lubricant performs efficiently at the operating parameters of the system pump or motor. It is useful to have a defined procedure to follow through the process. Consider a simple system with a fixed-displacement gear pump that drives a cylinder (Figure 2).

1. Collect all relevant data for the pump. This includes collecting all the design limitations and optimum operating characteristics from the manufacturer. What you are looking for is the optimum operating viscosity range for the pump in question. Minimum viscosity is 13 cSt, maximum viscosity is 54 cSt, and optimum viscosity is 23 cSt.

2. Check the actual operating temperature conditions of the pump during normal operation. This step is extremely important because it gives a reference point for comparing different fluids during operation. Pump normally operates at 92ºC.

3. Collect the temperature-viscosity characteristics of the lubricant in use. The ISO viscosity rating system (cSt at 40ºC and 100ºC) is recommended. Viscosity is 32 cSt at 40ºC and 5.1 cSt at 100ºC.

4. Obtain an ASTM D341 standard viscosity-temperature chart for liquid petroleum products. This chart is quite common and can be found in most industrial lubricant product guides (Figure 3) or from lubricant suppliers.

5. Using the viscosity characteristics of the lubricant found in Step 3, start at the temperature axis (x-axis) of the chart and scroll along until you find the 40-degree C line. At the 40-degree C line, track upward until you find the line corresponding to the viscosity of your lubricant at 40ºC as published by your lubricant manufacturer. When you find the corresponding line, make a small mark at the intersection of the two lines (red lines, Figure 5).

6. Repeat Step 5 for the lubricant properties at 100ºC and mark the intersection point (dark blue line, Figure 5).

7. Connect the marks by drawing a line through them with a straight edge (yellow line, Figure 5). This line represents the lubricant’s viscosity at a range of temperatures.

8. Using the manufacturer’s data for the pump’s optimum operating viscosity, find the value on the vertical viscosity axis of the chart. Draw a horizontal line across the page until it hits the yellow viscosity vs. temperature line of the lubricant. Now draw a vertical line (green line, Figure 5) to the bottom of the chart from the yellow viscosity vs. temperature line where it is intersected by the horizontal optimum viscosity line. Where this line crosses, the temperature axis is the optimum operating temperature of the pump for this specific lubricant (69ºC).

9. Repeat Step 8 for maximum continuous and minimum continuous viscosities of the pump (brown lines, Figure 5). The area between the minimum and maximum temperatures is the minimum and maximum allowable operating temperature of the pump for the selected lubricant product.

10. Find the normal operating temperature of the pump on the chart using the heat gun scan done in Step 2. If the value is within the minimum and maximum temperatures as outlined on the chart, the fluid is suitable for use in the system. If it is not, you must change the fluid to a higher or lower viscosity grade accordingly. As shown in the chart, the normal operating conditions of the pump are out of the suitable range (brown area, Figure 5) for our particular lubricant and will have to be changed.

Consolidating Hydraulic Fluids
The purpose of hydraulic fluid consolidation is to reduce complexity and inventory. Caution must be observed to consider all of the critical fluid characteristics required for each system. Therefore, fluid consolidation needs to start at the system level. Consider the following when consolidating fluids:

• Determine the specific requirements of each piece of equipment. Consider all the normal operating limits of your equipment.

• Talk to your preferred lubricant representative. You can gather and relay important information about the lubrication needs of your equipment. This will ensure that your supplier has all the products you require. Don’t sacrifice system requirements to achieve consolidation.

Also, observe the following hydraulic fluid management practices.

• Implement a procedure for labeling all incoming lubricants and tagging all reservoirs. This will minimize cross-contamination and assure that critical performance requirements are met.

• Use a First-In-First-Out (FIFO) method in your lubricant storage facility. A properly executed FIFO system reduces confusion and storage-induced lubricant failure.

Hydraulic systems are complicated fluid-based systems for transferring energy and converting that energy into useful work. Successful hydraulic operations require the careful selection of hydraulic fluids that meet the system demands. Viscosity selection is central to a correct fluid selection. There are other important parameters to consider as well, including viscosity index, wear resistance and oxidation resistance. Fluids can often be consolidated to reduce complexity and material storage cost. Caution should be exercised to avoid sacrificing fluid performance in an effort to achieve fluid consolidation.