Hydraulic systems are different animals. They require special care and feeding.
By Ray Thibault, CLS, OMA I, OMA II, MLT, MLT II, MLA II, MLA III, Contributing Editor
This article continues our ongoing series on the important components of lubrication certification examinations administered by the Society of Tribologists and Lubrication Engineers (STLE) and the International Council for Machinery Lubrication (ICML). Please refer to this article for more information on STLE and ICML certifications.
Hydraulics is defined as the transmittance of force from one point to another using the fluid as the transmitter. The advantages of hydraulics are:
- High power-to-size ratio
- Ability to achieve high variable speeds and forces
- Forces can be transmitted over long distances
- Allows large loads to be moved by small forces
- Instantly reversible
- High level of flexibility and simplicity
- High level of accuracy and control
- Reliable and maintainable
|Fig. 1. Pascal’s Law is the basic
principle governing hydraulics.
Pascal’s Law (as illustrated in Fig. 1) is the basic principle that governs hydraulics. Pascal’s Law notes that the pressure applied to a confined fluid is transmitted equally and undiminished in all directions throughout the confined vessel. For example, in Fig. 1, if 10 lbs. of force is applied on a 1-in2 piston face, the pressure exerted by the fluid is 10 psi in all directions at right angles to the surface of the confined vessel. Since force equals pressure times area, the 10-lb. force applied can generate a 100-lb. force on the bottom of the container.
Looking at a simple hydraulic system
Figure 2 depicts a simple hydraulic system. Fluid stored in a reservoir flows through the system with the aid of a pump. A valve controls the flow in the Fig. 2 system.
To prevent the type of system over-pressurization that can lead to safety issues and equipment damage, a pressure-relief valve is installed. It’s designed to open at a preset pressure and divert the fluid back to the reservoir. The pressurized fluid moves through the system and comes in contact with the linear actuator—which can be a piston in a cylinder or a hydraulic motor. The pressure of the fluid in contact with the face of the piston creates the force that moves the load. For example, in Fig. 2, a fluid pressure of 1000 psi in contact with a 5-in2 piston face can move a 5000-lb. load.
|Fig. 2. A simple hydraulic
system (click to enlarge)
The direction of high-pressure fluid flow is controlled by directional control valves—which are among the most critical components in a hydraulic system. As shown in Fig. 2, pressurized fluid can come in contact with each face of the piston to move the load in either direction. (There are many types of directional control valves; they’ll be discussed later.)
Most hydraulic systems incorporate the following components:
- A reservoir usually sized two to three times the pump fluid flow
- Fluid conductors (i.e., piping, tubing, hoses and fittings) to transport fluid through the system
- Hydraulic fluid
- A pump
- A filtration system to keep the fluid clean for the tight clearances in the directional control valves
- A pressure-relief valve
- Directional control valves to control direction of high-pressure fluid flow in the actuator (some can also control amount of flow)
- Flow-control valves to control actuator speed by controlling fluid flow
The heart of the hydraulic system is the pump. There is a popular misconception that pumps provide pressure. Pumps, however, provide flow—the resistance to that flow generates the pressure. Hydraulic-system pumps are generally positive displacement (PD) types:
- Fixed displacement
- Bent axis
- Variable displacement
- Bent axis
|Fig. 3. Vane pumps operate at a
maximum pressure of 3000 psi.
Tight clearances, however, make
these popular hydraulic system
pumps susceptible to particulate
The gear type—which operates at low pressures—is the simplest and one of the most common pumps in hydraulic systems. Compact, economical and low-maintenance, gear pumps have a high tolerance for particulate contamination.
The vane (illustrated in Fig. 3) and axial piston (Fig. 4) pump types are also commonly used in hydraulic systems.
Vane pumps can be either fixed or variable displacement (wherein fluid flow can be adjusted by adjusting the cam ring). These pumps are quiet and operate at a maximum pressure of 3000 psi. Based on their low cost and compact size, they’re among the most popular pumps for hydraulic systems. Tight clearances, though, make these units susceptible to particulate contamination. Because of the contact between the vanes and cam ring during boundary lubrication, an anti-wear hydraulic fluid is required.
There are two basic designs for axial piston pumps: inline and bent axis. Figure 4 is the inline type—compact and economical, but also susceptible to fluid particulate contamination. Pressures for inline axial piston pumps are between 3000 and 6000 psi. The bent axis axial piston pump is both fixed and variable displacement. The most efficient of all hydraulic-system pump types, these high-pressure units also operate between 3000-6000 psi. The highest-pressure hydraulic-system pump is the radial piston—a fixed-displacement design. These units can achieve pressures up to 9000 psi.
According to Eaton, the major failure modes for hydraulic system pumps are:
- Contamination (80%)
- Aeration & Cavitation (10-12%)
- Over Pressurization (5-6%), and
- Other (2-5%).
Fluid cleanliness of hydraulic fluids is critical to good performance both for pumps and valves.
|Fig. 4. This inline axial piston pump
operates between 3000 and 6000
psi. These compact, economical
units, though, are also susceptible
to fluid particulate.
There are three major control valves associated with hydraulic systems: flow, pressure-control and directional. The various types include:
- Pressure-relief is normally set 10-15% above normal system pressure.
- Unloading allows flow from a larger pump to be diverted to the reservoir when smaller volume pump is providing flow to the system.
- Sequence keeps actuator from moving before another moves.
- Counterbalance maintains control of loads subject to move by gravity.
- Pressure-reducing regulates pressure in branch circuit while maintaining higher pressure in remainder of circuit.
- Brake regulates flow when hydraulic motor is stopped.
- Directional-control valves
- Check valves permit flow in one direction.
- Solenoid (bang bang) valves permit flow in two directions and are activated by an electrical control magnet.
- Proportional valves allow movement in an infinite number of positions through the use of a solenoid.
- Servo valves (Fig. 5) allow movement in an infinite number of positions where an electrical sensor provides control. Servo valves, which also control flow, have the tightest clearances at 1-4 microns. This design provides the most precise control of any valve type. Oil cleanliness is critical in maintaining good operation of these components. (A minimum fluid cleanliness of 16/13/11 is required with servo valves, depending on pressure.)
|Fig. 5. Servo valves provide
the most precise control
of any valve type.
The major functions of a hydraulic fluid are power transmission, followed by lubrication of the pump. Other secondary functions include sealing, cooling and contaminant removal. Hydraulic fluids are classified into the following types:
- Mineral oils
- High water-based fluids typically are 95% water and 5% oil.
- Invert emulsions are typically 60% oil and 40% water.
- Water glycol fluids are typically 40-50% water.
- Phosphate esters are used as aviation and turbine hydraulic fluids.
- Polyol esters are used in many different industrial applications.
- Polyalkylene glycols
- Biodegradable fluids
- Vegetable oils
- Polyol esters
The major properties desired in a hydraulic fluid are:
- The ability to meet or exceed OEM specifications
- Minimum Viscosity Index (VI) of 95
- Good thermal and oxidative stability
- Good anti-wear protection for pumps
- Good water separability
- Low foaming
- Good air release
- Rust and corrosion protection
- Compatibility with seals
- High filterability
- Good shear stability
Given the fact that viscosity is the most important property of a lubricant, selection of the correct viscosity is key for proper pump operation in a hydraulic system. Typical viscosities for industrial applications are ISO 32, 46 and 68. The pump OEM will recommend the viscosity required. The viscosity has a minimum and maximum operating range based on operating temperature. Optimum temperature to run a hydraulic system is 110-140 F. The maximum viscosity allowed for pump startup also must be considered. As an example, Eaton has published the requirements in Table I for gear, vane and axial pumps in industrial applications:
Troubleshooting hydraulic systems
Table II lists basic troubleshooting tips for keeping a hydraulic system running optimally. Any problems should be identified as early as possible and corrected before permanent damage or safety issues are encountered.
The basic information presented here can be useful in pursuing lubrication certification through STLE and ICML. Bear in mind, though, it’s just that—very basic. For those considering certification, deeper knowledge of fluid power is required to pass an STLE or ICML exam.
Part V of this series (in the November/December issue) will address pneumatic systems and compressor lubrication. LMT
Ray Thibault is based in Cypress (Houston), TX. An STLE-Certified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training for operations around the world. Telephone: (281) 257-1526; email: firstname.lastname@example.org.
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