Selection of Hydraulic Fluid
Many petroleum and synthetic fluids are available and more are being formulated. The highly technical formulations of the fluids with their various pros and cons makes the selection of such fluids difficult for those who are not thoroughly acquainted with the latest improvements and new formulations.
Generally, hydraulic fluids are chosen based on considerations of the environment of the application and chemical properties of the fluid. Physical properties such as viscosity, density, and bulk modulus are not usually basic considerations. Viscosity is very important, but usually a variety of viscosity characteristics are available in each fluid type. Bulk modulus should be large, but this requirement usually yields to the high temperature capability of the fluid. For example, the low bulk modulus of silicone fluids is more than offset by their high temperature range.
A basic judgment in fluid selection is required concerning the Are and explosion hazard posed by the application. If the environment and high temperature limit of the application are within the range of petroleum base fluids, then any number of suitable oils are available from numerous manufacturers. If the application requires a fire-resistant fluid, a choice must be made between the chemically compounded and water base synthetics. Factors to be considered are temperature range, cost, lubricity, compatibility, chemical, and handling characteristics of the fluid. Once a fluid type is selected, a number of viscosity and viscosity-temperature characteristics are usually made available, and a suitable matching must be made to the requirements of the system hardware. Consultation with representatives of hardware and fluid manufacturers is essential to ensure satisfactory compatibility and performance.
Synthetic Hydraulic Fluids
Synthetic fluids on the whole have excellent fire resistant properties. Many of these fluids may be used at high temperatures, and some are quite expensive. Such fluids are named after their base stocks, that is, the predominant material, and their formulations are chemically involved.
Phosphate ester base fluids are used in both aircraft and industrial applications. Their thermal stability is rather poor for sustained operation at temperatures in excess of 300°F, but their lubricity is excellent [4], These fluids are solvents for many types of paints and seals so that care must be used to ensure compatibility with system materials. Examples of commercially available fluids include Skydrol 500A, Pydraul F-9 and 150, Cellulube 220, Houghto-Safe 1000 series, and Nyvac 200.
Silicate ester base fluids have excellent thermal stability which permits their use as high temperature fluids, but they have poor hydrolytic stability. Commercial fluids include Monsanto OS-45 and Oronite 8515. The halogens of chlorine and fluorine are united with hydrocarbons to form fluid base stocks of chlorinated hydrocarbons and fiuorinated hydrocarbons.
Such fluids have high thermal and oxidative stability required for high temperature applications, but relatively high freezing points limit their use at low temperatures. Commercial chlorinated hydrocarbons include Aroclor 1000 series and Pydraul A-200.
Silicone base fluids have excellent viscosity-temperature characteristics but are limited by their lubricating ability. Examples of commercial silicone fluids are Dow Corning F-60 and Versilube F-50.
The water base fluids are fire resistant and compatible with standard seal materials but have poor lubricating ability. tVaier glycols are a formulation of water and a glycol, which thickens the fluid to increase viscosity,
with various additives to improve lubricity and corrosion resistance. Commercial water glycols include Ucon Hydrolube 100 series, Houghto- Safe 600 series, and Cellugard. Water-in-oil emulsions are formed by a stable suspension of water particles in a hydrocarbon oil. However, the water and oil does tend to separate and, if allowed to stand, agitation is required to maintain the dispersion of water in the oil. Checking the fluid while in use is desirable to ensure that the water content is at a satisfactory level. Commercial examples are Shell Irus 902, Sunsafe, and Houghto-Safe 5000 series fluids. Although their high temperature range is limited because of the water content, the water base fluids offer a satisfactory and economical
industrial hydraulic fluid when properly used.
Petroleum Base Fluids
Petroleum base oils are by far the most commonly used hydraulic fluid. Petroleum, a complex mixture of chiefly hydrocarbons, must be highly refined to produce a fluid with viscosity characteristics suitable for hydraulic
control systems. Such mineral, turbine, or light oils, as they are often called, have a long history of satisfactory performance as a working fluid. Nearly all petroleum suppliers oflTer a wide variety of hydrocarbon fluids, ranging from straight refined petroleum to high formulated fluids containing additives to inhibit rust and oxidation, reduce foaming, and increase viscosity index and lubricity. A wide range of viscosity and viscosity-temperature characteristics are available from numerous manufacturers and should be consulted for specific properties. Military Specification MIL-H-5606B is the standard military specification for petroleum base hydraulic fluids.
Disadvantages of Hydraulic Control
Although hydraulic controls ofl’er many distinct advantages, several disadvantages tend to limit their use. Major disadvantages are thie following:
1. Hydraulic power is not so readily available as that of electrical powe:r This is not a serious threat to mobile and airborne applications but moist certainly afi’ects stationary applications.
2. Small allowable tolerances results in high costs of hydraulic comi- ponents.
3. The hydraulic fluid imposes an upper temperature limit. Fire am
Advantages of Hydraulic Control
There are many unique features of hydraulic control compared to other types of control. These are fundamental and account for the wide use of hydraulic control. Some of the advantages are the following:
1. Heat generated by internal losses is a basic limitation of any machine. Lubricants deteriorate, mechanical parts seize, and insulation breaks down as temperature increases. Hydraulic components are superior to others in this respect since the fluid carries away the heat generated to a convenient heat exchanger. This feature permits smaller and lighter components. Hydraulic pumps and motors are currently available with horsepower to weight ratios greater than 2 hp/lb. Small compact systems are attractive in mobile and airborne installations.
2. The hydraulic fluid also acts as a lubricant and makes possible long component life.
3. There is no phenomenon in hydraulic components comparable to the saturation and losses in magnetic materials of electrical machines. The torque developed by an electric motor is proportional to current and is limited by magnetic saturation. The torque developed by hydraulic actuators (i.e., motors and pistons) is proportional to pressure difference and is limited only by safe stress levels. Therefore hydraulic actuators develop relatively large torques for comparatively small devices.
4. Electrical motors are basically a simple lag device from applied voltage to speed. Hydraulic actuators are basically a quadratic reson.amcc from flow to speed with a high natural frequency. Therefore hydrauliic actuators have a higher speed of response with fast starts, stops, and spee:d reversals possible. Torque to inertia ratios are large with resulting high acceleration capability. On the whole, higher loop gains and bandwudths are possible with hydraulic actuators in servo loops.
5. Hydraulic actuators may be operated under continuous, intermit tenit, reversing, and stalled conditions without damage. With relief valwe protection, hydraulic actuators may be used for dynamic breaking. Larg
Fail-Safe Circuits
Fail-safe circuits are those designed to prevent injury to the operator or damage to the equipment. In general, they prevent the system from accidentally falling on an operator and also prevent overloading of the system. In following sections we shall discuss two fail-safe circuits: One is protection from inadvertent cylinder extension and other is fail-safe overload protection.
1. Protection from inadvertent cylinder extension: Figure 1.13 shows a fail-safe circuit that is designed to prevent the cylinder from accidentally falling in the event when a hydraulic line ruptures or a person inadvertently operates the manual override on the pilot-actuated DCV when the pump is not working. To lower the cylinder, pilot pressure from the blank end of piston must pilot open the check valve to allow oil to return through the DCV to the tank. This happens when the push button is actuated to permit the pilot pressure actuation of DCV or when the DCV is directly manually actuated when the pump operates. The pilot-operated DCV allows free flow in the opposite direction to retract the cylinder when this DCV returns to its offset mode.
2. Fail-Safe System with Overload Protection: Figure 1.14 shows a fail-safe system that provides overload protection for system components. The DCV V1 is controlled by the push-button three-way valve V2. When the overload valve V3 is in its spring offset mode, it drains the pilot line of valve V1. If the cylinder experiences excessive resistance during the extension stroke, sequence valve V4 pilot-actuates overload valve V3. This drains the pilot line of valve V1 causing it to return to its spring offset mode. If a person then operates the push-button valve V2 nothing happens unless overload valve V3 is manually shifted into its blocked-port configuration. Thus, the system components are protected against excessive pressure due to an excessive cylinder load during its extension stroke.
Speed Control of a Hydraulic Motor
Figure 1.12 shows the speed control circuit of a hydraulic motor using a pressure-compensated FCV.The operation is as follows:
– In a spring-centered position of the tandem four-way valve, the motor is hydraulically blocked.
– When the valve is actuated to the left envelope, the motor rotates in one direction. Its speed can be varied by adjusting the throttle of the FCV. Thus, the speed can be infinitely varied and the excess oil goes through the PRV.
– When the valve is deactivated, the motor stops suddenly and becomes locked.
– When the right envelope is in operation, the motor turns in the opposite direction. The PRV provides overload protection if, for example, the motor experiences an excessive torque load.
Meter-In Versus Meter-Out Flow-Control Valve Systems
A meter-out flow control system is one in which the FCV is placed in the outlet line of the hydraulic cylinder. Thus, a meter-out flow control system controls the oil flow rate out of the cylinder.
Meter-in systems are used primarily when the external load opposes the direction of motion of the hydraulic cylinder.
When a load is pulled downward due to gravity, a meter-out system is preferred. If a meter-in system is used in this case, the load would drop by pulling the piston rod, even if the FCV is completely closed.
One drawback of a meter-out system is the excessive pressure build-up in the rod end of the cylinder while it is extending. In addition, an excessive pressure in the rod end results in a large pressure drop across the FCV. This produces an undesirable effect of a high heat generation rate with a resulting increase in oil temperature.
Speed Control of a Hydraulic Cylinder
The speed control of a hydraulic cylinder circuit can be done during the extension stroke using a flow-control valve (FCV). This is done on a meter-in circuit and meter-out circuit as shown in Fig. 1.11. Refer to Fig. 1.11(a). When the DCV is actuated, oil flows through the FCV to extend the cylinder. The extending speed of the cylinder depends on the FCV setting. When the DCV is deactivated, the cylinder retracts as oil from the cylinder passes through the check valve. Thus, the retraction speed of a cylinder is not controlled. Figure 1.11(b) shows meter-out circuit; when DCV is actuated, oil flows through the rod end to retract the cylinder.
Cylinders in Parallel
Figure 1.10 shows a hydraulic circuit in which two cylinders are arranged in parallel. When the two cylinders are identical, the loads on the cylinders are identical, and then extension and retraction are synchronized. If the loads are not identical, the cylinder with smaller load extends first. Thus, the two cylinders are not synchronized. Practically, no two cylinders are identical, because of packing(seals)friction differences. This prevents cylinder synchronization for this circuit.
