Control of a Double-Acting Hydraulic Cylinder
The circuit diagram to control double-acting cylinder is shown in Fig. 1.2. The control of a double-acting hydraulic cylinder is described as follows:
1. When the 4/3 valve is in its neutral position (tandem design), the cylinder is hydraulically locked and the pump is unloaded back to the tank.
2. When the 4/3 valve is actuated into the flow path, the cylinder is extended against its load as oil flows from port P through port A. Oil in the rod end of the cylinder is free to flow back to the tank through the four-way valve from portB through portT.
3. When the 4/3 valve is actuated into the right-envelope configuration, the cylinder retracts as oil flows from port P through port B. Oil in the blank end is returned to the tank via the flow path from port A to port T.
At the ends of the stroke, there is no system demand for oil. Thus, the pump flow goes through the relief valve at its pressure level setting unless the four-way valve is deactivated.
Simple Pressure-Relief Valve
The most widely used type of pressure control valve is the pressure-relief valve because it is found in practically every hydraulic system. Schematic diagram of simple relief valve is shown in Fig. 1.1 and three-dimensional view is shown in Fig. 1.2. It is normally a closed valve whose function is to limit the pressure to a specified maximum value by diverting pump flow back to the tank. A poppet is held seated inside the valve by a heavy spring. When the system pressure reaches a high enough value, the poppet is forced off its seat. This permits flow through the outlet to the tank as long as this high pressure level is maintained. Note the external adjusting screw, which varies spring force and, thus, the pressure at which the valve begins to open (cracking pressure)(Fig. 1.3).
It should be noted that the poppet must open sufficiently to allow full pump flow. The pressure that exists at full pump flow can be substantially greater than cracking pressure. The pressure at full pump flow is the pressure level that is specified when referring to the pressure setting of the valve. It is the maximum pressure level permitted by the relief valve.
If the hydraulic system does not accept any flow, then all the pump flow must return to the tank via the relief valve. The pressure-relief valve provides protection against any overloads experienced by the actuators in the hydraulic system. Of course, a relief valve is not needed if a pressure-compensated vane pump is used. Obviously one important function of a pressure-relief valve is to limit the force or torque produced by hydraulic cylinders or motors.
The main advantage of direct-acting relief valves over pilot-operated relief valves is that they respond very rapidly to pressure buildup. Because there is only one moving part in a direct-acting relief valve, it can open rapidly, thus minimizing pressure spikes.
Telescopic Cylinder
A telescopic cylinder (shown in Fig. 1.6) is used when a long stroke length and a short retracted length are required. The telescopic cylinder extends in stages, each stage consisting of a sleeve that fits inside the previous stage. One application for this type of cylinder is raising a dump truck bed. Telescopic cylinders are available in both single-acting and double-acting models. They are more expensive than standard cylinders due to their more complex construction.
They generally consist of a nest of tubes and operate on the displacement principle. The tubes are supported by bearing rings, the innermost (rear) set of which have grooves or channels to allow fluid flow. The front bearing assembly on each section includes seals and wiper rings. Stop rings limit the movement of each section, thus preventing separation. When the cylinder extends, all the sections move together until the outer section is prevented from further extension by its stop ring. The remaining sections continue out-stroking until the second outermost section reaches the limit of its stroke;this process continues until all sections are extended, the innermost one being the last of all.
For a given input flow rate, the speed of operation increases in steps as each successive section reaches the end of its stroke. Similarly, for a specific pressure, the load-lifting capacity decreases for each successive section.
Double-Acting Cylinder with a Piston Rod on One Side
Figure 1.4 shows the operation of a double-acting cylinder with a piston rod on one side. To extend the cylinder, the pump flow is sent to the blank-end port as in Fig. 1.4(a). The fluid from the rod-end port returns to the reservoir. To retract the cylinder, the pump flow is sent to the rod-end port and the fluid from the blank-end port returns to the tank as in Fig.1.4(b).
Unloading Valves
Unloading valves are pressure-control devices that are used to dump excess fluid to the tank at little or no pressure. A common application is in high-low pump circuits where two pumps move an actuator at a high speed and low pressure.The circuit then shifts to a single pump providing a high pressure to perform work.
Another application is sending excess flow from the cap end of an oversize-rod cylinder to the tank as the cylinder retracts. This makes it possible to use a smaller, less-expensive directional control valve while keeping pressure drop low.
Flow Trough Orifice
Orifices are a basic means for the control of fluid power. Flow characteristics of orifices plays a major role in the design of many hydraulic control devices. An orifice is a sudden restriction of short length (ideally zero length for a sharp-edged orifice) in a flow passage and may have a fixed or variable area. Two types of flow regime exist (Fig. 3-10), depending on whether inertia or viscous forces dominate. The flow velocity through an orifice must increase above that in the upstream region to satisfy the law of continuity. At high Reynolds numbers, the pressure drop across the orifice is caused by the acceleration of the fluid particles from the upstream velocity to the higher jet velocity. At low Reynolds numbers, the pressure drop is caused by the internal shear forces resulting from fluid viscosity.
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