The power source is the key element in a fluid-power system. In a pneumatic system the power source is an air compressor, while in fluid-power systems it is a pump. These normally are driven by an electric motor or internal combustion engine. Various concepts are applied to convert the mechanical energy from a motor or engine to fluid energy in the system.
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Most systems can be made to work more efficiently when something is installed in the system to allow storage of temporarily unneeded fluid delivered from the pump or compressor. In hydraulic systems, the storage device, of course, is an accumulator; in pneumatic systems, it is a tank or receiver. However, most pneumatic systems are used with a receiver. The decision to use an accumulator in a hydraulic system is less clear-cut, but that decision can affect the choice and application of a hydraulic pump.
In some hydraulic systems, intensifiers or "boosters" replace pumps or compressors. They are most often used when one form of fluid energy is available, but a specific system must use another form. Most commonly, such need exists in a plant in which compressed air is readily available, and a hydraulic system is required for a specific small job. In that kind of task, an air-powered intensifier lets the pneumatic system power the hydraulic circuit, without requiring a new prime mover. Because these devices are directly competitive with pumps on some jobs, and complementary on others, they are included in this section.
Most hydraulic pumps receive fluid from a reservoir and pump it to a loaded actuator in such a fashion that the actuator can perform work. The pumps may deliver flows of less than one to as much as 600 gpm. They are capable of withstanding output pressures in the range of 500 to about 15,000 psi.
Pressure: Pump pressure rating is one of the major considerations in determining whether it can do the job. Nearly all hydraulic pumps work in rotary fashion. As a pump rotates, it develops a partial vacuum on the inlet ("suction") side, permitting fluid under atmospheric pressure in the reservoir to flow into the pump inlet. Then the pump ejects this fluid, usually at a pressure higher than atmospheric. It is worth noting that a pump does not create pressure. It merely moves fluid, causing the flow. Pressure is created by the load on the fluid; if no load exists, the fluid has very little pressure. As the load is placed on the fluid, the pressure at the outlet side of the pump increases to a value that is normally indicated as the pump maximum. Therefore, a 3,000-psi pump is a unit that can maintain flow against a load of 3,000 psi.
Pump pressure rating is generally limited by the capability of the pump to withstand pressure without undesirable increase in internal leakage, and without damage to the pump parts. Although many pumps can withstand pressures within the very wide range of 500 to 15,000 psi, ratings for maximum continuous service are often clustered in the 2,000 to 4,000-psi range. Typically, maximum pressure for external gear and vane pumps are from 2,000 to 4,000 psi. Internal-gear units run somewhat lower, with maximums in the range of 1,500 to 2,000 psi. Most piston pumps are designed for a maximum rating of 3,000 psi, although some are suitable for 5,000-psi service. A few permit higher pressures for intermittent peak loads
Flow: The second most important consideration in selecting a pump is its size and delivery. Size is usually expressed as volumetric flow output (gpm). Other words with the same meaning are flow, size, capacity, or delivery rate.
Flow rating of a pump is based on performance under a specific set of conditions. For example, pumps used in mobile applications are generally tested at 1,200 rpm, at an outlet pressure of 100 psi and atmospheric inlet pressure. Typically, manufacturer's literature states the conditions under which the rating is made.
Speed: A third consideration is the speed rating, which may be limited by the ability of the pump to fill without cavitating or by other mechanical considerations. The permissible speed range and inlet pressure requirements for any design are usually clearly defined.
- Volumetric efficiency is the ratio of actual to theoretical delivery. Difference between actual and theoretical delivery is normally due to internal leakage necessary to lubricate the pump (called "slippage") and other factors. Volumetric efficiency is typically very high, often in the mid to high 90s.
- Overall efficiency is the ratio of hydraulic power output to mechanical power input.
- Mechanical efficiency is the ratio of overall efficiency to volumetric efficiency. Mechanical losses are due principally to internal friction, and fluid compression.
Fluid compatibility: For years, petroleum oils have been the "standard" hydraulic fluid used in power circuits. This situation remains today, but is changing as safety considerations and government compliance agencies force greater acceptance of fire-resistant hydraulic fluids.
Most pumps used today were designed for petroleum fluids -- oil -- and nearly all worked well with them. When other fluids are used in these pumps, some suffer. Accordingly, a pump must be specially selected to operate with special fluids. Certain types of pumps do not work well with some hydraulic fluids.
Even when pump and fluid are basically compatible, pump seals must often be changed for compatibility with the other fluid. In addition, stability characteristics of fire-resistant fluids are often different than those of hydraulic oils. Different system operating temperatures and drain periods may be required for optimum performance. Maximum speed and pressure permitted may also be reduced, with pump life cut up to 50% with some fluids.
Wear of hydraulic pumps, motors, and valves is relatively low during full-film lubrication, but high during boundary lubrication. The simplest way to guard against excessive wear is to maintain reasonably high viscosity. Some manufacturers recommend that fluids have an adequate concentration of antiwear additives that can protect against wear, even during boundary lubrication.
While no all-encompassing standard for rating hydraulic fluid performance exists today, some component manufacturers and users have developed widely recognized specifications for fluids. The guidelines evolved to meet the needs of the particular manufacturer, and overcome deficiencies each saw when certain fluids were used in their equipment. Most premium hydraulic fluids used today meet the requirements of these rating systems. The design engineer, in general, should not be concerned with the exact nature of the tests, but merely that the fluid meets the recommendations for a particular type of pump and application.
Test data on "typical" pumps is readily available from most fluid suppliers, but there are some instances where field and bench experience do not correlate well. Subtle differences in pump design and conditions of use can drastically change wear rates. Size and weight: Straightforward comparison of size and weight characteristics by basic pump type is prevented by the overlap of individual designs. For instance, the axial-piston design that is widely used in industrial, marine, and aircraft applications can have many power/weight ratios, depending on the applications for which it is built. One common type of mobile pump has a ratio around 0.75 hp/lb; others may be 2.5 hp/lb. The additional expense of a highly refined piston pump capable of delivering 4 hp/lb is warranted for aircraft use, where every pound carries a double penalty. When reduced to miniature size for missile use (and when life is sacrificed for power), the same basic mechanism may deliver 8 hp/lb. Environment: Usually, effect of ambient temperature and altitude on performance is independent of the type of pump. Limits for satisfactory operation are established primarily by the effect of the environment on the fluid rather than by the type of pumping action. Humidity only affects requirements for the exterior casing.
When minimum or maximum temperatures are specified for a hydraulic system, the operating temperature of the fluid, not ambient temperature, is the critical factor. In most cases, it is possible to compensate for extremes of ambient temperature and to control fluid temperatures within a satisfactory range.
Minimum operating temperature is generally set by the increase in fluid viscosity as temperature falls. When fluid thickens to the point where inlet conditions can no longer keep the pump completely full, cavitation -- with possible pump damage -- occurs. Fire-resistant fluids have a higher specific gravity than petroleum oils, accompanied in some cases by higher viscosities at low temperatures. Many fire-resistant fluids contain water, which can vaporize if pressures are low or temperatures high. Thus, pump-inlet conditions are more sensitive when these fluids are used. High altitudes can produce a somewhat similar effect when the fluid reservoir is not pressurized. The usual solution is to supercharge the main pump with an auxiliary pump, or to flood the inlet by locating it below the fluid level in the reservoir.
Maximum allowable operating temperature depends on the properties of the fluid seals being used. Above allowable temperatures, many oils will be too thin to maintain proper lubrication at high-load points, and may progressively deteriorate as a result of oxidation. Under elevated temperatures, some seals may harden.